Stanford University: Polare Eisschilde stabiler als gedacht, Meeresspiegelanstieg weniger dramatisch als befürchtet

Die Sonne hat beim IPCC keinen guten Stand. Sie soll angeblich nur eine verschwindend geringe Klimawirkung besitzen. Sonnenforscher tun deshalb gut daran, möglichst weit in der Vergangenheit zurückzugehen, wenn sie den klimatischen Einfluss der Sonne dokumentieren. So zum Beispiel eine Gruppe um Florian Adolphi zu der auch Katja Matthes vom Geomar gehört. Die Wissenschaftler fanden eine starke solare Beeinflussung des Klimas während der letzten Eiszeit. Adolphi und Kollegen beschreiben einen solaren Verstärkermechanismus über die Stratosphäre, wobei sie vermuten, dass er auch heute noch in ähnlicher Weise wirkt, dies jedoch nicht in den gängigen Klimamodellen berücksichtigt wird. Nachzulesen in Nature Geoscience im August 2014:

Persistent link between solar activity and Greenland climate during the Last Glacial Maximum
Changes in solar activity have previously been proposed to cause decadal- to millennial-scale fluctuations in both the modern and Holocene climates1. Direct observational records of solar activity, such as sunspot numbers, exist for only the past few hundred years, so solar variability for earlier periods is typically reconstructed from measurements of cosmogenic radionuclides such as 10Be and 14C from ice cores and tree rings2, 3. Here we present a high-resolution 10Be record from the ice core collected from central Greenland by the Greenland Ice Core Project (GRIP). The record spans from 22,500 to 10,000 years ago, and is based on new and compiled data4, 5, 6. Using 14C records7, 8 to control for climate-related influences on 10Be deposition, we reconstruct centennial changes in solar activity. We find that during the Last Glacial Maximum, solar minima correlate with more negative δ18O values of ice and are accompanied by increased snow accumulation and sea-salt input over central Greenland. We suggest that solar minima could have induced changes in the stratosphere that favour the development of high-pressure blocking systems located to the south of Greenland, as has been found in observations and model simulations for recent climate9, 10. We conclude that the mechanism behind solar forcing of regional climate change may have been similar under both modern and Last Glacial Maximum climate conditions.

Vor der letzten Eiszeit gab es eine Warmzeit, die der heutigen Warmzeit in vielerlei Hinsicht ähnelt. Die sogenannte Eem-Warmzeit vor 120.000 Jahren war allerdings sogar noch wärmer als heute – ganz ohne menschliches Zutun. Eine Gruppe des Potsdamer PIK um R. Calov dokumentierte im Februar 2015 im Fachblatt The Cryosphere, dass der Meeresspiegel damals um fast anderthalb Meter über dem heutigen Niveau lag. Die Temperaturen während des Eem lagen über sieben Jahrtausende hinweg satte 5-8°C über dem heutigen Niveau. Während einer anderen Warmzeit vor 400.000 Jahren war es sogar noch wärmer und der Meeresspiegel stieg 6-13m über den heutigen Stand.

Im Großen und Ganzen blieb der grönländische Eissschid jedoch überraschend stabil während der letzten anderthalb Millionen Jahre des Pleistozäns, als Warm- und Kaltzeiten im 100.000-Jahrestakt wechselten. Eine Gruppe um den Geologen Paul Bierman von der University of Vermont fand kürzlich unter dem grönländischen Eis einen präglazialen organischen Boden, der vor 2,7 Millionen Jahren von den Gletschern überrollt wurde. Seit diesem Zeitpunkt war die Stelle stets von Eis überdeckt, also auch während aller pleistozäner Warmzeiten, die dem Eis nicht gefährlich werden konnten. Sci-News berichtete im April 2014 über die Entdeckung, die die Autoren in Science publiziert hatten.

