How the heatwave in Russia is connected to floods in Pakistan
Aug 12th 2010 | From The Economist print edition
AS RUSSIA burns to a crisp, thousands of kilometres to the south-west torrential storms visit unprecedented flooding on Pakistan. Both events can be attributed to the same large-scale pattern of atmospheric circulation. They are also both the sort of thing climate scientists expect more of in a warming world.
The upper atmosphere (the part through which the jet streams run) is gently rocked by what are known as Rossby waves—movements of air towards and away from the poles. These waves usually travel east or west, depending on various conditions. But they can also stand still, trapping the weather beneath them.
According to Brian Hoskins, a climate scientist at the University of Reading, this year’s anticyclones in the Atlantic have produced just such a gridlock in the world of the Rossby waves, with persistent troughs of low pressure over western and central Europe, a ridge of high pressure over Russia, and lows again farther east. The air itself doesn’t necessarily sit still, but the pressure patterns which dominate the weather persist. The troughs have seen rain—producing serious floods in central and eastern Europe and catastrophic ones in Pakistan. The pronounced and persistent high over Russia has seen record temperature after record temperature.
Like many atmospheric processes, heatwaves have a tendency to feed upon themselves. High pressure makes it hard for clouds to form, and thus for rain to fall. Under cloudless skies, the surface gives up its moisture, making the ground level hotter and drier while not increasing the chances of rain. As things get drier, fires start and spread. The still air keeps the smoke close to the surface, exacerbating its effects on health. The soot heats the air further. This is what has been happening in Russia for the past two months.
According to Geert Jan van Oldenborgh of the Royal Netherlands Meteorological Institute, a straightforward comparison of the temperatures seen in European Russia this summer with those of the past 60 years suggests that a lot of the country is experiencing temperatures which might be expected only once every 400 years or so. For parts of the patch, it is hotter than might be expected over several millennia.
If you take into account the warming trend of the past half century, however, the extraordinary heatwave starts to look less improbable: a once-in-a-century event, perhaps. As the warming trend continues in future, the chances of such events being repeated yet more frequently will become higher still.
Peter Stott, the head of climate monitoring at Britain’s Met Office, says that a change in the jet stream, which is part of the bigger pattern of gridlock in the upper atmosphere, has allowed more warm, moist monsoonal air to flow north to Pakistan. At the same time, says Professor Hoskins, cold air has been entering the region in the upper parts of the atmosphere, flowing south from Siberia as part of the same persistent pattern that is keeping Russia hot. The influx of cold air on top of warm, moist air favours the sort of deep convection that creates powerful storms, turning moisture in the air into water on the ground very efficiently.
How might the complex relationship between jet streams and Rossby waves change in a warmer world? At the moment, no one is sure. Climate change will shift the patterns of circulation in some ways, but there is no strong reason to believe that it will lead them to seize up more often. Yet the effects of these persistent patterns may get more unpleasant because the world will be warmer and have a more vigorous hydrological cycle.
Both heatwaves and heavy precipitation are more common everywhere than they were 50 years ago. Reflecting the latter trend, the Indian monsoon has been seeing more of its rainfall in extreme events than it did in the past. No single one of those events can be directly attributed to climate change; nor can Russia’s heatwave. The pattern of increases, though, fits expectations—and those expectations see things getting worse.