The amount of money, labor, and equipment we are willing to expend to avoid greenhouse warming depends in part on how we view the results of climate change and how much we are willing to risk possible negative consequences. Estimating all these outcomes is difficult, however, because we cannot predict with certainty what changes will occur globally and we cannot predict at all the effects in a given region. Regardless of what the changes will be, a necessary first step in determining the proper allocation of resources is to examine the ability of natural systems and humans to adapt.
Methods of Adaptation
Humans, animals, and plants are able to adapt to different climates. Animals and plants live in the Himalayas and in Death Valley, although not all species thrive in both. Human adaptability is shown by our living and working in both Riyadh and Barrow.
Human societies can and do thrive in many different climates, but it is the rate of climate change as much as its magnitude that could pose a threat. Disasters caused by severe weather and degradation of the environment illustrate the kinds of disruptions that could accompany rapid climate change. There are five alternative human responses: (1) modify the hazard, as by channeling rivers that are prone to flooding; (2) prevent or limit impacts, as by building dikes; (3) move or avoid the loss, as by implementing flood plain zoning; (4) share the loss, as by providing insurance; and (5) bear the loss, as by losing all or part of a crop. Thus we have a large menu of potential adaptation options, some of which are best made before an event and some after.
Plants and animals will always be found regardless of climatic changes in the ranges discussed here. The threat to the natural communities of plants and animals, called ecosystems, from greenhouse warming also comes from its projected rate of change as much as its magnitude. If the climate changes as rapidly as some computer models project, the present natural ecosystems may become fragmented and break up. New communities may replace them with different mixes of species. Long-lived plants like trees, for example, might persist. If ill adapted to the new conditions, however, they would fail to compete and reproduce. Species better fitted to the new climate would immigrate, sometimes hastened by disturbances of various kinds. Species well suited to the changing conditions may become more dominant, or pioneer species that could fill a particular niche may thrive in the new conditions. Certain ecosystems might vanish if the climate that currently sustains them disappears or changes its location faster than the key species are able to migrate.
The Role of Innovation
Much human adaptation involves the invention and diffusion of technological "hardware" or "software." Examples of technological hardware include air conditioners that make hot days comfortable and tractors that cultivate large tracts of land in a few days if spring is late. Software includes information, rules, and procedures like weather forecasts or insurance restrictions. Knowledge and new procedures are generally indispensable for adopting new hardware. Major breakthroughs like irrigation usually consist of innovations in social organization and financing as well as new machinery.
Many past innovations in hardware and software have helped people adapt themselves and their activities to climate and variable weather. Food preservation in warm weather, refrigeration and air conditioning, antifreeze for all-weather automobile travel, and weather satellites to aid prediction all help. Such innovations can occur rapidly in comparison to the 40 to 50 years envisioned for the equivalent doubling of atmospheric CO2. For example, in 1900 California had little crop production; in 1985 it produced twice as many dollars of crops as second-place Iowa. Penicillin was discovered in 1928; by 1945 it was saving thousands of lives.
The question frequently asked is how rapidly inventions can replace existing equipment and how fast other technology can be supplanted. About two-thirds of capital stock in most industrialized countries is in machinery, and one-third is in buildings and other structures. This capital stock turns over more rapidly than might be expected. Most current office space, for example, is in buildings built in the last 20 years. In Japan, the average period for virtually complete replacement of machinery and equipment ranges
from about 22 years in textiles to about 10 years in industries like telecommunications or electrical machinery. Replacement can be fast in agriculture, too. The estimated lifetime of particular strains for five major crops in the United States is less than 10 years and is expected to be even shorter in the future.
As societies have become more affluent, they have reduced their sensitivity to natural phenomena in many ways. Overall, the trend is toward systems of transportation, communication, and energy production and use that are less sensitive to climate. Improved technology and social organization also seem to have lessened the impacts of climate fluctuations on food supply over the last 100 years. In the time frame over which the effects of greenhouse warming are felt, more societies may become more robust with respect to climate change.
