Südpolarmeer entzieht der Atmosphäre große Mengen an CO2

Etwa die Hälfte des vom Menschen emittierten CO2 wird von den Ozeanen aufgenommen und verbleibt gar nicht in der Atmosphäre. Dabei gibt es einige Ozeanbereich, die besonders effiziente CO2-Senken bilden, andere Meeresgebiete hingegen sind weniger aufnahmebereit. Im Folgenden begeben wir uns auf einen Streifzug durch die neuere Literatur zum Thema.

Am 12. September 2015 hatte der Tagesspiegel zur Abwechslung einmal gute Klima-Nachrichten:

Ozean als Gegenspieler des Treibhauseffekts Südpolarmeer bremst den Klimawandel
Rund um die Antarktis nimmt der Ozean mal mehr, mal weniger Kohlendioxid aus der Atmosphäre auf. Seit 2002 wirkt der Effekt dem Treibhauseffekt entgegen.

Das Südpolarmeer nimmt seit 2002 wieder mehr Kohlendioxid auf, nachdem es in den 1980er Jahren deutlich langsamer arbeitete. Das berichten Peter Landschützer von der Eidgenössisch-Technischen Hochschule (ETH) in Zürich und seine Kollegen in der Zeitschrift „Science“. Die Weltmeere schlucken große Mengen des Treibhausgases Kohlendioxid aus der Luft, ein Viertel des Klimagasausstoßes der Menschheit verschwindet in ihren Tiefen. So bremsen die Ozeane den Klimawandel – aufgrund natürlicher Prozesse. Allerdings hat die Natur selten ein gleichmäßiges Tempo.

Weiterlesen im Tagesspiegel.

Auch Der Standard berichtete über die Science-Studie:

Das Südpolarmeer: Ein wankelmütiger Faktor im globalen Klimawandel
Das Meer rund um die Antarktis war bis zu den 1990ern – und ist wieder seit 2002 – die größte ozeanische Kohlenstoffsenke unseres Planeten. Was dazwischen geschah, konnten Forscher der ETH Zürich nun aufklären.

Weiterlesen im Standard

Wie funktionierts? Die Welt informierte am 21. September 2015 über eine Arbeit in ‘Current Biology’:

Kleine Tierchen futtern Kohlendioxid in Massen weg
Abschmelzendes Polareis hat auch eine gute Seite: Es trägt überraschenderweise dazu bei, der Luft Kohlendioxid zu entziehen. Weil in der westlichen Antarktis das Schelfeis zunehmend schrumpft, vermehrt sich das pflanzliche Plankton, von dem sich wiederum kleine Moostierchen (Bryozoa) am Meeresgrund ernähren. Letztere speichern somit große Mengen Kohlendioxid (CO2).

Seit den 80er-Jahren habe sich das Wachstum dieser Moostierchen (Bryozoa) in der Region fast verdoppelt, berichtet David Barnes vom British Antarctic Survey im Fachblatt „Current Biology“. Die durch diese und andere Bewohner des Meeresgrunds jährlich zusätzlich aufgenommene Menge des Treibhausgases entspreche der von 500 Quadratkilometern tropischen Regenwaldes, vermutet er.

Weiterlesen in der Welt.

Der September 2015 brachte wirklich einen großen Fortschritt in der Thematik. Am 18.9.2015 erschien nämlich auch ein Paper von David Munro und Kollegen in den Geophysical Research Letters, die den erfreulichen Trend zu verstärkter CO2-Aufnahme der antarktischen Meeresbereiche bestätigte. Auch die Drake Passage nimmt nun mehr CO2 auf als zuvor:

Recent evidence for a strengthening CO2 sink in the Southern Ocean from carbonate system measurements in the Drake Passage (2002–2015)
We present a 13 year (2002–2015) semimonthly time series of the partial pressure of CO2 in surface water (pCO2surf) and other carbonate system parameters from the Drake Passage. This record shows a clear increase in the magnitude of the sea-air pCO2 gradient, indicating strengthening of the CO2 sink in agreement with recent large-scale analyses of the world oceans. The rate of increase in pCO2surf north of the Antarctic Polar Front (APF) is similar to the atmospheric pCO2 (pCO2atm) trend, whereas the pCO2surf increase south of the APF is slower than the pCO2atm trend. The high-frequency surface observations indicate that an absence of a winter increase in total CO2 (TCO2) and cooling summer sea surface temperatures are largely responsible for increasing CO2 uptake south of the APF. Muted winter trends in surface TCO2 also provide temporary stability to the carbonate system that is already close to undersaturation with respect to aragonite.

Hier die Pressemitteilung der American Geophysical Union (AGU) zum Artikel:

Southern Ocean removing carbon dioxide from atmosphere more efficiently

Scientists compile densest carbon data set in Antarctic waters

Since 2002, the Southern Ocean has been removing more of the greenhouse gas carbon dioxide from the atmosphere, according to two new studies.

These studies make use of millions of ship-based observations and a variety of data analysis techniques to conclude that that the Southern Ocean has increasingly taken up more carbon dioxide during the last 13 years. That follows a decade from the early 1990s to 2000s, where evidence suggested the Southern Ocean carbon dioxide sink was weakening. The new studies appear today in the American Geophysical Union journal Geophysical Research Letters and the AAAS journal Science.

The global oceans are an important sink for human-released carbon dioxide, absorbing nearly a quarter of the total carbon dioxide emissions every year. Of all ocean regions, the Southern Ocean below the 35th parallel south plays a particularly vital role. “Although it comprises only 26 percent of the total ocean area, the Southern Ocean has absorbed nearly 40 percent of all anthropogenic carbon dioxide taken up by the global oceans up to the present,” says David Munro, a scientist at the Institute of Arctic and Alpine Research (INSTAAR) at the University of Colorado Boulder, and an author on the GRL paper.

