What is causing the thawing of Arctic permafrost? Are human activities responsible for at least some of the observed and projected thawing? What is the current extent or estimate of thawing? What do models suggest the extent of thawing will be later in the 21 st Century? What level of confidence exists, among experts, with respect to these model projections? Are the appearance and disappearance of small Arctic lakes a manifestation of a global warming and the thawing of the Arctic permafrost? What are the implications and real and potential impacts that the thawing of permafrost will have?
Dr. Anthony Socci, Senior Science Fellow, American Meteorological Society
Dr. Laurence C. Smith, Associate Professor, Departments of Geography and Earth & Space Sciences, University of California, Los Angeles, CA
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Dr. David M. Lawrence, Research Scientist, National Center for Atmospheric Research, Boulder, CO, USA
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A projection of severe degradation of nearsurface permafrost: Potential feedbacks on climate
In recent decades, the Arctic has witnessed significant climatic and other environmental changes ranging from decreases in sea ice extent, increases in shrub cover, to melting glaciers. Temperatures over Arctic land areas are rising at roughly twice the rate of the rest of the world. Permafrost, which is defined as soil or rock that remains below 0 o C for two or more years, is an archetypal component of the Arctic climate system. In harmony with other aspects of change, permafrost temperatures are rising and there have been reports of significant permafrost degradation in some locations.
Results from the NCAR Community Climate System Model (CCSM3) indicate that degradation of permafrost will continue and may accelerate during the 21 st century. The CCSM3 is a mathematical model of the global climate system that includes components representing the atmosphere, ocean, land, and seaice. The presentday global distribution of nearsurface permafrost in the CCSM3 compares well with observed estimates of continuous permafrost distribution both in terms of geographical extent and total area (~10.5 million km 2 ). Strict validation of the simulation of permafrost is difficult since temporal observations of permafrost at the largescale are not available. However, the CCSM3 reasonably replicates Arctic annual mean surface air temperatures, a key determinant of permafrost and captures the recent trend in Arctic temperatures and seaice extent during the latter half of the 20th century. Projections of the fate of nearsurface permafrost were assessed through simulations of 21 st century climate under various greenhouse gas emission scenarios provided by the Intergovernmental Panel on Climate Change (IPCC).
Under the “business as usual” emission scenario (A1B), the area containing permafrost in the nearsurface layer declines by ~80% by 2100. The land model component is limited to simulating the top 3.5m of the ground, though this is the ecologically and hydrologically important portion; deeper permafrost is not as vulnerable to thawing at the century timescale.
Although there are considerable uncertainties in the CCSM3 projections for nearsurface permafrost, both in terms of the magnitude and the timing, the model projection, in conjunction with the observed permafrost warming across the Arctic, suggests that largescale changes in permafrost are likely. The potential climate feedbacks associated with a degradation of nearsurface permafrost are diverse. Changes to Arctic vegetation, hydrology, and the carbon cycle are expected in the form of expanding shrub cover and northward forest migration, enhanced runoff to the Arctic Ocean as well as expanding and retreating lakes and wetlands, and the release of large quantities of soil carbon, currently frozen in permafrost soil, into the atmosphere. These feedbacks could contribute to an acceleration of global climate change.
Impacts of thawing permafrost on highlatitude
hydrology and carbon cycles
Scientific interest in the Arctic is at an alltime high, owing to a multitude of warminginduced changes now underway there and a growing appreciation for the region's importance to the global climate system. Recent studies using satellite and field data have revealed remarkable changes in the number and total area of Arctic lakes and wetlands in just the past few decades. A preliminary assessment is that they are growing in northern areas of continuous permafrost, but disappearing further south.
A proposed mechanism unifying these seemingly contradictory observations is thermokarst (slumped terrain and collapse features associated with melting ground ice) development and lake growth as permafrost begins to thaw, followed by enhanced infiltration and lake drainage as permafrost degrades still further – a wet phase followed by a dry phase. Throughout the Arctic, seasonal river and lake ice cover is breaking up earlier each year, increasing the openwater season and total exchanges or transfers of water vapor, carbon dioxide and methane from the land surface to the atmosphere.
Hydrology and terrestrial carbon cycles are particularly intertwined in Arctic environments. Moist, lowrelief areas promote accumulation of peatlands, which withdraw large quantities of carbon dioxide from the atmosphere while releasing methane, a powerful greenhouse gas. Over millennial time scales, such accumulation has resulted in significant longterm storage of atmospheric carbon in Arctic soils. With continued climate warming, the ultimate fate of this carbon depends crucially on whether wetter or drier conditions prevail.
In West Siberia, new data from both permafrost and permafrostfree areas suggests that thawing of carbonrich soils may trigger the release of substantial quantities of dissolved organic carbon to lakes, streams and rivers, which would subsequently be rapidly returned to the atmosphere. These and other Arctic physical, biogeochemical, and ecological processes are strongly influenced by permafrost, which is projected to experience widespread degradation in this century.
Dr. Laurence Smith joined the faculty of UCLA's Department of Geography in 1996, upon completion of his Ph.D. in Earth & Atmospheric Sciences at Cornell University. In 2000, he also became a member of the Department of Earth & Space Sciences at UCLA. Dr. Smith’s interests include Arctic hydrology, glaciology and carbon cycles, and their linkages to global climate change. His most recent projects include satellite sensing of changing permafrost lakes, river floods, ice breakup and glacial melt patterns, geomorphic impacts of Icelandic glacier outburst floods from field measurements, aircraft and satellites, and a major fieldbased study of carbon cycling in the vast peatlands of western Siberia.
Dr. Smith also serves in an advisory capacity on a variety of NASA science programs. He recently served as part of a Senior Review panel charged with reviewing all current NASA earthorbiting satellite missions. To date in his relatively young career, Dr. Smith has published more than thirtyfive scientific papers including two in the journal SCIENCE.
Dr. David Lawrence is a research scientist at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado. His research interests are centered around landatmosphere interactions and improving our understanding of the role of land surface processes in the Earth’s climate system and their influence on climate change. He is involved in the assessment and development of NCAR’s Community Climate System Model (CCSM) and is cochair of the CCSM Land Model Working Group. Previously he worked with Professor Julia Slingo in the Department of Meteorology at the University of Reading in the United Kingdom. He received his Ph.D. in 1999 from the Department of Atmospheric and Oceanic Science at the University of Colorado under the direction of Professor Peter Webster. He is author or coauthor of over 20 peerreviewed scientific articles on topics ranging from permafrost to landatmosphere interactions to monsoon dynamics.
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