Assessing Impacts and Adaptive Capacity
The data and analyses used in this study to assess impacts and adaptive capacity are drawn mostly, but not exclusively, from the United States. Few other countries share the United States' combination of wealth, low population density, and range of climates. Moreover, the panel recognizes that our domestic well-being is intimately tied to what happens in other countries. Major international shifts in trade flows, agricultural production, energy demand patterns, and more could profoundly affect this country. But a full analysis of such global interactions remains for future studies.
The assessment of impacts in this study examines separately the sensitivity of various human and ecologic systems to climate change. Not all interactions could be assessed, even though the panel recognizes that such interactions may be relevant. For example, unmanaged natural systems have important interactions with forestry. Although the assessment of forestry considers shifts in ranges of pests and other key species, major alterations in unmanaged natural systems may contain unforeseen problems for forestry. The assessment here is an initial appraisal of impacts and adaptive capabilities of affected human and natural systems in the United States; additional effort is necessary for a more complete understanding of these issues.
CO2 Fertilization of Green Plants
An increasing atmospheric concentration of CO2 would increase agricultural production by enhancing the use of sunlight and slowing transpiration in some plants. The overall production of organic material also depends on other factors such as temperature, moisture, and nutrients. It is difficult to anticipate the amount of increased organic production accompanying greenhouse warming because extrapolation from small-scale laboratory experiments
to whole fields of crops or to complete systems of unmanaged plants and animals is uncertain. The increases in photosynthesis and slowing of transpiration, however, would probably be somewhat less than observed in laboratory experiments. These effects would apply to plants in agriculture, managed forests, and unmanaged ecosystems.
Changes in average temperature are probably less important for agricultural productivity than changes in precipitation and evaporation. Whether the projected changes are calculated as precipitation and evaporation or the resulting changes in crop yields, the different climate scenarios produced by different general circulation models (GCMs) yield large variations for agriculture. But farming has always been sensitive to the weather, and experience suggests that farmers adapt quickly, especially in comparison to the rate at which greenhouse warming would occur. Countries like the United States, which encompass many climate zones and have active and aggressive agricultural research and development, would probably be able to adapt their farming to climatic changes deriving from greenhouse warming. Poorer countries with less wealth or fewer climate zones may have more difficulty avoiding problems or taking advantage of better conditions.
Managed Forests and Grasslands
Forests and grasslands each cover more than a quarter of the United States. Trees have long lifetimes, and are unlikely to adjust rapidly enough by themselves to accommodate rapid warming. Forests, however, can be managed to preserve ample forest products. Middle-aged forests are at most risk if climate changes, since young forests can be replaced cheaply and older ones are valuable to sustain. The adaptation of valuable forests by management is possible using methods that are flexible and work in many climates.
The natural landscape consists of unmanaged ecosystems that include many species of animals, plants, and microorganisms harvested as game, fruit, or drugs. Ecosystems absorb CO2, emit O2, and cleanse air and water. Ecosystems also emit CO2, CH4, and other hydrocarbons. For a variety of reasons, the adaptation of natural ecosystems to climate change is more problematic than that of managed systems like farms or plantation forests. The principal impacts of climate change are expected to be on plants. Impacts on animals would mostly be indirect, through changes in plant functioning
and vegetation dynamics, but significant direct effects of climate change are possible. Some species of birds appear especially responsive to temperature and may shift their ranges relatively rapidly. Climate change may make some species extinct, but the diversity of ecosystems would probably protect those functions that are carried out by many species. For example, diseases removed first the chestnut and then the elm from eastern forests, but the loss of their capacity to absorb CO2 was quickly made up by other species. Some ecological processes, however, are carried out by only a few species. Only a few species enhance soil productivity by fixing nitrogen, and the grazing of a single large species may alter a landscape. If climate change removed one of these species or encouraged another, even a diverse ecosystem could be affected. Even small climatic changes resulting from greenhouse warming would be likely to alter unmanaged ecosystems. The adaptation of the natural landscape can be helped by moving species when they are in trouble, providing corridors along which those that can may move, and intervening to maintain diversity of species in key ecosystems.
