A faster rise in drier years
Using these new satellite observations of water storage, Vincent Humphrey and his colleagues were able to measure the overall impact of droughts on photosynthesis and ecosystem respiration. They compared year-to-year changes in total water mass over all continents against global measurements of CO
2
increase in the atmosphere. They found that during the driest years such as 2015, natural ecosystems removed about 30% percent less carbon from the atmosphere than during a normal year. As a result, the concentration of CO
2
in the atmosphere increased faster in 2015 compared to normal years. At the other end of the scale, during the wettest year on record in 2011, CO
2
concentrations increased at a much slower rate due to healthy vegetation. These results help us understand why atmospheric CO
2
growth can vary a lot from one year to the other, even though CO
2
emissions from human activities are comparatively stable.
Crucial for monitoring emissions
During the last century, the concentration of CO
2
in the atmosphere has been steadily increasing because of human activities. “Now that most countries around the world have agreed they should limit CO
2
emissions, we are facing the challenge of monitoring human CO
2
emissions to a level of accuracy higher than ever before,” says Vincent Humphrey. In order to precisely evaluate the impact of climate policies, researchers must first develop vegetation models that can quantify and predict the perturbations introduced each year by natural ecosystems. “Thanks to our new results, we can now prove that the effects of droughts are stronger than has so far been estimated by vegetation models,” emphasises Sonia Seneviratne. Ultimately, these observations will be integrated into the next generation of models. They should improve the ability to track CO
2
emissions and verify that they meet the targets set in international climate agreements.
