What are the effect of changing climate on weather and human activities?

Contributed by:
It is now widely recognized that human activities are transforming the global environment. In the time it has taken for this book to come to fruition and be published, the evidence for climate change and its disruption of societal activities has become stronger.
Kevin E. Trenberth, Kathleen Miller,
Linda Mearns and Steven Rhodes
Kevin E. Trenberth, Kathleen Miller,
Linda Mearns and Steven Rhodes
National Center for Atmospheric Research
Boulder, Colorado
4. University Science Books
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Managing editor: Lucy Warner
Editor: Carol Rasmussen
Art and design: NCAR Image and Design Services
Cover design and composition: Craig Malone
Cover photo by Mickey Glantz
The cover photograph of the effects of drought on a farm in eastern Colorado in 1977 is
prototypical of scenes in the 1930s during the “dust bowl” era. The risk of such droughts
with global warming increases owing to increased drying of the landscape.
This book is printed on acid-free paper.
Copyright © 2000 by University Corporation for Atmospheric Research.
All rights reserved
Reproduction or translation of any part of this work beyond that
permitted by Section 107 or 108 of the 1976 United States Copyright Act
without the permission of the copyright owner is unlawful. Requests for
permission or further information should be addressed to UCAR
Communications, Box 3000, Boulder, CO 80307-3000.
Library of Congress Cataloging-in-Publication Data
Effects of changing climate on weather and human activities / Kevin Trenberth ... [et al.].
p. cm. – (The global change instruction program)
Includes bibliographical references and index.
ISBN 1-891389-14-9 (softcover : alk. paper)
1. Climatic changes. 2. Weather. 3. Human beings–Effect of climate on. I. Trenberth,
Kevin E. II. Series.
QC981.8.C5 E44 2000
Printed in the United State of America
10 9 8 7 6 5 4 3 2 1
5. A Note on the Global Change Instruction Program
This series has been designed by college professors to fill an urgent need
for interdisciplinary materials on global change. These materials are
aimed at undergraduate students not majoring in science. The modular
materials can be integrated into a number of existing courses—in earth
sciences, biology, physics, astronomy, chemistry, meteorology, and the
social sciences. They are written to capture the interest of the student who
has little grounding in math and the technical aspects of science but
whose intellectual curiosity is piqued by concern for the environment.
For a complete list of modules available in the Global Change Instruc-
tion Program, contact University Science Books, Sausalito, California,
univsci[email protected]. Information is also available on the World Wide Web
at http://www/uscibooks.com/globdir.htm or
6. Preface ix
Introduction 1
I. Climate 4
The Climate System 4
The Driving Forces of Climate 6
The Spatial Structure of Climate 7
II. The Weather Machine 9
III. Climate Change 14
Human-Caused Climate Change 14
The Enhanced Greenhouse Effect 14
Effects of Aerosols 15
The Climate Response and Feedbacks 15
IV. Observed Weather and Climate Change 17
Observed Climate Variations 17
Interannual Variability 19
V. Prediction and Modeling of Climate Change 21
Climate Models 21
Climate Predictions 21
Interpretation of Climate Change in Terms of Weather 23
VI. Impacts of Weather and Climate Changes on Human Activities 25
Weather Sequences 25
Location, Location, Location 25
Severe Weather Events 26
Societal Responses 26
Managing Risk 27
Impacts on Agriculture 27
Planning for Local Weather Changes 31
VII. The Need for More Research 33
Glossary 34
Suggested Readings 38
Discussion Questions 39
Index 40
7. It is now widely recognized that human activities are transforming the global
environment. In the time it has taken for this book to come to fruition and be
published, the evidence for climate change and its disruption of societal activi-
ties has become stronger. In the first 11 months of 1998, there were major floods
in China, Peru, and California, enormous damage from Hurricane Mitch in
Central America, record-breaking heat waves in Texas, and extensive drought
and fires in Indonesia; weather-related property losses were estimated at over
$89 billion, tens of thousands of lives were lost, and hundreds of thousands of
people were displaced. This greatly exceeds damage estimates for any other
year. The environment was ravaged in many parts of the globe. Many of these
losses were caused by weird weather associated with the biggest El Niño on
record in 1997–98, and they were probably exacerbated by global warming: the
human-induced climate change arising from increasing carbon dioxide and
other heat-trapping gasses in the atmosphere. The climate is changing, and
human activities are now part of the cause. But how does a climate change
manifest itself in day-to-day weather?
This book approaches the topic by explaining distinctions between weather
and climate and how the rich natural variety of weather phenomena can be sys-
tematically influenced by climate. Appreciating how the atmosphere, where the
weather occurs, interacts with the oceans, the land surface and its vegetation,
and land and sea ice within the climate system is a key to understanding how
influences external to this system can cause change. One of those influences is
the effect of human activities, especially those that change the atmospheric com-
position with long-lived greenhouse gases.
Climate fluctuates naturally on very long time scales (thousands of years),
and it is the rapidity of the projected changes that are a major source of concern.
The possible impacts of the projected changes and how society has responded
in the past and can in the future are also described. Everyone will be affected
one way or another. So this is an important topic, yet it is one about which a
certain amount of disinformation exists. Therefore it is as well to understand
the issues in climate change and how these may affect each and every one of us.
What we should do about the threats, given the uncertainties, is very much a
choice that depends upon values, such as how much we should be stewards for
the planet and its finite resources for the future generations. Many people favor
a precautionary principle, “better safe than sorry,” and err on the side of taking
actions to prevent a problem that might not be as bad as feared. This book helps
provide the knowledge and enlightenment desirable to ensure that the debate
about this can be a public one and carried out by people who are well informed.
Kevin E Trenberth
8. This instructional module has been produced by the Global
Change Instruction Program of the University Corporation
for Atmospheric Research, with support from the National
Science Foundation. Any opinions, findings, conclusions, or
recommendations expressed in this publication are those of
the authors and do not necessarily reflect the views of the
National Science Foundation.
This project was supported, in part, by the
National Science Foundation
Opinions expressed are those of the authors
and not necessarily those of the Foundation
9. Introduction
We experience weather every day in all its won- tion was wrong, major economic losses occurred
derful variety. Most of the time it is familiar, yet in both years and lives were disrupted.
it never repeats exactly. We also experience the These weather patterns and kinds of weather
changing seasons and associated changes in the constitute a short-term climate variation or fluc-
kinds of weather. In summer, fine sunny days tuation. If they repeat or persist over prolonged
are interrupted by outbreaks of thunderstorms, periods, then they become a climate change. For
which can be violent. Outside the tropics, as instance, in parts of the Sahara Desert we now
winter approaches the days get shorter, it gets expect hot and dry conditions, unsuitable for
colder, and the weather typically fluctuates from human habitation, where we know that civiliza-
warm, fine spells to cooler and snowy condi- tions once flourished thousands of years ago.
tions. These seasonal changes are the largest This is an example of a climate change.
changes we experience at any given location. How has the climate changed? What are the
Because they arise in a well-understood way factors contributing to climate and therefore to
from the regular orbit of the Earth around the possible change? How might climate change in
Sun, we expect them, we plan for them, and we the future? How does a change in climate alter
even look forward to them. We readily and will- the weather that we actually experience? How
ingly plan (and possibly adapt) summer swim- much certainty can we attach to any predic-
ming outings or winter ski trips. Farmers plan tions? What do we do in the absence of pre-
their crops and harvests around their expecta- dictability? Why are climate change and associ-
tion of the seasonal cycle. ated weather events important? What are the
By comparison with this cycle, variations in likely impacts on human endeavors and society
the average weather from one year to the next and on natural-resource-based economic activi-
are quite modest, as they are over decades or ties, such as agriculture? These are some of the
human lifetimes. Nevertheless, these variations questions we address in this module. Our dis-
can be very disruptive and expensive if we do cussion of impacts will focus on human activi-
not expect them and plan for them. For exam- ties. Although very important, the impacts of
ple, in summer in the central United States, the climate change on the natural environment and
major drought in 1988 and the extensive heavy the unmanaged biosphere are not dealt with
rainfalls and flooding in 1993 were at the here. Some of these consequences are discussed
extremes for summer weather in this region. (In further in the Global Change Instruction Pro-
the upper Mississippi Basin, rainfalls in May, gram module Biological Consequences of Global
June, and July changed from about 150 millime- Climate Change.
ters in 1988 to over 500 mm in 1993.) These two Many of these questions, although of con-
very different summers were the result of very siderable importance, unfortunately do not have
different weather patterns. We assumed, before simple answers. Also, many of the answers are
their occurrence, that the usual summertime not very satisfying. Because of the nature of the
mix of rain and sun would occur and that farm- phenomena involved, many outcomes can only
ers’ crops would flourish. Because this assump- be stated in a statistical or probabilistic way.
