Wetlands

Introduction

Wetlands are ecosystems with soils that are waterlogged seasonally or year-round. They are often located along borders between aquatic and terrestrial ecosystems, and may be fed by groundwater or surface water such as streams or rain, and contain vegetation adapted for life in saturated soils. Wetlands may be freshwater (inland) or brackish (coastal); they may also be artificial (rice paddies, reservoir shallows) or natural (swamps, coastal wetlands). They occur in diverse forms, from river edges to grassy areas to wetland forests such as cypress forests. They are both vulnerable to the effects of climate change and natural contributors to global warming, being the largest single source of the greenhouse gas methane (CH₄).

Historical Background and Scientific Foundations

Wetlands occupy only a few percent of the planet's surface. They are, however, the largest single source of methane, which is at least 21 times as potent a greenhouse gas as carbon dioxide (CO₂). Wetlands, including cultivated rice fields, account for 30–40% of all methane

emissions. Wetlands also have direct effects on climate, being an important part of the hydrologic cycle; they alter river flows, humidify the air through evaporation, and modify regional temperatures.

Wetlands absorb carbon dioxide and, in many areas, store it in the form of peat, that is, compacted, water-saturated dead plant matter. Peat currently holds about 25% of all carbon stored in soils and plants. Wetlands also produce methane through anaerobic (oxygen-free) digestion of organic matter. Methane-producing microbes found in soil, termed methanogens, thrive when water saturates soil, as is the case year-round or seasonally in wetlands. Methane excreted by methanogens bubbles upward through the soil, reaching the air by following plant stems or bubbling up through standing water. The bubbles that arise from pond mud when it is disturbed with a stick are methane.

Cool, dry weather causes wetlands to produce less methane than warm, wet weather does. Changes in climate that influence local temperatures and the amount of water entering wetlands therefore affect how much methane they produce. The overall effect of global warming is to act as a positive feedback on methane production from wetlands. A 2004 study by Drew T. Shindell and colleagues found that doubling pre-industrial CO₂ levels, as may occur by the end of this century, would increase annual global wetland methane emissions by 78%, which would be an increase over present-day methane emissions of about 20%. Most of this increase would be caused by the warming of existing tropical wetlands; the rest would come from the expansions of far-northern wetlands, which would almost triple summer methane emissions.

Although scientists have long appreciated the many ecological roles of wetlands, including their production of methane, only in the early 1990s did the global inventorying of wetland area begin in earnest. The first global, multi-year, month-by-month estimate of global wetland area was published in 2007. The satellite study found that from 1993 to 2000, global wetland area varied on average from 818,500 square mi (2.12 million square km) in December to 2.26 million square mi (5.86 million square km) in August. Accurate seasonal characterization of wetland extent is essential to accurate modeling of the relationship between wetlands and climate change.

Impacts and Issues

Global warming increases methane emission from wet-lands by warming tropical wetlands and increasing the extent of far-northern wetlands through the melting of permafrost. A study of thaw lakes dotting the North Siberian tundra, published in 2006, found that such thaw lakes, edged by wetlands, release 3.8 billion metric tons of methane per year, five times more than previously estimated and enough to raise estimates of methane emissions from northern wetlands by 10–63%. Methane emissions from this source increased by 58% from 1974 to 2000 because of global warming. In the wetlands of northern Canada, carbon uptake has increased with permafrost melting, which encourages plant growth, but methane emissions have increased thirty-fold from the same areas. The carbon flows of rapidly changing high-latitude ecosystems are still imperfectly understood.

WORDS TO KNOW

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

HYDROLOGIC CYCLE: The process of evaporation, vertical and horizontal transport of vapor, condensation, precipitation, and the flow of water from continents to oceans. It is a major factor in determining climate through its influence on surface vegetation, the clouds, snow and ice, and soil moisture. The hydrologic cycle is responsible for 25 to 30% of the mid-latitudes' heat transport from the equatorial to polar regions.

METHANE: A compound of one hydrogen atom combined with four hydrogen atoms, formula CH₄. It is the simplest hydrocarbon compound. Methane is a burnable gas that is found as a fossil fuel (in natural gas) and is given off by rotting excrement.

PEAT: A primitive form of coal with less heat content and pure carbon content than any form of coal. Also, soil composed chiefly of decaying plant matter.

THAW LAKES: Ponds that form on thermokarsts, that is, landscapes where permafrost (soil frozen year-round, often to hundreds of feet in depth) is melting. Also called thermokarst lakes. Thaw lakes are forming in large numbers in the Arctic due to climate warming.

Wetlands are also vulnerable to climate change. Rising temperatures and (in many areas) decreased rainfall will affect freshwater, inland wetlands directly. Coastal wetlands are threatened by rising sea levels, which will tend to flood them out: given a 1.18 ft (36 cm) rise in sea level, the Intergovernmental Panel on Climate Change (IPCC) has forecast a 33% loss of global coastal wetlands by 2080. Given a 2.36 ft (72 cm) rise, 44% of coastal wetlands are likely to be lost. Sea level rises of 3 ft (91 cm) or more by 2100 are thought increasingly likely by many scientists in light of the accelerated melting of Greenland's ice cap, first verified in 2006.

See Also Feedback Factors; Methane; Tundra.

BIBLIOGRAPHY

Books

Parry, M. L., et al, eds. Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. New York: Cambridge University Press, 2007.

Periodicals

Shindell, Drew T. “Impacts of Climate Change on Methane Emissions from Wetlands.” Geophysical Research Letters 31, no. 21 (2004): L21202.

Web Sites

Matthews, Elaine, and Catherine Prigent. “Wetlands' Outsize Influence on Climate.” NASA Science Briefs, October 2007. <http://www.giss.nasa.gov/research/briefs/matthews_02/> (accessed November 13, 2007).

Poff, N. Leroy, Mark M. Brinson, and John W. Day, Jr. “Aquatic Ecosystems & Global Climate Change: Potential Impacts on Inland Freshwater and Coastal Wetland Ecosystems in the United States.” The Pew Charitable Trusts, June 22, 2007. <http://www.pewtrusts.org/uploadedFiles/wwwpewtrustsorg/Reports/Protecting_ocean_life/env_climate_aquaticecosystems.pdf> (accessed November 13, 2007).

Larry Gilman