Stormy times ahead?
18 November 2003
Dr Jonathan Bamber, senior lecturer in the School for Geographical Sciences, is the lead investigator on a multi-institute project aimed at investigating how changes to the ice cover in the Arctic may affect the future climate of north-west Europe.
We are eternally fascinated by the weather – it is an essential topic of conversation. Huge sums of money are invested in trying to predict what it is going to do tomorrow, the day after or even a week into the future. But what about next year, the next decade or even the next century? What does the climate hold in store for us over these timescales? These are the sorts of questions that the project will try to tackle.
Funded by the Natural Environment Research Council under a scheme called RAPID, the multi-institute project aims to investigate potential triggers for abrupt climate change. These abrupt changes have been known to occur in the past, particularly during the last glacial period between about 120,000 and 12,000 years Before Present (BP). The changes then were as much as seven degrees centigrade in just a few decades. Such a large and rapid shift would have devastating socioeconomic consequences if it happened today, but during the relatively warm inter-glacial period that we are now in (known as the Holocene and covering the last 12,000 years) there is no evidence for such dramatic climate shifts. Does this mean that the Holocene is more stable than the glacial period that preceded it, or is the global warming that we apparently see today pushing the climate system towards an unstable threshold? This is one of the key questions that the project seeks to address.
Large and rapid climate shifts would have devastating socio-economic consequences
The density of sea water is controlled by its temperature (thermo) and its salinity (haline), thus the circulation of water driven by density differences is called the thermohaline circulation. The thermohaline circulation of the North Atlantic (of which the Gulf Stream is a part) keeps the UK several degrees warmer than it would otherwise be at these latitudes. But as this warm water mixes with cold water coming from the Arctic Ocean, it cools and becomes so dense that it sinks. Over time this dense water warms up and returns to the surface. The sinking regions and the return of water to the surface eventually form an enormous closed loop, termed the ‘global thermohaline conveyor belt’. It is widely believed that this thermohaline circulation of the oceans played a key role in the rapid changes that occurred during the last glacial period. During this time the ice sheet covering North America (called the Laurentide) would, from time to time, spew out huge armadas of icebergs into the North Atlantic, freshening the surface waters which eventually shut down the thermohaline circulation and triggered a major climate shift.
Today there is no Laurentide ice sheet, but there are other sources of freshwater, namely the Greenland ice sheet and Arctic sea ice. In the past few decades both of these ice masses have been observed to be shrinking and, if the predictions of global warming over the next century are accurate, this mass loss looks set to greatly accelerate. The Arctic is particularly sensitive to global warming because of a strong positive feedback between snow cover and temperature – as snow and ice disappear more solar radiation is absorbed by the much darker land and sea surface underneath, further enhancing the warming effect, which results in the disappearance of more snow. Thus most climate models predict that global warming will be substantially amplified in the Arctic. The key question Bamber and his team aim to address is: will the increase in freshwater from the Arctic be sufficient to reduce or shutdown the thermohaline circulation and, paradoxically, cause a cooling in northern Europe?
Although most of the glaciers were shrinking, the largest ice cap in the area was growing
Some surprising results have already been obtained. In 2002, Bamber and a team of NASA scientists carried out an airborne survey of several glaciers and ice caps in the Svalbard Archipelago in the high Arctic (Spitsbergen lies in this group of islands). They measured changes in the surface elevation of the ice masses over a six-year interval, with centimetre accuracy. To their surprise they found that although most of the glaciers were shrinking as expected, the largest ice cap in the area (the size of Devon) was growing. It turns out that the most likely explanation for this puzzling result is a rise in snowfall caused by an increase in areas of open water, due to the retreat of sea ice that normally covers the ocean, even in late summer. Arctic sea ice is predicted to retreat even more over the next few decades and it seems likely, therefore, to result in a more vigorous hydrological cycle. In other words, more rain! How far beyond the Arctic the effects of the increase in precipitation will be felt is currently being investigated using sophisticated computer-based climate models.
Bamber has teamed up with experts from several institutes including the Southampton Oceanography Centre, the Met Office Hadley Centre for Climate Prediction and Research, and the Department of Meteorology at the University of Reading. Together they will pool their expertise in observing and modelling how the oceans, atmosphere and cryosphere (glaciers, ice sheets and sea ice) interact in order to produce a suite of sophisticated computer models designed to reproduce present-day and future climates. The models exist in part, but need to be able to ‘talk’ to and interact with each other so that feedbacks between the different components of the climate system (as illustrated by the results obtained from Svalbard) are captured. The models also need to be fully validated against present-day observations to ensure that they realistically reproduce past climate trends.
When this validation phase is complete the team will use the models in two ways: first to help understand the underlying processes controlling the stability of the climate system and second to see how close to the edge we really are. Although it is beyond the team’s remit or capability to offer solutions, forewarned is at least forearmed!