Global warming is affecting the great British summer

An international research project has found evidence to suggest that the United Kingdom’s recent run of poor summers is the result of global warming and Arctic ice loss. Scientists from the University of Sheffield, Rutgers University, the University of Washington and the United States’ National Oceanographic and Atmospheric Administration (NOAA) contend that a change in summer Arctic wind patterns caused by global warming has resulted in more severe weather during British summertime.

The temperature increases that are being observed in the Arctic are comparatively greater than those occurring in other locations around the world. This process, known as Arctic amplification, has resulted in record low summer ice extent and thickness. In turn, this ice loss has led to shifts in wind patterns which have brought unseasonal weather to Greenland, Western Europe and North America.

In an interview with ScienceOmega.com, Dr Edward Hanna, one of the study’s authors and Reader in Climate Change at the University of Sheffield’s Department of Geography, explains why UK summers have not been the same since 2007…

In what ways have summer Arctic wind patterns changed during the last six years?
Over the last six summers, there appears to have been a regime shift. During this relatively short period of time, we have observed a wavier pattern in the jet stream, which now has more loops and branches. Over Greenland, for example, there seem to have been more winds blowing from south to north rather than from west to east – the direction in which they have previously blown. In the UK, on the other hand, we have had more north-westerly winds and there has been a more southerly branched jet stream over our country since 2007. Essentially, our weather patterns have been more unsettled over the last six summers than they were previously.

What do you and your colleagues think caused these wind patterns to change?
Global warming is amplified in Arctic areas. In fact, over the last two or three decades, it has been greater than warming in the rest of the world by a factor of about two. The whole process seems to have been accelerated by the melting of ice and by what are known as feedbacks in the Arctic climate system. These feedbacks speed up the absorption of heat at the surface. The melting of ice and the greater changes in temperatures at higher latitudes appear to be very strongly related to changes in the air patterns that control the wind across the entire hemisphere.

Since 2007, high pressure over Greenland has been much more persistent than it has been in previous decades. This trend is very striking. Winds move around pressure systems in particular ways, and these winds are no longer simply blowing from west to east. As I said, we have observed waves within the jet stream, but there have also been changes within the waves themselves.

Obviously, changes over Greenland affect places that are ‘downstream’ such as the UK. Greenland is, after all, a huge mountain. It is an ice dome capable of affecting atmospheric circulation and its blocking effect has been more pronounced over the last six summers. In 2007, for instance, the jet stream was bifurcated. One part of it went northwards over Greenland whilst the other branched southwards over the UK; much further south than it would normally reach. This event has been linked with the exceptional flooding and rainfall experienced in the UK six summers ago. This record level of rainfall has actually been exceeded in 2012, and in general, we have experienced some unusually wet periods since 2007.

So is the hot, dry British summer a thing of the past?
No, not at all. We are just pointing out that there now exists more variability within the system. I’m sure that the UK will get some dry, hot summers in the future but these might prove to come around less frequently. We have to remember that jet streams are influenced by multiple factors. Heat differences, for example, between lower and higher latitudes lead to heat redistribution. Heat surpluses that build at lower latitudes are driven to higher latitudes where there are heat deficits. A change in the heat gradient can result in a change in the jet stream’s behaviour; it can become wavier.

Of course, this is not the only factor that is influencing the jet stream. Changes in sea-surface temperatures and variations in solar energy also seem to play a role. We think that a combination of all of these factors causes the jet stream to vary. Global warming is not equal across all parts of the Earth’s surface, and its amplification at higher latitudes is throwing the whole climate system out of balance. When people talk about climate change – a horrible term – they really mean global warming. The climate is constantly changing because of natural variations. However, due to a combination of these variations and manmade, greenhouse warming, Arctic sea losses have been much greater than computer models anticipated. Even the models cannot replicate the late-summer ice losses that we’ve witnessed of late. The release of this much extra heat from the ocean into the atmosphere is bound to have a significant impact. We think that the amplified warming taking place at high latitudes is throwing the jet stream out of balance.

And 2012 has brought with it record levels of ice loss…
In the Arctic, this has certainly been the case. Satellite records show that sea-ice coverage is at an all-time low. At its lowest point this year, coverage fell to approximately 3,500,000 square kilometres. Previously, it had not fallen below 4,000,000 square kilometres. Since 2007, we have had a succession of record low years and this year is the lowest yet. Whilst we need to collect more data, there does seem to be a link between Arctic sea ice reductions and jet stream changes. Further modelling studies and observations will be necessary in order to verify our initial findings.

Are you and your colleagues able to predict how this situation might develop over the coming years?
Not at this time. Our results are new and follow-up work is needed. Some colleagues and I are now working on a project in collaboration with the Met Office. We will be looking at global climate data spanning from 1870 to the present day, and we will also be using Met Office simulations to look ahead to 2100. We hope that the comprehensive scope involved in this investigation will teach us more about the factors that are causing the jet stream to vary.