Wednesday, May 3, 2006
The introductory remarks in the announcement of today's American Meteorological Society's Environmental Science Seminar "Changes in Cold Places: A Look at the Greenland Ice Sheet, Arctic Sea Ice and the Antarctic Ice Sheet" is a noteworthy summary of the effects of global warming on the Greenland and Antarctic ice sheets and Arctic sea ice:
Are parts or all of the Greenland and Antarctic Ice Sheets undergoing a net melting? What is the nature of the observational evidence of melting? How far back in time do these observations extend? Are the melting and rates of melting consistent with model simulations and projections of melting? What are the factors deemed responsible for the melting? Is Arctic Sea Ice thinning? If so, by how much and over what period of time? What is the nature of the observational evidence for this thinning? Are the thinning and rates of thinning consistent with model simulations and projections of thinning? What factors are deemed responsible for the thinning of Arctic Sea Ice? Are there implications with respect to sea level rise? Is there any evidence of a fundamental change in the rate at which the Greenland ice sheet is changing and the manifestation of those changes? If so, why? How well do model simulations capture the dynamics of ice sheets and sea ice relative to observations? How well are these ice sheet models poised to project future changes in ice sheets, sea ice and sea level?
Introductory Perspective on Current Climate Changes
Over the last 30 years, the scientific understanding of the causes of the Earth's natural climate variability and the climatic impacts of human activity has advanced dramatically. In the early 1900's, global average temperatures increased from the predominately cooler temperatures of the previous several centuries, declined a little from 1940 to 1975, and then began to rise significantly. The climate warming of the early 1900's is now largely attributed to recovery from a period with more volcanic activity and consequent cooling. The small subsequent cooling in the mid-1900's is attributed mainly to increasing tropospheric aerosols from emission of sulfate pollutants, and the subsequent warming is attributed to increasing greenhouse gas (GHG) concentrations, the effect of which now overrides the aerosol cooling. About 0.6º C of the recent warming is attributed to the effect of GHG, which have been increasing exponentially (0.5%/year). Model projections of warming during the coming century range from about 1.5º C to 6º C depending on the GHG emission scenario. The models have also consistently projected that the warming will be greatest in the Arctic. During this century, we may be reaching or surpassing temperatures not seen since the last major warm period between ice ages 125-thousand years ago, when much of the Greenland ice sheet melted. Already, some of the largest effects of the global warming are becoming apparent in the Greenland ice sheet and the Arctic sea ice.
Observed Changes in Polar Ice Sheets and Sea Ice:
Responses to Climate Warming
Until recently, we did not know whether the Greenland and Antarctic ice
sheets were growing or shrinking, but significant climate-induced changes are now being detected. Satellites are detecting changes in ice
sheet mass by measuring changes in ice surface elevations and changes in
the sub-satellite gravity fields. Changes in ice velocity and ice
area are being determined from imaging radar and other high-resolution
imagery. The area and timing of surface melting are measured by
satellites, ice acceleration is measured by surface GPS, and surface
temperatures and other meteorological parameters are measured by
automatic weather stations on the ice. We now know that during 1992
to 2002 the Greenland ice sheet was thinning at the margins and growing
inland, probably with a small overall mass gain. Thinning at the
margins from increased melting and inland growth from increasing
precipitation are the expected responses to climate warming. The
area of surface melting during summer continues to increase, and the
acceleration of the ice flow (caused by the increased melting) has also
been increasing, since it was first detected in 1997. In the last 5
to 10 years, some of the outlet glaciers in Southern Greenland have been
accelerating and the number of associated ice-quakes has been
increasing. New satellite-gravity measurements indicate the mass
balance has now changed to a net loss, which suggests that the effects of
melting are becoming dominant over increasing precipitation, and that the
newly-discovered ice accelerations are very important.
During the 1990's, the ice sheet in West Antarctica was losing mass and the ice sheet in East Antarctica had a small mass gain. The ice loss in West Anatarctic is probably an ice-dynamic response to long-term climate change and perhaps past removal of adjacent ice shelves. The ice growth in the Antarctic Peninsula and parts of East Anarctica may be due to increasing precipitation. The floating ice shelves of the Antarctic had a corresponding mass loss in the West and a gain in the East. Continued ice shelf thinning from regional climate warming in West Antarctica may be a precursor to more loss of grounded ice. The recent contribution of the ice sheets to sea level rise has been a small part of the current rate of 3 mm/year (1 foot/century), but the ice sheets are likely to lose mass faster with additional warming.
