Most scientists believe that if significant climate change occurs it will take place gradually over a period of many decades. If change is gradual, the overall economic impact on wealthy countries such as the United States will probably be modest although some regions or groups may experience large costs and others may experience large benefits. After all, American society already exists very successfully in Alaska, Arizona, and Florida and these states span a range of climates much wider than any predicted changes. Farmers would have to adjust their crops, and in some cases, farming regions and other land use patterns would shift. Some water supply systems would have to be modified. Low coastal areas would have to make adjustments. But, our society regularly makes changes to adapt to natural and man-made fluctuations. It could probably handle these additional changes without much trouble, although nationally the total costs could add up to many billions of dollars.
While many of the impacts of climate change would be negative, some might be positive. Heating costs in northern areas might decline, agricultural productivity in places such as Canada, Scandinavia and northern Japan might be improved, and the amount of sunlight available for grain crops might increase as the regions where they grow shifts further north. Of course, not all northern regions would benefit. Some northern soils are not suitable for agriculture, some areas of permanently frozen ground (permafrost) might become large impassable bogs, and various insect pests and diseases might move north.
Several economists have tried to estimate the overall economic cost of climate change for the United States. For the sorts of gradual changes being predicted over the next century, they estimate costs in the vicinity of a quarter of one percent per year of GDP (gross domestic product). Such calculations are, of course, very uncertain.
There is some chance that climate change will be abrupt, perhaps brought on by a sudden shift in the general pattern of ocean circulation. If that happens, the economic costs to wealthy countries like the United States could be very large. Much new investment might be needed in a very short period of time. Agricultural and water systems might not easily be modified in just a few years, especially if uncertainty makes planning difficult. Most scientists believe that such catastrophic change is unlikely, but not impossible.
Of all human economic activities, agriculture is potentially most vulnerable to the effects of climate change. Here are brief descriptions of two recent studies:
Global Impacts: An international group of agricultural researchers used climate projections from three climate models (GCMs) to project regional climate changes at 112 locations in 18 countries under the assumption that the amount of carbon dioxide in the atmosphere had doubled. Average global temperature increased about 8°F (4.5°C). Regional agricultural experts projected the yields of wheat, corn, soybeans, and rice at each location. An economic model was then used to estimate patterns of world food prices and trade. Assuming that farmers employ simple adaptation practices, such as changing planting times and seed varieties to match the changed local climates, they estimate global food output to be unaffected for the case of one climate model, and to drop by 2% and 6% respectively for the other two climate models studied. The developing world is hit harder than the developed world. Including the effects of comparative costs in world trade, developed country output is predicted to rise between 4 and 14% and developing country output to fall by 9 to 12%. World food prices go up. The number of people at risk of hunger (due to higher prices) probably also goes up, perhaps by 50%. This analysis assumed that no major changes, such as construction of new irrigation projects, are undertaken. If such changes are included, the agricultural impact on all but the poorest developing countries probably becomes very small.
Source: C. Rosenzweig and M. L. Parry, "Potential Impact of Climate Change on World Food Supply," Nature, Vol. 367, pp. 133-138, 1994 January 13.
Impacts on the U.S. Great Plains: In a study of Missouri, Iowa, Nebraska and Kansas, called the MINK study, researchers studied both best-case and worst- case scenarios of the effect of a 2°F (1.2°C) rise in temperature, similar to the temperature change during the dust bowl era of the 1930s. In the best-case scenario, they assumed that higher carbon dioxide levels would speed plant growth while reducing water consumption. They also assumed that farmers would adapt with such strategies as earlier planting, using plant varieties with a longer growing season, and changing tillage and irrigation practices.
In the worst-case scenario, they assumed that crop growth would not be aided by increased concentrations of carbon dioxide and that farmers would not adapt successfully to the warmer climate. Here are the results:
The Worst-Case Scenario
The Best-Case Scenario
Source: N.J. Rosenberg and P.R. Crosson, The MINK Project: An overview, Washington, DC: U.S. Department of Energy, 1991.
What can we say about economic impacts in poor countries?
Whether it is fast or slow, climate change is likely to have greater economic impacts on poor countries than on rich countries. Two factors lead to this conclusion. First, poor countries are forced to live "closer to the edge" and have less capacity to adapt to changes. Compare the flooding by the Mississippi river in 1993 with various major floods you have heard about in developing countries such as Bangladesh. While the Mississippi floods were serious, the U.S. was able to adjust to them remarkably smoothly. Very few people died, aid was supplied by other parts of the country, food prices were hardly affected, and people got on with their lives. A similar flood in many poor countries would kill tens of thousands of people and cause massive disruptions in food supply, widespread disease, and economic dislocation for many years.