10. We first need to distinguish between weath- and their associated warm and cold fronts.
er and climate. An important concept to grasp is Tropical storms are organized, large-scale sys-
how weather patterns and the kinds of weather tems of intense low pressure that occur in low
that occur relate to climate. We refer to this rela- latitudes. If sufficiently intense these become
tionship as the “weather machine” because of hurricanes, which are also known as typhoons
the way the weather helps drive the climate sys- or tropical cyclones in other parts of the world.
tem. It is the sum of many weather phenomena Weather systems develop, evolve, mature, and
that determines how the large-scale general cir- decay over periods of days to weeks. From a
culation of the atmosphere (that is, the average satellite’s viewpoint, they appear as very large
three-dimensional structure of atmospheric eddies, similar to the turbulent eddies that
motion) actually works; and it is the circulation occur in streams and rivers, but on a much
that essentially defines climate. This intimate greater scale. Technically, they are indeed forms
link between weather and climate provides a of turbulence in the atmosphere. They occur in
basis for understanding how weather events great variety, but within certain bounds and
may change as the climate changes. over fairly short time frames.
There are many very different weather phe- Climate, on the other hand, can be thought
nomena that can take place under an unchang- of as the average or prevailing weather. The
ing climate, so a wide range of conditions word is used more generally to encompass not
occurs naturally. Consequently, even with a only the average, but also the range and
modest change in climate, many if not most of extremes of weather conditions, and where and
the same weather phenomena will still occur. how frequently various phenomena occur. Cli-
Because of this large overlap between the mate extends over a much longer period of time
weather events experienced before and after than weather and is usually specified for a cer-
some climate change, it may be difficult to per- tain geographical region. It has been said that
ceive such a change. Our perceptions are most climate is what we expect, but weather is what
likely to be colored not by the more common we get! Climate involves variations in which the
weather events but by extreme events. As cli- atmosphere is influenced by and interacts with
mate changes, the frequencies of different other parts of the climate system, the oceans, the
weather events, particularly extremes, will land surface, and ice cover. Climate can change
change. It is these changes in extreme condi- because of changes in any of these factors or if
tions that are most likely to be noticed. factors outside the Earth or beyond the climate
We normally (and correctly) think of the system force it to change.
fluctuations in the atmosphere from hour to The Earth’s climate has changed in the past
hour or day to day as weather. Weather is and is expected to change in the future. We will
described by such elements as temperature, air experience these changes through the day-to-
pressure, humidity, cloudiness, precipitation of day weather. It is natural to want to ascribe a
various kinds, and winds. Weather occurs as a cause to any perceived unusual weather, and
wide variety of phenomena ranging from small “climate change” is often espoused by the pop-
cumulus clouds to giant thunderstorms, from ular press as a possible cause. In some cases this
clear skies to extensive cloud decks, from gentle inference may be correct—but proving it to be
breezes to gales, from small wind gusts to torna- correct is exceedingly difficult. More often,
does, from frost to heat waves, and from snow extremes of weather occur simply as a manifes-
flurries to torrential rain. Many such phenome- tation of various interacting atmospheric
na occur as part of much larger-scale organized processes. In other words, extremes are general-
weather systems which consist, in middle lati- ly nothing more than examples of the tremen-
tudes, of cyclones (low pressure areas or sys- dous natural variability that characterizes the
tems) and anticyclones (high pressure systems), atmosphere.
These considerations make it essential to mean that the globe will warm everywhere at
understand and deal with the natural variability once. An example is the Northern Hemisphere
in the climate system. One way of thinking winter of December 1993 to February 1994. This
about the variability in the atmosphere is to con- winter was very cold and snowy, with many-
sider the inherent natural variability as being in more-than-normal winter storms in the
the realm of “weather,” while systematic northeastern part of the United States. How
changes in the atmosphere that can be linked to does this jibe with expectations of global warm-
a cause, such as interactions with the ocean or ing?
changes in atmospheric composition, are in the The pattern of exceptionally wintry weather
realm of climate. continued for several months, long enough to
For example, interactions between the atmo- heighten interest in its apparent climate implica-
sphere and the tropical Pacific Ocean result in tions. However, as part of this pattern, there
the phenomenon known as El Niño, which is were often mild and sunny conditions in the
responsible for disruptions in weather patterns western half of the United States and Canada,
all over the world. Technically, El Niño is a with above-average temperatures. Temperatures
warming of the eastern equatorial Pacific that were substantially above average in parts of
occurs every two to six years and lasts for sever- southeast Asia, northern Africa, the Mediter-
al seasons; it is a natural phenomenon and has ranean, and the Caribbean. The Northern Hemi-
occurred for thousands of years at least. It caus- sphere as a whole was 0.2°C above the average
es heavy rainfall along the western South Amer- for 1951 to 1980.
ican coast and southern part of the United Extensive regions of above and below nor-
States; drought or dry conditions in Australia, mal temperatures are the rule, not the exception,
Indonesia, southeastern Asia (including the even in the presence of overall warmer condi-
Indian subcontinent), parts of Africa, and north- tions. A bout of below-average temperatures
east Brazil and Colombia; and unusual weather regionally may not be inconsistent with global
patterns in other parts of the world. It can be warming, just as a bout of above-average tem-
thought of as a short-term climatic phenome- peratures may not indicate global warming.
non. In the following pages, a discussion is pre-
Other climate perturbations are more subtle sented of how the climate may change and the
and their effects on weather less obvious. reasons for possible changes. The primary rea-
Increases in heat-retaining gases called green- son for particular future climate change is the
house gases, the best known of which is carbon continuing influence of humans, especially
dioxide, are currently causing the climate to through changes in atmospheric composition
warm because of human activities. In this case, such as increases in greenhouse gases (notably
the climate change is very gradual and should carbon dioxide). We therefore pay particular
be noticeable only when the weather from one attention to these effects and attempt to trans-
decade is compared with that of another. Even late them into weather changes. A further issue
then, because of the background natural vari- is how these changes may in turn affect human
ability of the climate system, weather variations activities. Accordingly, we consider how possi-
specifically attributable to human influences ble changes in climate and weather affect vari-
may be extremely difficult to identify. ous economic sectors and human activities, and
While increasing greenhouse-gas concentra- we discuss some steps that can be taken to soft-
tions cause global-mean warming, this does not en the possible impacts.
12. I
The Climate System surface (or albedo). Water is a central element of
the climate system, and it appears in many
The Earth’s climate involves variations in a forms: snow cover, land ice (including glaciers
complex system in which the atmosphere inter- and the large ice sheets of Antarctica and Green-
acts with many other parts (Figure 1). The other land), rivers, lakes, and surface and subsurface
components of this climate system include the water.
oceans, sea ice, and the land and its features. Climate is also affected by forces outside
Important characteristics on land include vege- this system: radiation from the Sun, the Earth’s
tation, ecosystems, the total amount of living rotation, Sun-Earth geometry, and the Earth’s
matter (or biomass), and the reflectivity of the slowly changing orbit (Figure 2). Over long
Figure 1. Simplified schematic view of the components of the global climate system and their interactions. Components of the
climate system are indicated in bold type in boxes. Larger boxes at the top and bottom indicate the potential changes. Interac-
tions are shown by the arrows.
periods of time, the physical and chemical
makeup of the Earth’s surface also changes.
Continents drift, mountains develop and erode,
the ocean floor and its basins shift, and, in addi-
tion to water vapor changes, the composition of
the dry atmosphere also changes. These alter-
ations, in turn, change the climate.
