February 25, 2013, by Alex Kirby
EMBARGOED until 2000 GMT on Monday 25 February
Extreme weather is often the result of climate change, according to scientists in Germany, who say they have found how greenhouse gases are helping to trap the jet stream and the weather patterns it brings.
LONDON, 25 February – The cause of much of the recent extreme weather across the world is climate change triggered by human activities, scientists say.
The Earth has experienced a range of severe regional weather extremes in recent years, including the heat waves of 2011 in the US and 2010 in Russia, a year that also brought the unprecedented Pakistan floods.
Behind these distinct events, though, there is a common physical cause, according to a team at the Potsdam Institute for Climate Impact Research (PIK), Germany.
Their paper, which helps to explain the mechanism that is causing an increasing number of weather extremes, has the less than catchy title of Quasi-resonant amplification of planetary waves and recent Northern Hemisphere weather extremes, and will be published this week in the US Proceedings of the National Academy of Sciences (link provided on publication).
The paper suggests that man-made climate change is repeatedly disturbing the patterns of airflow around the northern hemisphere, bringing extreme conditions.
This airflow has always travelled in waves round the planet, the wave movements caused by the difference in temperature between the oceans and land surfaces. These in turn cause changes in the high altitude air currents, particularly the jet stream.
Lead author Vladimir Petoukhov explains: “An important part of the global air motion in the mid-latitudes of the Earth normally takes the form of waves wandering around the planet, oscillating between the tropical and the Arctic regions.
“So when they swing up, these waves suck warm air from the tropics to Europe, Russia, or the US, and when they swing down, they do the same thing with cold air from the Arctic.
“What we found is that during several recent extreme weather events, these waves almost freeze in their tracks for weeks. So instead of bringing in cool air after having brought warm air in, the heat just stays. In fact we observe a strong amplification of the usually weak, slow-moving component of these waves.”
Tested against the data
Time is critical here: two or three days of 30˚C are no problem, but 20 days or more lead to extreme heat stress, the authors say. Since many ecosystems and cities are not adapted to temperatures this high, prolonged hot periods can cause many deaths, as well as forest fires and serious crop losses.
Climate change caused by greenhouse gas emissions from the burning of fossil fuels does not mean uniform warming – in the Arctic the relative rise in temperatures, amplified by the loss of snow and ice, is higher than average.
This in turn reduces the temperature difference between the Arctic and, for example, Europe. Yet it is this temperature difference that is normally a main driver of airflow, including the jet streams.
Additionally, continents generally warm and cool more readily than the oceans. They always have. But, coupled with the new changes to the jet streams, the net result is to trap the energy of the slow-moving parts of the waves.
“These two factors are crucial for the mechanism we detected,” says Petoukhov. “They result in an unnatural pattern of the mid-latitude airflow, so that for extended periods the slow waves get trapped.”
The authors tested their assumptions using standard daily weather data from the US National Centers for Environmental Prediction (NCEP). During recent periods in which several major weather extremes occurred, they did observe the trapping and strong amplification of particular waves.
The data show an increase in the occurrence of these specific atmospheric patterns which is statistically significant at the 90% confidence level.
“Our dynamical analysis helps to explain the increasing number of novel weather extremes. It complements previous research that linked such phenomena to climate change, but had not yet identified a mechanism behind it”, says Hans Joachim Schellnhuber, director of PIK and co-author of the study.
“This is quite a breakthrough, even though things are not at all simple – the suggested physical process increases the probability of weather extremes, but additional factors certainly play a role as well, including natural variability.”
The 32-year period of the project provides a good indication of the mechanism involved but is too short to allow definite conclusions.
Still, scientists are surprised by how far beyond experience some of the recent extremes have been. The new data show that the emergence of extraordinary weather is not just a straightforward linear response to the average warming trend, and the proposed mechanism could explain that. – Climate News Network
Alex Kirby is a former BBC journalist and environment correspondent. He now works with universities, charities and international agencies to improve their media skills, and with journalists in the developing world keen to specialise in environmental reporting.