Verrücktes Amerika: Das wandernde Winter-Erwärmungsloch und die Abkühlung im Mittleren Westen

Die Welt wird immer wärmer und wärmer. Die ganze Welt? Nein! In den südöstlichen USA gibt es ein Gebiet, dass sich der Besetzung durch die Klimaerwärmung bisher erfolgreich widersetzt hat. Hier wird es sogar immer kälter. In den Staaten spricht man auch vom “Winter Warming Hole”. Eine Studie des Dartmouth College hat das Loch in der Erwärmung näher untersucht und herausgefunden, dass es sich bewegt. Es lebt! Schuld ist der polare Vortex. Nun ist alles klar. Wer es genau wissen will, der möge bitte die Pressemitteilung des College studieren.

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Es gab eine Zeit, da maß man jegliche registrierte Erwärmung und andere klimatische Unregelmäßigkeit dem CO2 zu: Man war stolz auf seinen One-Stop-Shop. Temperaturen hoch: CO2! Ein Sturm wirft einen Apfelbaum um: CO2! Der Rhein tritt über die Ufer: CO2! Endlich war die Weltformel für alle Probleme gefunden. Und die Schuldigen waren schnell ausgemacht: Wir alle. Und wir müssen für unsere Missetaten zahlen, Buße tun. Kling bekannt, ein uraltes Prinzip.

Allmählich kamen den Menschen aber Zweifel? CO2 als zentrale Erklärung für alles Böse auf der Erde? Ungläubige Wissenschaftler versündigten sich und fanden doch tatsächlich heraus, dass auch andere Faktoren außer den Treibhausgasen die Temperaturen nach oben treiben können. Zum Beispiel die wärmende Phase der Ozeanzyklen (1977-1998). Oder der städtische Wärmeinseleffekt. Oder der Ruß.

Und nun wird es ganz bitter. Im Mittleren Westen der USA sind die Sommer kälter und feuchter geworden. Wie kommts? Das Massachusetts Institute of Technology klärte am 13. Februar 2018 auf: Es ist die Intensivierung der Landwirtschaft! Sie lässt die Temperaturen fallen und den Regen üppiger werden:

Intensive agriculture influences US regional summer climate, study finds
An increase in corn and soybean production in the Midwest may have led to cooler, wetter summers there

Scientists agree that changes in land use such as deforestation, and not just greenhouse gas emissions, can play a significant role altering the world’s climate systems. Now, a new study by researchers at MIT and Dartmouth College reveals how another type of land use, intensive agriculture, can impact regional climate.

The researchers show that in the last half of the 20th century, the midwestern U.S. went through an intensification of agricultural practices that led to dramatic increases in production of corn and soybeans. And, over the same period in that region, summers were significantly cooler and had greater rainfall than during the previous half-century. This effect, with regional cooling in a time of overall global warming, may have masked part of the warming effect that would have occurred over that period, and the new finding could help to refine global climate models by incorporating such regional effects. The findings are being published this week in Geophysical Research Letters, in a paper by Ross Alter, a recent MIT postdoc; Elfatih Eltahir, the Breene M. Kerr Professor of Hydrology and Climate; and two others.

The team showed that there was a strong correlation, in both space and time, between the intensification of agriculture in the Midwest, the decrease in observed average daytime temperatures in the summer, and an increase in the observed local rainfall. In addition to this circumstantial evidence, they identified a mechanism that explains the association, suggesting that there was indeed a cause-and-effect link between the changes in vegetation and the climatic effects.

Eltahir explains that plants “breathe” in the carbon dioxide they require for photosynthesis by opening tiny pores, called stoma, but each time they do this they also lose moisture to the atmosphere. With the combination of improved seeds, fertilizers, and other practices, between 1950 and 2009 the annual yield of corn in the Midwest increased about fourfold and that of soybeans doubled. These changes were associated with denser plants with more leaf mass, which thus increased the amount of moisture released into the atmosphere. That extra moisture served to both cool the air and increase the amount of rainfall, the researchers suggest. “For some time, we’ve been interested in how changes in land use can influence climate,” Eltahir says. “It’s an independent problem from carbon dioxide emissions,” which have been more intensively studied.