Radiation is measured in Watts or per unit
area in Watts per meter squared (W/m2). Aver-
aged over day and night, as well as over all
parts of the world, the solar radiation received
at the top of the atmosphere is 342 W/m2 or 175
PetaWatts (175,000,000,000,000,000 Watts). For
comparison, a typical light bulb puts out 100
Watts, and a one-bar electric heater is 1,000
Atmospheric composition is fundamental to
the climate. Most of the atmosphere consists of
nitrogen and oxygen (99% of dry air). Sunlight
passes through these gases without being
absorbed or reflected, so the gases have no cli-
matic influence. The climate-relevant gases
reside in the remaining 1% of dry air, together
with water vapor. Some of these gases absorb a
portion of the radiation leaving the Earth’s sur-
face and re-emit it from much higher and colder
levels out to space. Such gases are known as
greenhouse gases, because they trap heat and
make the atmosphere substantially warmer than
it would otherwise be, somewhat analogous to
the effects of a greenhouse. This blanketing is
known as the natural greenhouse effect. The
main greenhouse gases are water vapor, which
varies in amount from about 0 to 2%; carbon Figure 2. Top: The Earth’s orbit around the Sun, illustrat-
dioxide, which is about 0.04% of the atmos- ing the seasons in both current times and 9,000 years ago.
phere; and some other minor gases present in Today the Earth is nearest the Sun in northern winter, and
the atmosphere in much smaller quantities. has an axial tilt of 23 1/2 degrees; in the past, the Earth was
The greatest changes in the composition of nearest the Sun in northern summer and tilted by 24
the atmosphere are entirely natural and involve degrees. Bottom: Changes in average Northern Hemisphere
water in various phases in the atmosphere: as solar radiation, in Watts per square meter, from 9,000 years
water vapor, clouds of liquid water and/or ice ago (ka) to the present over the annual cycle.
crystal clouds, and rain, snow, and hail. Other
constituents of the atmosphere and the oceans
can also change. A change in any of the climate
system components, whether it is initiated
inside or outside of the system, causes the
Earth’s climate to change.
14. The Driving Forces of Climate surface. (The fraction of solar radiation a planet
reflects back into space, and that therefore does
The source of energy that drives the climate not contribute to the planet’s warming, is called
is solar radiation (Figure 3). The Sun’s energy its albedo. So the albedo of the Earth is about
travels across space as electromagnetic radiation 31%.) Another 20% is absorbed by the atmos-
to the Earth and determines the energy avail- phere and clouds, leaving 49% to be absorbed by
able for climate. Infrared (or “thermal”) radia- the Earth’s surface.
tion, radio waves, visible light, and ultraviolet To balance the incoming energy, the planet
rays are all forms of electromagnetic radiation. and its atmosphere must radiate, on average,
The Earth’s atmosphere interferes with the the same amount of energy back to space (Fig-
incoming solar radiation (Figure 4, see page 7). ure 4). It does this by emitting infrared radia-
About 31% of the radiation is reflected away by tion. If the balance is upset in any way, for
the atmosphere itself, by clouds, and by the example, by a change in solar radiation, then
Heat Transport
Net Radiation
Night Day
90°S tgo on
in d i ati
g Lon Ra
Figure 3. The incoming solar radiation (right) illuminates only part of the Earth while the outgoing longwave radiation is
distributed more evenly. As the panel at left shows on an annual mean basis, the result is an excess (hatched) of absorbed
solar radiation over the outgoing longwave radiation in the tropics, while there is a deficit (stippled) at middle to high lati-
tudes. Thus there is a requirement for a poleward heat transport in each hemisphere (broad arrows, left) by the atmosphere
and the oceans. This radiation distribution results in warm conditions in the tropics but cold at high latitudes, and the tem-
perature contrast results in a broad band of westerlies in the extratropics of each hemisphere in which there is an embedded
jet stream (shown by the banded arrows) at about 10 km above the Earth’s surface. The flow of the jet stream over the differ-
ent underlying surfaces (ocean, land, mountains) produces planetary waves in the atmosphere and geographic spatial struc-
ture to climate.
the Earth either warms or cools until a new bal- act to cool the surface. While the two opposing
ance is achieved. (Solar radiation, the electro- effects almost cancel each other out, the net
magnetic spectrum, and the entire process of global effect of clouds in our current climate, as
energy transfer between Sun and Earth are dis- determined by space-based measurements, is to
cussed in greater detail in the GCIP module The cool the surface slightly relative to what would
Sun-Earth System.) Most of the radiation emitted occur in the absence of clouds. Consequently,
from the Earth’s surface does not escape imme- the bulk of the radiation that escapes to space is
diately into space because of the presence of the emitted either from the tops of clouds or by the
atmosphere and, in particular, because of the greenhouse gases, not from the Earth’s surface.
greenhouse gases and clouds in the atmosphere
that absorb and re-emit infrared radiation.
Clouds play a complicated role in the plan- The Spatial Structure of Climate
et’s energy balance. They absorb and emit ther-
mal radiation and have a blanketing effect simi- Some parts of the Earth’s surface receive
lar to that of the greenhouse gases. They also more radiation than others (Figure 3). The
reflect incoming sunlight back to space and thus tropics get the most, and actually gain more
Figure 4. The Earth’s radiation balance. The net incoming solar radiation of 342 W/m2 (top center) is partially reflected by
clouds and the atmosphere or by the Earth’s surface (a total of 107 W/m2, shown on the left-hand side of the figure). Of the
remainder, 168 W/m2 (49%) is absorbed by the surface. Some of that heat is returned to the atmosphere as sensible heating
(indicated by thermals, bottom center) and some as evapotranspiration that is realized as latent heat in precipitation. The
rest is radiated as thermal infrared radiation, and most of that is absorbed by the atmosphere and reemitted both up and
down, producing the greenhouse effect (bottom right). The radiation lost to space comes from three sources. Some of it is
emitted directly from the surface at certain wavelengths (40 W/m2); this region of the electromagnetic spectrum is called
the “atmospheric window.” Additional radiation is reflected to space from cloud tops (30 W/m2). The largest fraction (165
W/m2) comes from parts of the atmosphere that are much colder than the Earth’s surface.
16. energy than they lose to space. The midlatitudes equator, they are recognized as a vital part of
get less. The poles receive the least of all, emit- the weather machine.
ting more energy than they receive from the The continental land-ocean differences and
Sun. This imbalance sets up an equator-to-pole obstacles such as mountain ranges also play a
temperature difference or “gradient” that role by creating geographically anchored plane-
results, when coupled with the influence of the tary-scale waves in the westerlies (Figure 3).
Earth’s rotation, in a broad band of westerly These are the reasons why climate varies from,
winds in each hemisphere in the lower part of for instance, the west coast of the United States
the atmosphere. Embedded within these pre- to the east coast. These waves are only semiper-
vailing westerlies are the large-scale weather manent features of the climate system: they are
systems and winds from all directions (see Fig- evident in average conditions in any given year,
ure 6). These in turn, along with the ocean, act but may vary considerably in their locations
to transport heat poleward to offset the radia- and general character from year to year. Specifi-
tion imbalance (Figure 3). These weather sys- cally, changes in heating patterns can alter these
tems are the familiar events that we see every waves and cause substantial regions of both
day on television weather forecasts: eastward- above- and below-average temperatures in dif-
migrating cyclones and anticyclones (i.e., low- ferent places during any given season, such as
and high-pressure systems) and their associated the example given earlier for the winter of
cold and warm fronts. Because they carry warm 1993–94.
air toward the poles and cool air toward the
17. II
The Weather Machine
Weather phenomena such as sunshine, clouds of clouds, such as thunderstorm clouds, to form.
all sorts, precipitation (ranging from light driz- Note that the movement poleward of warm air
zle to rain to hail and snow), fog, lightning, and the movement equatorward of cold air usu-
wind, humidity, and hot and cold conditions ally go together as part of the same system
can all be part of much-larger-scale weather sys- because otherwise air would pile up in some
tems. The weather systems are cyclones (low- places, leaving holes elsewhere.
