Sea level rise may be an additional 10 centimeters (4 inches) higher by populated areas in northeastern North America than previously thought. Extreme northeastern North America and Greenland may experience even higher sea level rise. (Graphic courtesy Geophysical Research Letters, modified by UCAR.) Click to enlarge.
As areas with permafrost thaw and more old carbon is released, the carbon balance changes. Credit: Zina Deretsky, National Science Foundation. Click to enlarge.
28 May 2009
Permafrost thaw will make potentially significant contributions to atmospheric concentrations of carbon more rapidly that previously thought, according to a new study published in the 28 May issue of the journal Nature.
A separate study led by the National Center for Atmospheric Research (NCAR), which is being published 29 May in Geophysical Research Letters, concluded that the melting of the Greenland ice sheet this century may drive more water than previously thought toward the already threatened coastlines of New York, Boston, Halifax, and other cities in the northeastern United States and Canada.
Permafrost
Permafrost soils in boreal and Arctic ecosystems store almost twice as much carbon as is currently present in the atmosphere. This pool of carbon, deposited over thousands of years, remains locked in the heretofore perennially frozen ground. In recent years this area began to thaw, providing increased access to plants and microbes that could shift the carbon from the land to the atmosphere.
In earlier work we estimated that widespread permafrost thaw could potentially release 0.8-1.1 gigatons of carbon per year. Before this study, we didn’t know how fast that carbon could potentially be released from permafrost, and how this feedback to climate would change over time.
—Ted Schuur, University of Florida and lead author of the study
An understanding of the rate of carbon release is necessary to estimate the strength of positive feedback to climate change, a likely consequence of permafrost thaw. From 2004 to 2006, Schuur and his team used radiocarbon dating, a technique typically used to determine the age of artifacts, to track the movement of old organic carbon accumulated within the soils and permafrost at an Alaskan site. The ability to distinguish old carbon from newer carbon allowed the researchers to track current metabolism of old carbon in an area where permafrost thaw is increasing.
This research revealed that during the initial stages of permafrost thaw, plant growth and photosynthesis, which remove carbon from the atmosphere, increase. This increase offsets the release of old carbon from thawing. However, sustained thaw eventually releases more carbon than plants can uptake, overwhelming their compensatory capacities.
Our data document significant losses of soil carbon with permafrost thaw that, over decadal timescales, overwhelms increased plant carbon uptake at rates that could make permafrost a large biospheric carbon source in a warmer world.
—Vogel et al. (2009)
To put this in a global context, if the average global temperature continues to rise, current calculations predict that positive feedback from permafrost thaw could annually add as much carbon to the atmosphere as another significant source, land use change.
The Alaskan site where Schuur and colleagues carried out their research was monitored over the past two decades, with permafrost temperature measurements beginning before the permafrost began to thaw. This detailed record coupled with Schuur’s study of ecosystem carbon exchange and old carbon release provide a comprehensive picture of the dynamics of carbon exchange in response to permafrost thaw.
Records from this site exist on a decadal time scale, meaning we are able to more accurately account for the slow pace of change within the system. Overall, this research documents the long-term plant and soil changes that occur as permafrost thaws, thus providing a basis for making long term predictions about ecosystem carbon balance with increased confidence.—Ted Schuur
Melting Greenland Ice Sheets May Threaten Northeast United States, Canada
The NCAR study finds that if Greenland’s ice melts at moderate to high rates, ocean circulation by 2100 may shift and cause sea levels off the northeast coast of North America to rise by about 12 to 20 inches (about 30 to 50 centimeters) more than in other coastal areas. The research builds on recent reports that have found that sea level rise associated with global warming could adversely affect North America, and its findings suggest that the situation is more threatening than previously believed.
If the Greenland melt continues to accelerate, we could see significant impacts this century on the northeast US coast from the resulting sea level rise. Major northeastern cities are directly in the path of the greatest rise.
—NCAR scientist Aixue Hu, the lead author
A study in Nature Geoscience in March warned that warmer water temperatures could shift ocean currents in a way that would raise sea levels off the Northeast by about 8 inches (20 cm) more than the average global sea level rise. But it did not include the additional impact of Greenland’s ice, which at moderate to high melt rates would further accelerate changes in ocean circulation and drive an additional 4 to 12 inches (about 10 to 30 cm) of water toward heavily populated areas of northeastern North America on top of average global sea level rise. More remote areas in extreme northeastern Canada and Greenland could see even higher sea level rise.
