By Jon Kusler, New York Association of State Wetland Managers, and Virginia Burkett, National Wetlands Research Center, USGS

Climate change associated with increased carbon dioxide and other greenhouse gases poses significant threats to many of the world's coastal, estuarine and non-tidal wetland ecosystems. Tundra, prairie wetlands, bogs, swamps and other wetlands also play an important global role in reducing the amount and rate of increase in atmospheric carbon dioxide. As a result, destruction of wetland areas can result in a positive biotic feedback to global warming through the release of large amounts of stored carbon to the atmosphere. To date, there has been limited discussion in the US and internationally concerning the impacts of climate change and variability upon wetland ecosystems, or the role that wetlands may play in carbon sequestration.

Potential Impacts

Wetlands exist in the transition zone between aquatic and terrestrial environments, and can be dramatically affected by slight alterations in hydrology. Predictions summarized by the Intergovernmental Panel on Climate Change (IPCC, 1998) indicate a warmer climate over North America for the next century accompanied by changes in precipitation patterns. Such changes would strongly affect wetland ecological functions through changes in hydrology, biogeochemistry, and biomass accumulation.

Sea-level rise is regarded as one of the more certain consequences of global climate change. During the past 100 years sea level has risen at an average rate of about 1-2 mm per year (or 4 to 8 inches per century). The projected two- to five-fold acceleration of global average sea-level rise during the next 100 years will inundate low-lying coastal wetland habitats that cannot move inland or accrete sediment vertically at a rate that equals or exceeds sea-level rise.

A rise in mean global temperature of 1-3.5ºC over the next century, combined with reduced, stable, or even slightly increased total precipitation, would seriously impact some freshwater wetlands. Montane and alpine wetlands with temperature-sensitive plant and animal species may be particularly affected because they have little, if any, potential for migration. Increasing air and water temperatures are already resulting in permafrost degradation, declining water levels in boreal peatlands, and drying of wetlands at lower latitudes. Relatively small changes in precipitation, evaporation, or transpiration which alter surface or ground water level by only a few centimeters will be enough to reduce many wetlands in size, convert some wetlands to dry land or shift one wetland type to another. Changes in maximum and minimum temperature (not simply mean temperature) and in precipitation patterns (not simply total precipitation) may also have significant impacts. For example, reduced precipitation in the winter and spring may affect critical bird migration or nesting, although mean precipitation and water level remain constant.

Existing stresses and man-made alterations make wetlands more susceptible to changes in climate than most deep-water and upland habitats. Many wetlands in the lower 48 states have been drained and impounded for agricultural development; levees have been constructed around them to prevent flooding, and rivers that provide essential water and nutrients have been channelized, dammed, and diked. Approximately one half of the wetlands that existed in the lower 48 states at the time of European settlement have been converted to other uses. Due to fragmentation, wetland plants and animals cannot naturally "migrate" to other locations over time in response to temperature and water level changes. Many coastal or estuarine wetlands will be unable to migrate inland in response to sea-level rise, due to dikes, levees, fills, or other development.

Impacts of climate change will vary depending upon the types, magnitudes, and rate of changes in temperature, precipitation, and other factors. Each plant species (there are more than 6,000 listed wetland plants alone) may respond somewhat differently, although certain general responses may be expected. For example, increased CO2 will increase growth rates and biomass accumulation in most plants, but differential responses among species can influence plant competition and community structure. Conversely, a combination of increased temperature and reduced precipitation in some areas of the nation may result in decreased runoff and lowered groundwater levels, causing the drying of some wetlands and a change in wetland types for some others.

In summary, wetland types likely to be substantially impacted by climate change include:

• Coastal and estuarine wetlands - Coastal and estuarine wetland habitats may be destroyed if sea-level rise exceeds the rate of vertical sediment accretion and inland migration is not possible. Submerged aquatic vegetation, coastal marshes, baldcypress swamps, coastal bottomland hardwood forests, and other wetland types may all be affected by salt water intrusion.

