The humpback appeared to arrive just in time to join the Extinction Rebellion protests. And to highlight how these mighty mammals could help us fight climate change

For a few happy days, the appearance of a humpback whale in the Thames suggested that it is not only humans who are intent on an environmental uprising. As the founding emblem of the modern green movements, it seems apt that this leviathan appeared just in time to join this week’s Extinction Rebellion demonstrations.

Not only that, but in what may be the biggest goodwill gesture yet from the largest animals on our benighted planet, the whales may be about to help us out. If we give them the chance. The International Monetary Fund (IMF) has released a report detailing how great whales could sequester billions of tonnes of climate-threatening carbon from the atmosphere.

It is an exquisite and painful irony. In the 20th century, humans removed 3 million great whales from the oceans – their sublime beauty hunted, slaughtered, and processed. It was a terrible loss twice over, since scientists believe that the subtraction of this vast marine biomass may have hastened climate breakdown, not least by removing billions of tonnes of whale faeces that would have fertilised phytoplankton, which in turn would capture carbon released by human activities.

Now the international (if voluntary) moratorium on the hunting of great whales, implemented in 1986, is finally bearing fruit and some populations are recovering. There are even humpbacks off the coast of New York.

So, being human – we bring in the accountants. In its cost-benefit analysis, the IMF reports that if whales are allowed to return to their pre-whaling population estimate of 4 million to 5 million – from slightly more than 1.3 million today – they would capture as much carbon annually as four Amazon forests. Not only that, they make money for us: whale watching, as a relatively benign alternative to whale hunting, is worth $2bn globally, a year. And if that weren’t enough, even dead whales play their part. Each whale carcass, falling slowly to the ocean floor, will store the carbon captured in their carcasses for centuries.

Will these great grey giants be the saving of us? In 1976, as Save the Whales campaigning merchandise flooded popular consciousness, and not long after Greenpeace launched its first anti-whaling campaign by confronting whaling fleets in the north Pacific, the comedians Peter Cook and Dudley Moore asked, in one of the most obscene, and funniest recordings ever made, what have the whales ever done for us? “They’ve produced nothing in the way of literature,” the pair quip. “All they eat is fucking plankton. Can you imagine drifting along in the sea with your mouth open with a lot of plankton going in?”

Well, between them and their plankton, the joke may be on us. Whales are ready to rescue the world. It is an extraordinary notion – given what we have done to them – but perhaps that Thames whale, as lost as it was in our river, offered us a kind of hope.