For the first time, scientists have witnessed a direct connection between rising levels of atmospheric carbon dioxide and an increase in the amount of thermal radiation striking Earth’s surface. The work affirms a cornerstone of the theory that humans have contributed to worldwide warming in recent decades, the researchers report online February 25 in Nature.
Carbon dioxide, like other greenhouse gases, can absorb and reradiate infrared light back down to Earth. This process traps thermal energy around the planet that would otherwise escape into space. To uncover how large an effect recent CO2 increases have had on Earth’s energy balance, climate scientist Daniel Feldman of the Lawrence Berkeley National Laboratory in California and colleagues monitored the amount of thermal radiation hitting two sites in Alaska and Oklahoma on cloudless days. Because CO2 emits light within a signature range of wavelengths, the researchers could differentiate between energy balance changes caused by CO2 and those caused by other factors, such as water vapor.
Over 10 years of near-daily observations, the team found that a rise in CO2 concentrations of 22 parts per million boosted the amount of incoming thermal radiation by 0.2 watts per square meter, an increase of about 10 percent. The researchers say their results agree with the theoretical predictions of CO2-driven warming used in simulations of future climate.