AS TORRENTIAL storms visit unprecedented flooding on Pakistan, thousands of kilometres to the northwest Russia burns. The two events are linked by a large-scale pattern of atmospheric circulation which is producing a particularly persistent area of high pressure over Russia. They are also linked in both being the sort of events climate scientists predict more of in a warming world.Original article.
The immediate cause of the problems is the behaviour of the jet stream, a band of high-level wind that travels east around the world and influences much of the weather below it. Part of the jet stream’s meandering is tied to regular shifts of air towards and away from the pole, called Rossby waves. The Rossby waves set up wiggles in the jet stream, wiggles which, left to themselves, would move westward. Since the jet stream is flowing eastward, though, the net effect of the Rossby waves varies. When the waves are short, they go with the jet's flow and the resultant wiggling heads downstream to the east. When they are long they go against the flow, and the jet’s wiggling is transmitted upstream to the west. In between, there is a regime in which the waves move neither west nor east, and the weather stays put.
According to Brian Hoskins, a climate scientist at the University of Reading, the wave-generating activity of anticyclones in the Atlantic this year has been particularly well attuned to setting up these sorts of stationary waves, resulting in persistent troughs of low pressure over western and central Europe, a ridge of high pressure over Russia, and lows again to the east. The air itself does not necessarily sit still during all this, but the pressure patterns which dominate the weather persist. The high pressure over Russia is particularly pronounced and persistent, amounting to a block in the circulation of the atmosphere.
In Russia, this block provokes feedbacks that make things worse. High pressure makes it hard for clouds to form, and thus for rain to fall. Under cloudless skies the surface heats and gives up its moisture, making things at 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 malign effects on health. The sooty particles in the atmosphere heat it further.
The intensity of this heatwave has been remarkable. It is hotter than at any time in the instrumental record. According to an analysis by Geert Jan van Oldenborgh of the Royal Netherlands Meteorological Institute a straightforward comparison of the temperatures seen this summer with those of the past 60 years suggests that a large patch of Russia is experiencing temperatures which might be expected only once every 400 years or so. Some places within that patch are hotter than might be expected over several millennia.
Part of the straightforwardness of that analysis is that it treats all the previous years equally. When instead Dr van Oldenborgh takes into account that there has been a general warming trend over those past 60 years the heatwave starts to look less improbable—more like the sort of thing you might expect every century. As the warming trend continues in the future, the chances of such events being repeated more frequently will get higher. A single heatwave cannot be said to have been caused by global climate change; but what is known about climate change says such heatwaves are now more probable than they were.
According to Dr van Oldenborgh there is no obvious relationship in historical data between Russian heatwaves and particularly powerful monsoons. But that does not mean that in this particular case the same large-scale siezing-up of the weather playing a role in Russia is not also making the monsoon storms of Pakistan particularly damaging. Peter Stott, head of climate monitoring and attribution at Britain’s Met Office, points out that the amplified high over Russia has weakened the jet stream to the south, and a lot of moist monsoonal air is getting into the persistent trough over Pakistan. At the same time, says Dr Hoskins, cold air has been entering the region in the upper parts of the atmosphere, flowing south from the parts of Siberia that lie beyond the heatwave. 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.
One of the problems with the sort of blocking that is behind this weather (and was also, in another form, behind the abnormally cold winter in Western Europe) is that it is very hard to predict on a seasonal basis, or even just a few days out. Weather-forecasting models like it when big patterns move steadily round the planet. When they start to seize up a lot of non-linear effects—processes in which small causes have disproportionate effects—come into play, making it much harder to see what is going to happen next. So weather forecasts are not very good at capturing the imminent arrival of a blocking event or a stationary ridge. And climate models—which look at the average behaviour of possible future weather, rather than trying to predict a specific course of events—find it hard to give definitive answers about whether such events can be expected to be on the increase, or decrease.
In a world where greenhouse warming gets stronger, the tropics expand—an effect the beginning of which has already been observed. The paths of the jet streams to the north and south of the tropics will change in response to this. What that means for the interactions between jet streams and Rossby waves that lead to blocking, though, is unclear. Tony Lupo, an atmospheric scientist from the University of Missouri, has been looking at the question with some Russian colleagues. He says their climate modelling provides some reason to believe blocking effects might become more common in a warmer world, but also less forceful.
Most climatologists think that as yet it’s too early to say what might be expected. But some progress might be made quite soon. According to Tim Woollings, also of the University of Reading, new climate-modelling data being generated for the next report of the Intergovernmental Panel on Climate Change (IPCC) may make it easier to pick out patterns in blocking behaviour which have so far been hard to get to grips with.
Professor Hoskins does not expect a drastic change in the way blocking events happen, though their locations may alter a bit. The climate-change message from what is going on this summer, he says, is not to do with the blocking itself, but its consequences. In a cooler world, this sort of high pressure over Russia would be less likely to result in such a dramatic heatwave; nor, probably, would the floods in Pakistan have been so bad.
Episodes of heavy rain and snowfall are now more common around the world than they were 50 years ago, according to the IPCC’s 2007 review of the literature, which is to be expected in a warming world; warmer air can carry more water, and so more can be released when the conditions are right. Reflecting this trend, the Indian monsoon — which in terms of absolute levels of rainfall changed little over the 20th century—has been seeing more of its rainfall in extreme events than it used to. No one of those extreme events can be laid at the door of worldwide climate change; nor can the Russian heatwave. The pattern of increases, though, fits expectations—and those expectations see things getting worse.
Thursday, August 12, 2010
The Economist looks at the cost of Climate Change
The bottom line is uncertainty, as weather patterns become disrupted. This means food security evaporates, and a general state of flux takes place. Given high human populations, quite a lot of unanticipated attrition is likely due to disease, storms, droughts, floods and impacts these have on settlements built into previous climate systems that no long hold. Hence, this view from the Economist is sobering...