A study by Columbia University confirms the urgency of addressing climate change, since the limitation of the earth to absorb the excess CO2 You can accelerate a point of no return.
Although it is known that extreme weather events can affect inter-annual variability in carbon uptake, and some researchers have suggested that there may be longer-term effects, this new study is, according to its authors, the first to quantify the effects to throughout the 21st century and prove that the wetter years than normal do not compensate for the losses in carbon absorption during the drier than normal years, caused by events such as droughts or heat waves.
Anthropogenic CO2 emissions (emissions caused by human activities) raise the concentration of CO2 in the Earth's atmosphere and produce unnatural changes in the planet's climate system. The effects of these emissions on global warming they are only being partially reduced by land and ocean. Currently, the oceanic and terrestrial biosphere (forests, savannahs, etc.) is absorbing around 50 percent of these releases, which explains the bleaching of coral reefs and the acidification of the ocean, as well as the increase of carbon storage in our forests.
"However, it is not clear whether the land can continue to use anthropogenic emissions at current rates," says Pierre Gentine, associate professor of Earth and Environmental Engineering and affiliate of the Earth Institute, who led the study, published in Nature "If the earth reached a maximum rate of carbon consumption, global warming could accelerate, with important consequences for people and the environment. This means that we must all act immediately to avoid further consequences of climate change, "he adds.
Working with his doctoral student Julia Green, Gentine wanted to understand how variability in the hydrological cycle (droughts and floods and long-term drying trends) was affecting the ability of continents to catch some of the CO2 emissions. The research is particularly timely as climate scientists have predicted that extreme events will probably increase in frequency and intensity in the future, some of which we are already witnessing today, and that there will also be a change in rain patterns that will likely affect the ability of the earth's vegetation to capture carbon.
To define the amount of carbon stored in vegetation and soil, Gentine and Green analyzed the net productivity of the biome (NBP), defined by the Intergovernmental Panel on Climate Change as the net gain or loss of carbon. of a region, equal to the net production of the ecosystem minus the carbon lost by the disturbance, such as a forest fire or forest harvest.
The scientists used data from four Earth System models from the GLACE-CMIP5 experiments (global terrestrial atmosphere coupling experiment-inter-comparison model-coupled project) to perform a series of experiments to isolate reductions in NBP that they are strictly due to changes in soil moisture. They were able to isolate the effects of changes in long-term trends of soil moisture (ie, drying), as well as short-term variability (that is, the effects of extreme events such as floods and droughts) on the capacity of the earth to capture carbon.
"We saw that the value of NBP, in this case a net carbon gain at the surface of the earth, would actually be almost double if it were not for these changes (variability and trend) in soil moisture," says Green, Main author of the article. "This is a big problem, if the soil moisture continues to reduce the NBP at the current rate, and the rate of carbon capture by the land begins to decrease by the middle of this century, as we found in the models, potentially we could see a large increase in the concentration of atmospheric CO2 and a corresponding increase in the effects of global warming and climate change. "
Gentine and Green observe that the variability of soil moisture significantly reduces the carbon sink present in the earth, and their results show that both variability and drying trends will reduce it in the future. By quantifying the critical importance of soil-water variability for the terrestrial carbon cycle, and reducing carbon sequestration due to the effects of these changes in soil moisture, the study findings highlight the need to implement improved models. of vegetation response to water stress and earth-atmosphere coupling in terrestrial system models to limit future terrestrial carbon flow and better predict future climate.
"Essentially, if there were no droughts and heat waves, if there was no long-term drying during the next century, the continents could store almost twice as much carbon that now, "says Gentine. Because soil moisture plays such an important role in the carbon cycle, in the ability of the earth to capture carbon, it is essential that processes related to its representation in the models become a research priority. "
There is still much uncertainty about how plants respond to water stress, so Green and Gentine will continue their work to improve the representations of vegetation response to changes in soil moisture. Now they are focusing on the tropics, a region with many unknowns and the largest terrestrial carbon sink, to determine how vegetation activity is controlled as much by the changes in the humidity of the ground as by the atmospheric dryness. These findings will provide guidance to improve the representation of water stress in plants in the tropics.