The second reason that some poor countries are likely to be affected more severely is that the people in many poor countries live traditional lives in cultures that depend much more directly on a specific climate. Their agricultural practices, their housing, and many other aspects of their way of life, are adapted to local climate conditions. These traditional ways have been passed down for countless generations. Because of low education levels and strong cultural traditions, changing these ways in response to climate change may be very difficult.
On the positive side, some countries, such as India and China, may become more wealthy during the next century, and find it easier to cope with climate change. Other countries, that remain very poor, may have so little capital investment to loose that changing to new circumstances may be less costly for them than for partly developed countries.
How could climate change affect natural ecosystems?
We know that climate affects plants and animals in the natural environment. Many of us have seen the effects of such short-term climatic variations as droughts. On a longer time scale, scientists have reconstructed the history of past climates, such as ice ages, and shown that the ecology of entire continents has undergone profound shifts.
Of course, many factors other than climate can affect natural ecosystems. Among these, changes in human land use are probably the most important. For example, consider the enormous ecological impacts that were associated with the European settlement of the North American continent over the past 300 years.
While the ecological disruptions caused by climate change may not be as large as those caused by major changes in human land use, they still could be severe. How severe depends critically on how rapidly climate changes. Individual birds and animals can move. Plants and trees can only move from one generation to the next. If climate warms slowly, trees and plants will gradually migrate north. For example, most forests could probably cope with warming of 1-3°F over the next century. In contrast, while it is unlikely, warming of 5 to 10°F over the next century would undoubtedly cause great problems.
Not all species are likely to move at the same rate. Thus, even if change occurs relatively slowly, the various mixes of species that occur in the ecosystems of a warmer world might be somewhat different from the mixes of species that make up current ecosystems. Differences in things such as soils and soil microbes may also affect the ease with which different kinds of plants can move. Even if change is slow, some species may become trapped by natural barriers such as mountain ranges or large cities, and be unable to move. Unless humans intervene with preservation efforts, these species could be lost.
People value natural ecosystems partly in terms of what they have gotten used to. For example, many of today's New Englanders place a high value on the maples, birch, and white pines that make up their forests. The assurance that in the future such a forest may be preserved in Quebec or Ontario, while New England acquires a red pine and oak forest like that in the Carolinas, may offer small solace to these people! On the other hand, most of their great great grandchildren may not be aware that any change has occurred, just as today's New Englanders do not recall the deforested landscape of the 1860s.
The effects of climate change on tree growth.
Different species of trees grow best in different climates. Sugar maple trees do not grow in Florida, palm trees do not grow in New Hampshire. If climate changes, the regions in which different tree species occur may shift. The drawing shows estimates based on the results of two different climate models of how the locations where sugar maples can grow might shift in North America.
If they have the climate they need, and if they have plenty of water and nutrients, most trees are able to grow better in air that contains more carbon dioxide. The carbon dioxide acts as a fertilizer. However, it is not clear whether, as the amount of carbon dioxide in the atmosphere increases, natural forests will grow more vigorously. This is because natural forests often do not have enough water and nutrients to take advantage of the increased carbon dioxide.
As carbon dioxide levels increase, trees and many other plants may take up more carbon and offset much of the increase. Alternatively, as climate changes, many trees and other plants may die, and as they decay, the carbon they contain may be released to the atmosphere, accelerating the rate of carbon dioxide buildup. Scientists do not know which of these processes will be more important.
Since different plants respond differently both to different climates, and to different levels of carbon dioxide, the mixture of trees and other plant species that occur together may change over time. At a given location, some trees and other plants will find it easier to grow, some will find it harder. Balances may shift in complicated ways. This is particularly true when we add such complicating factors as pests and fire.
Learning from past climate change.
The earth's climate has changed continually through the past. If we can better understand some of these past changes, we may be better able to anticipate the impacts of possible future changes. The last ice age ended about 13,000 years ago. Studies of pollen in sediments suggest that stands of trees moved slowly north at a speed of between six and thirty miles per decade. The fastest rate recorded is about 125 miles per decade, for spruce moving back into northern Canada about 9000 years ago.
After a period of rapid warming, about 11,000 years ago there was a sudden and quite dramatic cooling which climatologists call the "Younger Dryas" cooling. This may have happened because of sudden changes in ocean circulation, perhaps triggered by fresh water runoff from melting glaciers. Temperatures may have dropped by up to several degrees in just a few years. Pollen in sediments, and other evidence, suggest that large-scale ecological disruptions may have occurred in Europe and North America.
More recently, there has been a smaller and more gradual period of cooling that began in about 1450 and ended in the late 1800s. Historical records suggest that the coolest periods were in the mid and late 1600s, and early and late 1800s. Known as the "little ice age," the beginning of this period of cooler weather and locally expanding glaciers was probably responsible for putting an end to Norse settlements in Greenland.