Auch Forscher der Stanford University gehen davon aus, dass das Grönlandeis viel stabiler ist, als man es ihm lange zugetraut hatte. Die Wissenschaftler untersuchten das warme Pliozän vor 3 Millionen Jahren, als das CO2 eine ähnliche Konzentration besaß wie heute. Lange hatte man gedacht, dass der Meeresspiegel damals bis zu 30 m über dem heutigen Niveau lag. Dies stellte sich jetzt jedoch als falsch heraus. Eine Neuberechnung auf Basis korrigierter Isotopenproxies ergab nun nur noch einen maximalen Meeresspiegl von 13,5 m über dem heutigen Stand, also weniger als die Hälfte. Es scheint also viel weniger Eis geschmolzen zu sein als lange angenommen. Die Stanford-Forscher vermuten, dass der ostantarktische Eisschild Garant für die größere Eisstabilität war. Im Folgenden die entsprechende Pressemiteilung vom 3. September 2015:

Ice sheets may be more resilient than thought, say Stanford scientists

Stanford study suggests that today’s ice sheets may be more resilient to increased carbon dioxide levels than previously thought.

By Miles Traer

Sea level rise poses one of the biggest threats to human systems in a globally warming world, potentially causing trillions of dollars’ worth of damages to flooded cities around the world. As surface temperatures rise, ice sheets are melting at record rates and sea levels are rising. But there may be some good news amid the worry. Sea levels may not rise as high as assumed. To predict sea level changes, scientists look to Earth’s distant past, when climate conditions were similar to today, and investigate how the planet’s ice sheets responded then to warmer temperatures brought on by increased carbon dioxide in the atmosphere.

In a recently published study in the journal Geology, PhD students Matthew Winnick and Jeremy Caves at Stanford School of Earth, Energy & Environmental Sciences explored these very old conditions and found that sea level might not have risen as much as previously thought – and thus may not rise as fast as predicted now. To better understand global sea level rise, Winnick and Caves analyzed the middle Pliocene warm period, the last time in Earth’s history, approximately 3 million years ago, when carbon dioxide levels in the atmosphere were close to their present values (350-450 parts per million). “The Pliocene is an important analogue for today’s planet not only because of the related greenhouse gas concentrations, but because the continents were roughly where they are today, meaning ocean and climate circulation patterns are comparable,” said Winnick. These similarities are why the Intergovernmental Panel on Climate Change (IPCC), the group responsible for global sea level rise projections, focuses on the mid-Pliocene warm period to inform their computer models.

Previous studies of the mid-Pliocene warm period used oxygen isotope records to determine the volume of Earth’s ice sheets and, by proxy, sea level. Effectively, the oxygen isotope records act as a fingerprint of Earth’s ice sheets. By combining the fingerprint with models of ice sheet meltwater, many previous researchers thought that sea level was likely 82 to 98 feet (25 to 30 meters) higher during the Pliocene. Such high sea level would require a full deglaciation of the Greenland Ice Sheet and the West Antarctic Ice Sheet, and as much as 30 percent of the East Antarctic Ice Sheet – enough to cover New York City under 50 feet of water. But these estimates arose because the researchers assumed that the Antarctic ice of the Pliocene had the same isotopic composition, that is, the same fingerprint, as it does today – an assumption that Winnick and Caves challenge in their new report.

To understand the isotopic composition of Pliocene ice, Winnick and Caves began in the present day using well-established relationships between temperature and the geochemical fingerprint. By combining this modern relationship with estimates of ancient Pliocene surface temperatures, they were able to better refine the fingerprint of the Antarctic ice millions of years ago. In re-thinking this critical assumption, and by extending their analysis to incorporate ice sheet models, Winnick and Caves recalculated the global sea level of the Pliocene and found that it was 30 to 44 feet (9 to 13.5 meters) higher, significantly lower than the previous estimate.

“Our results are tentatively good news,” Winnick said. “They suggest that global sea level is less sensitive to high atmospheric carbon dioxide concentrations than previously thought. In particular, we argue that this is due to the stability of the East Antarctic Ice Sheet, which might be more resilient than previous studies have suggested.” However, a rise in global sea level by up to 44 feet (13.5 meters) is still enough to inundate Miami, New Orleans and New York City, and threaten large portions of San Francisco, Winnick cautioned.

While the study helps refine our understanding of Pliocene sea level, both Winnick and Caves point out that it’s not straightforward to apply these results to today’s planet. “Ice sheets typically take centuries to millennia to respond to increased carbon dioxide, so it’s more difficult to say what will happen on shorter time scales, like the next few decades,” Winnick said. “Add that to the fact that CO2 levels were relatively consistent in the Pliocene, and we’re increasing them much more rapidly today, and it really highlights the importance of understanding how sea level responds to rising temperatures. Estimates of Pliocene sea level might provide a powerful tool for testing the ability of our ice sheet models to predict future changes in sea level.”