pressure systems) and anticyclones (high-pres- The process of warm air rising and cold air
sure systems) and the associated warm and cold sinking is pervasive in the atmosphere and is
fronts. Figure 5 gives a satellite image of a major also a vital part of the weather machine. Warm
storm system on the east coast of the United air is less dense than cold air and is thus natu-
States. Accompanying panels show the tempera- rally buoyant. As seen in Figure 4, warmth is
tures that delineate the cold front (see below) generally transferred from the surface to higher
and the sea-level pressure contours. It is sys- levels in the atmosphere, where the heat is
tems like these, and their associated weather eventually radiated to space. The process of
phenomena, that make up the weather machine. transferring heat upward is called convection. It
Weather systems exist in a broad band both gives rise to a vast array of weather phenomena,
separating and linking warm tropical and sub- depending on the geographic location, the time
tropical air and cold polar air. They not only of year, and the weather system in which the
divide these regions but also act as an efficient phenomena are embedded. Clouds that result
mechanism for carrying warmer air toward the from convection are called convective clouds.
poles and cold air toward the equator. Thus, in These range from small puffy cumulus clouds,
the Northern Hemisphere, southerlies (winds to multicelled cumulus that produce rain show-
from the south) are typically warm and norther- ers, up to large cumulonimbus clouds that may
lies (winds from the north) are cold. Within a produce severe thunderstorms.
weather system, the boundary of a region where Weather systems over the oceans have a
warm tropical or subtropical air advances pole- somewhat different character from those over
ward is necessarily a region of strong temperature land because of the abundant moisture over the
contrast. This boundary is called a warm front. As oceans which more readily allows clouds and
the warm air pushes cooler air aside, it tends also rain to form. Over land, storms are often more
to rise, because warm air is less dense. Because violent, in part because the land can heat and
the rising air also moves to regions of lower pres- cool much more rapidly than the ocean and also
sure it expands and cools, so that moisture con- because mountain ranges can create strong
denses and produces clouds and rain. winds and wind direction changes (called wind
The advancement equatorward of cold air shear) that can help facilitate the development
occurs similarly along a cold front, but in this of intense thunderstorms and even tornadoes.
case, the colder and therefore denser air pushes These conditions often occur in the United
under the somewhat warmer air in its path, States in spring to the east of the Rocky Moun-
forcing it to rise, often causing convective tains, where northward-moving air has an
Figure 5. Satellite imagery
of a major storm system on
the East Coast of the Unit-
ed States (Panel 3). Panel
1 shows the temperatures
that delineate the cold
front, and Panel 2 gives
the sea-level pressure con-
tours in millibars. In Panel
1, cold air over the United
States is pushing south and east, carried
by strong northwesterly winds. Panel 2
shows the low-pressure cyclone system
over the East Coast, which has a cold
front attached, indicating the leading
edge of the cold air. High pressures and
an anticyclone exist over the northern
Great Plains, accompanied by clear skies
(Panel 3). The cloud associated with the
cold front is also shown in Panel 3,
along with many other weather phenom-
ena typical in such cases, as marked on
the figure. From Gedzelman (1980).
abundant supply of moisture (a prerequisite for This aspect of atmospheric behavior is referred
cloud development) from the Gulf of Mexico. to as “nonlinear,” meaning that the relationships
Weather phenomena and the larger weather sys- are not strictly proportional. They cannot be
tems develop, evolve, mature, and decay largely charted by straight lines on a graph. The rela-
as turbulent instabilities in the flow of the tionships in nonlinear systems change in dispro-
atmosphere. Some of these instabilities arise portionate (and sometimes unpredictable) ways
from the equator-to-pole (i.e., horizontal) tem- in response to a simple change. A gust of wind
perature contrast (Figure 6). If, for some reason, may be part of a developing cloud that is
the contrast becomes too large, the situation
becomes unstable, and any disturbance can set
off the development of a weather system. Other
types of instability occur as a result of vertical
temperature gradients—often associated with
warm air rising and cold air sinking (convective
instability). These types of instability may be
related to the warming of the surface air from
below, or the pushing of warm and cold air
masses against one another as part of a weather
system developing. They may also occur as part COLD
of the cycle of night and day. Many other weath-
er phenomena arise from other instabilities or L
from breezes set up by interactions of the atmo-
sphere with complex surface topography. WARM
Weather phenomena and weather systems
mostly arise from tiny initial perturbations that
grow into major events. The atmosphere, like COLD
any other system, is averse to unstable situa-
tions. This is why many triggering mechanisms L
exist that will push the atmosphere back toward
a more stable state in which temperature con- WARM
trasts are removed. In general, therefore, once
the atmosphere has become unstable, some form
of atmospheric turbulence will take place and CLOUD
grow to alleviate the unstable state by mixing up
the atmosphere. It is not always possible to say
which initial disturbance in the atmosphere will L
grow, only that one will grow. There is, there-
fore, a large component of unpredictable behav- WARM
ior in the atmosphere, an unpredictability that is
exacerbated by and related to the underlying
random component of atmospheric motions. The Figure 6. Baroclinic instability is manifested as the develop-
processes giving rise to this randomness are now ment of a storm from a small perturbation in the Northern
referred to in mathematics as chaos. Because of Hemisphere with associated cold fronts (triangles) and
the above factors, weather cannot be accurately warm fronts (semicircles). The arrows indicate the direction
forecast beyond about ten days. of wind. The shading on the bottom panel indicates the
The processes and interactions in the atmo- extensive cloud cover and rain or snow region in the
sphere are also very involved and complicated. mature stage.
embedded in a big thunderstorm as part of a exact timing, location, and intensity of a single
cold front, which is attached to a low-pressure weather event more than ten days in advance,
system that is carried along by the overall west- because they are a part of the weather machine,
erly winds and the jet stream (an example is we should be able to predict the average statis-
given in Figure 5). All these phenomena interact tics, which we consider to be the climate. The
and their evolution depends somewhat on just statistics include not only averages but also
how the other features evolve. measures of variability and sequences as well as
Nevertheless, on average, we know that covariability (the way several factors vary
weather systems must behave in certain ways. together). These aspects are important, for
There are distinct patterns related to the climate. instance, for water resources, as described in
So, while we may not be able to predict the Weather Sequences (see page 22).
Figure 7. Many variables, such as temperature, have a
distribution or frequency of occurrence that is close to a
“normal” distribution, given by the bell-shaped curves
shown here. The center of the distribution is the mean
(average). The variability (horizontal spread) is
measured by the standard deviation. The values lie
within one standard deviation 68% of the time and
within two standard deviations 95% of the time.
Panel 1 (right) shows the distribution of temperature
for a hypothetical location. The axis shows the
departures from the mean in units of standard
deviation (vertical lines) and the temperature in °F,
with a mean of 50°F and a standard deviation of 9°F.
Panel 2 (left) shows, in addition, the distribution
if there is both an increase in mean temperature
of 5°F and a decrease in variability in the stan-
dard deviation from 9 to 7°F. Because of the
decreased variability, extremely high tempera-
tures do not increase in spite of the overall
warmer conditions, but note the decrease in inci-
dence of temperatures below 45°F.
To consider a more concrete example, sup- expected to fall within 50 plus or minus 18, or
pose that the average temperature in a month is between 32° and 68°F. We may also wish to
50°F. In addition to this fact, it is also useful to know that the lowest value recorded in that
know that the standard deviation of daily val- month is 22°F and the highest 79°F. Moreover, if
ues is 9°F (Figure 7). This is the statistician’s the temperature is above 60°F one day, we can
way of saying that 68% of the time the tempera- quantify the likelihood that it will also be above
tures fall within 50 plus or minus 9, or between 60°F the next day. And so on.
