MAMMOTHS WERE KEY. “When the … megaherbivores were killed off by humans ten millennia ago … the largest biome on earth, called the “mammoth steppe,” converted from grassland to boreal forest and tundra … thawing tundra is releasing greenhouse gases, whereas grassland fixes carbon.”
http://e360.yale.edu/feature/the_case_for_de-extinction_why_we_should_bring_back_the_woolly_mammoth/2721/