Dust from volcanos can produce brief periods of cooling. Dust from the 1991 Mt. Pinatubo eruption in the Philippines caused slight cooling for several years. A much more dramatic example is provided by the eruption of Tambora in Indonesia in April of 1815. This was followed by unusually cold temperatures for the next two years, 2 to 4.5°F (1-2.5°C) cooler than normal, and serious crop losses. In New England, 1816 was known as "the year without a summer."
Despite some disruptions such as the Younger Dryas cooling, the little ice age, and the year without a summer, overall the climate during the "interglacial" period since the last ice age has been pretty stable. The previous "interglacial" period may have been less stable. Evidence from ice cores collected in Greenland suggest that the climate may have hopped around between three different states: one similar to today's climate, one several degrees warmer than today's climate, and one several degrees colder than today's climate. Some of these changes occurred in periods of just a few decades. It is unclear if these were global- or hemispheric-scale changes, or just local changes in the vicinity of Greenland. If they were global, future studies may help us understand how rapid changes can occur and how ecosystems respond.
Scientists use a variety of methods to reconstruct past climates. On time- scales of thousands of years, they use both human records as well as tree rings and other direct biological evidence. On time-scales of tens of thousands of years, they use data collected from deep ice cores in arctic and antarctic glaciers. Tiny bubbles of gas trapped in the ice give some information about temperature as do grains of pollen and dust. They can also use data from sediments from stable locations such as the deep ocean. On time- scales of millions of years or more, scientists have to rely on evidence such as fossil remains in rocks.
Can you be more specific about some of the possible effects of climate change?
Because of the many uncertainties, it is not easy to be more specific. However, the paragraphs below provide some details about the topics of disease and health, insects, coral reefs, and mangrove swamps.
Disease and health are always issues of concern. Some scientists have suggested that diseases borne by insects, such as mosquitoes, might become more common in a warmer world, or shift their ranges into populations that do not have as many natural defenses. In a recent review of the available evidence, a scientific workshop conducted by the U.S. Environmental Protection Agency concluded that "...it is not well understood how changes in climates, particularly gradual changes, will affect disease patterns...Without knowing exactly what changes in climate will occur...it is impossible to predict what the impacts will be." Compared to current threats to human health such as viral epidemics and environmental pollution, risks from gradual climate change are likely to be modest.
At extremes of heat or cold, temperature itself can cause health effects such as heat stroke or frostbite. Studies of the patterns of deaths in U.S. cities suggest that the residents of very warm or cold climates take measures to adapt and protect themselves. Thus, it seems unlikely that temperature changes from global warming would have direct health consequences in the U.S.
Insect populations that feed on farms, forests, and natural ecosystems might be affected by climate change. In natural ecosystems, since different plant species would likely migrate at different speeds, and with different levels of success, the mix of pests with which they would have to cope might change significantly.
Under certain circumstances plants grow more rapidly in the presence of carbon dioxide. How this might interact with pest populations is unclear. On the one hand, more plant mass might mean more food for pests. On the other hand, if the ratio of carbohydrates to other nutrients in plant tissue were to change as a result of growth in a carbon dioxide rich atmosphere, pests might have to eat more in order to get the nutrient material they need to survive. How all these changes might interact and affect overall pest populations, or the levels of destruction they cause, is something that we will not be able to estimate until biological scientists have conducted many more studies.
Coral Reefs, which sustain two-thirds of all marine fish species and support human communities by providing fisheries and storm protection, may be affected in at least three ways. First, corals may "bleach." Corals thrive in a fairly narrow range of water temperatures. If the temperature becomes too high, corals expel the algae which give them their color and supply their food. With their food source gone, the corals stop growing and, if the algae do not become re-established, may die within a few months. In recent years, scientists have observed a number of instances of bleaching, but the causes are uncertain. Whether modest global warming would damage corals through bleaching is unclear. Second, if storms increase in a warmer world, corals may be physically broken up and be unable to re- establish themselves. Wave action is particularly damaging to branching corals. Third, sea level rise may affect corals, but it can be either beneficial or destructive, depending on how much and how rapidly it occurs. Some scientists believe that the rates of sea level rise currently predicted will be "moderately beneficial" to reefs, allowing some to expand their current boundaries while not adversely affecting the others.
Mangrove swamps are found in coastal tide-lands in Florida, India, Australia, Africa, and other subtropical and tropical zones. Mangrove trees grow above the water but have branching roots that are often flooded. They provide protective habitat for a wide variety of species and act as sediment traps, maintaining water quality, and both building up and protecting coastlines from erosion. Many coastal tropical fish are highly dependent on mangrove swamps for nursery, feeding, and spawning grounds.
