Around the North Pole are tundra areas with permanently frozen soil, which is high in carbon. When thawed, this carbon can convert to greenhouse gases, which in turn can increase global warming. One factor has been underestimated: the increasing role of sunlight.

There are currently about 1,000 billion tons of carbon in the frozen soils of the Arctic tundra. According to current estimates, 5 to 15 percent of this can be converted to CO2 at the end of this century, depending on the level of global warming. This is done by dehydration and oxidation, and by increasing the activity of bacteria in a warmer soil.

But researchers at the University of Michigan say there is a third way it can convert Arctic carbon into CO2: through direct contact with sunlight - using a process they call photomineralization in their journal Geophysical Research Letters .

CO2 emissions 14 percent higher due to erosion and sunlight

This conversion to CO2 can also increase considerably if the permanent ice layer thaws in the tundra soils. These then become much more sensitive to erosion due to the countless rivers that meander freely through the far north of Russia and Canada in the summer months. The rivers take small carbon particles with them and then deposit them in lakes, where in the summer the sun's rays shine on the dark carbon particles.

See also: Arctic ice rapidly deteriorating: Nearly three quarters has disappeared in 40 years

According to the Americans, this process would increase total CO2 emissions from permafrost thawing by 14 percent. In the scenario with the fastest warming of the Arctic, this would result in a further expansion of global warming of around 0.45 degrees.

"Climate models have only recently taken into account the greenhouse gases of thawing permafrost soils," said geochemist Rose Cory, the study's lead author. "But none of them include this feedback."

Old ice bottom thaws along

The researchers arrive at their calculations by illuminating soil samples from different tundra areas under water with light at different wavelengths, in order to simulate the Arctic conditions as closely as possible. They discovered that the direct conversion to CO2 depends, among other things, on the height of the sun and the amount of iron in the soil, which accelerates the conversion.

In addition, they could find out through carbon dating that carbon that has been frozen for thousands of years is currently entering the river and lake waters - and that this is not just a surface process where thawing is a natural seasonal phenomenon.