The earth’s land ecosystems with their soils and the vegetation growing on them play an important role in preventing climate change. They currently absorb about 30 percent of man-made CO 2 emissions and are like that as a carbon sink, an extremely important catchment basin for the greenhouse gas. This effect could turn into the opposite with further global warming, writes an American and New Zealand research team led by computer scientist Katharyn Duffy from Northern Arizona University.

Photosynthesis threshold Scientists see the ecosystems in rural areas in their function as carbon stores at one point -Control point. They would in turn increase to further sources for the CO 2 – emissions and the temperature rise on the globe.

The most important interaction between atmosphere and land generally takes place over the vegetation. Two processes in particular are important for the CO 2 balance of these systems: During photosynthesis, plants absorb the greenhouse gas, convert it into biomass and thus store the carbon. When they breathe, however, the plants release the gas again. Both processes are dependent on the warmth and the incidence of light.

While the metabolic process in the form of respiration runs faster and faster with increasing temperatures and thus If more CO 2 is released into the atmosphere, photosynthesis performance reaches a threshold value. At this temperature, their performance decreases. Thus, rising temperatures can lead to more CO 2 being exhaled than is stored via photosynthesis and partly converted into oxygen.

Observation network Fluxnet For their study published on Wednesday in the journal “Science Advances”, the authors analyzed recordings of data from the observation network Fluxnet, a worldwide network of stations , which measure the exchange of carbon dioxide, water vapor and energy between the biosphere and the atmosphere.

The scientists determined changes in photosynthesis and respiration that can be attributed exclusively to changes in temperature at each Flux Tower location. With this method they come to the conclusion that especially complex ecosystems such as rainforests and taiga forests, which store the most carbon, are already between 2040 and could lose more than 45 percent of their ability in the middle of the century. By the year 2100 around half of the terrestrial ecosystems would then reach the tipping point, from which the plants release carbon into the atmosphere faster than they bind it .

Warning of tipping points In summer 2018 researchers had already warned of other tipping points. They explained that such physical, mutually reinforcing feedback could cause the climate to get out of hand and trigger a hot period. But there is also hope for social counter-movements with opposing tipping points.

So far, global climate or general circulation models have suggested The Augsburg climate researcher Wolfgang Buermann knows that the “tipping point” that has now been described again will possibly be reached more in the second half of this century. That now from 20 to 30 years ago, it was said that had “important effects on the prognoses for climate change”. According to the professor, this is supported by new findings that point to a weakened CO 2 fertilization effect on plant productivity, which may be related to limitations in the available water or important nutrients. A shortcoming of the new analysis, however, is that a possible acclimatization of plant photosynthesis at high temperatures under the current – still relatively moderate – warming level may not be properly assessable.

Study criticism The study is “informative”, but raises many questions, says Matthias Forkel, professor for environmental remote sensing at the TU Dresden. For example, the authors either simplified or ignored various other processes, such as water availability, which influenced the carbon balance of ecosystems. Above all, dead organic matter such as leaves and a thick layer of vegetation could contribute to a cool microclimate and thus weaken soil respiration. On the other hand, the results are all the more worrying because other factors left out of the picture, such as the effect of droughts and forest fires, could additionally increase the release of CO 2 into the atmosphere.

Stefan Arndt, Professor of Ecological Ecosystems at the University of Melbourne, refers to a similar, as yet unpublished analysis by one of his doctoral students, according to which there is a temperature increase in Australia likely to have little impact on the carbon balance. The results of the current analysis are likely to be due to the falsified aggregation of the data and the models used. Markus Reichstein from the Max Planck Institute for Biogeochemistry in Jena also believes that the design of the study has opened up “many possibilities for extrapolation errors”.

(kbe)