EXECUTIVE SUMMARY

Although there are many different systems for defining and classifying wetlands, for this chapter we define wetlands generally as areas of land where the water table is at or near the surface for some defined period of time, leading to unique physiochemical and biological processes and conditions characteristic of waterlogged systems. Wetlands exist in both inland and coastal areas, covering approximately 4-6% of the Earth's land surface. They are found on every continent except Antarctica and in every climate from the tropics to the tundra. This chapter examines the possible impacts of climate change on non-tidal (primarily inland) freshwater wetlands.

Wetlands have many functions that have socioeconomic benefits: They provide refuge and breeding ground for many species, including commercially valuable ones; they are areas of high biodiversity; they control floods and droughts and improve water quality; and they are used for recreation and education. The direct economic value of these benefits varies between regions.

Human activities--such as the conversion of wetlands to agricultural and forest lands, construction of dams and embankments, and peat mining already pose a serious threat to wetlands worldwide. Mainly as a result of these activities, it is estimated that more than half of the world's wetlands have disappeared during the last century. These anthropogenic effects are most notable in densely populated areas and are expected to increase, especially in developing countries.

We are highly confident that climate change will have its greatest effect on wetlands by altering their hydrologic regimes. Any alterations of these regimes will influence biological, biogeochemical, and hydrological functions in wetland ecosystems, thereby affecting the socioeconomic benefits of wetlands that are valued by humans. Due to the heterogeneity of nontidal wetlands, and because their hydrologic conditions vary greatly within and among different wetland types and sites, the impacts of climate change on these ecosystems will be sitespecific. Impacts can be generalized for specific wetland types and, to some degree, wetland regions. However, generalization across wetland types is difficult and cannot be made in terms of locations or wetland categories.

We are highly confident that hydrologic changes or other disturbances that change the vegetation types in wetland areas will affect other wetland functions as well. However, many wetlands have inherently high spatial and temporal variability in plant communities due to climatic variations (e.g., seasonal flooding or drought) and variations in microtopography. We are confident that in some wetlands a changed plant community as a result of climate change will resemble at least some component of the existing community.

We are highly confident that climate change will affect the cycling of carbon in wetlands: Some carbon-sequestering wetlands will change from CO2 sinks to sources due to a lowering of the water table or increased temperature. Changes in the source/sink relationship of wetlands have already occurred in parts of the arctic region. Climate change leading to an alteration in the degree of saturation and flooding of wetlands would affect both the magnitude and the timing of CH, emissions. Drying of northern wetlands could lead to declines in CH, emissions.

We are confident that climate change will affect the areal extent and distribution of wetlands, although at present it is not possible to estimate future areal size and distribution of wetlands from climate-change scenarios. Regional studies from east China, the United States, and southern Europe indicate that the area of wetlands will decrease if the climate becomes warmer. Climate warming also would have severe impacts on wetlands in arctic and subarctic regions in this respect because it would result in a melting of permafrost, which is the key factor in maintaining high water tables in these ecosystems.

Adaptation, conservation, and restoration of wetlands in response to climate change varies among wetland types and the specific function being considered. For regional and global functions (e.g., trace-gas fluxes and carbon storage), there are no human responses that can be applied at the scale necessary. For wetland functions that are local in scale (habitat value, pollution trapping, and to some degree flood control), possibilities exist for adaptation, creation, and restoration. However, wetland creation and restoration technologies are just developing, and we do not yet have reliable techniques to create wetlands for many specific purposes.

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6.1. Introduction

6.1.1. Aims and Goals of the Chapter

This chapter examines the potential impacts of climate change on non-tidal wetland ecosystems and the possible options for responding to these changes. Tidal wetlands are covered in Chapter 9.

