In a joint project backed by the University of Maine, US, and Gulf of Maine Research Institute, Pershing compared whales to redwood trees as a store of carbon. Blue whales contain around nine tons of carbon; in the biological world, only large trees can store more than this. Using this analogy, whaling becomes equivalent to a forest fire, potentially releasing carbon dioxide to the atmosphere or "at the very least, removing it from the marine environment and putting it somewhere else," Pershing told a press conference at the AGU Ocean Sciences Meeting in Portland, US.

Populations of whales, it turns out, are in some ways better at storing carbon than forests. Once a forest has finished growing or regenerating and reached a steady state it tends not to store any additional carbon - each tree that dies and releases its carbon to the atmosphere is replaced by another. But the bodies of whales that die naturally tend to sink to the ocean floor. If the whale dies in a deep area of ocean the carbon from its body could remain locked away for a couple of hundred years.

Whaling, on the other hand, has in the past led to use of the whale's body parts in products, such as lamp oil and animal feed. This probably enabled an earlier release of carbon to the atmosphere. Even if the entire whale was not removed from the ocean, its hacked-up remains were likely to have been consumed by smaller organisms in surface waters, rather than sinking to the depths. This would again have led to quicker recycling of the carbon.

Pershing calculated that one hundred years of whaling have added 105 million tonnes of carbon to the atmosphere. This is equivalent to the carbon released by burning 130,000 sq km of temperate forest or by 128,000 Hummers driving for 100 years. To perform the sums, Pershing assumed that all the carbon in a whale killed by humans is released to the atmosphere.

Although the carbon released by whaling is just a small fraction of the 7 billion tonnes that man emitted in 2005, Pershing says it is comparable to the amount that would be saved by other proposed carbon management and storage schemes. "Whales were the oil of the 1700s and 1800s," he added.

Not everyone agrees that killing whales has affected the level of carbon dioxide in the Earth's atmosphere. "It is obvious that the dramatic decrease in the whale populations has greatly altered the world's ocean ecosystem. However, I don't think that whale hunting has had any major impact on the level of carbon dioxide in the Earth's atmosphere," said Vasily Spiridonov, a consultant of the Marine Program of the World Wildlife Fund (WWF).

Having compared whales to trees, Pershing also introduced poodles to illustrate that larger animals are more efficient at storing carbon. Zelda, his wife's 6 pound toy poodle, eats one cup of food a day. His standard poodle Padawan, meanwhile, which is ten times heavier than Zelda, eats just five cups of food per day. So Padawan is using relatively less carbon, in the form of food, to store the carbon already in his body.

Because of this greater efficiency of larger animals, Pershing says his findings provide an incentive to focus on conserving large species of marine organisms, such as whales, sharks and tuna. Populations of some of these animals and fish are down by 90% or more.

For the same reason it makes sense to focus on large individuals of each species. In the case of fish, large individuals also tend to produce more eggs, so this approach should increase the population's recovery rate.