41° and 59°F, and 95% of the time the values are
22. III
Climate Change
We have shown how climate and weather are on buildings and roads quickly runs off into
intimately linked and explained how climate gutters and drains, and so the ground is not
may be considered as the average of weather moist, as it would be if it were an open field.
together with information about its variability By contrast, when the Sun shines on a farmer’s
and extremes. Climate, however, may be forced field, heat usually goes into evaporating sur-
to change, not through internal weather effects, face moisture rather than increasing the tem-
but due to the influence of external factors. And, perature; the presence of water acts as an air
if the climate changes in this way, so too will its conditioner. In fact, in some places a reverse of
underlying statistical nature, as characterized by urban warming, a suburban cooling effect, has
the weather we experience from day to day. We been found because of lawns and golf courses
now address this possibility. that are excessively watered. Changes in the
properties of the surface because of changes in
land use give rise to these aforementioned cli-
Human-Caused Climate Change mate changes. Nevertheless, these effects are
mostly rather limited in the areas they influ-
The climate can shift because of natural ence.
changes either within the climate system (such as
in the oceans or atmosphere) or outside of it (such
as in the amount of solar energy reaching the The Enhanced Greenhouse Effect
Earth). Volcanic activity is an Earth-based event
that is considered outside of the climate system Of most concern globally is the gradually
but that can have a pronounced effect on it. changing composition of the atmosphere caused
An additional emerging factor is the effect by human activities, particularly changes aris-
of human activities on climate. Many of these ing from the burning of fossil fuels and defor-
activities are producing effects comparable to estation. These lead to a gradual buildup of sev-
the natural forces that influence the climate. eral greenhouse gases in the atmosphere, with
Changes in land use through activities such as carbon dioxide being the most significant. They
deforestation, the building of cities, the storage also produce small airborne particulates—
and use of water, and the use of energy are all aerosols—that pollute the air and interfere with
important factors locally. The urban heat island radiation. Because of the relentless increases in
is an example of very local climate change. In several greenhouse gases, significant climate
urban areas, the so-called concrete jungle of change will occur—sooner or later. The green-
buildings and streets stores up heat from the house-gas component of this change in climate
Sun during the day and slowly releases it at is called the enhanced greenhouse effect. While
night, making the nighttime warmer (by sever- this effect has already been substantial, it is
al degrees F in major cities) than in neighbor- extremely difficult to identify in the past record.
ing rural regions. Appliances, lights, air condi- This is because of the large natural variability in
tioners, and furnaces all generate heat. Rainfall the climate system, which is large enough to
have appreciably masked the slow human-pro- plants. The latter inject sulfur dioxide into the
duced climate change. atmosphere, a molecule that is oxidized to form
The amount of carbon dioxide in the atmo- tiny droplets of sulfuric acid. In terms of their
sphere has increased by more than 30% (Figure climate impact, these sulfate aerosols are
8) since the beginning of the industrial revolu- thought to be extremely important; they form
tion, due to industry and the removal of forests. the pervasive milky haze often seen from air-
In the absence of controlling factors, projections craft windows as one travels across North
are that concentrations will double from pre- America. Because aerosols are readily washed
industrial values within the next 60 to 100 years. out of the atmosphere by rain, their lifetimes are
Carbon dioxide is not the only greenhouse gas short—typically a few days up to a week or so.
whose concentrations are observed to be Thus, human-produced aerosols tend to be con-
increasing in the atmosphere from human activi- centrated near industrial regions.
ties. The most important other gases are Aerosols can help offset, at least temporari-
methane, nitrous oxide, and the chlorofluoro- ly, global warming arising from the increased
carbons (CFCs). greenhouse gases. However, their influence is
regional and they do not cancel the global-scale
effects of the much longer-lived greenhouse
Effects of Aerosols gases. Significant climate changes can still be
Human activities also put other pollution
into the atmosphere and affect the amount of
aerosols, which, in turn, influences climate in The Climate Response and Feedbacks
several ways. From a climate viewpoint, the
most important aerosols are extremely small: in Some climate changes intensify the initial
the range of one ten-millionth to one millionth effect of greenhouse gases and some diminish it.
of a meter in diameter. The larger particles (e.g., These are called, respectively, positive and nega-
dust) quickly fall back to the surface. tive feedbacks, and they complicate the way the
Aerosols reflect some solar radiation back to climate responds. For example, water vapor is a
space, which tends to cool the Earth’s surface.
They can also directly absorb solar radiation,
leading to local heating of the atmosphere and,
to a lesser extent, contributing to an enhanced
greenhouse effect. Some can act as nuclei on
which cloud droplets condense. Their presence
therefore tends to affect the number and size of
droplets in a cloud and hence alters the reflec-
tion and absorption of solar radiation by the
Aerosols occur in the atmosphere from nat-
ural causes; for instance, they are blown off the
surface of deserts or dry regions. The eruption
of Mt. Pinatubo in the Philippines in June 1991 Figure 8. Annual carbon dioxide concentrations in parts
added considerable amounts of aerosol to the per million by volume (ppmv). The total values are given at
stratosphere, which scattered solar radiation, left, and the departures from the 1961–90 average (called
leading to a global cooling for about two years. anomalies) are given at right. The solid line is from meas-
Human activities that produce aerosols include urements at Mauna Loa, Hawaii, and the dashed line is
biomass burning and the operation of power from bubbles of air in ice cores.
powerful greenhouse gas and therefore absorbs clear just how clouds may change with changing
infrared radiation, so when a warmer climate climate. Other important feedbacks occur
causes more moisture to evaporate, the resulting through atmospheric interactions with snow and
water vapor increase will make the temperature ice, the oceans, and the biosphere. Quantifying
even warmer. Clouds can either warm or cool these various feedbacks is perhaps the greatest
the atmosphere, depending on their height, type, challenge in climate science, and the uncertain-
and geographic location. Hence they may con- ties in their magnitude are the primary source of
tribute either positive or negative feedback uncertainty in attempts to predict the large-scale
effects regionally; their net global effect in a effects of future human-induced climate change.
warmer climate is quite uncertain as it is not
25. IV
Observed Weather and
Climate Change
Observed Climate Variations toward a larger increase in minimum than in
maximum daily temperatures. The reason for
Scientists expect climate change, but what this difference is apparently linked to associated
changes have they observed? Analysis of global increases in low cloudiness and to aerosol
observations of surface temperature show that effects as well as the enhanced greenhouse
there has been a warming of about 0.6°C over effect. Changes in precipitation and other com-
the past hundred years (Figure 9). The trend is ponents of the hydrological cycle are deter-
mined more by changes in the weather systems
and their tracks than by changes in temperature.
Because such weather systems are so variable in
both space and time, patterns of change in pre-
cipitation are much more complicated than pat-
terns of temperature change. Precipitation has
increased over land in the high latitudes of the
Northern Hemisphere, especially during the
cold season.
Figure 10 shows changes observed in the
United States over the past century. Note espe-
cially the trend for wetter conditions after about
the mid-1970s in the first panel (a). Panel b
reveals that the main times of drought in the
United States were in the 1930s and the 1950s. In
the 1930s there was extensive drying in the
Great Plains, referred to as the Dust Bowl
because of the blowing dust and dust storms
characteristic of that time. In part, the Dust Bowl
was exacerbated by poor farming practices.
Naturally, times of moisture surplus tend to
alternate with times of extensive drought. Panel c
reveals the increasing tendency for rainfall to
occur in extreme events of more than two inches
Figure 9. Average annual mean temperatures, expressed as of rain per day over more of the country. Thus,
anomalies from the 1961–90 average, over the Northern and heavy rainfalls tend to occur more often or over
Southern Hemispheres (middle and bottom panels) and for more regions than previously, a steady and sig-
the globe from 1860 to 1998. Mean temperatures for nificant trend of about a 10% increase in such
1961–90 are 14°C for the globe, 14.6°C for the Northern events. Temperatures have also increased in gen-
Hemisphere, and 13.4°C for the Southern Hemisphere. eral (Panel d), but the warmest years tend to be
Based on Jones et al. (1999). those associated with the big droughts, which
26. (a) U.S. average annual precipitation (b) Dry (drought) and wet (flood) conditions in the U.S.