Marine and Coastal Environments
Concern about coastal swamps and marshlands comes from their special ecological value and the fact that they are already under stress from human development and pollution. Wetlands have persisted in the past despite slowly changing sea levels. Greenhouse warming could induce sea level rise, however, faster than new wetlands could form. In addition, human activity could constrain such movement if wetlands are bounded by dikes, bulkheads, or other structures. Climate change also could alter upwelling of deep ocean waters or paths of major currents and thus wind and precipitation patterns. Areas of upwelling are among the biologically most productive ocean habitats, and such changes could affect fisheries substantially. We do not understand these phenomena well enough, however, to predict the ecological consequences of coastal or ocean changes with confidence. At present, the potential for human intervention to ease adaptation in marine ecosystems seems limited.
Climate change affects natural seasonal and yearly variations in water resources by changing precipitation, evaporation, and runoff. The first indications that the demand for water is exceeding the supply usually come during drought. Changes in water supply due to greenhouse warming could be moderated, for example, by storage (in natural aquifers or constructed reservoirs) or joint operation of water systems. Demand for water can be reduced through a variety of management techniques, including conservation and
price incentives. Constructing dams, canals, and other facilities takes time, and so such adaptation actions need to be taken well in advance. Actions to deal with current variability of water supply should help prepare for the possible consequences of greenhouse warming.
Industry and Energy
Most industrial sectors, including electric power generation, are only moderately sensitive to climate change. Access to regular water supplies is the largest single problem. In most sectors, the planning horizon and lifetime of investments is shorter than the rates of change we could expect from greenhouse warming. In general, industry in the United States will likely adapt to greenhouse warming without much difficulty.
Tourism and Recreation
Tourism and recreation are more sensitive to climate change than some other sectors because part of the industry is closely associated with nature. This part of tourism and recreation will necessarily migrate as the attractive conditions and areas move. Although specific regions will be adversely or favorably affected, for a country as large as the United States, the overall effect will probably be negligible.
Settlements and Coastal Structures
Direct climatic changes of greatest importance to human settlements are changes in the extremes and seasonal averages of temperature, and in the geographic and seasonal distributions of rainfall. Although these direct climatic changes may be important, the secondary effects of greenhouse warming on the levels of water bodies are much more important. Urban areas will probably choose to protect existing sites rather than move. Adaptations can be encouraged by changing building codes and land use planning. Allowances should be made for climate change when long-lived structures or facilities are constructed or renovated.
Humans have successfully adjusted to diverse climates. Human health could be affected by greenhouse warming because people are sensitive to climate directly as well as being susceptible to diseases whose carriers, or "vectors," are sensitive to climate. In the United States, however, the rate of improvements in health due to better technology and its application should greatly exceed the threat to health due to climate change. These improvements
would not, of course, result from choices about costs and benefits of responding to greenhouse warming as such. The health consequences may be worse in countries with fewer resources.
Historical evidence suggests that migration over long distances, such as occurred in the United States during the Dust Bowl period, is not an automatic response to climate change. Migrations typically follow established routes and cover relatively short distances. While economic and other stresses will continue to provide incentives for migrants to move to the United States or other industrialized countries, there is unlikely to be climate-driven migration on a scale that could not be managed, at least in the next two decades. What happens over the course of a decade or two, however, can set the stage for developments over the longer term. Nevertheless, taking steps now to prevent future migration would not be justified given human adaptability to change and uncertainties about which areas would be affected.
Concern about political tranquility stems from fear that the occasional disaster of today might become persistent tomorrow and that accumulation of problems may become overwhelming. Countries outside the industrial world may lack the institutions or resources to manage additional environmental crises. Difficulties of organizing coordinated, multilateral responses to problems such as hunger are already evident. Greenhouse warming could aggravate present economic, political, and social problems, swamping national governments and international assistance activities and programs.
Some Important Indices
The same diversity that illustrates how humanity and nature adjust to environmental conditions shows that global averages are inappropriate as foundations for thinking about impact or adaptation. Because most adaptations are local, their cost cannot be calculated until such factors as future water supply can be predicted in specific regions. Strategic indices of greenhouse warming for agencies to monitor and scientists to predict include the following:
1. Seasonal and yearly variation in regional supplies of water to streams and soils.
2. Variability of ocean currents, particularly those affecting regional habitability and coastal life.
3. Variation in regional sea level and inshore height of waves.
4. Variability of the timing of such biological events as blooms and migrations.
Since even future global averages are uncertain, we will not soon know what these four regional indices will indicate and therefore will not be able to predict local impacts and to design specific adaptations. Nonetheless, monitoring the local climate, including the water in streams and seasonal events, is crucial over time and will eventually lay the foundation for designing and selecting these specific adaptations.
