Greece’s wildfires are burning through the country’s natural carbon sinks in the latest sign of how heat waves have the potential to undermine a key climate change solution.

More than 100 wildfires in the country have emitted over a million metric tons of carbon dioxide into the atmosphere in July, the most in at least two decades, according to the Copernicus Climate Change Service, which uses the European Union’s satellites to observe the earth.

That’s equivalent to a third of how much Greece’s natural carbon sinks — the bulk of which tend to be forests — usually remove from the atmosphere in a year, according to data from the European Environment Agency.

While the emissions make up a small proportion of Greece’s overall pollution, it’s the latest sign of the dangers posed by increasingly extreme weather. The EU is currently working on a carbon certification scheme, which would help credit carbon removals, but one of the key challenges is how to make sure that the CO2 that is captured isn’t released back into the atmosphere.