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A report in The Sunday Times, citing research by physicist Alex Wissner-Gross of Harvard University, claimed that a typical search on a desktop computer generates about 7g of CO2.

The figure was attributed to the huge energy demands of running Google's worldwide data centres, and the fact that Google sends each request to multiple servers in a bid to complete the search as quickly as possible.

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"Google is very efficient but its primary concern is to make searches fast and that means it has a lot of extra capacity that burns energy," Wissner-Gross claims.

Google says the CO2 figure is "many times" too high. "Google is fast - a typical search returns results in less than 0.2 seconds," the company claims in a blog post.

"Queries vary in degree of difficulty, but for the average query, the servers it touches each work on it for just a few thousandths of a second. Together with other work performed before your search even starts (such as building the search index) this amounts to 0.0003 kWh of energy per search, or 1 kJ."

"In terms of greenhouse gases, one Google search is equivalent to about 0.2 grams of CO2," the company adds. "In the time it takes to do a Google search, your own personal computer will use more energy than Google uses to answer your query."

Google claims it's continuing to invest in improving the energy efficiency of its data centres.