The GRL paper focuses on one region of the Southern Ocean extending from the tip of South America to the tip of the Antarctic Peninsula (see Figure 1). “The Drake Passage is the windiest, roughest part of the Southern Ocean,” says Colm Sweeney, lead investigator on the Drake Passage study, co-author on both the GRL and Science papers, and a CIRES scientist working in the NOAA Earth System Research Laboratory in Boulder, Colorado. “The critical element to this study is that we were able to sustain measurements in this harsh environment as long as we have—both in the summer and the winter, in every year over the last 13 years. This data set of ocean carbon measurements is the densest ongoing time series in the Southern Ocean.”

The team was able to take these long-term measurements by piggybacking instruments on the Antarctic Research Supply Vessel Laurence M. Gould. The National Science Foundation-supported Gould, which makes nearly 20 crossings of the Drake Passage each year, transporting people and supplies to and from Antarctic research stations. For over 13 years, it’s taken chemical measurements of the atmosphere and surface ocean along the way.

By analyzing more than one million surface ocean observations, the researchers could tease out subtle differences between the carbon dioxide trends in the surface ocean and the atmosphere that suggest a strengthening of the carbon sink. This change is most pronounced in the southern half of the Drake Passage during winter (see Figure 2). Although the researchers aren’t sure of the exact mechanism driving these changes, “it’s likely that winter mixing with deep waters that have not had contact with the atmosphere for several hundred years plays an important role,” says Munro.

The Science paper, led by Peter Landschützer at the ETH Zurich, takes a more expansive view of the Southern Ocean. This study uses two innovative methods to analyze a dataset of surface water carbon dioxide spanning almost three decades and covering all of the waters below the 35th parallel south. These data—including Sweeney and Munro’s data from the Drake Passage—also show that the surface water carbon dioxide is increasing slower than atmospheric carbon dioxide, a sign that the Southern Ocean as a whole is more efficiently removing carbon from the atmosphere. These results follow previous findings that showed that the Southern Ocean carbon dioxide sink was stagnant or weakening from the early 1990s to the early 2000s.

In addition to the Drake Passage measurements, the Science paper uses datasets that represent a significant international collaboration, including carbon dioxide sampling from NOAA’s Ship of Opportunity Program. This program, led by Rik Wanninkhof of NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) who is also a coauthor of the Science paper, is the world’s largest coordinated ocean carbon dioxide sampling operation. Despite all these efforts, the Southern Ocean remains undersampled. “Given the importance of the Southern Ocean to the global oceans’ role in absorbing atmospheric carbon dioxide, these studies suggest that we must continue to expand our measurements in this part of the world despite the challenging environment,” says Sweeney.

Die Temperatur in der Südpolarregion hat einen starken Einfluss auf das CO2-Aufnahmevermögen des Südpolarmeeres. Hierauf wies die University of New South Wales am 28. September 2015 mit Hinweis auf ein Nature Geoscience Paper hin:

How ocean circulation changed atmospheric CO2

Scientists have struggled for the past few decades to understand why air temperatures around Antarctica over the past one million years were almost perfectly in synch with atmospheric CO2 concentrations. Both dipped down during glacial ice ages and back up again during warm interglacials. By contrast, temperature in the tropics and Northern Hemisphere was less closely tied to atmospheric CO2 concentrations. “This relationship between Antarctica temperature and CO2 suggested that somehow the Southern Ocean was pivotal in controlling natural atmospheric CO2 concentrations,” said Dr Maxim Nikurashin from the ARC Centre of Excellence for Climate System Science. “The key that unlocked the mystery was the colder atmosphere and extensive sea ice around Antarctica during the glacial period. Together they fundamentally changed top to bottom ocean circulation and enabled more CO2 to be drawn from the atmosphere.”

The researchers found in a paper published today in Nature Geoscience that during glacial periods when the atmosphere was colder and sea ice was far more extensive, deep ocean waters came to the surface much further north of the Antarctic continent than they do today. This meant that the nutrients brought up from the bottom of the ocean spent more time on the surface of the ocean as the currents moved them southwards before the flow encountered Antarctica and circled back down to the bottom of the ocean. Because the upwelled waters ran along the surface for a longer period of time, nutrients spent more time near the surface of the ocean where phytoplankton could feed on them for longer.

The biological processes that result from phytoplankton blooms directly take carbon out of the atmosphere. Some of this carbon then sinks to the bottom of the ocean when the phytoplankton die, locking it away in the deep sea for thousands of years. “The biological processes that take up carbon from the atmosphere even take place in and under the ice, if that ice is not too thick, which is why the biological processes persisted for a lot longer during cooler periods,” the authors said. “Our results suggest that this change in circulation and the consequent extended biological activity by itself took 30-60ppm of CO2 out of the atmosphere. That’s about one half of the glacial-interglacial change.”

However, when temperatures warm over the Antarctic regions, deep waters rise from the floor of the ocean much closer to the continent. This means nutrients are near the surface for a shorter time before returning to the deep ocean floor. With less time on the surface there is less time for the biological processes to take place and less carbon is taken out of the atmosphere. This is the situation we see today. “This finding is a major advance in understanding the natural carbon cycle, gained by applying a new understanding about how the “overturning circulation” of the Southern Ocean works,” said lead author Dr Andrew J Watson from the University of Exeter.

Paper: Southern Ocean buoyancy forcing of ocean ventilation and glacial atmospheric CO2. Nature Geoscience. doi:10.1038/ngeo2538.