Evaluating Adaptation Options
It is difficult to evaluate adaptation options in the face of uncertainties. Consider a hypothetical bridge over an estuary as an example. An added meter of height above sea level might add $100,000 to current construction costs. If that additional clearance were not included at the time of construction, and the sea level rose enough to require it after 50 years, the retrofit raising of the bridge might cost $5 million. Discounted at 6 percent per year, the present value of that $5 million is $271,000. If we were certain the sea would rise, we could realize a benefit of $171,000 in this example by adding the meter of clearance today rather than waiting.
This kind of comparison of current and future investment should be performed when each adaptation option is considered. There are three key elements in this approach: the probability that the outcome will require adaptive action, the discount rate, and the time at which future spending would have to take place. Obviously, reducing our uncertainty about future climate would justify larger investments in adaptations.
Economical adaptation that lessens sensitivity to climate is desirable. Developing drought-resistant crops or using water more efficiently should enable us to deal with weather variability today and position us to cope with future climate change. Poorer countries may have greater difficulties. They typically lack money, information, and expertise. Often they are sorely stressed by current weather extremes, and additional strains accompanying climate change may make their lot worse. If greenhouse warming improves their situation, they may have difficulty taking advantage of their good fortune because of the limits on their capacity to respond.
In general, there are four limits on human responses to greenhouse warming. One is time. Time is needed for people to adapt in a location to a new climate, to design and build new infrastructures, or to adapt by moving to a region where the climate is preferred. Although time is needed to adapt managed things like farming, the historical evidence suggests that farmers can respond, especially in developed countries. The second limit is water. Some uses, like irrigation or cooling, use water in large quantities. Transporting
large quantities of water over great distances is possible but expensive. The third limit, and a common one, is money. Adaptations like furnaces and air conditioners, sea walls and canals take money. The fourth limit is techniques and information that are used to make decisions and set priorities.
The recommendations in this report address these areas. It is important that we incorporate these limitations into our thinking when we imagine the effects of the climate of 2030 imposed on the people and landscape of that time.
Adapting to Climate Change
Just as strategic planning requires ranking greenhouse warming with all the other changes ahead, it also demands sorting human activities and nature into classes of sensitivity and adaptability to greenhouse warming alone. Then the more sensitive and serious consequences of greenhouse warming can be ranked within the whole spectrum of changes, and adaptational responses can be decided accordingly. The Adaptation Panel (see Part Four) developed the classifications presented in Table 5.1, which are used here to categorize adaptation options with respect to the United States.
Activities with Low Sensitivity
Fortunately, several human activities have low sensitivity, allowing us to concentrate on others. Machinery and buildings are renewed faster than the projected pace of greenhouse warming, and so industry should have little trouble adapting. In general, the decision-making horizons in industry are shorter than the time at which most climatic impacts would emerge. Most industries in countries like the United States can thus be expected to adapt as the climate changes.
The expected climatic changes are within the range people now experience where they live and to which those who move usually learn to adapt. In industrial countries, public health should be less at risk than it is elsewhere. The pace of improvements in health from better technology and public measures can and likely will exceed any deterioration from greenhouse warming. Epidemics from causes already known, failure to control population, and chemical pollution are more serious threats to health than greenhouse warming.
Activities that are Sensitive but can be Adapted at a Cost
As the most valuable outdoor human activity, farming would have the greatest impact on national income due to greenhouse warming. Average warming would not greatly affect yields, but seasonal variations in precipitation
TABLE 5.1 The Sensitivity and Adaptability of Human Activities and Nature
and evaporation would. Experience shows, however, that farming must continually adapt to cope with, and even exploit, the stresses and fickle nature of climate. Adaptations to climate change would be required in both rich and poor countries to protect crops, substitute new ones, and protect their foundations of soil and water.
