The Earth´s climate is seesawing

The research group, currently consisting of Svante Björck, Karl Ljung and Dan Hammarlund, has retrieved cores of lake sediments and peat along a north-south transect of Atlantic islands and adjacent land areas: Greenland, Iceland, Faroes, Azores, Tristan da Cunha, Isla de los Estados, and the Antarctic Peninsula. Based on detailed analyses of geochemistry, mineral magnetism and pollen content, hitherto unknown details of Atlantic climate dynamics have been resolved. Extensive radiocarbon dating and rapid sedimentation rates in the terrestrial deposits allow a much higher temporal resolution of the data than provided by marine sediment cores.

— Our records reflect details of the climatic evolution in the Atlantic region since the end of the last Ice Age to the present day. We would have liked to compare our results to similar data sets from other parts of the South Atlantic, but no other records provide the same degree of resolution, says Professor Björck. After the end of the last Glacial both Hemispheres became warmer as a result of melting ice sheets, but during the last 9000 years we can identify a persistent “seesaw” pattern. When the South Atlantic was warm it was cold in the North Atlantic and vice verse.

— This is most certainly related to large-scale ocean circulation in the Atlantic Ocean. The main current system – “the Great Ocean Conveyor” – is driven by sinking of dense, relatively cold and salty water in the northern North Atlantic. This results in southward-flowing deep-water that is replaced by warm surface water brought to high northern latitudes from the tropics and ultimately from the South Atlantic, says Svante Björck, and continues:

— The deep-water formation in the north is dependent on cooling of surface water with a high salt content. If sufficiently large amounts of fresh water are supplied to the North Atlantic, such as from melting ice-sheets or major increases in precipitation, the deep-water formation, and hence the transport of warm surface water from the south, may cease or at least decrease substantially. This is known to have happened repeatedly during the present Interglacial (the warm period since the last Ice Age). Minor disturbances have taken place in recent time, such as the Great Salt Anomaly in the 1970s, which seriously affected the cod population around the Faroe Islands. Our results from Nightingale Island in the Tristan da Cunha island group, between South Africa and Argentina, for the first time give evidence of warming of the South Atlantic associated with cooling in the north. This is a major breakthrough in palaeoclimate research.

In agreement with most other climate researchers, the Lund group is not concerned about a complete shut-down of the Gulf Stream as envisioned in the apocalyptic film “The day after tomorrow”. However, future warming induced by anthropogenic greenhouse-gas emissions may influence the system.

— We don't know with certainty what will happen. Some attempts at measuring ocean currents suggest a recent weakening of the Gulf Stream, and the transport of heat to the North Atlantic region may well decrease in the future as a result of increased precipitation. Such a scenario might lead to less warming in Europe than predicted by the IPCC, but we will probably not face an arctic climate, summarizes Svante Björck.

Further information can be obtained from Svante Björck +46 46 222 79 85, Karl Ljung +46 46 222 78 88 or Dan Hammarlund +46 46 222 79 85