Humans Are Producing Too Much Carbon Dioxide for Forests to Absorb

A recent study shows that, while intact forests are playing a large role in absorbing CO2, it's only a fraction of the amount human activity creates.
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Steam and exhaust rise from different companies on a cold winter day on January 6th, 2017, in Oberhausen, Germany.

Steam and exhaust rise from different companies on a cold winter day on January 6th, 2017, in Oberhausen, Germany.

Forests around the world are absorbing more carbon dioxide from the atmosphere, but they still can't keep up with the sheer volume of the global-warming gas being emitted through human activity, a new study has found.

"Intact forests are playing a large role in absorbing the CO2 we're emitting," says Benjamin Gaubert, the lead author of the study and a scientist with the National Center for Atmospheric Research. "This means that global forest are helping to mitigate climate change or at least helping to mitigate the impacts of carbon emissions in the atmosphere."

The study, published in the journal Biogeosciences, suggests forest growth is becoming more robust as atmospheric carbon concentrations increase, and therefore taking more CO2 out of the air. Nevertheless, the concentration of heat-trapping carbon in the atmosphere continues to intensify as the forests can only capture a fraction of total human-caused emissions, which in 2018 totaled 37.1 billion tonnes.

The study looked at models of atmospheric inversion from research institutions across the world. It combined these with surface observations to estimate carbon fluctuations over northern and southern forests and verified using aircraft observations. The study relied on supercomputers to run simulations of the climate models.

The scientists started with data on atmospheric CO2 levels measured all over the world since 1985. From there, they implemented the models to predict how much CO2 had been sourced and sunk in different regions of the world to make sure they matched the measurements.

While northern forests, which cover a greater landmass, account for the majority of the CO2 absorption, the study suggests that tropical forests in the global south are most effective, per given area, at trapping carbon.

"We found that carbon movements in the tropics were nearly neutral," Gaubert says. "That was interesting for us because we know that rapid deforestation and urbanization would lead one to believe there would be more carbon outflows if [the forests] weren't sequestering carbon faster than before."

Gaubert says forests in the tropics were likely more effective at capturing carbon than northern forests because of favorable growing conditions, such as year-long sunlight and more rainfall. According to Gaubert, around 30 percent of annual carbon emissions are captured by global net forest growth.

"Both the southern tropics and the northern temperate forests are taking up more carbon than in the past, but the amount being taken up by intact tropical forests was a particularly interesting revelation for the team," he says.

The Path Ahead

While the study suggests global forests are growing more vigorously as a result of higher concentrations of CO2 in the atmosphere, Gaubert says "many uncertainties" remain about the effects of climate change on global forests and their ability to sequester carbon in the long term.

In a study published January in the journal Nature, researchers found that, rather than absorbing more greenhouse gas emissions, plants and soils may start absorbing less when the climate heats up past a certain point.

"There [is] a large spread in the models and great deal of uncertainty remains," Gaubert says. "This study adds to the scientific debate over what effects of increasing CO2 concentrations in the atmosphere will have on forests and their ability act as carbon sinks or sources."

The study was funded by the National Science Foundation, NASA, the European Union's Copernicus Atmosphere Monitoring Service, the Japanese Environment Ministry's Environment Research and Technology Development Fund, Environment Climate Change Canada, and the Canadian Space Agency.

According to an analysis by Global Forest Watch, tropical forest loss currently accounts for 8 percent of the world's annual CO2 emissions. In comparison to the world's largest emitters, tropical deforestation would the third-largest contributor to global warming if it were considered a country, outpacing the entire E.U.

Since the 2015 Paris Agreement, forest-related emissions have continued to increase as forests are devastated by wildfires—increasingly exacerbated by a changing climate—or cleared to make way for agriculture and pasturelands.

Study co-author Britton Stephens says the findings should increase concern for conserving what's left of the world's forests, especially in the tropics. "The forests we aren't cutting down in the tropics are taking up a lot of carbon," he says.

The loss of tropical forests also poses a serious threat to global biodiversity. While they cover less than 2 percent of the world's surface, tropical forests are home to an estimated 50 percent of terrestrial life.

This story originally appeared at the website of global conservation news service Mongabay.com. Get updates on their stories delivered to your inbox, or follow @Mongabay on Facebook, Instagram, or Twitter.

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