Natural ecosystems--These may be the systems most vulnerable to climate change. We are least able to intervene and help with adaptation of natural ecosystems because of limited knowledge about ecological processes (see chs. 2 and 3). The shift in climate zones may far exceed the ability of vegetation such as forests, to adapt through migration (see fig. 1-7). Climate zones may shift hundreds of miles in a century whereas natural rates of dispersal and colonization may be on the order of tens of miles in the same time period (35). In addition fire and disease could result in rapid die back of many existing forest and other terrestrial ecosystems(157). Helping plants to migrate through such activities as widespread seed dispersal would be very expensive and have dubious prospects for success(188). These Issues are discussed in detail In "Forests" (vol. 2 ch. 6).
Climate change could also lead to a loss of diversity. Isolated ("island") species may find themselves in climate zones that are no longer suitable for their survival (132). The potential for migration of plants and animals to new suitable habitats is not known, but barriers such as water bodies or development could impede migration (see fig 1- 6). Species in mountainous terrain could migrate to higher elevations. This creates reduced habitat areas which are correlated with reductions in species diversity. For example a study of a 5 deg.F (3 deg.C) warming in the Great Basin National Park in eastern Nevada concluded that it would cause 20 to 50 percent of species in individual mountain ranges to go extinct (108). The ability for human intervention to maintain species diversity in the face of climate change is currently limited. Selected species could be transplanted to new habitats but this could be very resource intensive and would only be feasible in certain cases; little research has actually been done on transplantation of multiple-species systems. Migration corridors could be created, but their chances of success are limited because migration rates are slow and the direction of species migration is unknown. In addition, the creation of corridors may be relatively expensive compared with setting aside new protected areas (154). These issues are discussed further in "Preserves: Federally Protected Natural Areas (vol. 2 ch. 5).
Climate change can result in the loss of coastal wetlands directly through sea level rise and indirectly, through interaction with societal response to sea level rise. Many coastal wetlands will likely be inundated because the sea will rise faster than wetland sediments accrue (161). Some wetlands will adapt to climate change by migrating upland with the rising tides The areas with the greatest risk of wetland loss are along the Gulf and East Coasts of the United States (see fig. 1-4). This will result in a net loss of wetlands because vast areas of tidal flats such as in the Mississippi Delta will be inundated while inland migration will create new wetlands having only a fraction of the area of today's wetlands. [2] This net loss of wetlands will be even larger where coastal structures such as bulkheads or levees block the inland migration of wetlands (162).
Even it were feasible to create new coastal wetlands the costs of this would be so high that large-scale restoration programs would become unattractive. The average cost of creating wetlands has been estimated at roughly $20,000 to $45,000 per acre ($50,000 to S100,000 per hectare) [3] not including land acquisition costs.[4] This figure however can vary from just a few hundred dollars per acre to many hundreds of thousands of dollars per acre. Though technology is improving (see vol. 2 box 4-A) attempts to recreate wetland structure and function fully have been limited. Prohibiting the construction of or removing coastal structures would enable more wetlands to colonize upland areas. It may not be feasible to move some existing coastal structures that impede wetland migration. For example it is unlikely that areas of dense development would be relocated.
Prairie pothole and riparian wetlands in regions that get drier may be at greater risk than those in regions that get wetter. For example, in the North Central States, increases in temperatures and evaporation rates could cause many prairie potholes to shrink or disappear, leading to further declines in already diminished continental waterfowl-populations (9). Tundra may shrink as increased temperatures allow the permafrost to thaw and drain (see box 1-G). In addition, wetlands of any type that are already degraded by pollution, water diversions, or fragmentation may also be particularly vulnerable (198, 199). The status and vulnerability of coastal, riparian, depressional, and tundra wetlands are discussed in "Wetlands" (vol. 2, ch. 4).
Fisheries--The potential effects of climate change on aquatic ecosystems have been studied very little to date, and could vary significantly. In some cases, marine fish may be able to migrate to new, more suitable habitats, depending on several factors, if food sources are available (80). Some freshwater fish in open waters, such as the Great Lakes, could benefit from a larger thermal niche (98). Fish in small lakes and streams, however, may suffer from increases in temperature that adversely affect survival, reproduction, or their ability to migrate to cooler locations (101). Changes in water quality will also affect the survival of aquatic organisms. Climate change may alter circulation patterns in many lakes, reducing dissolved-oxygen concentrations. Higher temperatures will also act to reduce dissolved-oxygen concentrations (71). Sea level rise will increase saltwater intrusion of estuaries, potentially benefiting marine fish at the expense of freshwater species (80). However, changes in estuaries could have broad impacts on the U.S. fishery. By far, the greatest portion of commercial catches, with the exception of those from Alaskan fisheries, are composed of estuarine-dependent species (139). Ongoing alterations of critical habitat (such as those caused by geographic fragmentation and pollution) may be exacerbated by climate change. Box 2-C (ch. 2) discusses, by region, the condition and value of fisheries today, current problems, and the potential impacts of climate change.
