Updating Our Understanding of Climate Change in the North Atlantic: The Role of Global Warming and the Gulf Stream
O'Hare, Greg, Geography
It is well accepted that the temperate nature of UK/Europe climate is heavily dependent on the Gulf Stream. Many modelled studies have shown that ocean freshening in the North Atlantic by global warming will reduce the strength of the Gulf Stream - an ocean current thought to be principally driven by ocean density (i.e. temperature and saltiness) differences in the high North Atlantic (O'Hare et al., 2005). If weakening in the Gulf Stream occurs gradually over the 21st century (IPCC, 2007), a possible amelioration of the rising temperatures brought about by global warming could occur over the UK and Western Europe. However, if the Gulf Stream collapses in just a few decades, there could be an abrupt onset of very cold conditions over the region (Jenkins, 2007). In recent years a major paradigm shift has occurred in our understanding of the processes that transport not only the Gulf Stream but also ocean currents in general. It is becoming clear that winds in the atmosphere drive most of the circulation in the ocean. Indeed, references to the idea that density differences in the ocean, caused by thermohaline variations in temperature and saltiness, have diminished in recent oceanographic literature (Toggweiller and Russell, 2008). These new ideas concerning winds and on how currents are transferred in the oceans have reduced the theoretical threat of Gulf Stream collapse and respective cooling over the North Atlantic region. They have also unfortunately diminished the possibility of a scaling down of the future impact from global warming over the region.
Our understanding of North Atlantic climate and the dual processes of global warming and Gulf Stream action which affect it, is as uncertain today as in the early 2000s when it was summarised in Geography by O'Hare et al. (2005). Today, as then, modelled projections of future temperature rise for the region, as for instance by the prestigious InterGovernmental Panel on Climate Change (IPCC), are still highly dependent, among other factors, on estimated greenhouse gas emissions. Also as then, the IPCC are not completely certain about Gulf Stream behaviour. The Panel continues to maintain, however, that observable weakening in the Gulf Stream system is 'very likely' towards the latter part of the 21st century (IPCC, 2007) and will bring an element of cooling to the global heating of the North Atlantic and its adjacent eastern shore-lands. Figure la shows climate change over the North Atlantic according to the IPCC in relation to a moderately low future greenhouse gas emission scenario called the Bl (IPCC, 2007). This scenario conservatively projects total accumulations of global carbon dioxide (CO2) by the end of the present century at around 1100 GtC (Giga or 109 tonnes carbon). With the B1 scenario global warming will occur over the region, but it will be ameliorated by Gulf Stream weakening. For instance, net surface temperatures in the central high North Atlantic may only rise by about 0.5°C with average rises over the UK around 1.5-2.0°C. This contrasts with regions beyond the effect of the weakening Gulf Stream such as central North America, Eastern Europe and the high Arctic, which will be subjected to higher regional temperature increases. A much higher greenhouse gas emission scenario (A2) is shown in Figure lb by the IPCC (2007) projecting a possible total accumulation of CO2 from today of around 1750 GtC by the end of the century. With the A2 scenario temperature increases by global warming are higher than with the Bl, but Gulf Stream weakening may also be stronger under greater warming. The net result is a fairly strong cooling fingerprint within a more highly warmed North Atlantic region. As shown in Figure lb, temperatures in the UK are projected to increase by around 2-3°C with much higher globally warmed temperature rises over North America, central Europe and the high polar latitudes. It should be acknowledged that despite a possible net warming in the UK of only around 2-3X by 2090, this still represents what scientists call a dangerous level of temperature rise. …