Researchers attribute thinning of Greenland glacier to ocean warming preceded by atmospheric changes


The sudden thinning in 1997 of Jakobshavn Isbræ, one of Greenland’s largest glaciers, was caused by subsurface ocean warming, according to research published in the journal Nature Geoscience. The research team traces these oceanic shifts back to changes in the atmospheric circulation in the North Atlantic region.



The study, whose lead author was David Holland, director of the Center for Atmosphere Ocean Science, part of New York University’s Courant Institute of Mathematical Sciences, suggests that ocean temperatures may be more important for glacier flow than previously thought.



The project also included scientists from the Wallops Flight Facility, Canada’s Memorial University, the Danish Meteorological Institute, and the Greenland Institute of Natural Resources.



Jakobshavn Isbræ, a large outlet glacier feeding a deep-ocean fjord on Greenland’s west coast, went from slow thickening to rapid thinning beginning in 1997. Several explanations have been put forward to explain this development. The scientists in the Nature Geoscience study sought to address the matter comprehensively by tracing changes in ocean temperatures and the factors driving these changes.



In doing this, they relied on previous results published by others that used NASA’s Airborne Topographic Mapper, which has made airborne surveys along a 120-kilometer stretch in the Jakobshavn ice-drainage basin nearly every year since 1991. While many other glaciers were thinning around Greenland, these surveys revealed that Jakobshavn Isbræ thickened substantially from 1991 to 1997. But, after 1997, Jakobshavn Isbræ began thinning rapidly. Between 1997 and 2001, Airborne Topographic Mapper surveys showed an approximately 35-meter reduction in surface elevations on the glacier’s 15-kilomater floating ice tongue. This is far higher than thinning rates of grounded ice immediately upstream.


The researchers reported that these changes coincided with jumps in subsurface ocean temperatures. These temperatures were recorded by the Greenland Institute of Natural Resources from 1991 to 2006 over nearly the entire western Greenland continental shelf. These data indicate a striking, substantial jump in bottom temperature in all parts in the survey area during the second half of the 1990s. In particular, they show that a warm water pulse arrived suddenly on the continental shelf on Disko Bay, which is in close proximity Jakobshavn Isbræ, in 1997. The arrival coincided precisely with the rapid thinning and subsequent retreat of Jakobshavn Isbræ. The warm water mass remains today, and Jakobshavn Isbræ is still in a state of rapid retreat.



The remaining question, then, is what caused the rise in water temperatures during this period.



The researchers traced these oceanic changes back to changes in the atmospheric circulation in the North Atlantic region. The warm, subsurface waters off the west Greenland coast are fed from the east by the subpolar gyre-or swirling water-of the North Atlantic, by way of the Irminger current. The current flows westward along the south coast of Iceland. Since the mid-1990s, observations show a warming of the subpolar gyre and the northern Irminger Basin, which lies south of Greenland. The researchers attributed this warming to changes in the North Atlantic Oscillation (NAO), which is a large-scale fluctuation in the atmospheric pressure system situated in the region. The surface pressure drives surface winds and wintertime storms from west to east across the North Atlantic affecting climate from New England to western Europe.



Specifically, they noted a major change in the behavior of the NAO during the winter of 1995?, which weakened the subpolar gyre, allowing warm subpolar waters to spread westward, beneath colder surface polar waters, and consequently on and over the west Greenland continental shelf.



“The melting of the ice sheets is the wild card of future sea level,” Holland explained, “and our results hint that modest changes in atmospheric circulation, possibly driven by anthropogenic influences, could also cause future rapid retreat of the Antarctic Ice Sheet, which holds a far greater potential for sea level rise.”

An accurate picture of ice loss in Greenland





Researchers to develop a method for creating an accurate picture of Greenland's shrinking ice cap
Researchers to develop a method for creating an accurate picture of Greenland’s shrinking ice cap

Researchers from TU Delft joined forces with the Center for Space Research (CSR) in Austin, Texas, USA, to develop a method for creating an accurate picture of Greenland’s shrinking ice cap. On the strength of this method, it is now estimated that Greenland is accountable for a half millimetre-rise in the global sea level per year. These findings will be published in the scientific journal Geophysical Research Letters in early October.



