The melting of the vast ice sheets that covered parts of North America and Europe during glacial periods was complex, with impacts on ocean circulation and Northern Hemisphere climate, suggests two review articles published online this week in Nature Geoscience.
Anders Carlson and Kelsey Winsor reconstructed the pattern of ice-sheet breakup across eastern North America, western Europe and Scandinavia during the two most recent transitions out of a glacial state, about 20,000 and 140,000 years ago. They found that the speed and pattern of ice-sheet retreat depended on where the ice sheet was situated: ice sheets sitting solely on land broke up steadily in response to increasing amounts of incoming sunlight and rising greenhouse gas concentrations. Those that reached into the ocean exhibited a delayed breakup marked by periods of rapid retreat.
Torbjorn Tornqvist and Marc Hijma found that two rapid pulses of meltwater release from the decaying North American ice sheet about 8,500 and 8,200 years ago caused a sea-level rise of between 0.4 and 2 metres. The meltwater pulses were also associated with changes in North Atlantic circulation that led to an abrupt cooling in Northern Hemisphere temperatures. They suggest that in warm climate states, abrupt changes in Atlantic Ocean circulation require a prolonged release of freshwater into the North Atlantic Ocean.
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