The giant mammals help remove about 400,000 tonnes of carbon from the atmosphere a year by releasing large quantities of liquid faeces into the upper layers of the ocean, Australian researchers have calculated.

Trish Lavery, of Flinders University, said whale poo is rich in iron and stimulates the growth of phytoplankton - microscopic plants that soak up carbon dioxide. ''When the phytoplankton die, the trapped carbon sinks to the deep ocean.''

Ms Lavery and her team have estimated that industrial culling of sperm whales has resulted in an extra 2 million tonnes of carbon dioxide a year remaining in the atmosphere. ''It makes a compelling case for an immediate ban on whaling,'' she said.

Whales had previously been accused of having a large carbon footprint because they exhale a lot of carbon dioxide, a greenhouse gas. But the study shows they more than offset these emissions by defecating.

Sperm whales, of which there are an estimated 12,000 in the Southern Ocean, dive deep to consume a diet of squid and fish.

The researchers calculated they release about 50 tonnes of iron near the surface in their faeces, which floats around and fertilises the iron-poor waters, increasing phytoplankton blooms.

Although whales exhale about 200,000 tonnes of carbon a year, the net gain is 200,000 tonnes of carbon locked in the ocean for hundreds or thousands of years.

The sperm whales in the Southern Ocean represent only about three per cent of the global population, and those elsewhere could also make a significant contribution to carbon removal from the atmosphere, the researchers said.

So could other kinds of whales and sea creatures that feed deep in the ocean and poo in the surface layers where light is available for photosynthesis.

''Seals and sealions often consume prey at depth, but whether the waste is liquid and buoyant requires further investigation,'' the researchers said.

Before whaling began there used to be about 10 times as many sperm whales in the Southern Ocean.