The research was based on data from the German-American GRACE (Gravity Recovery and Climate Experiment) satellites, two satellites that have been orbiting the earth behind each other since mid-2002. Deviations in the earth’s gravitational field cause fluctuations in the distance between the satellites, which is measured to a precision of a millionth of a metre. As gravity is directly related to mass, these data can be used to plot changes in the earth’s water balance, such as the disappearance of the ice caps. Satellite data of this kind are ideal for measuring areas such as Greenland, where the extreme conditions make local measurements very difficult. With this in mind, researchers from TU Delft and the CSR devised a method that would create a more accurate picture of the changes taking place in Greenland than had previously been possible.


Sea level rise



Greenland lost an average of 195 cubic kilometres of ice per year between 2003 and 2008, which is enough to cause an annual increase in the global sea level of half a millimetre, or 5 cm over the course of the next century. A report recently published by the Dutch Delta commission estimated that the melting ice cap in Greenland would cause the sea level to rise by 13 to 22 cm by 2100. But these two figures do not necessarily contradict each other: whereas the first two years of the study showed a loss of 131 cubic kilometres of ice per year, during the last two years this figure had risen to 222 cubic kilometres per year, an increase of 70 percent. This sharp increase was mainly caused by the extremely warm summer of 2007, when more than 350 cubic metres of ice melted in just two months. However, it is not yet clear whether the ice will continue to melt at this rate during the next few years, as ice loss varies greatly from summer to summer. Long-term observations are needed to compile a reliable estimate of Greenland’s contribution to the rising sea level during the next century.

Patterns



The method used also enables scientists to plot the loss of mass per region, thereby providing new insight into the patterns of ice loss. For example, for the first time since measurements were started, the extremely warm summer of 2007 saw a decrease in the ice mass at high altitudes (above 2,000 metres). It also became clear that the ice loss is advancing towards the North of Greenland, particularly on the west coast. The areas around Greenland, particularly Iceland, Spitsbergen and the northern islands of Canada, seem to be particularly badly affected. A follow-up study will focus on the influence of these smaller glaciers on the sea level.


More information



Wouters, B., D. Chambers, and E. J. O. Schrama (2008), GRACE observes small-scale mass loss in Greenland, Geophys. Res. Lett., doi:10.1029/2008GL034816, in press.

Paleozoic ‘sediment curve’ provides new tool for tracking sea-floor sediment movements


Follows sea-level rise and fall between 542 and 251 million years ago



As the world looks for more energy, the oil industry will need more refined tools for discoveries in places where searches have never before taken place, geologists say.



One such tool is a new sediment curve (which shows where sediment-on-the-move is deposited), derived from sediments of the Paleozoic Era 542 to 251 million years ago, scientists report in this week’s issue of the journal Science. The sediment curve covers the entire Paleozoic Era.



“The new Paleozoic sea-level sediment curve provides a way of deriving predictive models of sediment migration on continental margins and in interior seaways,” said Bilal Haq, lead author of the Science paper and a marine geologist at the National Science Foundation (NSF). The paper’s co-author is geologist Stephen Schutter of Murphy Oil International in Houston, Tx.



“The sediment curve is of interest to industry, and also to scientists in academia,” said Haq, “as the rise and fall of sea-level form the basis for intepretations of Earth history based on stratigraphy.”


Through stratigraphy, the study of rock layering (stratification), scientists can derive a sequence of time and events in a particular region. Recent advances in the field of stratigraphy, including better time-scales for when sediments were deposited, and availability of data on a worldwide basis, are allowing scientists to reconstruct sea level during the Paleozoic.



The rises and falls of sea level during this period form the basis of stratigraphic interpretations of geology not only in the sea, but on land. These sea level increases and decreases are used extensively, Haq said, in predictive models of sediment movements.



