4. Gulf Stream shutdown

The Gulf Stream, and its extension the North Atlantic Drift, are part of a system of currents known as the Atlantic Thermohaline Circulation (ATHC) (Figure 11a), which is in turn an element in the worldwide system of currents known as the global conveyor (Figure 11b). The Gulf Stream and the North Atlantic Drift transport warm, tropical, waters from the Gulf of Mexico as far north as the Arctic Circle. During the journey, evaporation due to winds blowing across the sea surface cools these warm waters, and also makes them saltier and therefore denser. Under the freezing conditions of the Arctic, sea ice forms, further concentrating salt in the water and increasing density to such a degree that the water starts to sink, eventually returning southwards in a current known as the North Atlantic Deep water (NADW), which connects with the rest of the global conveyor.

Figure 11: (a) The Atlantic Thermohaline Circulation (ATHC) carries warm tropical waters (red) to high latitudes and returns cold, deep water (blue) that feeds the Global Conveyor (b). Here, warm waters are shown in light blue and cold waters in dark blue. Courtesy: Greg Holloway (a) and Wikipedia (b).

(a)

(b)

The Gulf Stream and its associated currents transport an enormous amount of heat northwards, roughly equivalent to the thermal output of a million large power stations (one million billion Watts). This heat plays a critical role in maintaining a relatively benign climate in the UK and NW Europe, keeping temperatures up to 8° C higher than in comparable latitudes such as northern Canada or the Kamchatka Peninsula of eastern Siberia. The North Atlantic Drift provides the balmy conditions that allow palms and other sub-tropical plants to thrive in western Ireland, SW England and parts of western Scotland, and keeps the Lofoten Islands ice-free despite their location within the Arctic Circle.

A weakening of the Gulf Stream and associated warm currents has been linked in the past with a deterioration of the North Atlantic climate and most global climate models predict that current global warming may have such an effect at some point in the future. In the recently published landmark volume, Avoiding Dangerous Climate Change, which summarises the results of a DEFRA-sponsored meeting held in Exeter in 2005, Michael Schlesinger of the University of Illinois and colleagues highlight the risk of collapse of the ATHC this century. They conclude that the chance of shutdown if we do nothing to mitigate greenhouse gas emissions is more than 50 percent. Even if we make drastic cutbacks, the probability remains more than 25 percent.

Notwithstanding these predictions, there is now considerable evidence that dramatic changes have occurred in the North Atlantic in recent decades, and the first signs that a serious slowdown of the ATHC may already have started. A number of pieces of research have indicated a significant freshening of Arctic waters, due to a combination of increased precipitation, accelerating melting of the Greenland Ice Sheet and greater northward flow of freshwater from Canadian and Siberian rivers – all a consequence of climate change. The concern is that this would reduce the salinity of the Gulf Stream’s extension – the North Atlantic Drift – to such a degree that they would no longer be sufficiently dense to sink. Such a situation would short-circuit the ATHC by stopping the return flow of cold, dense, water that makes up the NADW. Evidence that this return current might also be slowing could lie in observations made by Faroe Island and British scientists, which suggest that the return of deep, cold water between Greenland and Scotland has slowed by 20 percent in the last 50 years. Most significantly, in December 2005, Harry Bryden and his team at the Southampton Oceanography Centre in the UK published evidence for a 30 percent slowdown in the warm waters heading towards the Arctic in the North Atlantic Drift. These results are based upon indirect measurements of the circulation and based upon just five ‘snapshots’ of data acquired in 1957, 1981, 1992, 1998 and 2004, the latter by Bryden’s team. As a consequence, some oceanographers have questioned the validity and significance of the results, claiming that the observations may just represent a blip and that circulation may increase again. Having examined the earlier data, however, Bryden and co-workers point out that the flow was steady between 1957 and 1992, but dropped off before 1998 and has remained low since.

While the seas off Europe show no sign of cooling – another argument used by scientists sceptical of the results – and are in fact slightly warmer than a decade ago, Bryden forecasts that if the slowdown persists, temperatures in the UK and Europe could be expected to fall by about 1° C over the coming decade. This may not sound like much, but it could bring conditions similar to those that gripped the region during the Little Ice Age, between the 15^th and 19^th centuries, which saw sea ice in the Channel, frost fairs on the Thames, and skating on the Dutch canals. More worrying is what will happen if the ATHC shuts down completely, as it has done in the past, and which is likely to happen with very little warning. The increased freshwater flow into the Nordic seas can initially be expected to weaken the circulation slowly – as we may now be seeing. At a particular threshold, however, it is likely to jump to a new state in which there is little or no heat flux to the North West Atlantic. If this happens, temperatures across Europe and eastern North America would fall by ~ 4° C within 20 years, bringing chillier summers and appalling winters. UK Met Office simulation suggest that within 5 or 6 years of shutdown winter temperatures would regularly fall far below minus 10° C (Figure 12). Such a situation would bring massive disruption and upheaval, with agriculture, health, energy supply and travel worst affected. In the UK, winters as bad as or worse than those of 1946-47 and 1962-63 would be the norm, with snow on the ground for months at a time, ice storms and blizzards ubiquitous and ports frozen in by pack ice. The ramifications of complete shutdown would also reach far beyond the North Atlantic region, with predicted effects farther afield including a weakening of the Indian Monsoon and halving of rainfall in parts of Central and South America. This could slash agricultural productivity and place what remains of the Amazon Rainforest in peril.

Figure 12: UK Met Office simulation suggests that within 5 or 6 years of a shutdown of the ATHC, winter temperatures would frequently fall far below minus 10 °C . Courtesy: UK Met Office Hadley Centre.

While much speculation remains, our understanding of the situation should improve in the next 10 years or so, when data begins to come in from a British array of instruments strung out between Africa and America and designed to monitor a range of ocean properties including temperature and salinity. If the reduced flow is shown to be continuing or accelerating, then a serious rethink will be required in relation to how climate change will impinge upon life and business in the UK and Europe.

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