For the first time, scientists have shown a direct link between rising levels of carbon dioxide — or CO2 — in Earth’s atmosphere and an increase in how much solar energy warms the ground. The finding supports a key theory about what’s behind the recent worldwide warming of Earth’s climate. It links a measurable share of that warming to human activities that release CO2. These include the burning of fossil fuels (coal, oil and gas) for heating, power and transportation.
CO2 is known as a greenhouse gas. By that, scientists mean that this gas allows the sun’s visible light to pass through. But when that light hits Earth’s surface, it can be transformed to heat (infrared light). CO2 now traps that heat (like a greenhouse window) and holds much of it within the lower atmosphere — right down to Earth’s surface.
Daniel Feldman is a climate scientist at Lawrence Berkeley National Laboratory. It’s a Department of Energy research center in Berkeley, Calif. He and his colleagues sought to uncover how large the effect of recent increases in CO2 have been on Earth’s near-surface warming. To do that, they monitored the sunlight hitting two sites on cloudless days. One was in Alaska, the other in Oklahoma.
CO2 absorbs some wavelengths of the infrared light now being radiated from Earth’s surface. Then it releases very specific wavelengths of this infrared light. This infrared radiation goes in all directions — including back to Earth’s surface.
Knowing this, the researchers could look at the wavelengths of infrared light and, like a fingerprint, link it to what share was from a CO2 buildup in the air, and what share was due to other things, such as water vapor.
Feldman’s group reviewed more than 10 years of near-daily observations of sunlight and temperature for the two locations. After sifting through these data, the team showed that a rise in CO2 levels of 22 parts per million in air boosted the amount of the sun’s heat on the ground by 0.2 watts per square meter. That’s an increase of about 10 percent.
The researchers say their results agree with predictions of CO2-driven warming created by computer models. Those models have been used to forecast future climate conditions. Feldman’s team reported its findings online February 25. They appear in the journal Nature.