(c) Percent U.S. much above normal rainfall from 1 (d) U.S. Temperatures
day extreme events (>2”)
(e) Much above and much below normal U.S. temper- (f) Number of hurricanes making landfall
Figure 10. (a) The variations in U.S. average annual precipitation from the long-term average (mm), (b) the incidence of
droughts and floods expressed as percentages of the U.S. land area, (c) the percentage of the United States that receives more
than 2 inches (50.8 mm) of rainfall in one day, (d) the variation of the average annual U.S. temperature from the long-term
average (°C), (e) the incidence of much-above and much-below normal temperatures expressed as percentage of U.S. land
area, and (f) the number of hurricanes making landfall. In panels b, c, and e, the definitions of drought, flood, much above
normal, and much below normal all correspond to the top or bottom 10% of all values on average. In panels b and e, the
extents of the much-above and much-below normal areas are plotted opposite one another as they tend to vary inversely. This
is not guaranteed, however, as wet (warm) conditions in one part of the country can be and often are experienced at the same
time as dry (cold) conditions elsewhere (see Figure 11). From Karl et al. (1995).
contribute to many heat waves (because water is the variations about this value indicate the
no longer present to act as a natural air condi- extent to which the country was experiencing
tioner). In the United States, some of the warmest an unusual number of extremes of one sort or
years occurred in the 1930s. The warmth of the another. The major droughts of the 1930s and
1980s and 1990s, especially compared with the 1950s again are evident in this figure. In more
1900 to 1920 period, cannot simply be explained recent decades, the increase in extremes comes
by heat waves and changes in drought, however. from the increases in much-above normal tem-
In Panel e, we see that most of the temperatures peratures and the increase in extreme one-day
much below average occurred in the early part of rainfall events exceeding two inches.
this century, while most of the temperatures well
above average occurred either in the past 15
years or in the 1930s. Hurricanes naturally vary Interannual Variability
considerably in number from year to year (Panel
f). Since they are so variable, and relatively rare, A major source of variability from one year
no clear trends emerge. to the next is El Niño. The term El Niño (Spanish
Figure 11 shows a consolidation of these fac- for the Christ child) was originally used along
tors into a U.S. climatic extremes index (CEI). It the coasts of Ecuador and Peru to refer to a
is made up of the annual average of whether warm ocean current that typically appears
several indicators are much-above or much- around Christmas and lasts for several months.
below normal, where these categories corre- Fish yields are closely related to these currents,
spond to the top and bottom 10% of values. A which determine the availability of nutrients, so
value of 0% for the index would mean no por- the fishing industry is particularly sensitive to
tion of the country experienced extreme condi- them. Over the years, the term has come to be
tions in any category. A value of 100% would reserved for those exceptionally strong warm
mean the entire country was under extreme intervals that not only disrupt the fishing indus-
conditions throughout the year under all cate- try but also bring heavy rains.
gories. The average value, because of the way El Niño events are associated with much
the index is defined, must be around 10%, and larger-scale changes across most of the Pacific
Figure 11. The CEI is the
sum of two numbers. The
first reflects the percentage
of the United States, by
area, where maximum and
minimum temperatures,
moisture, and days of pre-
cipitation were much-above
or much-below normal. The
second number is twice the
percentage of the United
States, by area, where the
number of days of very
heavy precipitation (more
than two inches) was much
greater than normal. From
Karl et al. (1995).
Ocean. These changes in turn alter weather extensively in the module El Niño and the Peru-
patterns around the globe through changes in vian Anchovy Fishery.)
the atmospheric circulation. They can alter the Because the magnitude of El Niño events is
atmospheric waves (Figure 3) and thus the relatively large compared with climate change
tracks of storms across North America and else- on the slower decadal time scale, El Niño is
where. The major floods in the summer of 1993 manifested much more readily than global
in the upper Mississippi River basin were partly warming in the weather we experience and in
caused by El Niño. Recent floods in California the regional climate variations. This is a prime
(winters of 1994–95 and 1997–98) were also example of interannual variability of climate,
linked to El Niño as the storm track continually which, in general, tends to mask the climate
brought weather systems onto the west coast of change associated with global warming.
the United States. (El Niño is discussed more
29. V
Prediction and Modeling of
Climate Changes
In general, climate changes cannot be predicted mate of the accompanying climate change.
simply by using observations and statistics. To make a true prediction of future climate
They are too complex or go well beyond condi- it is necessary to include all the human and nat-
tions ever experienced before. For the most ural influences known to affect climate (cf. Fig-
detailed and complicated projections, scientists ures 1 and 12). Because future changes in sever-
use computer models of the climate system al external factors, such as solar activity and
called numerical models. These models are volcanism, are not known, these must be
based on physical principles, expressed as assumed to be constant until such time as we
mathematical formulas and evaluated using are able to predict their changes.
Climate Predictions
Climate Models
The climate is expected to change because
Global climate models attempt to include the of the increases in greenhouse gases and
atmospheric circulation, oceanic circulation, land aerosols, but exactly how it will change depends
surface processes, sea ice, and all other processes a lot on our assumptions concerning future
indicated in Figure 1. They divide the globe into human actions. When developing countries
three-dimensional grids and perform calculations industrialize, they burn more fossil fuels, gener-
to represent what is typical within each grid cell. ate more electricity, and create industries, most
For climate models, owing to limitations in of which produce some form of pollution.
today’s computers, these grid cells are quite Developed countries are currently the largest
large—typically 250 kilometers in the horizontal sources of pollution and greenhouse gases.
dimension and a kilometer in the vertical dimen- Because future changes are not certain, climate
sion. As a result, many physical processes can models are used to depict various possible “sce-
only be crudely represented by their average narios.” These are not really predictions but pro-
effects. jections of what could happen. If a projection
One method used to predict climate is to indicates that very adverse conditions could
first run a model for several simulated decades happen, policy actions could be taken to try to
without perturbations to the system. The quality change the outcome. The following are some
of the simulation can then be assessed by com- features of possible future climate changes cre-
paring the average, the annual cycle, and the ated by human activities. Greatest confidence
variability statistics on different time scales with exists on global scales; regional climate changes
observations. If the model seems realistic are more uncertain.
enough, it can then be run including perturba- 1. The models indicate warming of 1.5 to
tions such as an increase in greenhouse-gas con- 4.5°C for a climate with atmospheric CO2 con-
centrations. The differences between the climate centrations doubled from preindustrial times,
statistics in the two simulations provide an esti- when they were 280 parts per million by vol-
Figure 12. Schematic model of the fluid and biological Earth that shows global change on a time scale of decades to cen-
turies. A notable feature is the presence of human activity as a major inducer of change; humanity must also live with the
results of change from both anthropogenic and natural factors. From Trenberth (1992).
ume. An effective doubling of CO2, taking into is expected to lead to an increase in extremely
account aerosols and other greenhouse gases, is hot days and a decrease in extremely cold days.
likely to occur around the middle of the 21st So far, over the past century, during which
century. Corresponding manifestations of North- time carbon dioxide has increased from 290–300
ern Hemisphere climate change will take place to 360 parts per million by volume (roughly a
some 20 to 50 years later because it takes the 20% increase), the observed temperature increase
oceans at least that long to respond. The lag is has been fairly modest, about 0.5°C (see Figure
likely to be greater over the Southern Hemi- 9). This temperature increase is reasonably con-
sphere because of the influence of the larger sistent with model predictions when effects of
ocean area. Aerosols are also expected to aerosols are included. But large uncertainties
increase in areas undergoing industrialization remain, particularly because of questions about
(such as China) and to decrease in North Ameri- how clouds might change.
ca and Europe, where steps are being taken to 2. The hydrological cycle is likely to speed
decrease acid rain by decreasing sulfur emis- up by about 10% with CO2 doubling, bringing
sions. The effects of aerosols will complicate cli- increased evaporation and increased rainfall in
mate change and will most likely change the general. With warming, more precipitation is apt
regional distribution of the temperature increase. to fall as rain in winter instead of snow, and,
When effects of aerosols and greenhouse gases with faster snowmelt in spring, there is likely to
are combined, one estimate puts the average rate be less soil moisture at the onset of summer over
of temperature increase in the next century at midlatitude continents. When this change is
about 0.15 to 0.25°C per decade. Such a warming combined with increased evaporation in sum-
mer, any natural tendency for a drought to occur to simulate rudimentary El Niño cycles. It seems
is likely to be enhanced. However, there is not likely that El Niño will continue to exist in a
good agreement among the models on this warmer world. Because El Niño and its cool
aspect. An enhanced hydrological cycle also counterpart La Niña create droughts and floods
implies increased intensity of rainfall, such as in different parts of the world, and because
has been found for the United States (Figure 10). global warming tends to enhance the hydrologi-
Increases in rainfall in winter but drier condi- cal cycle, there is a real prospect that future such
tions in summer would challenge future water events will be accompanied by more severe
managers to avoid flood damage and keep up droughts and floods. In the tropics, in particular,
with the demand for fresh water. because of the great dependence on thunder-
3. Because warming causes the ocean to storm rainfall and its tendency to fall at certain
expand and snow and glacial ice to melt, one times of the year (during the wet or monsoon
real threat is a rise in sea level. There may be season) the main prospect that looms is one of
some compensation through increased snowfall larger variability and larger extremes in weather
on top of the major ice sheets (Greenland and events.