Already under severe pressure from human activities such as coastal development and water pollution, mangrove ecosystems are further threatened by the sea level rise associated with global warming. The IPCC predicts that global warming would cause sea level to rise just under 2 inches (4.5 cm) per decade. Most mangrove ecosystems can at most tolerate a rise of only about 0.5 inches (1.3 cm) per decade. Furthermore, it would be difficult if not impossible to protect mangrove ecosystems from the rising seas because traditional methods of coastal protection such as sea walls cut off the circulation of nutrients and sea-water necessary for mangrove survival. In a recent survey, the World Wildlife Federation concluded that, because of the impact of human activities, the rate of sea level rise and the very limited options for protection, "the world's mangroves are likely to face severe disruption in the next few decades."
How much might the sea level rise?
Tides and winds move the level of the oceans up and down all the time. "Sea level" refers to the ocean's average level over a long time. In many parts of the world, sea level changes gradually as the coast or the ocean floor rises or falls due to natural geological changes or human actions such as pumping large amounts of oil out of the ground. In addition to these often large local changes, over the past century the average sea level has been rising at a rate of between 0.4 to 0.8 inches (.5 to 1 cm) per decade. Scientists are uncertain why this is occurring. There is no persuasive evidence that the rate of rise has increased in recent years.
If the climate becomes warmer, the oceans will warm. As water warms, it expands slightly. Thus, in addition to the local variations that now occur in sea level, global warming might cause a general increase in sea level all around the world. As Part 2 of the main brochure explains, scientists estimate that a warming of 3.5°F (2°C), which will probably not occur before about 2075, will cause sea level to rise between 8 and 30 inches (30-76 cm). If warming were to continue long enough, and become large enough, mountain glaciers and polar ice caps might melt. This could release large amounts of additional water into the oceans and result in significantly greater sea level rise. This is unlikely to occur any sooner than 150 years from now. Nor is it clear that if warming continues, glaciers will melt. A somewhat warmer climate causes more precipitation. In polar regions, this means more snow. Modest warming could actually work to build glaciers and slow or even reverse sea level rise.
What might happen if sea level rises?
If global warming were to cause sea level to rise a couple of feet over the next century, two types of problems would result: permanent flooding of very low lying areas, and increased storm damage. Permanent flooding could pose problems for certain coastal ecosystems, for highly vulnerable cities such as Venice, and for some coastal drinking water supplies. However, the larger problems are likely to come with storms. When storm winds blow onto shore they cause water to "pile up." If the sea level rises, the amount of this "storm surge" may increase, with the result that coastal ecosystems may be flooded more often, some beaches may be eroded more rapidly, and building and other structures along the coast may suffer greater and more frequent damage. The box below presents a case study of this problem for Ocean City, Maryland.
Developed countries like the U.S., and even low lying developed countries like the Netherlands, can use a combination of land use laws, and technologies such as dikes and storm surge barriers to minimize damage. In contrast, heavily populated coastal areas in developing countries such as Bangladesh might suffer enormous losses of life and property.
In the long run, if sea level continued to rise, even developed countries might begin to experience serious costs. Many of the world's biggest cities are in low lying coastal locations. If, as seems likely, these cities respond to sea level rise by building dikes, rather than by gradually relocating, the result over hundreds of years could be that a growing proportion of the world's population would live in locations below sea level that are vulnerable to sudden catastrophic floods.
A case study of the impacts of rising sea level.
Ocean City, Maryland is a beach resort located on Fenwick Island, a long thin barrier island half a mile off the Maryland coast. In the center of town, on the ocean-side of the island, there is a boardwalk that runs north-south along the wide sandy beach with hotels, restaurants and gift shops. Along the bay-side of the island there are sheltered marinas and private residences. There are also private beach-front residences both to the north and south of the central strip. As of today, the annual economic benefit from recreational activity in Ocean City is estimated to be $28 million per year.
Even without climate change the beach gets washed away by storms. On several occasions, the U.S. Army Corps of Engineers has had to dredge sand from offshore to rebuild the beach. Occasionally, large storms also cause property damage through wave action and flooding. Without any sea level rise, over the next fifty years, it is expected that the property losses from these storms will average about $7 million per year. We say "average" because in some years there will be no losses, and in other years, large losses.
Over the next 100 years scientists estimate that sea level at Ocean City may rise between 4 and 35 inches (10-90 cm). If this happens, and if storm patterns remain unchanged, computer studies conducted by Anand Patwardhan at Carnegie Mellon University, suggest that the additional property losses due to storms will range between $0.5 and $2 million per year. In addition, he estimates that the economic losses due to impacts on recreational use will average between $2 and $5 million per year. If, as some scientists predict, climate change produces more frequent or more intense storms, these costs would probably rise.
Ocean City might reduce these costs by moving or abandoning structures, or by building bulkheads. However, Dr. Patwardhan's studies find that for reasonable planning assumptions (discount rates of less than 10%), the best strategy is to continue the practice of rebuilding the beach with sand from offshore.