'This Is a Big Deal': Plants and Soil Reaching Limit of CO2 Absorption

The soil and plants that soak up carbon dioxide in the Earth's atmosphere could be reaching their limit of absorption of human emissions—pushing our planet closer to the brink of no return when it comes to climate change. That's the latest message from top scientists.

Currently, the terrestrial biosphere—which is made up of the Earth's vegetation and land—soaks up around 25 percent of carbon dioxide emissions caused by humans. However, moisture levels in the biosphere impact the amount it can absorb. As we experience more periods of extreme dryness and wetness, these natural sinks are being put at risks, found the authors of a study published in the journal Nature. In turn, the subsequent build-up of carbon dioxide could cause global warming to speed up.

Read more: Greenland's ice sheet reaching 'tipping point,' melting four times faster than 2003

To arrive at their conclusion, the researchers at the Columbia University School of Engineering and Applied Science studied what is known as net biome productivity (NBP). That is defined by the United Nations' Intergovernmental Panel on Climate Change, tasked with combating global warming, as: "the net gain or loss of carbon in an area, equal to the net ecosystem production, minus the carbon lost from events such as a forest being cut down, or setting on fire.

Using results from the Global Land Atmosphere Coupling Experiment-Coupled Model Intercomparison Project (GLACE-CMIP5), which measures the effects of soil moisture on the Earth's climate, the researchers mapped out the impact of possible scenarios, such as flooding or droughts.

Julia Green, a PhD student at the Columbia University School of Engineering and Applied Science and lead author of the study, explained: "We saw that the value of NBP, in this instance a net gain of carbon on the land surface, would actually be almost twice as high if it weren't for these changes (variability and trend) in soil moisture.

"This is a big deal! If soil moisture continues to reduce NBP at the current rate, and the rate of carbon uptake by the land starts to decrease by the middle of this century—as we found in the models—we could potentially see a large increase in the concentration of atmospheric CO2 and a corresponding rise in the effects of global warming and climate change."

She told Newsweek the tipping point for maximum carbon absorption could be reached as soon as 2060. Also, she was shocked to find "just how large the effects of soil moisture are."

"We knew that soil moisture anomalies could effect year to year variability of the Earth's carbon uptake—but we didn't know it had such a large effect on the mean long-term uptake."

Dr. Pierre Gentine, associate professor of earth and environmental engineering at Columbia University who led the study, commented: "Essentially, if there were no droughts and heat waves, if there were not going to be any long-term drying over the next century, then the continents would be able to store almost twice as much carbon as they do now."

"Because soil moisture plays such a large role in the carbon cycle, in the ability of the land to uptake carbon, it's essential that processes related to its representation in models become a top research priority."

Dave Reay, Professor of Carbon Management and Education at the U.K's University of Edinburgh told Newsweek: "This study further confirms what we have long feared. Climate change impacts like drought are already eroding the natural sinks for carbon around the world. Each forest transformed to dry savannah or peat land desiccated by drought means less carbon is locked up and more is left in the atmosphere—enhancing warming yet further."

However, he suggested the study was also limited in some ways as the way global sinks respond to changes is different depending on the region.

"Climate change may increase the frequency and intensity of drought, but more CO2 will also help some plants use water more efficiently," he suggested. "Changes in land use and the deposition of nitrogen will also alter these global carbon sinks at a whole range of scales, and in a wide array of ways—our models still struggle to encompass all of these interactions."

Bob Ward, policy and communications director at the Grantham Research Institute on Climate Change and the Environment at the London School of Economics and Political Science, told Newsweek: "This is a single study reporting the results of models that are attempting to project future changes in soil moisture and the likely consequences, so there are still large uncertainties. But these uncertainties are connected to potentially huge risks, so the study should not be ignored."

He agreed that losing the natural sinks would "could have very serious consequences for the rate of global warming."

Reay went on to stress it would be easy to dismiss the depletion of natural sinks as nothing to do with the actions of individuals.

"In fact, it has everything to do with all of us," he said. "The huge carbon sinks that are global forests and soils have served as a planetary pressure valve for our own burgeoning emissions over the last century. If we fail to cut emissions deep and fast then these vast sinks could falter and increasingly switch from carbon-absorbing friend to big-emitting foe."

This article has been updated with comment from Julia Green.

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Scientists have warned plants and soil slowing climate change may stop absorbing enough C02 to avoid dangerous global warming. Getty Images