The words “climate change” tend to invoke images of heat and storms and smog-filled air. But one of the greatest global warming risks may start in the ground.

A team of 50 scientists throughout the world spent more than 25 years studying soil carbon loss and its connection to climate change. Their work, published on Wednesday in the journal Nature, shows that global warming will lead to the loss of about 120 trillion pounds of carbon by mid-century. The resulting gain in carbon dioxide would be equivalent to adding another industrialized nation the size of the United States to the planet.

Let’s back up. Soil acts as a “sink,” or a place where large stores of carbon reside. When plants grow and die, their carbon is added to the soil. This carbon is converted into carbon dioxide by microorganisms in the soil, which then gets released into the air. That carbon dioxide then gets absorbed by plants, which convert the carbon dioxide into oxygen through photosynthesis.

But with increased temperatures due to global warming, these microorganisms are forced to work overtime, releasing more carbon dioxide than plants can absorb. Scientists estimate that the amount of carbon dioxide released from this carbon cycle alone between now and 2050, assuming a one degree global rise in temperature, would constitute 17 percent more emissions than the exhaust from all vehicles and manufacturing worldwide during that same period.

“Ecosystems are not passive bystanders under climate change,” said John Harte, an ecosystem scientist and co-author of the Nature study. “They are active players and can influence the future climate.”

The Arctic will be hardest hit by this soil carbon loss, the scientists found. The permafrost in that region is a repository for carbon, and the loss of that carbon could lead to accelerated melting of these ice-filled regions.

“This again highlights Arctic permafrost carbon as a significant feedback that will accelerate climate change,” said Ted Schuur, an ecologist who studies terrestrial ecosystems and was not involved in this study.

Schuur said that while human emissions from fossil fuels and deforestation are the main triggers of climate change, this nature-driving carbon feedback is something we can’t ignore.

“People should understand that the natural carbon cycle is much bigger than human emissions, and we have the potential to push it out of its current quasi-equilibrium,” Schuur said. “And, if we do so, it will be harder to slow or stop, even if we curb human carbon emissions.”

Jonathan Sanderman, a scientist studying soil carbon at Woods Hole Research Center and not a part of this study, said that while the soil is part of the problem, it can also be part of the solution.

“Soil carbon has been called a ticking time bomb in the climate change research field due to its vast size and potential vulnerability,” Sanderman said. “There is currently a global effort to help farmers better manage their land for soil carbon sequestration to simultaneously improve food production and mitigate climate change.”

Scientists have been working for years to develop strategies that would remove carbon dioxide from the air and reuse it. One example in the farming industry is known as biochar, which involves heating dead plant matter in an oven with no air. This creates a charcoal that — when added to soil to fuel crop growth — stores carbon dioxide, preventing it from entering the atmosphere.

Sanderman said that we need to pay attention to all of the factors impacting climate change, from the air down to the dirt.

“Climate change isn’t only about protecting trees,” Sanderman said. “The hidden half of the carbon cycle – the soil – is enormously important in regulating the climate.”