University of Toronto: Vielleicht sogar weniger Stürme durch Klimaerwärmung

Unser Thema heute: Stürme in mittleren Breiten, also dort, wo die meisten unserer Leser wohnen. Die University of Toronto meldete 2015, dass der Klimawandel wohl nicht zu mehr Stürmen führen wird, aber möglicherweise die Intensitäten veschieben könnte. Schwere Stürme könnten seltener werden. Pressemitteilung der University of Toronto:

Global warming research: strong storms to become stronger, weak storms to become weaker

University of Toronto study finds atmosphere will adapt to hotter, wetter climate

A study led by atmospheric physicists at the University of Toronto finds that global warming will not lead to an overall increasingly stormy atmosphere, a topic debated by scientists for decades. Instead, strong storms will become stronger while weak storms become weaker, and the cumulative result of the number of storms will remain unchanged. “We know that with global warming we’ll get more evaporation of the oceans,” said Frédéric Laliberté, a research associate at U of T’s physics department and lead author of a study published this week in Science. “But circulation in the atmosphere is like a heat engine that requires fuel to do work, just like any combustion engine or a convection engine.”

The atmosphere’s work as a heat engine occurs when an air mass near the surface takes up water through evaporation as it is warmed by the sun and moves closer to the equator. The warmer the air mass is, the more water it takes up. As it reaches the equator, it begins to ascend through the atmosphere, eventually cooling as it radiates heat out into space. Cool air can hold less moisture than warm air, so as the air cools, condensation occurs, which releases heat.  When enough heat is released, air begins to rise even further, pulling more air behind it producing a thunderstorm. The ultimate “output” of this atmospheric engine is the amount of heat and moisture that is redistributed between the equator and the North and South Poles.

“By viewing the atmospheric circulation as a heat engine, we were able to rely on the laws of thermodynamics to analyze how the circulation would change in a simulation of global warming,” said Laliberté. “We used these laws to quantify how the increase in water vapour that would result from global warming would influence the strength of the atmospheric circulation.” The researchers borrowed techniques from oceanography and looked at observations and climate simulations. Their approach allowed them to test global warming scenarios and measure the output of atmospheric circulation under warming conditions.

“We came up with an improved technique to comprehensively describe how air masses change as they move from the equator to the poles and back, which let us put a number on the energy efficiency of the atmospheric heat engine and measure its output,” said Laliberté. The scientists concluded that the increase in water vapour was making the process less efficient by evaporating water into air that is not already saturated with water vapour. They showed that this inefficiency limited the strengthening of atmospheric circulation, though not in a uniform manner. Air masses that are able to reach the top of the atmosphere are strengthened, while those that can not are weakened.

“Put more simply, powerful storms are strengthened at the expense of weaker storms,” said Laliberté. “We believe atmospheric circulation will adapt to this less efficient form of heat transfer, and we will see either fewer storms overall or at least a weakening of the most common, weaker storms.” The findings are reported in the paper “Constrained work output of the moist atmospheric heat engine in a warming climate” published January 30 in Science. The work was supported by grants from the Natural Sciences and Engineering Research Council of Canada.

Paper: F. Laliberte, J. Zika, L. Mudryk, P. J. Kushner, J. Kjellsson, K. Doos. Constrained work output of the moist atmospheric heat engine in a warming climate. Science, 2015; 347 (6221): 540 DOI: 10.1126/science.1257103

Das gefällt dem PIK-Institut natürlich gar nicht. Sofort schlüpfte es in die Rolle der ungeliebten Nörgeltante und gab zu bedenken, dass die Verringerung der Stürme eine höhere Gefahr von Hitzewellen nach sich ziehe. Einfach großartig dieses PIK. Als nächstes werden sie Lottogewinner davor warnen, nicht vom schweren Portmonnaie erdrückt zu werden. Eine köstliche Truppe, die sich da in Potsdam zusammengefunden hat, mit freundlicher Unterstützung der Bundesregierung. Der PIK-Fanclub verbreitete die Nachricht in Windeseile, z.B. Christopher Schrader bei der Süddeutschen Zeitung.