This chapter gives particular emphasis to the possible impacts of climate change on the areal extent, distribution, and functions of non-tidal wetlands, in the context of other natural or anthropogenic stressors that are likely to affect these ecosystems simultaneously. In addition to describing the importance of different climate variables and the range of factors that determine the sensitivity of individual wetlands, the chapter uses four case studies to illustrate the effects of climate change on certain defined wetland areas: the Sahel, northern boreal wetlands, Kalimantan (Indonesia), and the Florida Everglades.

This is the first time that IPCC has attempted a detailed assessment of the potential impacts of climate change on the structure and function of wetlands. Previous assessments briefly touched upon wetlands in a qualitative discussion of methane (CH,) sources and sinks (Melillo et al., 1990) and discussed wetlands in the context of ecosystem responses to increased CO2 concentrations, illustrated by case studies of the arctic tundra and a salt marsh.

The present assessment is hindered because the literature on wetlands is highly variable in quality and coverage and large gaps in knowledge remain regarding many of their regulating processes. In particular, relatively few studies exist on the impacts of climate change on inland wetlands; most that do exist have been carried out on specific wetland sites and/or have tended to focus on the Northern Hemisphere. These factors are reflected in the examples and conclusions in this chapter and in the emphasis on case studies. Recently, wetlands and wetland-related topics have begun to receive increasingly greater attention, and new information is expected to be published in the near future.

6.1.2. Definition

Wetlands exist in both inland and coastal areas, covering approximately 4-6% of the Earth's land surface. A wide variety of wetland definitions are found in the literature. Cowardin et al. (1979) argue that there is no single, correct, indisputable, ecologically sound definition for wetlands, primarily because of the diversity of wetlands and because the demarcation between dry and wet environments lies along a continuum. In general, a wetland describes any area of land where the water table is at or near the surface for some defined period of time, leading to unique physiochemical and biological processes and conditions characteristic of shallowly flooded systems (Mitsch and Gosselink, 1993). This chapter will discuss both permanent and temporary wetlands.

This chapter covers inland wetlands, or those not subject to tidal influences including peatlands, swamps, marshes, and floodplains. Peatlands consist of bogs and fens, which may be forested, and are peat-accumulating wetlands in moist climates (peat is partially decomposed plant material). Bogs are acidic, poor in nutrients, and receive water from precipitation only, whereas fens are generally circumneutral, richer in nutrients, and receive water primarily from overland flow and/or groundwater. Swamps or forested wetlands are areas with little or no peat accumulation. Marshes or herbaceous wetlands and floodplains are flooded areas along rivers or lakes (Zoltai and Pollet, 1983).

More than seventy global classification schemes exist internationally. Because the response of wetlands to climate change tends to be site- or region-specific, no existing scheme is useful for this chapter in relating geographic or physical features with climate responses. For this reason, this chapter will focus on describing the climate and other variables that determine the response of individual wetland sites, rather than attempting to correlate responses with particular wetland types.

Many studies have shown that hydrologic parameters are strong controllers of wetland ecosystem structure and function (Gosselink and Turner, 1978; Novitzki, 1989; Kangas, 1990). The source, renewal rate, and timing of the water regime directly control the spatial and temporal heterogeneity of wetland ecosystem structure and function. The hydroperiod-defined as the depth, frequency, duration, and season of flooding is usually the single most important regulator in wetlands, controlling many of their important characteristics (Lugo et al., 1990a). The hydroperiod is determined by the climate, topography, catchment area, soils, and geology of the region in which the wetland is situated (Armentano, 1990).

For this assessment, we focus on climate-change effects on hydrology as an integrative tool for our analysis. However, these effects are highly site-specific, and there are few general, categorical conclusions that can be drawn. There is extreme hydrological variation between and even within individual wetlands, such as differences in the direction of water flow (vertical, unidirectional, or bidirectional; Lugo et al., 1990b). This variability, coupled with the resolution at which these hydrological differences can be found, reinforces the need to describe wetland responses on a site-by-site basis. It is possible to generalize impacts for specific wetland types and, to some degree, wetland regions, but it is difficult to generalize across different wetland types.

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