Recent claims estimates that a typical search uses "half the energy as boiling a kettle of water" and produces 7 grams of CO2. Urs Hölzle explains over at Google blog about the energy efficiecy and CO2 emissions of Google Search and Google Data center:

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.[…]

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