California has set a goal of reducing its greenhouse-gas emissions to 1990 levels over the next 13 years, but finding out whether they're reaching that goal could be tricky. So a coalition of agencies and labs, coordinated by the Lawrence Berkeley National Laboratory in Berkeley, is putting up sensors on towers over metropolitan areas around the state. The sensors will report twice daily on the greenhouse-gas concentrations in the air, and will help Californians know if their state's moving in the right direction.

Source: Greenhouse gas sensors tower over California. Nature, October 24, 2007. Photo: the Sutro Tower in San Francisco, location of one of the sensors.