With Speed and Violence Fred Pearce - Global Commons Institute

REFLECTING ON WARMING:
WHAT'S WATTS?
Planet Earth's energy imbalance
Jim Hansen knows about the atmosphere from top to bottom. He began his
career as an atmospheric physicist, studying under James van Allen, after
whom the Van Allen Belts of the upper atmosphere are named. He published
papers on the Venusian atmosphere before he moved on to our own. So
when Hansen stops talking about degrees of temperature and starts
counting how many watts of energy reach Earth's atmosphere and how
many leave it, I recognize that we are getting down to the nitty-gritty of what
sets Earth's thermostat.
I know about watts. I have a 6o-watt bulb in the lamp over my desk. At
school almost forty years ago, my physics teacher had a stock line for any
lesson on electricity. "It's the watts what kill," he said, meaning that they are
what matters. When Hansen says the sunlight reaching the surface of Earth
in recent centuries has been about 240 watts for every 10.8 square feet, I can
visualize that. It is four 60-watt bulbs shining on a surface area the size of
my desk. That figure ever changes only slightly, because the sun itself is
largely unchanging. If the sun were to grow stronger, more radiation would
reach Earth, and we would warm up. But only so much. A warmed surface