Extreme summers driven by human-caused global warming: study

Extreme weather events, such as unprecedented summer heat, flooding, drought and torrential rain, are an effect of human-caused global warming, a new study has found.
Researchers from Pennsylvania State University in the US looked at a combination of roughly 50 climate models from around the world.
They looked at the historical atmospheric observations to document the conditions under which extreme weather patterns form and persist.
Scientists found a link between extreme weather events and the impact climate change is having on the jet stream.
The unusual weather events such as the 2003 European heat wave, the 2010 Pakistan flood and Russian heatwave, the 2011 Texas and Oklahoma heat wave and drought and the 2015 California wildfires piqued our interest, researchers said.

"Most stationary jet stream disturbances, however, will dissipate over time," said Michael Mann of Penn State University.

"Under certain circumstances the wave disturbance is effectively constrained by an atmospheric wave guide, something similar to the way a coaxial cable guides a television signal," Mann said.
"Disturbances then cannot easily dissipate, and very large amplitude swings in the jet stream north and south can remain in place as it rounds the globe," he said.
This constrained configuration of the jet stream is like a rollercoaster with high peaks and valleys, but only forms when there are six, seven or eight pairs of peaks and valleys surrounding the globe.

The jet stream can then behave as if there is a waveguide-
Uncrossable barriers in the north and south - and a wave with large peaks and valleys can occur.
Theoretically, standing jet stream waves with large amplitude north/south undulations should cause unusual weather events, researchers said.
The researchers looked at real-world observations and confirmed that this temperature pattern does correspond with the double-peaked jet stream and waveguide patter associated with persistent extreme weather events in the late spring and summer such as droughts, floods and heat waves.
They found the pattern has become more prominent in both observations and climate model simulations.

"If the same weather persists for weeks on end in one region, then sunny days can turn into a serious heat wave and drought, and lasting rains can lead to flooding," said Stefan Rahmstorf of Potsdam Institute for Climate Impact Research (PIK) in Germany.
"We are now able to connect the dots when it comes to human-caused global warming and an array of extreme recent weather events," said Mann.
The study was published in the journal Scientific Reports.