Nature’s Solution to Climate Change, 2019, International Monetary Fund

Basically the argument is that phytoplankton in the oceans is a bigger user of CO2 and releaser of O2 than trees, so if we had more phytoplankton that would help mitgate climate change.

The limiting factor to the amount of phytoplankton is often minerals in the water, and whale excreta is an important source of this, so if we have more whales, we have more phytoplankton, and therefore less CO2 in the atmosphere. Also the whale's body itself is quite a large store of CO2, which, when it dies, sinks to the bottom of the ocean.

So the argument is that to help mitigate climate change, money should be spent on saving the whale.