Agriculture--This system is very sensitive to climate, but climate change impacts may be offset by intense management over short time frames. High temperatures and drought could reduce crop yields, although this effect could be counteracted by higher atmospheric concentrations of carbon dioxide and longer growing seasons in higher latitudes (129). The potential for agricultural adaptation, particularly at the farm level, is very high (30). Changes in management practices (e.g., changing planting dates or using irrigation or crop-switching) can reduce or eliminate many of the potentially negative impacts of climate change. Shifts in climate zones would result in changes in relative productivity levels, with some areas increasing output, and other areas reducing output due to increased competition(1). See "Agriculture" (vol. 1, ch. 6) for further discussion.
Coastal resources--Cities, roads, airports, and other coastal resources are vulnerable to flooding from sea level rise and hurricanes. The population near the coast is growing faster than populations in any other region of the country, and the construction of buildings and infrastructure to serve this growing population is proceding rapidly. As a result, protection against and recovery from hazards peculiar to the coastal zone, such as hurricanes and sea level rise, are becoming ever more costly (11). The combination of popularity and risk in coastal areas has important near-term consequences for the safety of coastal residents, protection of property, maintenance of local economies, and preservation of remaining natural areas. These points are discussed further in "Coasts" (vol. 1, ch. 4).
Water resources--These resources are vulnerable to several climate change impacts. Changes in precipitation and higher levels of evapotranspiration can combine to affect surface-water and groundwater supplies, flood and drought frequency, and hydropower production. Arid basins could experience the largest relative change in water flow from climate change (67). Numerous studies have been conducted on the relative vulnerability of the major U.S. river basins to flood and drought, supply distributions, hydropower reductions, groundwater overdrafts, and extreme events (48, 49, 88, 188). They conclude that the water resource regions most vulnerable to some or all of these events are the Great Basin; California, Missouri, Arkansas, Texas Gulf, Rio Grande, and Lower Colorado (see fig. 1-5). See "Water" (vol. 1, ch. 5) for more information; Appendix 5.1 Lists State-by-State problems.
Human health--Climate change could affect human health, but there is a great deal of uncertainty about whether mortality and morbidity would actually increase and about the potential for adaptive measures (such as the use of air conditioning) to offset any negative impacts. Several studies have concluded that the potential range of infectious diseases could shift with climate change, but the exact nature of these shifts is uncertain (94). Even if the range of disease-carrying vectors, such as mosquitoes, changes enhanced pest-control measures could nullify the increased threat of disease. Effects of climate change in other countries could displace some populations. If "Environmental refugees" lead to an increase in immigration, there is the potential for increased importation of communicable diseases into the United States (164). Other studies have shown that climate change could lead to increased cases of heat-stress mortality (74). Uncertainties about changes in human physiological and behavioral response make it difficult to draw conclusions about the risks of climate change to human health.
Energy--Higher temperatures will no doubt increase energy demand for cooling and decrease energy demand for heating. This would result in an increase in the demand for electricity (primarily for air conditioning) and for electric-generating capacity (93). This new demand would not be completely offset by reductions in the use of oil and gas for heating (86). The largest capital costs would be associated with increased power plant construction which could cost as much as $170 to $320 billion about 12 percent more than the increases in capacity needed to meet population and economic growth through the middle of the next century (93). As with sea level rise adapting to increased energy demand could involve significant costs.
Transportation--Some forms of transportation could be positively or negatively affected by climate change Inland shipping maybe the most sensitive to climate change. On the one hand, warmer winters would likely result in less ice cover and a longer shipping season. For example ice cover on the Great Lakes could be reduced by 5 to 13 weeks (4) lowering shipping and related costs (78). On the other hand lower river flow and lake levels could increase shipping costs by reducing shipping tonnage capacity or blocking shipping (143). Some roads near the coast may have to be moved or protected from sea level rise. In many instances, adaptation is highly probable in transportation at some cost to the economy (see vol. 1 box 5-E "Navigating the Mississippi through Wet and Dry Times").(see summary table)
1 This box is a compendium of information drawn from previous studies, recent research, and OTA's assessment. The back chapters of this report discuss a subset of these issues. 2 S. Leatherman, University of Maryland at College Park, personal communication, November 1992. To convert acres to hectares, multiply by 0.405. 3 To convert acres to hecters, multiply by 0.405. 4 D. King, Chesapeake Biological Laboratory, University of Maryland, personal communication, November 1992.
