Climate change sceptics lose vital argument

The New Scientist
November 29, 2006

By Zeeya Merali

The "hockey stick" graph, which shows a rapid rise in world temperatures over recent decades, has been both poster child for the dangers of human- induced global warming and prime target for climate change sceptics. They cite an anomaly in the graph - it does not record a dip in temperature between 1200 and 1850 - as reason to ditch the whole thing. Now new data may help explain why the graph does not record the "little ice age".

Ocean currents in the North Atlantic, dominated by the Gulf Stream, usually keep winter temperatures in western Europe mild by carrying warm water north from the tropics towards Europe and heating the westerly winds travelling from North America. Climate scientists have suspected that a weak Gulf Stream may have caused the little ice age, but until now there has been no direct evidence for this theory.

Jean Lynch-Stieglitz at the Georgia Institute of Technology in Atlanta and her colleagues calculated the strength of the Gulf Stream during the little ice age by looking at marine fossils in sediment cores taken from the Straits of Florida. Ocean circulation is driven by variations in water density caused by differences in temperature and salinity. These variations also affect the ratio of oxygen isotopes in marine fossil shells. By measuring these isotope ratios, the team calculated that the Gulf Stream was 10 per cent weaker during the little ice age (Nature, vol 444, p 601).

Moreover, the cooling that resulted was confined mainly to the northern hemisphere, says Lynch-Stieglitz - which indicates it was a regional effect. Michael Mann at Pennsylvania State University in University Park, who constructed the hockey stick graph, has always argued that if this were so, the little ice age would not show up on a global temperature record (New Scientist, 18 March, p 40).

"We're seeing a rearrangement of heat around the globe - so globally overall it's not colder," says Lynch-Stieglitz.

Last month, climate scientists played down the possibility of a mini ice age in the near future (New Scientist, 7 November, p 13). "We usually look for a much more dramatic weakening - an almost total shutdown of the circulation - when considering the impact on future climate," says Meric Srokosz at the National Oceanography Centre in Southampton, UK. By contrast, the 10 per cent weakening found by the Georgia team is surprisingly small, says Lynch-Stieglitz. She says that climate scientists have to incorporate this finding into their models to better understand how such a small change brought about the little ice age.

Tim Hermach
Native Forest Council
PO Box 2190 Eugene, OR 97402 541.688.2600 541.461.2156 fax web page: DEFENDING NATURE

Informant: Scott Munson


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