The Day After Tomorrow

New research suggests something like the movie The Day After Tomorrow may have happened in the past, with sobering implications for the future. In this blockbuster, melting Arctic ice led to dramatic cooling across the Northern Hemisphere, highlighting the paradox that in a warming world, some regions may indeed cool (or at least not warm as much as expected). Whilst no one is suggesting this could lead to a new ice age, climate model projections do suggest some parts of the world may respond differently to long-term warming. One of the big uncertainties is the impact a melting Greenland ice sheet might have across the North Atlantic. At a simple level, the world’s oceans are connected as though by one enormous conveyor belt. At one end of the loop, warm tropical waters popularly known as the Gulf Stream drift up into the North Atlantic where over the course of a year the evaporating surface delivers heat downwind equivalent to the output from a million power stations. It’s a major reason for the starkly different temperatures experienced along the northern 50th parallel; why Europe is over 15˚C warmer than Newfoundland and Labrador on the other side of the Atlantic. Eventually, however, this northward-flowing current becomes too cold and salty to stay afloat, and sinks, heading south kilometres below the surface. If enough of Greenland warms, the fresh water released by a melting ice sheet may be sufficient to dilute the surface ocean waters and stop the formation of salty, dense water, breaking the loop. One way to resolve future uncertainty is to look into the past when conditions were warmer than today. Regular Intrepid Science followers will have read previous work we’ve done on the Last Interglacial but the short story is this was a period around 127,000 to 116,000 years ago when polar regions were some 2˚C warmer than present day - a potential scenario for the future [thanks to long-term changes in the Earth’s orbit, our planet received extra warmth from the Sun, triggering a host of changes that include less Arctic sea ice and carbon release from the oceans and permafrost, amplifying warming. These so-called ‘positive feedbacks’ are a real concern for future climate change but that’s another story]. 

Where the Greenland Ice Sheet meets the North Atlantic. Icebergs in high summer in Sermilik Fjord, one of the largest fjords in southeast greenland. Helheim glacier, which drains into the fjord, has seen some of the highest acceleration of ice velocity recorded across the Greenland Ice sheet over the past decade (credit: C.J. Fogwill).

By looking at the recent geologic record we can reconstruct what happened in the North Atlantic during the Last Interglacial. A few years ago, a great mate, Richard Jones, and I produced the first global temperature reconstruction for the Last Interglacial (published in Journal of Quaternary Science as ‘Does the Agulhas Current amplify global temperatures during super-interglacials?’). With an international team of friends from the universities of Exeter, UNSW, Lund and the Natural History Museum (London), we’ve now published a new study in the journal Geology called ‘Delayed maximum northern European summer temperatures during the Last Interglacial as a result of Greenland Ice Sheet melt’. Drilling some 5 metres of Last Interglacial lake muds in Denmark we’ve been able to extract fossil remains of what are known as non-biting midges (something referred to as ‘chironomids’). Chironomids are wonderful for reconstructing past climate. The surface lake water temperatures they inhabit dictate what species can exist at any one time. Some species thrive in warm waters, others prefer more frigid conditions. The practical upshot is by extracting the minute remains of chironomids preserved in the lake muds layer-by-layer, it’s possible to reconstruct how temperatures have changed over time. To our amazement we found the warmest temperatures didn’t happen at the start of the Last Interglacial but some 3000 years later. And when we compared our reconstruction to ocean and ice core records across the North Atlantic, we reached a startling conclusion. The summer temperatures mapped onto what was happening with the Greenland ice sheet: when Greenland was melting, summer temperatures effectively downstream over Europe were relatively cool. And when the Greenland ice sheet stopped melting, Europe warmed dramatically. Worryingly, recent work by Stefan Rahmstorf at the Potsdam Institute for Climate Impact Research and colleagues suggest this may already be happening. 

This work was funded by the Australian Research Council (ARC) and we gratefully acknowledge their support.