The last ten years has seen an extraordinary rise in the number of extreme weather conditions in summer - the record heat wave in Eastern Europe in 2010 is one example. Whereas anthropogenic global warming can be the reason behind a gradual increase in the number of such heat-waves, the extreme strength and length of these events are not so easy to explain. Scientists of the Potsdam Institute for Climate Impact Research, PIK, have investigated large sets of global weather data and related them to the development of giant waves in the atmosphere. The findings of the study were published in the Proceedings of the US National Academy of Sciences in July 2014.
A major part of global air movements in the mid latitudes usually takes the form of atmospheric waves that wander around the world - called Rossby waves. When the waves swing to the north they suck-in warm air from the tropics towards Europe, Russia or the USA; when they swing south the same happens with cold air from the arctic. Under specific resonance conditions, unusually slowly wandering, very strong waves develop in the atmosphere, and this leads to extreme weather conditions on the ground. A major finding of the study is that resonance events of this type have become more frequent. Their occurrence has almost doubled since 2000.
The reason for this increase could lie in processes taking place in the arctic. Since the year 2000, arctic warming has taken place at a pace almost double that of the rest of the planet. One cause is the shrinkage of light coloured marine areas - meaning that less sunlight is reflected back into space, because open ocean is darker and heats up more intensely. As temperatures in the arctic rise, the difference in temperature compared to other regions dwindles - and it is this temperature difference that is the main driver of air streams in the atmosphere, which determine our weather.
Coumou, D., Petoukhov, V., Rahmstorf, S., Petri, S., Schellnhuber, H.J. (2014): Quasi-resonant circulation regimes and hemispheric synchronization of extreme weather in boreal summer. Proceedings of the US National Academy of Sciences (PNAS) [DOI: 10.1073/pnas.1412797111]: