"When the herds of northern megaherbivores were killed off by humans ten millennia ago, Zimov says, the largest biome on earth, called the “mammoth steppe,” converted from grassland to boreal forest and tundra. In these days of global warming, thawing tundra is releasing greenhouse gases, whereas grassland fixes carbon." [0]

There is a really great documentary about Stewart Brand, We Are as Gods, that also covers this project. [1]

[0]https://e360.yale.edu/features/the_case_for_de-extinction_wh... [1]https://www.weareasgods.film

Like the vast majority of all ice-age megafauna, they could not co-exist with the rapidly expanding human population and were driven from the land that sustained them and/or hunted to extinction.

The basic argument runs like this: reintroduce still living species (and then megafauna), megafauna turn forests into grasslands (see what modern elephants do to trees), grasslands keep snow on the ground longer and contribute to a cooler climate.