Southern Ocean sperm whales are an unexpected ally in the fight against global warming, removing the equivalent carbon emissions from 40,000 cars each year thanks to their faeces, a study found yesterday.

The cetaceans have been previously fingered as climate culprits because they breathe out carbon dioxide (CO2), the commonest greenhouse gas.

But this is only a part of the picture, according to the paper, published in the British journal Proceedings of the Royal Society B.

In a heroic calculation, Australian biologists estimated that the estimated 12,000 sperm whales in the Southern Ocean each defecate around 50 tonnes of iron into the sea every year after digesting the fish and squid they hunt.

The iron is a terrific food for phytoplankton - marine plants that live near the ocean surface and which suck up CO2 from the atmosphere through photosynthesis.

As a result of faecal fertilisation, the whales remove 400,000 tonnes of carbon each year, twice as much as the 200,000 tonnes of CO2 that they contribute through respiration.

By way of comparison, 200,000 tonnes of CO2 is equal to the emissions of almost 40,000 passenger cars, according to an equation on the website of the US Environmental Protection Agency.