Droughts increasingly reduce CO2 uptake in the tropics

June 1 () –

ETH Zurich researchers have found that droughts and terrestrial water variability have had an increasing effect in the carbon cycle in the tropics over the last sixty years.

Most climate models do not capture this observation. This could mean that terrestrial ecosystems could absorb less CO2 than expected in their role as future carbon sinks.

Plants take CO2 to grow. They extract it from the atmosphere and use it to build organic compounds through photosynthesis and water. Terrestrial ecosystems have absorbed an average of about 32 percent of CO2 emissions caused by human activity over the past six decades. Whether terrestrial vegetation can continue to function as a carbon sink in a changing climate and to what extent is a key question in climate science and has vital policy relevance.

Earth’s climate system has numerous feedback loops. These are processes triggered by global warming that have a feedback effect on climate change, amplifying or diminishing it. Such carbon system feedback loops are difficult to measure and model, and represent a significant factor of uncertainty in climate projections. “Therefore, it is difficult to quantify exactly how terrestrial carbon sinks will respond to additional human-induced climate change,” says lead study author Sonia Seneviratne, Professor of Earth Climate Dynamics at ETH Zurich.

Until now, the available literature suggested that terrestrial carbon sinks could only be clearly affected by a high or very high increase in global warming, ie above 2 to 4 degrees Celsius. Now a team of researchers led by Seneviratne has found evidence that terrestrial ecosystems may be less resilient to climate change than previously thought.

“We found that tropical carbon sinks are increasingly vulnerable to water scarcity,” he says. it’s a statement Laibao Liu, a postdoctoral researcher in Seneviratne’s group and first author of the study, which the researchers publish in the journal Nature.

The results suggest that droughts have had an increasing impact on the carbon cycle in the tropics over the past 60 years, with vegetation absorbing less and less CO2 during drought events, an effect that most climate models cannot capture.

However, the observed change appears to be based on a known feedback loop: in hot, dry conditions, plants stop taking up CO2 to prevent water loss. In addition, there may also be an increase in plant mortality and fires, leading to the additional release of CO2 into the biosphere. If such conditions were to occur more frequently, it could lead to a reduction in the terrestrial CO2 sink and thus a further increase in global warming.

Already in 2018, Seneviratne’s team demonstrated on a global scale how stressed ecosystems absorb less carbon during severe droughts, that is, how the concentration of CO2 in the atmosphere increases significantly in dry years. In fact, the growth rate of atmospheric CO2 varies from year to year depending on the availability of terrestrial water. The biggest challenge was figuring out where droughts were occurring around the world. Since then, sophisticated satellite observation of Earth’s water reservoirs has made it possible to determine this more precisely.

In this study, the researchers wanted to find out if there was a change in the correlation between water availability and CO2 growth rate over time. “Given that annual fluctuations in CO2 growth rates are clearly dominated by carbon fluxes between land and atmosphere in the tropics, we were able to investigate this global question using tropical climate data from the last sixty yearsLiu explains.

In particular, the researchers were able to show that the coupling between tropical water availability and CO2 growth rates intensified in the 30-year period between 1989 and 2018 compared to the period from 1960 to 1989.

In other words, tropical water, or more precisely its scarcity, has become an increasingly limiting factor shaping the annually fluctuating carbon cycle and its feedback loops.