The current Science paper is a shorter version of the results of a global synthesis of Paleozoic stratigraphy on which the authors have worked for many years.



“We hope that the publication of a sediment curve for this entire era will enhance interest in Paleozoic geology,” said Haq, “and help the exploration industry in its efforts to look at older and deeper sediments.”

NASA data show Arctic saw fastest August sea ice retreat on record


Following a record-breaking season of arctic sea ice decline in 2007, NASA scientists have kept a close watch on the 2008 melt season. Although the melt season did not break the record for ice loss, NASA data are showing that for a four-week period in August 2008, sea ice melted faster during that period than ever before.



Each year at the end of summer, sea ice in the Arctic melts to reach its annual minimum. Ice that remains, or “perennial ice,” has survived from year to year and contains old, thick ice. The area of arctic sea ice, including perennial and seasonal ice, has taken a hit in past years as melt has accelerated. Researchers believe that if the rate of decline continues, all arctic sea ice could be gone within the century.



“I was not expecting that ice cover at the end of summer this year would be as bad as 2007 because winter ice cover was almost normal,” said Joey Comiso of NASA’s Goddard Space Flight Center in Greenbelt, Md. “We saw a lot of cooling in the Arctic that we believe was associated with La Niña. Sea ice in Canada had recovered and even expanded in the Bering Sea and Baffin Bay. Overall, sea ice recovered to almost average levels. That was a good sign that this year might not be as bad as last year.”



The 2008 sea ice minimum was second to 2007 for the record-lowest extent of sea ice, according to a joint announcement Sept. 16 by NASA and the University of Colorado’s National Snow and Ice Data Center (NSIDC) in Boulder, Colo. As of Sept. 12, 2008, the ice extent was 1.74 million square miles. That’s 0.86 million square miles below the average minimum extent recorded from 1979 to 2000, according to NSIDC.


Contributing to the near-record sea ice minimum in 2008 was a month-long period in the summer that saw the fastest-ever rate of seasonal retreat during that period. From August 1 to August 31, NASA data show that arctic sea ice extent declined at a rate of 32,700 square miles per day, compared to a rate of about 24,400 square miles per day in August 2007. Since measurements began, the arctic sea ice extent has declined at an average rate of 19,700 miles per day at the point when the extent reaches its annual minimum.



Observations of changes to sea ice over time are possible due to a 30-year record of data from NASA and other agency satellites, including Nimbus-7, Aqua, Terra and the Ice, Cloud, and land Elevation Satellite (ICESat).



Researchers say that the recent seasonal acceleration could be in part due to conditioning going on in the Arctic. For example, research by Jennifer Kay of the National Center for Atmospheric Research in Boulder, Colo., and colleagues reported this April in Geophysical Research Letters that reduced cloud cover in 2007 allowed more sunlight to reach Earth, contributing to a measureable amount of sea ice melt at the surface. Reduced cloud cover also contributed to warmer ocean surface temperatures that led to melting of the ice from below.



“Based on what we’ve learned over the last 30 years, we know that the perennial ice cover is now in trouble,” Comiso said. “You need more than just one winter of cooling for the ice to recover to the average extent observed since the measurements began. But the trend is going the other way. A warming Arctic causes the surface water to get warmer, which delays the onset of freeze up in the winter and leads to a shorter period of ice growth. Without the chance to thicken, sea ice becomes thinner and more vulnerable to continued melt.”

Ancient Arctic Ice Could Tell Us About Future Of Permafrost





Duane Froese examines an ancient ice wedge. (Credit: Image courtesy of University of Alberta)
Duane Froese examines an ancient ice wedge. (Credit: Image courtesy of University of Alberta)

Researchers have discovered the oldest known ice in North America, and that permafrost may be a significant touchstone when looking at global warming.



“Previously it had been thought that permafrost completely melted out of the interior of Yukon and Alaska about 120,000 years ago, when climate was warmer than today,” said Duane Froese, an assistant professor in the Department of Earth and Atmospheric Science and lead author of the study.



