Arctic warming to trigger intense heatwaves and droughts in Europe and USA

Temperatures in the Arctic are rising because of human-induced global warming, which will lead to Europe, the US and other regions within the Northern hemisphere’s mid-latitudes having prolonged periods of extreme heat and droughts during the summers months, scientists from the Potsdam Institute for Climate Impact Research reported.

Northern Hemisphere mid-latitude nations are northern countries with a temperature climate, and include most of Europe (excluding the far north of Scandinavia), the US (except the far south and northern Alaska), Canada (excluding the far north), Mongolia, northern China, most of Japan (except for its most southerly island), and most of Russia (excluding the far north).

The researchers published their study findings in the journal Science (citation below).

Extreme heat and periods of prolonged drought will become more common in the summer months in the northern countries with a temperate climate.

Lead author, Dr. Dim Coumou, said:

“When the great air streams in the sky above us get disturbed by climate change, this can have severe effects on the ground. While you might expect reduced storm activity to be something good, it turns out that this reduction leads to a greater persistence of weather systems in the Northern hemisphere mid-latitudes.”

“In summer, storms transport moist and cool air from the oceans to the continents bringing relief after periods of oppressive heat. Slack periods, in contrast, make warm weather conditions endure, resulting in the buildup of heat and drought.”

Climate change may affect warm season circulation further

Previous studies had concentrated primarily on winter storms, given that they tend to cause much more damage. Compared to the summer, average storm activity during the winter months remains largely steady, the authors pointed out.

The incidence of overall storm activity during the summer months in Northern Hemisphere mid-latitude regions has been declining significantly, according to observational data gathered from weather stations and satellites. This means summer storms are either occurring less frequently, less intensely, or both.

The scientists studied synoptic eddies, specific types of turbulences, and calculated the total energy in their wind speeds.

This energy, which reflects the level of interaction between the intensity and frequency of low and high pressure systems in the atmosphere, is ten percent lower now than in 1979.

Co-author, PhD candidate Jascha Lehmann, said:

“Unabated climate change will probably further weaken summer circulation patterns which could thus aggravate the risk of heat waves.”

“Remarkably, climate simulations for the next decades, the CMIP5, show the same link that we found in observations. So the warm temperature extremes we’ve experienced in recent years might be just a beginning.”

Rising arctic temperatures may be driving circulation changes

The authors believe that the accelerated warming of the Arctic may be the cause of the observed changes in circulation.

We all know that greenhouse-gas emissions from burning fossil fuels is raising global temperatures, but few of us are aware that in the far north the warming is occurring at a faster rate.

Far northern regions of our planet that were covered in ice but now are ice free are darker in colour (than white ice), and consequently reflect less of the Sun’s light back into space.

This warms up the water which raises the temperature of the air. This in turn reduces the atmospheric temperature difference between the cold polar region and the rest of the Northern Hemisphere.

For the air to move (circulation) there must be a temperature difference – the bigger the difference the more air movement there is. If this difference shrinks the jet-stream weakens, the scientists explain. They suggest this weakening is causing the observed decline in summer storm activity.

Dr. Coumou said:

“From whichever angle we look at the heat extremes, the evidence we find points in the same direction. The heat extremes do not just increase because we’re warming the planet, but because climate change disturbs airstreams that are important for shaping our weather.”

“The reduced day-to-day variability that we observed makes weather more persistent, resulting in heat extremes on monthly timescales. So the risk of high-impact heat waves is likely to increase.”

Citation: Dim Coumou, Jascha Lehmann, and Johanna Beckmann. “The weakening summer circulation in the Northern Hemisphere mid-latitudes.” Science. Published online 12 March, 2015. DOI:10.1126/science.1261768.