When Michael Mann goes before Congress Wednesday to testify on global warming, he'll be armed with one more piece of evidence that greenhouse gases from fossil fuel burning are fundamentally altering the climate and leading to life-threatening and costly extreme weather.
Mann is the lead author of a new study showing that the greenhouse gas buildup is slowing down planetary atmospheric waves, which results in regional summer climate extremes. That includes a deadly 2003 European heat wave, as well as extensive wildfires in Siberia and severe flooding in Pakistan that took place simultaneously in 2010.
Pennsylvania State University climate scientist Mann said he and his fellow scientists discovered, by studying extensive climate data, "a particular type of jet stream pattern that is associated with many of the extreme events we've seen in recent years." He added that there is every reason to expect "these persistent weather events to become more prominent over time...with increasing greenhouse gas concentrations."
Mann is a high-profile scientist whose advocacy of climate action has made him a lightning rod for criticism from the right. He will testify Wednesday before the House Science, Space and Technology Committee, led by Texas Republican Lamar Smith, in a hearing that is stacked with climate change skeptics.
The study, published March 27 in the journal Scientific Reports, examines temperature data related to the jet stream and winds that flow around the Northern Hemisphere from west to east and that loop from north to south between the tropics and the Arctic. The pattern is called Rossby waves.
"We identified particular temperature patterns that occur when these large planetary waves slow down, and we found that, in the course of the past 100 years, this pattern is becoming more frequent," said study co-author Stefan Rahmstorf, a scientist with the Potsdam Institute for Climate Impact Research.
The Rossby waves shape day-to-day and seasonal weather, said Rutgers University climate researcher Jennifer Francis, who was not involved in the study but has done related research on changes in hemispheric wind patterns. The northward loops carry warm air up from the tropics and help form high pressure systems, associated with warm and dry weather. The southward dips pull cold Arctic air down, generating stormy low pressure areas, she explained.
The study found that as greenhouse gases have increased in the atmosphere, those waves have lingered longer over particular regions. "Anything that makes those waves more persistent means the weather is going to be more persistent too, and summer extreme events are associated with these persistent patterns," Francis said.
The 2010 summer heatwave over Siberia and simultaneous widespread flooding in Pakistan was a classic example of such a "stuck" pattern, according to Francis. "What we're learning is there are multiple ways that global warming is going to affect weather systems in different regions and different seasons."
The culprit in the climate change associated with the Rossby waves is the decreasing temperature contrast between the Arctic and the tropics and between sea surface and land surface areas, the study finds. High latitudes are warming much faster than mid-latitudes so the contrast is decreasing, which causes weather patterns to slow down or get stuck. Since the start of the industrial age, conditions favoring Rossby wave stalling have increased 70 percent, with most of that change in the past 40 years, according to co-author Kai Kornhuber. He is also with the Potsdam Institute for Climate Impact Research.
Previous studies by Francis and other scientists have linked changes in Northern Hemisphere summer wind patterns to the shrinking of Arctic sea ice and the rapid spring meltdown of Northern Hemisphere snow cover. Both reduce the north-south temperature contrast that drives the winds. "The temperature pattern changes they found are very consistent with the loss of spring snow cover in high latitudes," Francis said, explaining that bare ground heats up faster than a reflective white shield of snow.
One recent study linked widespread melting of the Greenland Ice Sheet in 2015 with a significant and sustained shift in hemispheric winds that carried warm air north. Those winds had never before intruded so far into the Arctic during summer, researchers reported in the journal Nature Communications.
A similar pattern this past winter led to several weeks of temperatures far above the average in the Arctic, where sea ice set a record for low maximum extent the third year in a row. That was just the latest sign that fundamental changes in the Earth's climate system are literally in the wind.
ICN reporter Zahra Hirji contributed to this story.