Zombie fires - also known as overwintering fires - are a rare phenomenon that occur in countries like Canada, Alaska, and Russia.
In June and July 2019, temperatures around the world were some of the hottest on record, and more than 100 wildfires burned in the Arctic circle.
They emitted a record 244 megatonnes of carbon dioxide, and destroyed millions of hectares of forests across Siberia, Alaska, Greenland, and Canada.
The researchers behind the report are worried that climate change, and the rising of the Earth's temperatures could increase the number of zombie fires in the future.
After a wildfire has been extinguished on the surface, some of it can still burn belowground in secret, fuelled by peat and methane.
These fires can continue to burn all through winter, hidden under a layer of snow, and in spring as the temperature rises, the snow melts and the soil dries out, the wildfires can re-ignite and spread once again.
"With low oxygen levels under the snowpack, overwintering fires smoulder slowly, only to flare up again when the snow melts and dry conditions arrive in the spring." said Rebecca C Scholten, the lead author of the study and a PhD student at the Vrije Universiteit Amsterdam in the Netherlands.