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.