Scientists have been cautious about estimating average sea level rise this century in part because of complex processes within ice sheets. The 2007 assessment of the Intergovernmental Panel on Climate Change projected that sea levels worldwide could rise by an average of 7 to 23 inches (18 to 59 cm) this century, but many researchers believe the rise will be greater because of dynamic factors in ice sheets that appear to have accelerated the melting rate in recent years.
The new research was funded by the US Department of Energy and by NCAR’s sponsor, the National Science Foundation. It was conducted by scientists at NCAR, the University of Colorado at Boulder, and Florida State University.
To assess the impact of Greenland ice melt on ocean circulation, Hu and his coauthors used the Community Climate System Model, an NCAR-based computer model that simulates global climate. They considered three scenarios: the melt rate continuing to increase by 7% per year, as has been the case in recent years, or the melt rate slowing down to an increase of either 1% or 3% per year.
If Greenland’s melt rate slows down to a 3% annual increase, the study team’s computer simulations indicate that the runoff from its ice sheet could alter ocean circulation in a way that would direct about a foot of water toward the northeast coast of North America by 2100. This would be on top of the average global sea level rise expected as a result of global warming. Although the study team did not try to estimate that mean global sea level rise, their simulations indicated that melt from Greenland alone under the 3% scenario could raise worldwide sea levels by an average of 21 inches (54 cm).
If the annual increase in the melt rate dropped to 1%, the runoff would not raise northeastern sea levels by more than the 8 inches (20 cm) found in the earlier study in Nature Geoscience. But if the melt rate continued at its present 7% increase per year through 2050 and then leveled off, the study suggests that the northeast coast could see as much as 20 inches (50 cm) of sea level rise above a global average that could be several feet. However, Hu cautioned that other modeling studies have indicated that the 7% scenario is unlikely.
In addition to sea level rise, Hu and his co-authors found that if the Greenland melt rate were to defy expectations and continue its 7% increase, this would drain enough fresh water into the North Atlantic to weaken the oceanic circulation that pumps warm water to the Arctic. Ironically, this weakening of the meridional overturning circulation would help the Arctic avoid some of the impacts of global warming and lead to at least the temporary recovery of Arctic sea ice by the end of the century.
The northeast coast of North America is especially vulnerable to the effects of Greenland ice melt because of the way the meridional overturning circulation acts like a conveyer belt transporting water through the Atlantic Ocean. The circulation carries warm Atlantic water from the tropics to the north, where it cools and descends to create a dense layer of cold water. As a result, sea level is currently about 28 inches (71 cm) lower in the North Atlantic than the North Pacific, which lacks such a dense layer.
If the melting of the Greenland Ice Sheet were to increase by 3% or 7% yearly, the additional fresh water could partially disrupt the northward conveyor belt. This would reduce the accumulation of deep, dense water. Instead, the deep water would be slightly warmer, expanding and elevating the surface across portions of the North Atlantic.
Unlike water in a bathtub, water in the oceans does not spread out evenly. Sea level can vary by several feet from one region to another, depending on such factors as ocean circulation and the extent to which water at lower depths is compressed.
The oceans will not rise uniformly as the world warms. Ocean dynamics will push water in certain directions, so some locations will experience sea level rise that is larger than the global average.
—NCAR scientist Gerald Meehl, a co-author of the paper
Resources
· Jason G. Vogel, Kathryn G. Crummer, Hanna Lee, James O. Sickman, T. E. Osterkamp, Edward A. G. Schuur (2009) The effect of permafrost thaw on old carbon release and net carbon exchange from tundra. Nature 459, 556-559 doi: 10.1038/nature08031
· Aixue Hu, Gerald Meehl, Weiqing Han, and Jianjun Yin (2009) Transient Response of the MOC and Climate to Potential Melting of the Greenland Ice Sheet in the 21st Century. Geophysical Research Letters
May 28, 2009 in Climate Change.
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