There has been an increased number of extreme-weather events in the last few years which scientists have struggled to explain.

A research study conducted at the Potsdam Institute for Climate Impact Research (PIK) was recently able to track the extreme weather back to trapped air waves in the atmosphere. The team's research showed that air wave-trapping is on the rise. Their research paper will be published in the Proceedings of the US National Academy of Science (PNAS).

"The large number of recent high-impact extreme weather events has struck and puzzled us," says Dim Coumou, the study's lead author. "Of course we are warming our atmosphere by emitting CO2 from fossil fuels, but the increase in devastating heat waves in regions like Europe or the US seems disproportionate."

A possible cause for the weather changes that they found was changing circulation patterns in the atmosphere. They studied global weather data and found a rise in the phenomenon known as Rossby Waves, airstreams that wander around the globe. When they are in the north, they bring warm air from the tropical areas to Europe, Russia and North America. Then, when they move south, they bring cold air from the Arctic.

The new study showed that times of extreme weather correlate to times when these Rossby Waves are trapped in one region and become amplified, causing heat waves.

"Behind this, there is a subtle resonance mechanism that traps waves in the mid-latitudes and amplifies them strongly," says Stefan Rahmstorf, who co-authored the study.

The team found that under certain conditions the atmosphere develops unusually slow-moving Rossby Waves which are also stronger than usual. These waves can cause extreme weather on land. The team also found that more of these waves are forming more often. The waves are occurring almost twice as often now as they were before the year 2000.

"Evidence for actual changes in planetary wave activity was so far not clear. But by knowing what patterns to look for, we have now found strong evidence for an increase in these resonance events," Rahmstorf said.

One reason the team postulated for these waves occurring more often now is that due to greenhouse gas emissions, the Arctic ice is melting. As the ice melts, less light from the sun is reflected back into the atmosphere; once the ice melts, the exposed ocean is dark, which attracts more heat.

"This melting of ice and snow is actually due to our lifestyle of churning out unprecedented amounts of greenhouse gases from fossil fuels," says Hans Joachim Schellnhuber, who also co-authored the study.

The temperature differences between the Arctic and the rest of the world were a driving force for atmospheric circulation patterns, so as the Arctic becomes less cold the global atmosphere patterns have been thrown out of balance, the study suggests.

"The planetary waves topic illustrates how delicately interlinked components in the Earth system are." Schellnhuber says. "And it shows how disproportionately the system might react to our perturbations."