Rapid industrialisation, population growth and air travel expansion have all been blamed for contributing to global warming and climate change.
But now it appears we have something else to hold responsible.
Squirrels. Yup, squirrels.
American scientists believe the impact of wildlife upon the release of carbon from the permafrost that occupies around a quarter of the northern hemisphere has been underestimated.
Dr Sue Natali, from Woods Hole Research Center in Massachusetts, and Nigel Golden, from the University of Wisconsin, spent eight days in the Kolyma River watershed in north-east Siberia, Russia, studying the burrows of arctic ground squirrels.
They found that when squirrels made their burrows in the permafrost they mixed soil layers, increased aeration, moisture and temperature, as well as redistributing soil nutrients - all of which could contribute to an increased thawing of the permafrost and release of organic carbon.
As the climate warms and permafrost thaws, the fate of this large [organic carbon] pool will be driven not only by climatic conditions, but also by ecosystem changes brought about by arctic animal populations.