Eltahir, Alter, and their co-authors noticed that records showed that over the course of the 20th century, “there were substantial changes in regional patterns of temperature and rainfall. A region in the Midwest got colder, which was a surprise,” Eltahir says. Because weather records in the U.S are quite extensive, there is “a robust dataset that shows significant changes in temperature and precipitation” in the region. Over the last half of the century, average summertime rainfall increased by about 15 percent compared to the previous half-century, and average summer temperatures decreased by about half a degree Celsius. The effects are “significant, but small,” Eltahir says.

By introducing into a regional U.S. climate model a factor to account for the more intensive agriculture that has made the Midwest one of the world’s most productive agricultural areas, the researchers found, “the models show a small increase in precipitation, a drop in temperature, and an increase in atmospheric humidity,” Eltahir says — exactly what the climate records actually show. That distinctive “fingerprint,” he says, strongly suggests a causative association. “During the 20th century, the midwestern U.S. experienced regional climate change that’s more consistent with what we’d expect from land-use changes as opposed to other forcings,” he says. This finding in no way contradicts the overall pattern of global warming, Eltahir stresses. But in order to refine the models and improve the accuracy of climate predictions, “we need to understand some of these regional and local processes taking place in the background.”

Unlike land-use changes such as deforestation, which can reduce the absorption of carbon dioxide by trees that can help to ameliorate emissions of the gas, the changes in this case did not reflect any significant increase in the area under cultivation, but rather a dramatic increase in yields from existing farmland. “The area of crops did not expand by a whole lot over that time, but crop production increased substantially, leading to large increases in crop yield,” Alter explains. The findings suggest the possibility that at least on a small-scale regional or local level, intensification of agriculture on existing farmland could be a way of doing some local geoengineering to at least slightly lessen the impacts of global warming, Eltahir says. A recent paper from another group in Switzerland suggests just that.

But the findings could also portend some negative impacts because the kind of intensification of agricultural yields achieved in the Midwest are unlikely to be repeated, and some of global warming’s effects may “have been masked by these regional or local effects. But this was a 20th-century phenomenon, and we don’t expect anything similar in the 21st century,” Eltahir says. So warming in that region in the future “will not have the benefit of these regional moderators.”

Siehe auch Beitrag dazu in Spektrum der Wissenschaft.

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Vor kurzem rauschte es kräftig im medialen Blätterwald. Ein Forscherteam behauptete, der Meeresspiegelanstieg würde sich immer weiter beschleunigung. Potsdam stand Kopf. Die fünfte Jahreszeit war angebrochen. Im Internet bestellten einige Zeitgenossen bereits Tickets für die Arche Noah vor, um auf Nummer sicher zu gehen.

Au weia. Offensichtlich hat die Welt die Kunst des Lesens verloren, und damit ist das Lesen der Originalfachpublikation gemeint. Dabei hätte schon der Abstract gereicht:

Climate-change–driven accelerated sea-level rise detected in the altimeter era
Using a 25-y time series of precision satellite altimeter data from TOPEX/Poseidon, Jason-1, Jason-2, and Jason-3, we estimate the climate-change–driven acceleration of global mean sea level over the last 25 y to be 0.084 ± 0.025 mm/y2. Coupled with the average climate-change–driven rate of sea level rise over these same 25 y of 2.9 mm/y, simple extrapolation of the quadratic implies global mean sea level could rise 65 ± 12 cm by 2100 compared with 2005, roughly in agreement with the Intergovernmental Panel on Climate Change (IPCC) 5th Assessment Report (AR5) model projections.

Eine simple Trendextrapolation eines quadratischen Trends. Science at it’s best!? Dabei wissen wir, dass Ozeanzyklen mit einer Periodenlänge von 60 Jahren hier eine maßgebliche Rolle spielen. Dafür sind die Satellitendatenreihen aber viel zu kurz. Das ist so ein bisschen, als wenn man die Temperaturentwicklung von März bis August misst und dann für die kommenden 10 Jahre extrapoliert. Hilfe, wir werden verbrennen!

Offenbar haben die Autoren bei der “Enso Korrektur” getrickst um den quadratischen Trend hinzubekommen, den sie dann über 80 Jahre extrapolieren. Wenn man es richtig macht, kommt kein solcher heraus. Ein User bei WUWT hat das mit MEI gemacht, frankclimate mit einem SST-basierten Index bei Judith Curry.