Climatologist James Hansen of the NASA Goddard Institute for Space Studies warns that atmospheric carbon dioxide levels above 350 parts per million—lower than current readings—may trigger catastrophic rises in sea level. Image: FRED CONRAD New York Times/Redux Pictures (Hansen)
The basic proposition behind the science of climate change is so firmly rooted in the laws of physics that no reasonable person can dispute it. All other things being equal, adding carbon dioxide (CO2) to the atmosphere—by, for example, burning millions of tons of oil, coal and natural gas—will make it warm up. That, as the Nobel Prize–winning chemist Svante Arrhenius first explained in 1896, is because CO2 is relatively transparent to visible light from the sun, which heats the planet during the day. But it is relatively opaque to infrared, which the earth tries to reradiate back into space at night. If the planet were a featureless, monochromatic billiard ball without mountains, oceans, vegetation and polar ice caps, a steadily rising concentration of CO2 would mean a steadily warming earth. Period.
But the earth is not a billiard ball. It is an extraordinarily complex, messy geophysical system with dozens of variables, most of which change in response to one another. Oceans absorb vast amounts of heat, slowing the warm-up of the atmosphere, yet they also absorb excess CO2. Vegetation soaks up CO2 as well but eventually rereleases the gas as plants rot or burn—or, in a much longer-term scenario—drift to the bottom of the ocean to form sedimentary rock such as limestone. Warmer temperatures drive more evaporation from the oceans; the water vapor itself is a heat-trapping gas, whereas the clouds it forms block some of the sun’s warming rays. Volcanoes belch CO2, but they also spew particulates that diffuse the sun’s rays. And that’s just a partial list.