I have been curious about the carbon footprint of a Google search for a while, and the Google Blog comes through with the information:
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.The post is in response to a Times article that stated incorrectly that a search had a footprint of 7g. Both Wired Science and TechCrunch gave their take as well.
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.
That means a typical individual's Google use for an entire year would produce about the same amount of CO2 as just a single load of washing.
While a Google search seems like an ephemeral thing with no impact on the environment, in fact they require many servers which require energy both to build and operate them. It has been estimated that computer servers use 1.2% of total US electricity.
But, the overall impact of a Google search is very small. It is likely that the impact of running your personal PC for the 10 seconds that it takes to review the results is greater than that of the Google servers. And, if the information gleamed from the search saves you a car trip than it more than pays for itself in saved energy. If you are trying to reduce your carbon footprint, there are definitely better things to do than to try and reduce the number of Google searches you make.