Researchers have found a common physical cause behind recent severe weather extremes, such as the heat waves in the United States in 2011 and Russia 2010, and the 2010 Pakistan flood. They say that man-made climate change repeatedly disturbs the patterns of atmospheric flow around the world’s northern hemisphere, through a subtle resonance mechanism.
Atmospheric ‘waves’ between tropical and Arctic regions are an important part of the global air motion. When they swing up, they suck warm air from the tropics to Europe, Russia, or the US, and when they swing down, they bring cold air from the Arctic. These planetary waves almost froze in their tracks, for weeks, during several recent extreme weather events. So instead of bringing in cool air after having brought warm air in before, the heat just stays. Time is critical: two or three days of 30 degrees Celsius are no problem, but two or three weeks can lead to extreme heat stress. Many ecosystems and cities are not adapted to this, and prolonged hot periods can bring a high death toll, forest fires, and dramatic harvest losses.
Climate change is not uniform across the world. Greater warming in the Arctic, amplified by loss of snow and ice, is reducing the temperature difference between the Arctic and, for example, Europe – and temperature differences are a main driver of air flow. Also, continents generally warm and cool more readily than the oceans. Combining these two things results in an unnatural pattern of mid-latitude air flow, so that the normal slow waves get trapped for extended periods.Scientists have been surprised by how far outside past experience some of the recent extremes have been. As an explanation of mechanism behind them, this research shows that the emergence of extraordinary weather is not just a linear response to the average warming trend.
The research, by V Petoukhov, S Rahmstorf, S Petri and H J Schellnhuber of the Potsdam Institute for Climate Impact Research, will be published on line in the Proceedings of the National Academy of Sciences.