“What we found is that even within the discontinuous permafrost zone-the area where permafrost is warm and within a few degrees of 0C and shallow, only a few to tens of metres thick-it has survived at some locations for more than 700,000 years.” Because of the potential longevity of the permafrost, it tells the story of climate changes over the course of hundreds of thousands of years, which Froese says is immeasurably valuable.


When permafrost thaws, much of the carbon that was formerly locked up becomes available for decomposition in thawed soil or beneath lakes and is released as carbon dioxide or methane. “Based on the incredible antiquity of the ice wedges we documented, we think that permafrost that is more than several metres below the surface is more resilient to climate warming that previously thought,” said Froese.



However, Froese and his colleagues emphasize that their study is not an invitation to ignore the potentially serious impacts of climate warming, particularly in the North.



“Permafrost is like the glue that holds the Arctic together,” said University of Alberta graduate student Alberto Reyes. “Widespread deep thaw would be bad news for northern infrastructure and economic development, and may have dramatic effects on ecosystems that are adapted to the presence of shallow permafrost.”

The green Sahara, a desert in bloom





Dr. Rik Tjallingii investigates the Earth's climatic past by analysing sediment cores from the sea floor; here at the Institute of Geosciences at Kiel University. - Photo: CAU, J. Haacks
Dr. Rik Tjallingii investigates the Earth’s climatic past by analysing sediment cores from the sea floor; here at the Institute of Geosciences at Kiel University. – Photo: CAU, J. Haacks

Reconstructing the climate of the past is an important tool for scientists to better understand and predict future climate changes that are the result of the present-day global warming. Although there is still little known about the Earth’s tropical and subtropical regions, these regions are thought to play an important role in both the evolution of prehistoric man and global climate changes. New North African climate reconstructions reveal three ‘green Sahara’ episodes during which the present-day Sahara Desert was almost completely covered with extensive grasslands, lakes and ponds over the course of the last 120.000 years.



The findings of Dr. Rik Tjallingii, Prof. Dr. Martin Claussen and their colleagues will be published in the October issue of Nature Geoscience.


Scientists of the MARUM – Center for Marine Environmental Research in Bremen (Germany) and the Alfred-Wegener-Institute in Bremerhaven (Germany) studied a marine sediment core off the coast of Northwest Africa to find out how the vegetation cover and hydrological cycle of the Sahara and Sahel region changed. The scientists were able to reconstruct the vegetation cover of the last 120.000 years by studying changes in the ratio of wind and river-transported particles found in the core. “We found three distinct periods with almost only river-transported particles and hardly any wind dust particles, which is remarkable because today the Sahara Desert is the world’s largest dust-bowl,” says Rik Tjallingii. He now works at Kiel University, researching within the cluster of excellence ‘The Future Ocean’. The scientists explain these periods by an increase of the precipitation that resulted in a much larger vegetation cover resulting in less wind dust and stronger river activity in the Sahara region. The green Sahara episodes correspond with the changing direction of the earth’s rotational axis that regulates the solar energy in the tropical Atlantic Ocean. Periods of maximum solar energy increased the moisture production while pushing the African monsoon further north and increasing precipitation in the Sahara.



To validate their interpretations, the scientist compared their geological reconstruction with a computer model simulation of the Sahara vegetation cover, performed by the research group of Prof. Dr. Martin Claussen. Dr. Claussen is Director of the Max-Planck-Institute of Meteorology in Hamburg and chairs the cluster of excellence ‘Integrated Climate System Analysis and Prediciton’ at the University of Hamburg. The computer model simulation shows three periods with an almost completely vegetated Sahara at the same time as seen in the geological record. This supports the interpretation of geologists and, in turn, demonstrates the value of computer model results. Additionally, the computer model indicates that only a small increase in precipitation is sufficient to develop a vegetation cover in the Sahara.



Computer model simulations for the future suggest an expansion of the vegetation cover in the Sahara Desert if human-driven climate change leads to aggressive global warming. However, it is difficult to conclude that the Sahara will actually become greener than it is today, as the simulations do not account for the influence of human activity in this area.