Describing his research in the Sunday Times, the Harvard academic said that despite Google's secrecy over its carbon footprint, he had calculated on the basis of publicly available information that each Google search generates an estimated 5-10 grams of CO2.

The newspaper reported that two Google searches releasing 7g of CO2 were almost the equivalent of boiling a kettle for a cup of tea. Wissner-Gross later said his work was focused on the web overall, and the example of tea kettles was not one of his.

The scale of the search engine's carbon footprint is due to the fact that the request is sent to multiple servers across the world, but only the quickest response reaches the user, according to Wissner-Gross.

"Google isn't any worse than any other data centre operator. If you want to supply a really great and fast result, then it's going to take extra energy to do so," he told the BBC.