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.
1. GLOBAL CHANGE INSTRUCTION PROG R A M EFFECTS OF CHANGING CLIMATE ON WEATHER AND HUMAN ACTIVITIES Kevin E. Trenberth, Kathleen Miller, Linda Mearns and Steven Rhodes
2. EFFECTS OF CHANGING CLIMATE ON WEATHER AND HUMAN ACTIVITIES
3. EFFECTS OF CHANGING CLIMATE ON WEATHER AND HUMAN ACTIVITIES Kevin E. Trenberth, Kathleen Miller, Linda Mearns and Steven Rhodes National Center for Atmospheric Research Boulder, Colorado UNIVERSITY SCIENCE BOOKS SAUSALITO, CALIFORNIA
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, univscibks@igc.org. Information is also available on the World Wide Web at http://www/uscibooks.com/globdir.htm or V
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 IX
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. 1
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. 2
11. EFFECTS OF CHANGING CLIMATE ON WEATHER AND HUMAN ACTIVITIES 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. 3
12. I Climate 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. 4
13. EFFECTS OF CHANGING CLIMATE ON WEATHER AND HUMAN ACTIVITIES 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. 5
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 90°N 60 30 Heat Transport Net Radiation Solar 0 Night Day Radiation 30 60 Ou 90°S tgo on in d i ati g Lon Ra gwave 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. 6
15. EFFECTS OF CHANGING CLIMATE ON WEATHER AND HUMAN ACTIVITIES 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. 7
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 8
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 9
18. THE WEATHER MACHINE 1 2 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 3 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). 10
19. EFFECTS OF CHANGING CLIMATE ON WEATHER AND HUMAN ACTIVITIES 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 COLD 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 WARM 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 COLD 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. 11
20. THE WEATHER MACHINE 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. 12
21. EFFECTS OF CHANGING CLIMATE ON WEATHER AND HUMAN ACTIVITIES 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 13
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 14
23. EFFECTS OF CHANGING CLIMATE ON WEATHER AND HUMAN ACTIVITIES 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 present. 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. 15
24. CLIMATE CHANGE 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 16
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 17
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). 18
27. EFFECTS OF CHANGING CLIMATE ON WEATHER AND HUMAN ACTIVITIES 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). 19
28. OBSERVED WEATHER AND CLIMATE CHANGE 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 20
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- 21
30. PREDICTION AND MODELING OF CLIMATE CHANGES 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- 22
31. EFFECTS OF CHANGING CLIMATE ON WEATHER AND HUMAN ACTIVITIES 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 23
32. THE SUN-EARTH SYSTEM 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 24
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 25
34. IMPACTS OF WEATHER AND CLIMATE CHANGES ON HUMAN ACTIVITIES 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- 26
35. EFFECTS OF CHANGING CLIMATE ON WEATHER AND HUMAN ACTIVITIES 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, 27
36. IMPACTS OF WEATHER AND CLIMATE CHANGES ON HUMAN ACTIVITIES 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. 28
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