As Monbiot says in the video, the effect that whales have on the oceans - and on the entire atmosphere - is so powerful that it could even mitigate some of the effects of climate change. By releasing nutrients in their fecal plumes to fertilize plankton and by mixing up the water column to allow plankton to reach the photic zone, whales create what's called a "trophic cascade" throughout the entire ocean ecosystem.
"The return of the great whales, if they're allowed to recover, could be seen as a benign form of geoengineering. It could undo some of the damage we've done, both to the living systems of the sea, and to the atmosphere.
Monbiot produced a similar video last February, called "How Wolves Change Rivers":