Sunday, November 04, 2018

Global Warming Is Messing with the Jet Stream. That Means More Extreme Weather.

By Bob Berwyn, InsideClimate News
Oct 31, 2018

Greenhouse gases are increasingly disrupting the jet stream, a powerful river of winds that steers weather systems in the Northern Hemisphere. That's causing more frequent summer droughts, floods and wildfires, a new study says.

The findings suggest that summers like 2018, when the jet stream drove extreme weather on an unprecedented scale across the Northern Hemisphere, will be 50 percent more frequent by the end of the century if emissions of carbon dioxide and other climate pollutants from industry, agriculture and the burning of fossil fuels continue at a high rate.

In a worst-case scenario, there could be a near-tripling of such extreme jet stream events, but other factors, like aerosol emissions, are a wild card, according to the research, published today in the journal Science Advances.

The study identifies how the faster warming of the Arctic twists the jet stream into an extreme pattern that leads to persistent heat and drought extremes in some regions, with flooding in other areas.

The researchers said they were surprised by how big a role other pollutants play in the jet stream's behavior, especially aerosols—microscopic solid or liquid particles from industry, agriculture, volcanoes and plants. Aerosols have a cooling effect that partially counteracts the jet stream changes caused by greenhouse gases, said co-author Dim Comou, a climate and extreme weather researcher at the Potsdam Institute for Climate Impacts Research and the Vrije Universiteit Amsterdam.


The jet stream is a powerful high-altitude wind that shapes and moves weather systems from west to east. Different branches of the jet stream undulate from the subtropics to the edge of the Arctic. In the past 15 years at least, the jet stream has been coiling up more, slithering farther north and south. When it gets stuck in the extreme pattern identified by the scientists, it leads to more deadly and costly weather extremes.

That extremely wavy pattern, called "quasi-resonant amplification," was evident during the extreme summer of 2018, Mann said.


That brought blazing temperatures and wildfire conditions to California, flooding over the Eastern U.S. and unprecedented heat to the Scandinavian Arctic region, as well as a six-month heat wave and drought across parts of Central Europe, all events showing a clear global warming fingerprint, according to scientists.

The new study focuses on summer extremes, while other research has looked at how global warming affects the jet stream in winter.


Polar Vortex: How the Jet Stream and Climate Change Bring on Cold Snaps

It might seem counterintuitive, but global warming plays a role in blasts of bitter cold weather. The reason: It influences the jet stream. Here’s how.

By Bob Berwyn, InsideClimate News
Feb 2, 2018


The northern polar jet stream (it has a counterpart in the Southern Hemisphere) is driven partly by the temperature contrast between masses of icy air over the North Pole and warmer air near the equator. Climate change, true to the predictions of the past half century, has led to faster warming in the Arctic than in the temperate zones. So the temperature difference between the two regions has been lessening.

Research suggests that this reduction in the temperature difference is robbing the jet stream of some of its strength, making it wobblier and contributing to more temperature extremes.

What's the jet stream's role in extreme weather?

The jet stream is strongest in winter, when it has the greatest effect on weather in more densely populated parts of North America and Eurasia.

When it rolls along in relatively steady waves, normal weather ensues, with spells of cold, snow and intermittent warm-ups.

But when it coils far to the south, bitter cold Arctic air spills southward along with it.

Wriggling like a garden hose, each southward kink in the wind tends to be balanced out by a northward bend somewhere else. That can lead to the western states, even Alaska, being unusually warm and dry while the middle of the country and the eastern states freeze.

How is the jet stream changing?

Research shows that over the past several decades, the jet stream has weakened. There's also evidence that as it wobbles, it can get stuck out of kilter, which can lead to more persistent weather extremes, including heat waves, cold snaps, droughts and flooding.

Scientists say there is strong evidence that human-caused global warming has altered the strength and path of the powerful winds.


They did find that cold spells aren't as cold as they would be in a climate unaltered by greenhouse gases.

"Because of Arctic amplification, the cold air coming south is not as cold as it used to be," said Geert Jan van Oldenborgh, a Dutch climate scientist involved in the World Weather Attribution analysis. The decline in cold spells, he said, is "the big signal."

I've noticed that. People complain about how long some cold spells last now, but the lowest temperatures are not nearly as cold as they used to be, and when it gets to cold temperatures that used to be expected at some time every winter, people consider them extreme.

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