Gimme swelter: we explain the UK and US heatwaves

After the cold, the heat. High pressure spreading across the UK from Siberia last spring brought record cold temperatures. Now more high pressure, this time from the tropical Atlantic, is bringing a sweltering heatwave. These high-pressure zones are blocking the jet stream which usually brings the country's normal changeable weather. "Blocking highs" are an increasing theme of North American weather reports too, bringing concern of a long-term shift. New Scientist looks at the issues.

What's going on?

Blocking highs are a regular feature of the weather in mid-latitudes. They cut out the normal rain-bearing weather systems that are dragged from west to east by the jet stream, a near-continuous high-speed wind flow in the upper atmosphere. The result of blocking highs is episodes of stable and often extreme weather. That is why, with temperatures repeatedly exceeding 30?C in the last week, the UK has been under a "level three" heatwave alert ? one level short of a national emergency.

How unusual is the sweltering British weather?

It's very unusual. This is the first significant heatwave in the UK for seven years. Older British readers may remember the long, hot summer of 1976, which saw five days with temperatures above 35?C, which is still a record. That year also brought water shortages, standpipes in the streets and widespread forest and heathland fires. But the current heatwave is not far behind. The UK Met Office issued wildfire alerts on Friday after a series of small blazes in south-east England.

Is it part of a trend?

It's hard to be sure. There is no firm statistical evidence of more or longer-lasting blocking highs. But we have had some nasty ones recently, widely blamed for the Russian forest fires during the extreme heatwave of 2010, some extreme hot and cold in the US, and even floods in Pakistan, also in 2010.

Jennifer Francis at Rutgers University, New Jersey, reported last year that the jet stream appears to have slowed down, by about 14 per cent in the past three decades. It is also meandering more, looping north towards the Arctic and south towards the tropics ? perfect conditions for blocking highs. The weather, she says, is getting "more stuck".

Is climate change to blame?

It could be. It is well established that the jet stream is sustained by the temperature difference between Arctic and lower latitudes. And because the Arctic is warming faster than elsewhere, that temperature difference is declining. So we should expect a weakening jet stream and more blocking highs in the decades ahead.

Not everyone agrees, though. Earlier this year, a paper co-authored by Ian Simmonds of the University of Exeter, UK, suggested that the jet stream may not be meandering more than before. So everyone is now confused. Simmonds concluded that the "possible implications for European summer weather" were "complex". Our advice? Keep buying the sunscreen.

What do the big climate models say?

They, too, have a downer on the Francis's theory. Most ? including the latest CMIP5 model, developed for the Intergovernmental Panel on Climate Change (IPCC) ? actually predict a decrease in blocking highs, at least over Europe. The most likely reason for such a decrease is that global warming pushes the jet stream further north, according to Elizabeth Barnes, an atmospheric physicist at Colorado State University in Fort Collins. This shorter track will leave less room for blocking highs to form.

So where does that leave us?

Sweating. We all know now that global warming has not been as vigorous in the past decade as it was in the 1980s and 1990s. Probably natural cycles, such as the movement of heat between the oceans and atmosphere, affected both the rapid warming before and the temperature plateau since. But the underlying trend, as the IPCC will report in September, is still upwards. The basic physics of greenhouse gases more or less dictates that.

So, whether or not blocking highs become more frequent, the chances of temperatures pushing up beyond 30?C and triggering Met Office alerts seem set to increase.

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