Sorge um Objektivität bei Bild der Wissenschaft: Redakteur präsentiert fragwürdige Klimaaktivisten-Studie zum Ende der letzten Eiszeit, ohne ernstzunehmende anderslautende Resultate zu erwähnen

Bild der Wissenschaft berichtete am 21. August 2015 über eine seltsame neue Studie:

Eiszeit-Gletscherschmelze durch Treibhausgase
Was verursachte den Rückgang der Gletscher am Ende der letzten Eiszeit? Zu dieser Frage gibt es noch immer Unklarheiten – war verstärkte Sonnenstrahlung, Veränderungen der Meeresströmungen oder aber tatsächlich der Anstieg der Treibhausgaskonzentration für den Wärmeeffekt hauptverantwortlich? Eine Neudatierung, wann Felsen vom Gletschereis einst freigegeben wurden, schafft nun Klarheit: Die Schmelze am Ende der Eiszeit begann doch zeitgleich mit einem natürlichen Anstieg der Treibhausgas-Konzentration.

Das Ende der letzten Eiszeit vor 15.000 Jahren durch einen Anstieg der Treibhausgase bedingt? Eine Vielzahl von früheren Studien fand das genaue Gegenteil, nämlich zunächst eine Erwärmung und dann mit zeitlicher Verzögerung von einigen hundert Jahren ein Anstieg der CO2-Konzentration. Die überwiegende Mehrheit der Wissenschaftler vertritt diese Ansicht und kann den Zusammenhang auch gut belegen:

Daher wundert es doch sehr, dass Redakteur Martin Vieweg von Bild der Wissenschaft (BdW) die Debatte aufgrund eines einzigen neuen Papers plötzlich für beendet erklärt, die angeblich nun endlich “Klarheit schafft”. Was hat die neue Arbeit nun wirklich gefunden und um welche Forscher handelt es sich? Hierzu lesen wir in BdW etwas weiter:

“Wir haben festgestellt, dass diese Alterseinstufungen der Gesteine ungenau waren. Die neuen Daten bestätigen nun hingegen: Zeitgleich mit dem Anstieg der Treibhauskonzentration begannen die Gletscher zu schmelzen und sich zurückziehen”, sagt Jeremy Shakun vom Boston College.

Shakun, Shakun, den Namen hatten wir doch schon einmal gehört? Genau, mit genau der selben Masche hatte Shakun 2012 schon einmal versucht, die Verhältnisse auf den Kopf zu stellen. Siehe unseren Blogartikel “Statistik-Trick befördert CO2 vom Beifahrer zum Chauffeur: Fragwürdiger neuer Shakun-Artikel in Nature“. Damals erntete die Gruppe in der Fachwelt viel Unverständnis für ihre Ausarbeitung. Nun versucht es Shakun in ähnlicher Gruppenzusammensetzung also erneut. Die Erfolgsaussichten sind jedoch gering. Denn wer einmal murskst, dem glaubt man nicht. Skepsis ist auch angebracht, da zur Gruppe erneut der IPCC-Hardliner Peter Clark gehört, der bereits mehrfach mit extremen und alarmistischen Sichtweisen negativ aufgefallen ist.

Bild der Wissenschaft ist hier vorzuwerfen, seine Leserschaft über die Vorgeschichte und IPCC-Lastigkeit der Forschergruppe nicht informiert zu haben. Sind jetzt alle Vorgängerarbeiten an anderen Orten und anderem Material plötzlich hinfällig, nur weil eine Gruppe ein anderes Ergebnis herausbekommt? Wohl kaum. Wir haben in der BdW-Redaktion nachgefragt, wie es zu der vorschnellen und einseitigen Darstellung kommen konnte.

Im Folgenden die Pressemitteilung der Oregon State University vom 21. August 2015:

Greenhouse gases caused glacial retreat during last Ice Age

A recalculation of the dates at which boulders were uncovered by melting glaciers at the end of the last Ice Age has conclusively shown that the glacial retreat was due to rising levels of carbon dioxide and other greenhouse gases, as opposed to other types of forces.

