North Atlantic circulation may be more sensitive to Greenland melting than thought
North Atlantic circulation may be more sensitive to Greenland melting than thought
Universitat Autonoma de Barcelona
May 8, 2007
According to two international-research studies on the last ice age, studies with the participation of Dr Rainer Zahn, research professor in the ICREA at the UAB Institute of Environmental Science and Technology (ICTA), before the great ice sheets of the Arctic Ocean began to melt, early sporadic episodes of melting of the old ice sheet which covered the British Isles had already begun to affect the circulation of the ocean currents, which played a key role in the climatic stability of the planet. Based on this observation, scientists consider that the acceleration of the melting of the Greenland ice cap could play an important role in the future stability of ocean circulation and, hence, in the development of climate change.
The magnitude of possible climate change in the future will depend to a large degree on the response of ocean circulation to global warming, as the ocean currents distribute an immense quantity of heat around our planet and, besides, determine levels of humidity and energy. Any variation in ocean circulation may lead to substantial and abrupt climate changes (that is to say over less than 30 years) on a global scale.
Deep ocean sediments offer a record of ocean circulation in the past. By studying these sediments, we can see that abrupt changes in ocean circulation and the subsequent climate change are not a new phenomenon, but have happened on several occasions in the past. When the great ice sheets covering North America and Scandinavia melted at the end of the last ice age, the subsequent flow of fresh water into the North Atlantic caused the greatest natural disturbance in ocean circulation in the last 20,000 years. This episode provides an excellent model to examine the relation between ocean disturbance and climate instability.
Greenland’s coast is riddled with fjords—glacier-carved canyons flooded by seawater. In several of the fjords, the water appears pale turquoise, in contrast to the dark blue of the ocean water. This lighter shade results from fine sediment. The sediment particles are small enough to remain suspended in water, lightening its color. East of the network of fjords, pale blue sea ice swirls along Greenland’s coast in a paisley pattern. These intricate shapes in the sea ice result from ocean currents moving in opposite directions. Immediately east of the coast is a southward current. East of that, a different ocean current heads north. The currents brushing past each other constantly push the sea ice in different directions. NASA image created by Jesse Allen, Earth Observatory, using data obtained from the Goddard Earth Sciences DAAC. |
According to a revision article published in Science, ocean circulation during the last ice age was very different to present day circulation. The formation of deep water currents in the North Atlantic was much weaker and the flow of warm water from the Gulf Stream decreased. This led to a cooling of the northern hemisphere and contributed to the formation of the great ice caps which covered North America, Scandinavia and Europe.
In a similar study, the marine sediments of the North Atlantic were observed in order to document the sequence of events that led to that disturbance. The melting caused a significant decrease in the Gulf Stream, which transports warm water from the Gulf of Mexico to the North. This submerged the region of the North Atlantic into a period of glacial cold which lasted at least 1,200 years.
Nevertheless, the slowing down of the ocean circulation in the North Atlantic began about 700 to 1,200 years before this great melting of the ice caps and the subsequent flow of fresh water into the ocean took place. The very first stage of this change coincided with brief and isolated periods of melting of the small British Ice Sheet (BIS). The authors of the study have come to this conclusion from an observation of the fine layers of sediment (formed by grains of quartz) coming from successive waves of icebergs which, when they melted dumped their load of sediments onto the sea bed. These icebergs came from the edges of the ice which surround and stabilised the BIS.
These results show that the disturbances caused by melting may in turn cause substantial changes in ocean circulation without the need for a catastrophic dumping of fresh water. This seems to indicate that an acceleration in the melting of the Greenland ice cap, could, in fact, play a key role in the future stability of ocean circulation and climate change in the whole North Atlantic region.
Knutz, P.C., Zahn, R. and Hall, I.R., 2007. Centennial-scale variability of the British Ice Sheet: Implications for climate forcing and Atlantic meridional overturning circulation during the last deglaciation. Paleoceanography (American Geophysical Union), Volume 22, doi:10.1029/2006PA001298.
Lynch-Stieglitz, J., J. F. Adkins, W. B. Curry, T. Dokken, I. R. Hall, J. C. Herguera, J.-M. Hirschi, E. V. Ivanova, C. Kissel, O. Marchal, T. M. Marchitto, I. N. McCave, J. F. McManus, S. Mulitza, U. Ninnemann, F. Peeters, E.-F. Yu, R. Zahn, 2007. Atlantic meridional overturning circulation during the last glacial maximum. SCIENCE, 316, 66-69, 2007.
This is a modified news release from Universitat Autonoma de Barcelona.