Antarctica) so that they could increase in height
even as they melt around the edges. Currently,
sea level is observed to be rising by 1 to 2 Interpretation of Climate Change in Terms
mm/year, and this rate should increase, so that of Weather
there are prospects for about a 50-cm rise in sea
level by 2100, but the main impacts are not like- For assessing impacts, what is most needed
ly to be felt until the 22nd century. are projections of local climate change. However,
4. With warming, increases in water vapor producing such projections represents a consider-
(a greenhouse gas) and decreases in snow cover able challenge. Climate predictions are especially
and sea ice (lower albedo) provide positive difficult regionally because of the large inherent
feedbacks that should enhance the warming as natural variability on regional scales. We have
time goes on. The land should warm more than discussed changes in climate mostly in terms of
the oceans, and the largest warming should changes in average conditions. But we experience
occur in the Arctic in winter. those changes mainly through changes in the fre-
5. Stratospheric cooling is another likely quency of extreme weather events, e.g., how hot
effect of increased greenhouse gases. This cool- it gets on a daily basis, or how frequent and vio-
ing has important implications for ozone deple- lent thunderstorms become. An average monthly
tion, because the chemistry responsible for the change in temperature of 3°C (5°F) may not
Antarctic ozone hole is more effective at lower sound like very much, but it has a very dramatic
temperatures. The loss of ozone also increases effect on the daily frequency of extreme tempera-
stratospheric cooling. tures, e.g., see Figure 7. For example, currently in
6. Because of increased sea-surface tempera- Des Moines, Iowa, the likelihood that the maxi-
tures, there may be changes in tropical storms mum temperature on any day in July will exceed
and hurricanes. Hurricanes sustain themselves 35°C (95°F) is about 11%. However, with an
at temperatures above 27°C, feeding on the increase in the average monthly maximum tem-
extra water vapor and latent heat those temper- perature of 3°C the likelihood almost triples, to
atures create. However, natural variability of about 30%. Small changes in the average can
hurricanes is large (Figure 10), so any effect bring about relatively large changes in frequen-
from climate change will be hard to detect for cies of extremes.
many decades. In addition to a change in the average cli-
7. Coupled ocean-atmosphere general circu- mate, the variability itself could also change. If
lation models have only very recently been able the daily variability of temperature increases in
Des Moines, then an even greater portion of countries will continue to experience weather
days would exceed 35°C. If, on the other hand, much as they have before. In some places they
the variability decreases, the temperature from may notice that the time between major snow
one day to another would be more similar than storms is longer, heat waves are more frequent
before. Changes in variability affect changes in and debilitating, the intensity and frequency of
the frequency of extremes and have more effect thunderstorms are changed, coastal damage to
than changes in averages (see Figure 7). There is beaches is more common, prices of some com-
some evidence that with climate warming, daily modities increase while others decrease, water
variability of temperature might decrease so conserving practices in certain communities are
that there might be fewer cold extremes in win- intensified, and so on. Areas where the cumula-
ter. Variability of temperature could decrease in tive effects of weather are important, such as
some seasons (e.g., winter) but increase in oth- water resources and agriculture, may be more at
ers. risk.
Changes in variability of precipitation are Many of the effects may be rather subtle
also anticipated and will tend to be associated most of the time, and the actual impact may
with changes in the average precipitation. Varia- originate through other pressures (increasing
bility generally increases as average precipitation population, as an example) and may only be
increases. In the United States, precipitation exacerbated by the changes in climate. But there
extremes have been found to increase in the past are also likely to be dramatic effects. As an
few decades (see Figure 10 Panel c). The picture example, during a drought a string of wide-
for precipitation is more complicated, however. spread heat waves may put increased demand
For example, climate change is likely to alter the on air conditioning, causing brownouts and
jet stream and associated location of storm tracks, even blackouts as the electricity demand exceeds
so that some places will experience an increase in available capacity; or there may be more medical
storminess while others, not very far away, will emergencies, such as heat stroke, involving those
experience a decrease. Such opposite changes who do not have or cannot afford air condition-
over short distances should be expected and are ing. Ironically, the extra use of air conditioning
an inherent part of climate for rainfall, but this is leads to increased fossil fuel use and hence a
likely to be confusing to many people. greater emission of greenhouse gases.
It is likely that most people in developed
33. VI
Impacts of Weather and Climate
Changes on Human Activities
Human activities and many sectors of eco- As an example, suppose place A has 0.5
nomic activity depend on weather and climate inches of gentle rain every three days, for a
in different ways. Some rely on average condi- monthly average of 5 inches, and place B has 2.5
tions. Others are sensitive to extremes. Yet oth- inches of rain on two consecutive days of the
ers depend upon variety and so weather month but with all other days dry, again for a
sequences can be important. Aside from choos- monthly total of 5 inches. The monthly total is
ing the climate by selecting the right location, the same, but the sequence differs greatly and
there are other ways we can attempt to cope the climates would be quite different. At place A,
with climate change and its consequences for the rain would replace the evaporation and use
agriculture, fisheries, and so forth. of moisture by plants; there would be few pud-
dles, so there would be no runoff into streams.
As a rule of thumb, anytime there is more than 3
Weather Sequences inches of rain in a day, there will be fairly exten-
sive flooding. So at place B it is likely that low-
Conditions may be altered not only by indi- lying parts of roads would be flooded, culverts
vidual weather events but also by sequences of would overflow, basements would flood, and
weather events. Weather sequences, for exam- there would be substantial damage from all the
ple, play a big role in determining stream runoff runoff during the two rainy days. But then the
and soil moisture, and can result in prolonged rest of the month, the ground would dry out and
periods of abnormal temperatures and sun- plants would become stressed and wilt unless
shine. These are important determinants of agri- they had very deep and extensive roots. The dif-
cultural yields, and the responsiveness of yields ferent sequences of weather make for very dif-
to such other inputs as fertilizer depends on the ferent impacts.
growing conditions supplied by a sequence of
weather events.
Runoff to surface streams and groundwater Location, Location, Location
recharge, or replenishment, depend on extended
sequences of weather events so that the contribu- Climate and weather contribute to personal
tion of individual rainstorms to runoff depends satisfaction. For example, the satisfaction provid-
on whether previous conditions were wet or dry. ed by a walk in the park varies according to
In addition, the timing of runoff in mountainous whether conditions are balmy or blustery. A sim-
river basins is strongly dependent on snowpack ple economic model of the allocation of time
accumulation and rate of melt. Mountain runoff, between walks in the park and other activities
thus, is quite sensitive to temperature variations. predicts that parks will become more crowded as
The quantity and timing of runoff, in turn, deter- the weather improves. Casual observations con-
mine the availability of water for competing agri- firm that prediction. Many people also express a
cultural, municipal, industrial, hydropower, willingness to pay to live where they can expect
recreational, and ecological uses. to enjoy particular climatic characteristics, such
as frequent mild, sunny weather. Their valua- physical environment. It can damage property,
tions of those characteristics may be expressed as cause loss of life and population displacement,
a willingness to accept a somewhat lower real destroy or sharply reduce agricultural crop
wage or to pay more for housing of comparable yields, and temporarily disrupt essential servic-
quality in order to live in a preferred climate. es such as transportation, telecommunications,
Climates are tied to particular locations, so and energy and water supplies. Society has
that when individuals decide to move them- developed various methods to avoid or mini-
selves and their productive activities to a certain mize adverse impacts of weather and has also
place, they are also choosing the climate in developed means to facilitate recovery from
which they will live and operate. For most eco- extreme weather phenomena. Yet, because
nomic activities, climate is only one of many severe weather events repeatedly disrupt
factors influencing choice of location. For some socioeconomic activities and cause damage,
activities, the characteristics of climate are a cen- society continues to search for new ways to pro-
tral factor in location decisions. The expected tect lives and property. Some of these involve
availability of snow is an important concern for behavioral adjustments based on past societal
the location of ski resorts. A sufficiently low risk experience, such as educating citizens about
of severe freezes is a critical consideration in the what to do in the event of a tornado warning.
location of orange groves, and crop selection Others involve the application of new meteoro-
decisions and farm management strategies are logical research findings for improving the pre-
heavily influenced by probable growing-season diction of where and when severe weather will
conditions. occur (see page 28).