The most significant change in sea ice has been a 9%/decade decline in the area of sea ice surviving the summer melting on the Arctic Ocean. Now, for the first time the laser altimeter on NASA's ICESat is making comprehensive measurements of sea-ice freeboards, from which ice thickness maps are being derived. These new results show a fundamental change in the character of the sea-ice thickness distribution in the Arctic, indicating a loss of the thicker ice that was characteristic of earlier decades.
Causes of Changes in Arctic Sea Ice
The Arctic Ocean has been warming in recent decades and that warming
appears to have accelerated during the last several years as observed by
satellites and in situ measurements and as projected by models.
The main manifestation of such a trend has been melting of the Arctic ice
pack and the dramatic reduction of summer sea ice cover. Satellite
records of the Arctic sea ice cover show a decreasing trend in ice
concentration since 1979, with large seasonal and interannual
variability. This trend has been coincident, in part, with the
high-index polarity of the Northern Hemisphere Annular Mode or NAM
(a.k.a., the North Atlantic Oscillation or NAO, or the Arctic Oscillation
or AO) represented by a reduced winter weather regime over mid- to
high-latitude continental regions of the Northern Hemisphere. However,
some of the recent variability clearly points to other modes of large
scale climate forcing as well. Especially overlooked appears to be the
oceanic thermodynamic control of sea ice through the under-ice ablation
and lateral melt along marginal ice zones. Those ice-ocean interactions
may act to accelerate summer melt and significantly reduce the
correlation between sea ice cover change and AO/NAO forcing.
Some global climate models project up to a 50% reduction of summer sea ice cover in the Arctic Ocean by 2100, as a result of an amplified response to global warming. Unfortunately, the majority of such models can not adequately reproduce past and present variability in the Arctic sea ice and ocean circulation, which diminishes their accuracy of future climate prediction. An arguably more realistic representation of the Arctic Ocean and its sea ice, using regional high resolution simulations over the past decades, implies that during the last decade sea ice thickness and volume might be shrinking at a much higher rate than predicted by climate models or determined from the satellite-derived trend of sea ice extent. Positive ice-albedo feedback and increased oceanic advection of heat via Pacific and Atlantic Water may in reality lead to an accelerated reduction or even to the complete removal of summer Arctic sea ice within a few decades. Such a change may result in additional significant changes to the global ocean thermohaline circulation and climate, especially over northern Europe. In addition, the warming trend, if it continues along its present trajectory, will likely not only significantly affect global climate but is also likely to change the strategic and economic importance of the Arctic Ocean through its use for commercial shipping routes and increased exploration of natural resources.
Model Estimates of Ice-Sheet Thinning
Ice-sheet thinning in response to warming probably is contributing to sea-level rise a century earlier than indicated by previous model estimates, suggesting that future sea-level rise may be faster than formerly projected.
Earth-system models are increasingly skillful at simulating the atmospheric and oceanic changes that have occurred over recent decades, and projections from earlier earth-system models are proving to have been fairly accurate. However, the 2001 IPCC assessment indicated that the most-likely response of the Greenland and Antarctic ice sheets to warming through the year 2100 would be to remove water from the oceans, with snowfall increasing more than melting and with little change in ice flow, although assessed uncertainty was large and included the possibility of ice-sheet mass loss. Just five years later, best estimates from numerous recent studies indicate that the ice sheets are losing mass in response to warming, although again with some uncertainty.
Numerous barriers exist to accurate projection of ice-sheet changes in response to specified climate changes. The ice-sheet thickness is not even known everywhere, nor is the depth of ocean water beneath floating extensions called ice shelves that expose ice to potentially rapid melting from below. Basal thawing of ice on land allows faster ice flow, especially if the ice rests on smooth rocks or soft muds, but the distribution of these substrates is not well-mapped. Most earlier ice-flow models simplified the representation of ice flow for computational efficiency, but the recent data show that the simplifications omitted important processes that can contribute to rapid ice-sheet changes. Coupled ocean-atmosphere models do not resolve the processes and heat fluxes very near the ice sheets well, complicated by difficulties in representing sea ice that often occurs near ice sheets.
Consideration of history over many time scales shows that warming generally melts ice and raises sea level. The recent ice-sheet changes point in the same direction. Model improvements and additional data will be required to learn how rapidly this might occur in the future.
Dr. Anthony Socci, Senior Fellow, American Meteorological Society
Dr. H. Jay Zwally, ICESat Project Scientist, National Aeronautics and Space Administration, Goddard Space Flight Center, Greenbelt, MD
Dr. Wieslaw Maslowski, Research Associate Professor, Department of Oceanography, Graduate School of Engineering and Applied Sciences, Naval Postgraduate School, Monterey, CA
Dr. Richard B. Alley, Evan Pugh Professor of Geosciences and Associate of the Earth and Environmental Systems Institute, Pennsylvania State University, University Park, PA