Carbon dioxide levels are now significantly higher than they were at that time, as a result of the Industrial Revolution and other human activities since then. Because of that, the study confirms predictions of future glacial retreat, and that most of the world’s glaciers may disappear in the next few centuries. The findings were published today in Nature Communications by researchers from Oregon State University, Boston College and other institutions. They erase some of the uncertainties about glacial melting that had been due to a misinterpretation of data from some of these boulders, which were exposed to the atmosphere more than 11,500 years ago.

“This shows that at the end of the last Ice Age, it was only the increase in carbon dioxide and other greenhouse gases that could have caused the loss of glaciers around the world at the same time,” said Peter Clark, a professor in the OSU College of Earth, Ocean and Atmospheric Sciences, and co-author on the study. “This study validates predictions that future glacial loss will occur due to the ongoing increase in greenhouse gas levels from human activities,” Clark said. “We could lose 80-90 percent of the world’s glaciers in the next several centuries if greenhouse gases continue to rise at the current rate.”

Glacial loss in the future will contribute to rising sea levels and, in some cases, have impacts on local water supplies. As the last Ice Age ended during a period of about 7,000 years, starting around 19,000 years ago, the levels of carbon dioxide in the atmosphere increased from 180 parts per million to 280 parts per million. But just in the past 150 years, they have surged from 280 to about 400 parts per million, far higher than what was required to put an end to the last Ice Age.

The new findings, Clark said, were based on a recalculation of the ages at which more than 1,100 glacial boulders from 159 glacial moraines around the world were exposed to the atmosphere after being buried for thousands of years under ice. The exposure of the boulders to cosmic rays produced cosmogenic nuclides, which had been previously measured and used to date the event. But advances have been made in how to calibrate ages based on that data. Based on the new calculations, the rise in carbon dioxide levels – determined from ancient ice cores -matches up nicely with the time at which glacial retreat took place.

“There had been a long-standing mystery about why these boulders were uncovered at the time they were, because it didn’t properly match the increase in greenhouse gases,” said Jeremy Shakun, a professor at Boston College and lead author on the study. “We found that the previous ages assigned to this event were inaccurate. The data now show that as soon as the greenhouse gas levels began to rise, the glaciers began to melt and retreat.” There are other forces that can also cause glacial melting on a local or regional scale, the researchers noted, such as changes in the Earth’s orbit around the sun, or shifts in ocean heat distribution. These factors probably did have localized effects. But the scientists determined that only the change in greenhouse gas levels could have explained the broader global retreat of glaciers all at the same time.

In the study of climate change, glaciers have always been of considerable interest, because their long-term behavior is a more reliable barometer that helps sort out the ups-and-downs caused by year-to-year weather variability, including short-term shifts in temperature and precipitation.

Hier der Abstract des Papers:

Regional and global forcing of glacier retreat during the last deglaciation

Jeremy D. Shakun, Peter U. Clark, Feng He, Nathaniel A. Lifton, Zhengyu Liu, Bette L. Otto-Bliesner

The ongoing retreat of glaciers globally is one of the clearest manifestations of recent global warming associated with rising greenhouse gas concentrations. By comparison, the importance of greenhouse gases in driving glacier retreat during the most recent deglaciation, the last major interval of global warming, is unclear due to uncertainties in the timing of retreat around the world. Here we use recently improved cosmogenic-nuclide production-rate calibrations to recalculate the ages of 1,116 glacial boulders from 195 moraines that provide broad coverage of retreat in mid-to-low-latitude regions. This revised history, in conjunction with transient climate model simulations, suggests that while several regional-scale forcings, including insolation, ice sheets and ocean circulation, modulated glacier responses regionally, they are unable to account for global-scale retreat, which is most likely related to increasing greenhouse gas concentrations.