The location of other industries is tied to the
availability of particular natural resources. The
lumber and paper industries require trees. Societal Responses
Hydropower dams are located where stream
gradients and rates of flow offer significant One way of reducing vulnerability to
potential generation. Fishing fleets and process- weather is to reduce damage to property,
ing capacity are based to allow access to expect- through such strategies as stricter construction
ed concentrations of commercially valuable fish. standards, tighter building codes, and restric-
Such resources are themselves tied to climate. tions on development in floodplains and on
The connections are obvious for hydropower, coastal barrier islands. The construction of
where drought conditions can quickly lead to storm sewers can help minimize short-term
reduced generation. The impacts of climatic flood damage in highly developed areas where
variations on the timber industry are less imme- there is substantial impermeable surface such as
diate, although prolonged droughts can signifi- pavement. The casualty and hazard insurance
cantly reduce the stock of healthy standing trees industry in more developed countries helps
and often create favorable conditions for forest insured parties rebuild and replace property
fires. damaged by severe weather. Of course, insur-
ance does not physically protect property from
weather-related damage, but it does facilitate
Severe Weather Events recovery and replacement in the aftermath of
extreme weather events such as tornadoes, hur-
The most dramatic impact of weather on ricanes, and floods. The insurance industry
human endeavors is often through severe itself has been altered by perceptions of climate
weather events that may alter as the climate change, such as rates for coastal insurance in
changes. Severe weather has always affected Florida. Reservoirs increase resilience to short-
human activities and settlements as well as the term fluctuations in streamflows and thus pro-
tect the water supply and hydropower produc- Managing Risk
tion. Electric utilities also increase resilience to
variable hydropower output and variable Climate and day-to-day weather variations
demand by maintaining backup generation affect a wide variety of economic activities. Cli-
capacity (e.g., coal-fired plants) and by buying mate influences the spatial distributions of pop-
or selling power over interconnected transmis- ulation and of industrial, agricultural, and
sion grids. resource-based production activities, while
A second way of reducing vulnerability to weather can affect levels of production and
weather is through technology. Many technolo- production costs. In addition, severe weather
gies are so common that they have become part can damage or destroy property.
of society’s everyday affairs and activities. For In gambling, even the most astute players
example, modern tires, windshield wipers, and will occasionally lose. In economics, if climate-
fog lights have helped reduce the hazard of induced loss reveals new information on the
driving in bad weather conditions. Indoor heat- nature of the climatic risk or on the vulnerabili-
ing and air conditioning provide comfort and ty of affected activities, or if it alters people’s
protection from extreme temperatures in winter perceptions of the risk, then they will readjust
and summer. The invention of shelter itself was their risk-management strategies. If not, they
probably prompted by human desires to have will go back to the status quo. For example,
protection from the extremes of weather and cli- towns that are hit by tornadoes are usually
mate as well as from predators and human ene- rebuilt in the same location because one hit does
mies. not signal any change in the long-term risk. A
Modern weather forecasting, which has pro- series of extreme events, on the other hand, may
gressed rapidly over the past half-century, can be taken as a signal that previously available
give advance warning of possibly dangerous information provided an inaccurate picture of
weather conditions. Forecasters can frequently the true risk, or that the climate has changed. In
provide information minutes to several days that situation, a town might not rebuild in the
ahead of possible severe weather conditions. In same location.
many cases, decisions may be made based on
forecasts to reduce or eliminate potential vulnera-
bility to severe weather. For example, on a con- Impacts on Agriculture
struction site, concrete deliveries may be resched-
uled to ensure that snow, ice, and cold tempera- Humans have been interested in under-
tures do not interfere with its proper curing. Busi- standing and predicting the effects of climate on
nesses may alter trucking schedules and routes in crop production since the rise of agriculture,
response to anticipated foul weather. In certain because food production is critical to human
circumstances, farmers may be able to harvest all survival. A classic Biblical example is in Gene-
or part of their crops in advance of what could be sis, where Joseph interprets a dream of the
destructive weather. The usefulness of weather Pharaoh’s as a portent of seven coming years of
forecast information varies among economic sec- good grain harvests followed by seven years of
tors. While a reliable weather forecast may help a crop failure.
farmer to efficiently schedule crop irrigation, for Crop yields are strongly affected by changes
example, it cannot help that farmer protect a crop in technological inputs such as fertilizer, pesti-
from imminent hail damage. Other coping mech- cides, irrigation, plant breeding, and manage-
anisms, such as crop insurance, preparedness, ment practices, but the major cause of year-to-
and routine maintenance of flood levees and year fluctuations in crop yield is weather fluctu-
storm sewers, also help society manage its vul- ations. Agricultural crops are mainly sensitive to
nerability to extreme weather events. fluctuations in temperature and precipitation,
although solar radiation, wind, and humidity Effects of Temperature and Precipitation on
are also important. In general a crop grows best Crop Yield. The temperature regime of a
and produces maximum yield for some opti- particular locale will affect the timing of
mum value of the relevant climate variable; as planting and harvesting and the rate at which
conditions depart from the optimum, the plants the crop develops. With adequate moisture, the
suffer stress. The responsiveness of yields, and potential growing season is largely determined
therefore the financial return, to such inputs as by temperature; in temperate mid-latitude
fertilizer and pesticides varies with weather regions this generally extends from the last frost
conditions, so that it is prudent for farmers to in the spring to the first frost in the fall. The rate
make adjustments depending on the weather. at which plants develop and move through their
Pacific Salmon
In the case of private production and invest- surface temperatures along the Pacific coast of
ment decisions, the climate-related risks fall North America and changes in near-shore cur-
largely on the parties making the decisions rents associated with more frequent and per-
unless they have chosen to purchase some sistent El Niño events appear to have con-
form of insurance, allowing the sharing of the tributed to remarkable increases in the pro-
risk with others. To the extent that the deci- ductivity of Alaskan salmon stocks and to
sion makers bear the risk, they have the declining runs of some salmon spawning in
incentive to engage in appropriate risk-man- Washington, Oregon, and California. In the
agement strategies and to make efficient use early 1990s, these trends culminated in a
of available climate- and weather-related series of record Alaskan salmon harvests and
information. severe declines in once-thriving Coho and
Many climate-sensitive natural resources Chinook fisheries in Washington and Oregon.
are managed as public property, and decisions These fluctuations in northern and southern
regarding their use are made by government salmon stocks contributed to the breakdown
agencies, often with considerable input from of international cooperation under the Pacific
the interested public. In such cases, the effects Salmon Treaty. Under pressure from commer-
of climatic variability often complicate the cial, sport, and Indian fishing interests within
already difficult task of balancing the conflict- their respective jurisdictions, British Colum-
ing demands of competing interests. Fisheries bia, Alaska, and the West Coast states were
are sensitive to climatic variations, but the unable to come to a consensus over a fair and
true impacts of climate are often complex and biologically sound division of the harvest for
difficult to separate from the impacts of other six years. The resulting inability to control
factors (such as fishery management, over- Alaskan and Canadian exploitation of deplet-
fishing, spawning habitat degradation, water ed stocks migrating to the southern spawning
diversions, building of dams, and pollution) areas contributed to their further decline.
influencing the survival, growth, and spatial Finally, in June 1999, the governments
distribution of fish populations. responded to the imperiled state of the stocks
The Pacific salmon fishery provides an by implementing a new agreement that
example. Since the mid-1970s, warmer sea- adjusts harvests to changes in abundance.