BBC has a post on the Carbon Cost of Google Search.
'Carbon cost' of Google revealed
The research found a google search produced 7g of carbon dioxide
Two search requests on the internet website Google produce "as much carbon dioxide as boiling a kettle", according to a Harvard University academic.
US physicist Alex Wissner-Gross claims that a typical Google search on a desktop computer produces about 7g CO2.
However, these figures were disputed by Google, who say a typical search produced only 0.2g of carbon dioxide.
A recent study by American research firm Gartner suggested that IT now causes two percent of global emissions.
Dr Wissner-Gross's study claims that two Google searches on a desktop computer produces 14g of CO2, which is the roughly the equivalent of boiling an electric kettle.
Google’s Urs Hoelzle has a response.
Recently, though, others have used much higher estimates, claiming that a typical search uses "half the energy as boiling a kettle of water" and produces 7 grams of CO2. We thought it would be helpful to explain why this number is *many* times too high. Google is fast — a typical search returns results in less than 0.2 seconds. 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. For comparison, the average adult needs about 8000 kJ a day of energy from food, so a Google search uses just about the same amount of energy that your body burns in ten seconds.
In terms of greenhouse gases, one Google search is equivalent to about 0.2 grams of CO2. The current EU standard for tailpipe emissions calls for 140 grams of CO2 per kilometer driven, but most cars don't reach that level yet. Thus, the average car driven for one kilometer (0.6 miles for those in the U.S.) produces as many greenhouse gases as a thousand Google searches.
We've made great strides to reduce the energy used by our data centers, but we still want clean and affordable sources of electricity for the power that we do use. In 2008 our philanthropic arm, Google.org, invested $45 million in breakthrough clean energy technologies. And last summer, as part of our Renewable Energy Cheaper than Coal initiative (RE<C), we created an internal engineering group dedicated to exploring clean energy.