Another Manntastic claim: Extreme weather events linked to climate change impact on the jet stream

From Penn State, and the “close but no cigar” department (see bold in text) comes this modelspalooza masquerading as science:

Unprecedented summer warmth and flooding, forest fires, drought and torrential rain — extreme weather events are occurring more and more often, but now an international team of climate scientists has found a connection between many extreme weather events and the impact climate change is having on the jet stream.

“We came as close as one can to demonstrating a direct link between climate change and a large family of extreme recent weather events,” said Michael Mann, distinguished professor of atmospheric science and director, Earth System Science Center, Penn State. “Short of actually identifying the events in the climate models.”

The unusual weather events that piqued the researchers’ interest are things 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.

The researchers looked at a combination of roughly 50 climate models from around the world that are part of the Coupled Model Intercomparison Project Phase 5 (CMIP5), which is part of the World Climate Research Programme. These models are run using specific scenarios and producing simulated data that can be evaluated across the different models. However, while the models are useful for examining large-scale climate patterns and how they are likely to evolve over time, they cannot be relied on for an accurate depiction of extreme weather events. That is where actual observations prove critical.

The researchers looked at the historical atmospheric observations to document the conditions under which extreme weather patterns form and persist. These conditions occur when the jet stream, a global atmospheric wave of air that encompasses the Earth, becomes stationary and the peaks and troughs remain locked in place.

“Most stationary jet stream disturbances, however, will dissipate over time,” said Mann. “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. 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.”

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.

“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, Potsdam Institute for Climate Impact Research (PIK), Germany.

The structure of the jet stream relates to its latitude and the temperature gradient from north to south.

Temperatures typically have the steepest gradients in mid-latitudes and a strong circumpolar jet stream arises. However, when these temperature gradients decrease in just the right way, a weakened “double peak” jet stream arises with the strongest jet stream winds located to the north and south of the mid-latitudes.

“The warming of the Arctic, the polar amplification of warming, plays a key role here,” said Mann. “The surface and lower atmosphere are warming more in the Arctic than anywhere else on the globe. That pattern projects onto the very temperature gradient profile that we identify as supporting atmospheric waveguide conditions.”

Theoretically, standing jet stream waves with large amplitude north/south undulations should cause unusual weather events.

“We don’t trust climate models yet to predict specific episodes of extreme weather because the models are too coarse,” said study co-author Dim Coumou of PIK. “However, the models do faithfully reproduce large scale patterns of temperature change,” added co-author Kai Kornhuber of PIK.

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.

“Using the simulations, we demonstrate that rising greenhouse gases are responsible for the increase,” said Mann. The researchers noted in today’s (Mar. 27) issue of Scientific Reports that “Both the models and observations suggest this signal has only recently emerged from the background noise of natural variability.”

“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.

While the models do not reliably track individual extreme weather events, they do reproduce the jet stream patterns and temperature scenarios that in the real world lead to torrential rain for days, weeks of broiling sun and absence of precipitation.

“Currently we have only looked at historical simulations,” said Mann. “What’s up next is to examine the model projections of the future and see what they imply about what might be in store as far as further increases in extreme weather are concerned.”

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If Mann’s press release wasn’t heavy on alarmism enough, read the press release by fellow RealCimateer Stefan Rahmstort

Weather extremes: Humans likely influence giant airstreams

POTSDAM INSTITUTE FOR CLIMATE IMPACT RESEARCH (PIK)

The increase of devastating weather extremes in summer is likely linked to human-made climate change, mounting evidence shows. Giant airstreams are circling the Earth, waving up and down between the Arctic and the tropics. These planetary waves transport heat and moisture. When these planetary waves stall, droughts or floods can occur. Warming caused by greenhouse-gases from fossil fuels creates favorable conditions for such events, an international team of scientists now finds.

“The unprecedented 2016 California drought, the 2011 U.S. heatwave and 2010 Pakistan flood as well as the 2003 European hot spell all belong to a most worrying series of extremes,” says Michael Mann from the Pennsylvania State University in the U.S., lead-author of the study now to be published in Scientific Reports. “The increased incidence of these events exceeds what we would expect from the direct effects of global warming alone, so there must be an additional climate change effect. In data from computer simulations as well as observations, we identify changes that favor unusually persistent, extreme meanders of the jet stream that support such extreme weather events. Human activity has been suspected of contributing to this pattern before, but now we uncover a clear fingerprint of human activity.”

How sunny days can turn into a serious heat wave

“If the same weather persists for weeks on end in one region, then sunny days can turn into a serious heat wave and drought, or lasting rains can lead to flooding”, explains co-author Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research (PIK) in Germany. “This occurs under specific conditions that favor what we call a quasi-resonant amplification that makes the north-south undulations of the jet stream grow very large. It also makes theses waves grind to a halt rather than moving from west to east. Identifying the human fingerprint on this process is advanced forensics.”

Air movements are largely driven by temperature differences between the Equator and the Poles. Since the Arctic is more rapidly warming than other regions, this temperature difference is decreasing. Also, land masses are warming more rapidly than the oceans, especially in summer. Both changes have an impact on those global air movements. This includes the giant airstreams that are called planetary waves because they circle Earth’s Northern hemisphere in huge turns between the tropics and the Arctic. The scientists detected a specific surface temperature distribution apparent during the episodes when the planetary waves eastward movement has been stalling, as seen in satellite data.

Using temperature measurements since 1870 to confirm findings in satellite data

“Good satellite data exists only for a relatively short time – too short to robustly conclude how the stalling events have been changing over time. In contrast, high-quality temperature measurements are available since the 1870s, so we use this to reconstruct the changes over time,” says co-author Kai Kornhuber, also from PIK. “We looked into dozens of different climate models – computer simulations called CMIP5 of this past period – as well as into observation data, and it turns out that the temperature distribution favoring planetary wave airstream stalling increased in almost 70 percent of the simulations since the start of the industrial age.”

Interestingly, most of the effect occured in the past four decades. “The more frequent persistent and meandering Jetstream states seems to be a relatively recent phenomenon, which makes it even more relevant,” says co-author Dim Coumou from the Department of Water and Climate Risk at VU University in Amsterdam (Netherlands). “We certainly need to further investigate this – there is some good evidence, but also many open questions. In any case, such non-linear responses of the Earth system to human-made warming should be avoided. We can limit the risks associated with increases in weather extremes if we limit greenhouse-gas emissions.”

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Article: Michael E. Mann, Stefan Rahmstorf, Kai Kornhuber, Byron A. Steinman, Sonya K. Miller, Dim Coumou (2017): Influence of Anthropogenic Climate Change on Planetary Wave Resonance and Extreme Weather Events. Scientific Reports [DOI: 10.1038/srep45242]

Weblink to the article once it is published: http://www.nature.com/articles/srep45242

To all this modeling sans empirical evidence I say:

Nature had an editorial five years ago that remains germane today:

Better models are needed before exceptional events can be reliably linked to global warming.

Having 50  models (as Mann’s PR says) isn’t necessarily better, but it does help convince people who believe that consensus is more important than actual evidence.