In 2004, the European Climate Forum highlighted more specific indicators of dangerous climate change, including circumstances that could lead to global and unprecedented consequences, extinction of iconic species (e.g. the Polar Bear), loss of entire ecosystems or human cultures, a threat to water resources, and a significant rise in mortality rates. Dangerous climate change is likely to happen suddenly in response to the crossing of specific thresholds or achievement of so-called tipping points.
Two major resulting threats relate to rapidly rising sea-levels and the shutting down of the Atlantic Thermohaline Circulation (ATHC) - the Gulf Stream and associated currents. Concerns about catastrophically rising sea-levels have increased in the light of recent observations of accelerated ice loss at the poles.
The rate of melting of the Greenland Ice Sheet has doubled in the last 10 years, from 96 km3 in 1996 to 220 km3 in 2005, with three of the biggest glaciers draining the ice sheet doubling their rates of sliding seawards in the last 7 years. A local temperature rise of just 2.7° C (corresponding to a global rise of less than 2° C) is predicted to result in irreversible melting of the Greenland Ice Sheet and an eventual ~ 7 m sea-level rise.
This threshold could be reached as early as 2050. Increased ice loss is also occurring in Antarctica, where the West Antarctic Ice Sheet is now losing about 150 km3 a year. Over the last 50 years, an area of ice shelves the size of Jamaica have broken up and melted, while on the Antarctic Peninsula, the Pine Island and Thwaites glaciers are now moving three times faster than 10 years ago. Melting of these glaciers alone would raise sea-levels by more than a metre, with complete melting of the West Antarctic Ice Sheet raising global sea-levels by ~ 5 m. Sea-levels are currently rising at ~ 3 mm a year, but if catastrophic melting of the Greenland and/or West Antarctic ice sheets occurs, some climate scientists now forecast a rise as high as 1-2 m this century and several metres by 2200.
Another aspect of dangerous climate change involves the shutdown of the Gulf Stream and associated currents (making up the Atlantic Thermohaline Circulation or ATHC), which keep the UK and Europe up to 8 degrees warmer than comparable latitudes such as northern Canada and eastern Siberia. Climate models predict that if nothing is done to mitigate greenhouse gas emissions, one recent study estimated that the probability of ATHC shutdown this century could be higher than 50 percent. Recent measurements suggest, however, that the ATHC may have slowed by 30 percent since 1992. This could result in a 1° C fall in UK and European temperatures within 10 years, bringing a return to Little Ice Age conditions, while complete shutdown of the ATHC could result in a 4° C fall, with winter temperatures frequently far lower than minus 10° C within 5-6 years of shutdown.
The latest in a series of papers over the last few years adds further fuel to the idea that global warming may be driving major changes in the waters of the North Atlantic. Harry Bryden and colleagues, at Southampton's National Oceanography Centre, recently reported (Nature 438, 655-57) a 30 percent slowing of the Atlantic meridional overturning circulation (AMOC) between 1958 and 2004.
The AMOC, of which the Gulf Stream is a part, carries warm upper waters into high latitudes and returns cold, deep waters southwards to the equator. Shutting off the AMOC has happened in the past, most notably at times during the ice ages, and is predicted in some global climate models as a consequence of increased greenhouse gas emissions and accelerated warming. Some researchers put the probability of a dramatic slow-down or shutdown as high as 45 percent with a 2 - 3º C temperature rise; an increase that is virtually certain to take place this century.
Without the warming influence of the Gulf Stream, western Europe, including the UK, is likely to be around 4º C cooler, with chillier summers and bitterly cold winters a result. While admitting that the changes may be part of a natural cycle Bryden and his co-workers are concerned that the dramatic modifications may well reflect the influence of global warming and a freshening of North Atlantic waters due to rapidly melting Arctic ice.
Posted on 10th August 2006
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