Although less thoroughly managed than farming and growing a crop with a long life, regeneration and management techniques are available that should enable needed forest products to be sustained.
Should climate warm, most cities would try to adapt rather than abandon their sites. Although the adaptation might be costly, the costs would in most cases be lower than the cost of moving the city. By far the highest costs would be in coastal cities, where added protection would be needed in response to storms if the sea rises. Where the coast is thinly settled, protective zoning or even retreat may be sensible.
For the nation as a whole, tourism and recreation seem adaptable to greenhouse warming at little net cost. Adaptation within a country or a region, however, may entail switching a function or activity from one geographical area to another. Some regions may win a new activity, while the same activity becomes untenable and is lost in another. The gradual changes foreseen in this study will combine these pluses and minuses, with a likely small net change for a nation of our size and diversity.
Activities that are Sensitive with Questionable Adjustment or Adaptation
In the unmanaged systems of plants and animals that occupy much of our landscape and oceans, however, the rate of change of some key processes may be slower than the pace of greenhouse warming, making their future questionable. Unmanaged ecosystems respond relatively slowly, and hence their adaptability to greenhouse warming is more questionable than that of the managed systems of crops on a farm or timber in a plantation.
This slow response comes from the long lives of some of their components, like trees that last longer than the ones planted for timber. It comes from the slow and chancy arrival of seed and birds traveling on the wind, in currents, or along corridors rather than being intentionally transported and planted by farmers. Response is slow because the replacement of plants and animals on an acre of wild land or in an estuary can take decades or centuries and because evolution takes centuries or millennia.
Greenhouse warming would not likely make land barren except at the arid extremes of existing climates if climate became drier. What is likely are changes in the composition of ecological communities in favor of those species that are able to move rapidly and far and the disappearance of some species that move slowly. Marine plants and animals inhabiting intertidal regions of rocky shores undoubtedly would be affected by rising sea level. Coral reefs, which are breeding and feeding areas for many of the world's tropical fisheries, could suffer because they appear to be particularly sensitive to water temperature changes.
Although the impacts of the whole range of climatic changes on the functioning of ecosystems cannot be predicted with confidence, the risk of their happening justifies some of the adaptation strategies recommended in Chapter 9 and adds to the justification for some of the mitigation strategies.
Cataclysmic Climatic Changes
Large changes in climate have happened in the past. Desperate masses of people have fled drought or flood in places with marginal farming and growing population. These disasters occurred before greenhouse gases began
increasing, and they could occur again. The panel knows of no convincing attempt, however, to compute the probability of cataclysmic changes such as the stopping of the current that warms Europe. Because the probability and nature of such unexpected changes are unknown, the panel cannot project their impacts or devise adaptations to them.
As discussed in Chapter 3, a rise in global average temperatures in the next century above those of any period in the last 200,000 years cannot be excluded. Unfortunately, there currently is no way to reliably determine the effects of such global changes for particular regions. These changes will probably be gradual. People in the United States likely will have no more difficulty adapting to such future changes than to the most severe conditions in the past, such as the Dust Bowl.* Other countries may have more difficulty, especially poor countries or those with fewer climate zones. Some natural systems of plants and animals would be stressed beyond sustainability in their current form, a prospect some people may find unacceptable. The stronger the concern about these various changes, the greater the motivation to slow greenhouse warming.
In addition, the panel has not found it possible to rule out or rule in such major disturbances as sudden and major changes in regional climates, ocean currents, atmospheric circulations, or other natural or social phenomena. At present, it is not possible to analyze their likelihood or consequences.
Human societies and natural systems of plants and animals change over time and react to changing climate just as they react to other forces. It would be fruitless to try to maintain all human and natural communities in their current forms. There are actions that can be undertaken now, however, to help people and natural systems adjust to some of the anticipated impacts of greenhouse warming. The panel recommends action now (see Chapter 9) based on gradual climate change. Such action would be more important if climate change proved to be sudden and unanticipated rather than smooth and predictable.
*See dissenting statement by panel member Jessica Mathews at the end of Part One.