Land Use, Land-Use Change and Forestry

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4.4.6. Wetlands Management

Wetlands are defined as areas of land that are inundated for at least part of the year, leading to physico-chemical and biological conditions characteristic of shallowly flooded systems (IPCC, 1996b). Anaerobic conditions associated with inundation slow decomposition rates and allow accumulation of large stores of carbon over long time scales, even in systems of relatively low productivity. Although wetlands occupy only 4-6 percent of the Earth's land area (~0.53-0.57 Gha) (Matthews and Fung, 1987; Aselmann and Crutzen, 1989), they store an estimated 20-25 percent of the world's soil carbon (350-535 Gt C) (Gorham, 1995). The rates of carbon accumulation in peats (organic soils commonly associated with wetlands) vary with age (Armentano and Menges, 1986; Tolonen and Turunen, 1996) but eventually reach equilibrium when inputs equal losses (slow decomposition rates applied to very large carbon stores) (Clymo, 1984). Most of the wetland area and associated carbon storage is in peatlands in temperate and boreal regions; roughly 10-30 percent is in the tropics.

Decomposition under anaerobic conditions produces methane-a greenhouse gas. Wetlands are the largest natural source of methane to the atmosphere, emitting roughly 0.11 Gt CH4 yr^-1 of the total of 0.50-0.54 Gt CH4 yr^-1 (Fung et al., 1991). Using a Global Warming Potential (GWP) of 21 for CH4, emissions of ~1.7 g CH4 m^-2 yr^-1 will offset the CO2 sink equivalent to a 0.1 Mg C ha^-1 yr^-1 accumulation of organic matter. The range of CH4 emissions from freshwater wetlands ranges from 7 to 40 g CH4 m^-2 yr^-1; carbon accumulation rates range from small losses up to 0.35 t C ha^-1 yr^-1 storage (Gorham, 1995; Tolonen and Turunen, 1996; Bergkamp and Orlando, 1999). Most freshwater wetlands therefore are small net GHG sources to the atmosphere. Two exceptions are forested upland peats, which may actually consume small amounts of methane (Moosavi and Crill, 1997) and coastal wetlands, which do not produce significant amounts of methane (e.g., Magenheimer et al., 1996). Wetlands appear to be relatively small sources of N2O to the atmosphere, except when they are converted for agricultural use. Although methane emissions from wetlands are now reported as part of a nation's GHG emissions, we compare methane and CO2 emissions equivalently here to demonstrate the impact of various wetland activities on atmospheric GHGs.
Wetlands are vulnerable to future climate change (see IPCC, 1996b, Chapter 6). Increased decomposition rates in warmer temperatures-if associated with drier conditions-may lead to large carbon losses to the atmosphere, particularly from northern peatlands (Gorham, 1995); warmer temperatures may also lead to enhanced CH4 emissions. Changes in regional hydrology caused by precipitation changes may cause loss or new growth of wetlands locally. Changes in permafrost extent and depth will alter the extent and dynamics of tundra wetlands. Sea-level rise will impact coastal wetland areas.

Wetlands management takes several forms: conversion to agriculture, drainage for forestry or agriculture, conversion for urban/industrial land uses, creation through construction of dams (energy uses), direct harvesting, and wetlands reconstruction.

Table 4-10 lists major practices that impact wetlands, along with associated processes affecting carbon storage and methane emission. Most practices affect CH4 and CO2 emissions in opposite ways. Data for the area of total wetlands, the area of human-impacted wetlands, and effects on GHGs are largely unknown for many regions. Hence, Table 4-10 gives qualitative rather than quantitative estimates for the net GHG effect of different management practices. Drainage of wetlands is associated with potentially large carbon losses as organic matter that has accumulated slowly over centuries to millennia is oxidized. Methane emissions from drained wetlands will be reduced (drained systems may even consume methane), offsetting some of the net GHG emission. For wetlands that do not emit significant methane (coastal wetlands), carbon stock changes will dominate. For many freshwater wetlands, methane emissions roughly balance the effect of carbon stock changes in CO2-equivalent emissions.

To calculate the effect of wetlands activities on GHG emissions, information on areal conversion by wetland type is needed, along with net changes in GHG emissions. The area of global wetland converted for human use is poorly known, and estimates are mostly unavailable at the country level. Global estimates range from 6 percent (Armentano and Menges, 1986) to 50 percent (Moser et al., 1996), with most conversion in temperate and tropical regions.

Wetland extent and duration of inundation are observable through remote sensing (e.g., Johnston and Barson, 1993). Carbon accumulation and methane emission rates can potentially be modeled, although these models are still in developmental stages for most wetland types. Validation of changes in carbon stores through field sampling is challenging because of the large size of the pools (e.g., the depth of many peaty soils) and the difficulty of physical access. Methane measurements on a wide scale would be difficult and expensive.

Several non-GHG impacts are associated with land practices that affect wetlands. Wetlands are specialized habitats that have distinct and often valuable flora and fauna; their loss is a biodiversity issue. Wetlands sequester many pollutants at the local level; in several countries, wetlands are constructed to treat wastewater. They act as a buffer for rapid changes in hydrology, and removal of wetlands can lead to increased flooding in some areas. Harvested organic matter from organic soils and peatlands is used as a fuel in some regions. Several international negotiations pertain to these aspects of wetlands-in particular, the Convention on Wetlands, the Convention on Biodiversity, and the Marine and Coastal Work Program (Bergkamp and Orlando, 1999).