• Permafrost wetlands - Vast expanses of tundra, marshes, and wet meadows underlain by permafrost may be dramatically altered by changes in hydrology associated with increased temperature. A warming of 5º C would melt virtually all of the subarctic permafrost in Alaska, which would affect more wetland acreage than is presently found in all other states combined. Massive wetland systems of the Yukon-Kuskokwim delta in western Alaska are vulnerable to both permafrost degradation and sea-level rise.

• Peatlands - Bogs, fens, and other largely organic wetlands at lower latitudes are highly vulnerable to subtle changes in ground water level, which plays a crucial role in the accumulation and decay of organic matter.

• Alpine wetlands near the tops of mountains - Even small amounts of warming may destroy "relic" plant and animal species in alpine wetlands because there will be little opportunity to migrate to other locations.

• Prairie pothole wetlands - Reductions in wetland size and the disappearance of some wetlands can be expected with increases in temperatures and/or reduced precipitation in the prairie pothole region. Recent work suggests that the predicted increase in temperatures in the Northern Great Plains over the next 50 years will result in more frequent droughts and declines in the numbers of both prairie wetlands and ducks.

• Other "drier end" depressional, slope, flats, river and lake fringe wetlands - Drying, decrease in wetland size, and conversion to uplands can be expected for most freshwater wetlands where precipitation decreases or remains steady while temperatures are increased because these wetlands are very sensitive to subtle changes in precipitation and groundwater level.

On the other hand, some riverine, lake fringe, and other wetlands in regions of the nation with increased rainfall will increase in size, and vegetation biomass may increase in wetlands overall due to rising CO2 levels. This could happen in the southeast and the northeast, where precipitation is likely to continue to increase. Wetland expansion is not likely, however, where shorelines have been "hardened" by bulkheads or where drainage is improved to prevent flooding. There may be exceptions where water levels fall as well, such as the Great Lakes where lowering of water levels may expose wide flats or benches which will be colonized by wetland vegetation.

Mitigation Options

There are no practical options for protecting wetlands as a whole from increased temperature, changes in precipitation, or rapidly rising sea level - although a variety of management measures could be applied on a wetland by wetland basis to increase the resiliency of specific wetlands or to reduce or partially compensate for impacts. Many of these measures would be considered "no risk" or "low risk" and could be justified based upon non-climate threats to wetlands alone. For example, increased protection for existing wetlands and removal of stresses (e.g., water pollution) may not only reduce the sensitivity of plants and animals to small changes in temperature or precipitation, but also achieve broader wetland protection and restoration goals.

Other "no risk" measures for achieving broader objectives and reducing climate change impacts include: development setbacks for coastal and estuarine wetlands; sediment diversions for dams; linking presently fragmented wetlands and waters to provide the passageways and corridors needed for plant and animal migration; using water control structures for some wetlands to enhance particular functions and address decreased precipitation and/or increased evaporation; increasing management programs for exotic species; and implementing various wetland restoration measures.

Federal, state, and local governments should, on a regional basis, identify and target for active management those wetlands that are expected to be most susceptible to small changes in climate. Wetlands which will meet not only present but future needs (e.g., waterfowl production) under various climate change scenarios should receive high priority for protection, acquisition, and management. Wetland restoration and creation may also be used to offset some of the impacts of climate change. For example, salt marsh restoration might be implemented in tidally restricted or degraded wetlands. New peatlands might be created through impoundment in some areas. But, there will be both economic and other practical limits (e.g., limited availability of land) upon use of such methods.

Note: A follow-on article by the authors entitled "Climate Change in Wetland Areas Part II: Carbon Cycle Implications" will be published in the next issue of Acclimations.

For more information, contact:

Jon Kusler, Director, Association of State Wetland Managers, P.O. Box 269, Berne, NY 12023-9746 (518-872-1804); Virginia Burkett, Chief, Forest Ecology Branch, National Wetlands Research Center, US Geological Survey, Lafayette, LA 70506 (318-266-8636)