Grasping at straws...

It may be that "the major carbon repository in Earth is probably the mantle, rather than the atmosphere or biosphere, but it is the least well understood," Walter told OurAmazingPlanet. "The mantle reservoir might affect the global cycle over Earth's history."

From here.   The grasping at straws part comes from the wild hope that the solution to our global warming problem somehow lies in harnessing the mantle.