Researchers at ETH (Switzerland), the Laboratory of Environmental and Climate Science (LSCE: CEA/CNRS/UVSQ, Paris-Saclay) and the University of Exeter (United Kingdom) have shown that the atmospheric concentration of carbon gas increased more rapidly during dry years, because ecosystems suffering from water stress absorb less carbon dioxide. Their results help explain why the increase in atmospheric CO2 can vary so much from one year to the next, even if emissions related to human activities remain relatively stable.
This effect needs to be factored into global climate models in the future.
Terrestrial ecosystems absorb an average 30% of anthropogenic CO2 emissions, a phenomenon which moderates increasing concentrations of this greenhouse gas in the atmosphere. However, plants need water to grow. During drought conditions, plants slow down their metabolism to protect themselves. That means that their uptake of atmospheric CO2 is lower: their role as a 'carbon sink' is thus diminished.
In their study, published in Nature , the researchers at ETH Zurich, the LSCE and the University of Exeter employed an innovative approach to measure, by satellite, the overall sensitivity of Earth's ecosystems to water stress.
The latest satellites can measure extremely slight variations in the Earth's gravitational field, including those caused by variations in the quantity of water stocked on the continents.
In a predominantly dry year, such as 2015 (at global scale), the uptake of CO2 by natural ecosystems was around 30% lower than in a normal year. In 2015, this caused the atmospheric concentration of CO2 to increase at a faster rate. 2011, on the other hand, was generally a very wet year; vegetation growth caused a slower increase in atmospheric CO2.
Severe droughts in the Northern hemisphere cause significant reductions in crop yields, reduced carbon uptake by the forests, and accelerated increases in atmospheric concentrations of CO2. Plants respond to drought by partially closing their stomatal pores (openings in plants used in gas exchange), to minimize water loss through evaporation, to the detriment of carbon uptake through photosynthesis. This maximizes the efficient use of water, as confirmed by the laboratory measurements and field experiments published in Nature Geoscience by an international team which includes researchers at the LSCE .
To quantify this phenomenon, the 13C/12C ratio of stable carbon isotopes was measured across millions of square kilometers and over a period of around ten years affected by recent climate change. The researchers found the enhancement of water use efficiency and the reduction in atmospheric carbon uptake in the Northern hemisphere during the droughts that hit Europe, Russia and the USA in 2001–2011 to be consistent both spatially and temporally.
Such droughts have a much greater impact than that predicted by the six most powerful models currently used in the field. That suggests that these models underestimate feedback between climate and the carbon cycle caused by drought: they need to take the plants' response to water stress into account more effectively, primarily using measurements of stable carbon isotopes.
The mass of carbon stored on continental land surfaces (in other words, the terrestrial carbon sink) increased during the period 1998-2012, a period of slow climate warming. Why this value tripled in comparison to the preceding period (1980–1998) remains unclear. This enhancement of the carbon sink cannot be explained solely by either fertilization associated with the increase in atmospheric CO2, or by climate change.
In a study in Nature Geoscience , an international team including researchers from the LSCE used modeling to show that changes in land use are the main cause of this phenomenon. It is explained by increased reforestation in temperate regions in the Northern hemisphere, together with decreased tropical forest area loss.
Estimates based on reverse modeling of atmospheric data support this scenario. However, another model fails to reproduce this increase in the carbon sink, probably because it does not factor in decreased tropical deforestation.
These studies show how important it is to improve the quantification of changes in carbon emissions due to land use in order to better understand recent terrestrial carbon sink trends.
During droughts, plants suffer water stress and absorb less carbon dioxide. The measured global effect is greater than previous estimates would suggest. © Pixabay
 Sensitivity of atmospheric CO2 growth rate to observed changes in terrestrial water storage. Nature, 08/30/2018.
 Increased water-use efficiency and reduced CO2 uptake by plants during droughts at a continental scale. Nature Geoscience, 08/27/2018.
 Lower land-use emissions responsible for increased net land carbon sink during the slow warming period, Nature Geoscience, 08/20/2018: https://www.nature.com/articles/s41561-018-0204-7