The last few weeks have seen some extraordinary weather events around the world: relentless extreme heat in Russia, biblical flooding in Pakistan and devastating landslides in China. Tens of millions of people have had their lives disrupted and thousands have died, and — beyond reasonable doubt — global warming is playing a part in creating these extremes. But how much of a part? Michael Tobis asked this question in a recent post:
Are the current events in Russia “because of” “global warming”? To put the question in slightly more formal terms, are we now looking at something that is no longer a “loading the dice” situation but is a “this would, practically certainly, not have happened without human interference” situation?
The answer, at least in the case of the current extremes, would appear to be yes.
Jeff Masters at Weather Underground has (as usual) been providing exemplary coverage of the Russian heat wave, and in a post on August 6th he described it as “one of the most remarkable weather events of my lifetime”. Over the month of July, Moscow’s mean daily temperature was 7.8ºC above normal (the previous record, set in 1938, was 5.3ºC above normal), and since the beginning of August the daily maximum has been consistently 15ºC above average, which Masters describes as “a truly extraordinary anomaly”. At the time of writing, Moscow had experienced 29 successive days with temperatures over 30ºC, easily the longest and most intense heat wave since records began. Masters quotes Alexander Frolov, head of Russia’s weather service:
Our ancestors haven’t observed or registered a heat like that within 1,000 years. This phenomenon is absolutely unique.
What’s particularly striking about this event is the large margin by which previous long-standing records are being smashed. The Economist, in an excellent article on climate change and extreme weather, quotes Dutch meteorologist Geert Jan van Oldenborgh on the odds:
…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.
Those numbers assume the climate isn’t warming. When van Oldenburgh assumes warming:
…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.
Van Oldenborgh did a similar analysis of the heat and cold anomalies of last northern hemisphere winter, which I covered back in April. The key point is that if the climate were not changing, an event as dramatic as the Russian heatwave would be very, very unlikely. If we factor in the warming trend, it remains unusual, but less so. And if that warming trend continues (and it will), then we can expect more record-breaking heat waves around the world.
Global warming impacts the weather we experience in two ways: by increasing the probability of new records — when a heat wave happens, you are likely to get more warmth (see the graph in my post on rainfall). But there is a second impact: the potential for changes in the circulation of the atmosphere. The climate of any part of the planet depends on lots of factors, but the flow of weather systems is crucial. As an example, consider the South Island of New Zealand. The prevailing (or normal) wind is westerly, and when that wind bumps into the Southern Alps it drops rain, and lots of it. Hence rainforest, speedy glaciers and tourism. On the other side of the Alps, we get warm dry winds and little rain. Now imagine that the frequency of easterly winds increases and westerlies decrease. The east coast gets wetter, and the west coast dryer. Cue big change in climate, even if the temperature doesn’t warm. There’s actually a hint of this happening in the modelling toward the end of the century — though for the east coast of the north island, not down here.
So can we draw a line between Russian heat, flooding in Pakistan and China and changes in the pattern or shape of weather? Perhaps a combination of the after-effects of El Niño, record sea-surface temperatures in the tropical Atlantic and a reduction in Arctic sea ice are affecting the way the jet stream meanders around the northern hemisphere, creating a persistent ridge of high pressure over Russia — a blocking pattern that has flow on effects for Asia. Wired asked Kevin Trenberth if the heat and floods could be linked:
“The two things are connected on a very large scale, through what we call an overturning or monsoonal circulation,” he said. “There is a monsoon where upwards motion is being fed by the very moist air that’s going onshore, and there are exceptionally heavy rains. That drives rising air. That air has to come down somewhere. Some of it comes down over the north.”
Rob Carver at Weather Underground explains more here, New Scientist discusses the “frozen” jet stream here and UK Met Office scientist Peter Stott (who wrote the definitive paper on the record-breaking European heat wave of 2003) offers his thoughts at the Guardian.
What I find scary in all this is the multiple coincidence of record heat and catastrophic flooding in Pakistan and China — in a year where the first six months had already set a record for insurance losses on extreme events. The last 12 months have been the warmest in the global record. A modest El Niño event has boosted temperatures and affected weather patterns in an eery echo of events that followed the great El Niño of 1997-98. Back in 1999, Kevin Trenberth reviewed the extreme weather events of 1997-98. It was, as he suggests, a wild ride:
In early August, for example, major floods devastated parts of Korea, and in August and September 1998, extensive monsoon-related flooding struck heavily-populated eastern India and Bangladesh. Widespread heavy rains in China, at about the same time, released the mighty Yangtze River from its banks, with ensuing reports of more than 3,000 deaths, some 230 million people homeless, and over $30 billion in flood damage. In the summer of 1998 heat waves and air pollution episodes plagued many regions of the world, particularly in Egypt and other Mediterranean countries, and in southern Europe. In New Zealand, record floods in July and October 1998 were the worst in 100 years. But the costliest disaster of them all, in terms of human life, struck the Caribbean in late October. Hurricane Mitch caused the deaths of more than 11,000 people in Honduras, Nicaragua, Guatemala and El Salvador, primarily through the extensive flooding that followed prolonged and heavy rains.
This time round the floods are in Pakistan instead of India and Bangladesh, and the heat has moved north from the Mediterranean to Russia. The Atlantic hurricane season has not yet really got going, but we can only hope there won’t be another Mitch. In the 12 years since that El Niño, the climate system has continued to accumulate energy. When an ENSO event releases that energy it has to go somewhere, and that’s into heatwaves, floods, hurricanes and melting ice.
As the years go by and the warming continues, those extremes are only going to get worse. To me, it looks very much as though it won’t be a gradual warming that causes us the biggest problems, it’ll be the direct and indirect effects of increasing weather extremes. Hot years are going to be hard years for humanity.
[Update: The Wonk Room covers the same subject, but includes an interview with Rob Carver:
I agree with Michael Tobis's take at Only In It For the Gold that something systematic has changed to alter the global circulation and you'll need a coupled atmosphere/ocean global model to understand what's going on. My hunch is that a warming Arctic combined with sea-surface-temperature teleconnections altered the global circulation such that a blocking ridge formed over western Russia leading to the unprecedented drought/heat wave conditions. Without contributions from anthropogenic climate change, I don't think this event would have reached such extremes or even happened at all.
[Update 2 (in quick succession): Stu Ostro at The Weather Channel posts on Russian heat, extremes and 500mb anomalies:
The upshot: Whether with temperatures, precipitation, or storms (tropical or otherwise), and regardless of in which direction the extremes are, it's a case of Weather Gone Wiggy, and this is happening at the time when the Earth's climate is at an exceptionally warm level compared to that of at least the past century. There have been extremes for as long as there has been weather; it's their nature which is changing along with changing atmospheric moisture, stability, and circulation patterns.
[Update 3 (on a roll!): World Meteorological Organisation on recent extremes]
[Update 4: The Guardian expands on Weather Underground's list of new temperature records.]
[Update 5, 14/8/10: Jeff Masters posts on the jetstream and its influence on the heatwave: The Great Russian Heat Wave of 2010 is one of the most intense, widespread, and long-lasting heat waves in world history.]