A Google R&D post describes the ecological perspective of Google search: Powering a Google search.
The Google post says: "Tools like email, online books and photos, and video chat all increase productivity while decreasing our reliance on car trips, pulp and paper." And "In terms of greenhouse gases, one Google search is equivalent to about 0.2 grams of CO2."
An important but omitted part of the equation is what people do as a result of running a Google search query.
Some folks may decide to sprinkle a pasture with carbon-friendly windmills, but others will decide to buy that gas-guzzling SUV they've been hankering for, or plan their next trip abroad by jet.
The post also glosses over other Google products such as Google Maps, Find Businesses, and Get Directions. These apps are probably pollution facilitators. Surely some Google Maps users will walk, jog, zip-line, or bicycle to their destinations. However, most users will probably drive a polluting car.
I use Google search quite often, and I am glad that Google is trying to organize the world's knowledge - however I'm not convinced the net impact of Google is green.