Klimawandel bedeutet auch die Verschiebung von Wind- und Sturmgürteln. Das war in vorindustrieller Zeit so, und auch heute. Das Weizmann Institute of Science gab hierzu am 15. November 2017 die folgende Pressemitteilung heraus:

Off Track: How Storms Will Veer in a Warmer World

Weizmann Institute of Science research uncovers the internal mechanisms driving storms toward the poles

Under global climate change, Earth’s climatic zones will shift toward the poles. This is not just a future prediction; it is a trend that has already been observed in the past decades. The dry, semi-arid regions are expanding into higher latitudes, and temperate, rainy regions are migrating poleward. In a paper that that was recently published in Nature Geoscience, Weizmann Institute of Science researchers provide new insight into this phenomenon by discovering that mid-latitude storms are steered further toward the poles in a warmer climate. Their analysis, which also revealed the physical mechanisms controlling this phenomenon, involved a unique approach that traced the progression of low-pressure weather systems both from the outside — in their movement around the globe — and from the inside — analyzing the storms’ dynamics.

Prof. Yohai Kaspi of the Institute’s Earth and Planetary Sciences Department explains that Earth’s climatic zones roughly follow latitudinal bands. Storms mostly move around the globe in preferred regions called “storm tracks,” forming over the ocean and generally traveling eastward and somewhat poleward along these paths. Thus, a storm that forms in the Atlantic off the East Coast of the US at a latitude of around 40N will reach Europe in the region of latitude 50N. Until recently, however, this inclination to move in the direction of the nearest pole was not really understood. Dr. Talia Tamarin in Kaspi’s group solved this fundamental question in her doctoral research.

Kaspi: “From the existing climate models, one can observe the average storm tracks, but it is hard to prove cause and effect from these. They only show us where there are relatively more or fewer storms. Another approach is following individual storms; however, we must deal with chaotic, noisy systems that are heavily dependent on the initial conditions, meaning no storm is exactly like another. Talia developed a method that combines these two approaches. She applied a storm-tracking algorithm to simplified atmospheric circulation models in which thousands of storms are generated, thus eliminating the dependence on initial conditions. This allowed her to understand how such storms develop over time and space, and what controls their movement.” Even such simplified models involve calculations that require several days of computation in one of the Weizmann Institute’s powerful computer clusters.

In the present study, to understand how the movement of storms may change in a warmer world, Tamarin and Kaspi applied the same method to full-complexity simulations of climate change predictions. Their analysis showed that the tendency of storm tracks to veer in the direction of the poles intensifies in warmer conditions. They discovered that two processes are responsible for this phenomenon. One is connected to the vertical structure and circulation near the tops of these weather systems. A certain type of flow that is necessary for them to grow also steers the storms toward the pole, and these flows are expected to become stronger when average temperatures rise.

The second process is connected to the energy tied up in the water vapor in such storms. In global warming, the hotter air will contain more water vapor, and thus more energy will be released when the vapor condenses to drops. “The hottest, wettest air is circulating up the eastern flank of the storm — to the northern side — and releasing energy there,” says Tamarin. “This process pushes the storm northward (or southward in the southern hemisphere), and this effect will also be stronger in a warmer climate.”

The models of climate change predict that if average global temperatures rise by four degrees over the next 100 years, storms will deviate poleward from their present tracks by two degrees of latitude. The research performed at the Weizmann Institute of Science shows that part of this will be due to the mechanism they demonstrated, and the other part is tied to the fact that storms are born at a higher latitude in a warmer world. “The model Talia developed gives us both qualitative information on the mechanisms that steer storms toward the poles and quantitative means to predict how these will change in the future,” says Kaspi. “Although two degrees may not sound like a lot, the resulting deviation in temperature and rain patterns will have a significant effect on climate zones,” he adds.