A new balance for the global carbon balance

Researchers determine how rates of photosynthesis and ecosystem respiration depend on the climate in order to obtain a better assessment of the consequences of climate change.

Climate predictions could become more accurate and more reliable in the future - thanks to new findings on the role of terrestrial ecosystems in the global carbon cycle. International teams of researchers headed by the Max Planck Institute for Biogeochemistry in Jena now present comprehensive data analyses in two related studies. The analyses also lead to more precise estimates of how the ecosystems could react to climate change. In most ecosystems, the photosynthesis rate at which plants fix carbon dioxide from the atmosphere changes relatively little as temperature varies. Over 40 percent of the Earth’s vegetated surface reacts very sensitively to changes in the amount of precipitation, however. The respiration of the ecosystems, when flora and fauna release carbon dioxide, also increases to a lesser extent than has recently often been assumed when the temperature rises. Moreover, this temperature dependence is the same all over the world - even in ecosystems as different as the tropical savannah and the Finnish needleleaf forest, for example. (Science Express, July 5, 2010)

The climate is quite temperamental: countless factors are involved and many feedback mechanisms enhance effects such as the anthropogenic greenhouse effect. This makes it difficult to make predictions, especially as many processes in the Earth system are still not completely understood. More light is now being shed on the part played by terrestrial ecosystems in the global carbon cycle. This applies to the role of photosynthesis, whereby plants fix carbon dioxide, as well as the process of respiration, during which plants release carbon dioxide once again. The scientists are thus making an important contribution to understanding how the global carbon cycle reacts to global warming and climate change. "Our results suggest that the availability of water, in particular, plays a decisive role for the carbon cycle in ecosystems. It is often more important than temperature," says Markus Reichstein, a scientist at the Max Planck Institute for Biogeochemistry who has investigated these issues together with his colleagues and two international teams.

In one of the current studies, the researchers involved in the Fluxnet initiative measured how the respiration of ecosystems reacts to short-term variations in temperature at 60 stations spread across the globe. They found that the rate at which plants and microorganisms convert sugar into carbon dioxide does not even double when the temperature increases by ten degrees from one week to the next, for example. "With the aid of suitable models it is then possible to calculate how climate change could affect the respiration of the ecosystems and the global carbon cycle," says Markus Reichstein.

Some earlier investigations at the ecosystem level resulted in threefold to fourfold accelerations, which would enhance the greenhouse effect. It was not possible to reconcile these data with global models and atmospheric measurements of carbon dioxide concentrations and their seasonal variations, however. "We can now settle obvious contradictions between experimental and theoretical studies," says Miguel Mahecha, who played a crucial role in coordinating and evaluating the new measurements on ecosystem respiration. His colleague Markus Reichstein adds: "Particularly alarmist scenarios for the feedback between global warming and ecosystem respiration thus prove to be unrealistic."

These measurements also contradict a further assumption which earlier investigations seemed to suggest: that the respiration of the ecosystems in the tropics and temperate latitudes is influenced to a lesser degree by temperature than at higher latitudes. As the Jena scientists have now discovered, the respiration of very different ecosystems intensifies to the same extent when it becomes hotter. The factor which determines the acceleration of the respiration thus obviously does not depend on the local temperature conditions and the specific characteristics of an ecosystem. "We were very surprised that different ecosystems react relatively uniformly to temperature variations," says Miguel Mahecha. "After all, for example, we analysed savannahs, tropical rain forests, and also central European broadleaf and needleleaf forests and agricultural ecosystems."

Climate researchers must now examine how the new findings affect the predictions for the carbon dioxide balance and climate change. "It is still not possible to predict whether this attenuates the positive feedback between carbon dioxide concentration and temperature," says Markus Reichstein. "The study shows very clearly that we do not yet have a good understanding of the global biogeochemichal cycles and their importance for long-term developments."

The researchers want to change this with their investigation of the rate of photosynthesis. According to this, the terrestrial ecosystems store 450 billion tonnes of carbon dioxide annually. "Although a similar value had been assumed before, it was only an hypothetical estimate," says Christian Beer, who was in charge of the study. Sixty percent of the carbon dioxide which plants globally take up from the atmosphere is swallowed up by the tropical rain forests and savannahs. The savannahs owe their comparatively important role to the huge area which they cover. The rain forests, in contrast, take up particularly large amounts of carbon dioxide over relatively small areas in order to produce biomass.

The global analysis has also enabled the researchers to establish that the amount of carbon dioxide which is fixed by photosynthesis in leaves is influenced by different climatic factors in different vegetation zones. Sometimes the temperature plays a more important role, sometimes the intensity of the solar radiation, and sometimes the amount of water which the plants can take up from the ground.

From a global point of view, however, water is the factor which has the strongest effect: over 40 percent of Earth’s vegetated surface plants photosynthesize more when the supply of water increases, and less during droughts. In temperate grasslands and shrublands, the amount of carbon dioxide which plants fix as sugar depends to 69 percent on their water supply, in the tropical rain forest this figure is only 29 percent. The researchers call the amount of carbon dioxide which ecosystems annually take up primary production. "We were surprised to find that the primary production in the tropics is not so strongly dependent on the amount of rain," says Markus Reichstein. "Here, too, we need to therefore critically scrutinize the forecasts of some climate models which predict the Amazon will die as the world gets drier."

The data forming the basis of the two reports now published originate from an international network of over 250 measuring stations which undertake long-term observations in different ecosystems across the world. Teams of researchers have high measurement towers above grasslands or forests to record the water and carbon dioxide concentrations in the air and the wind turbulences all over the world. This enables them to calculate how much carbon dioxide is taken up and released by a certain ecosystem. They relate these values to the local climate and divide the Earth’s surface into quadrants measuring 50 by 50 kilometres. The researchers derive the global primary production by using satellite data to extrapolate the photosynthesis rates of the individual ecosystems. The satellites measure how much light energy the vegetated surface of the Earth absorbs on a global basis. Until now, climate researchers have extrapolated the interplay of climate and photosynthesis or ecosystem respiration on the basis of hypotheses.

The new Fluxnet initiative, which Markus Reichstein and colleagues from Italy and North America initiated, brings together the results of the global measurements for the first time. "We are gaining new insights into Earth’s climate system because various research groups are prepared to disclose their data," says Reichstein. "The current studies, in particular, prove the added value of long-term observations of the Earth system for research. They help to reduce the uncertainty of current climate predictions and exclude some model predictions."