As the Earth's surface and lower atmosphere continue to warm, another part of the planet's atmosphere is doing the opposite. Far above the ground, the upper atmosphere has been cooling significantly for decades. Scientists have long recognized this unusual contrast as one of the clearest signals of human driven climate change, but the exact physics behind it remained uncertain. Now, researchers at Columbia University say they have finally uncovered the mechanism responsible. Their new study explains how carbon dioxide (CO 2 ) interacts with different wavelengths of light in ways that cool the upper atmosphere while warming the planet below. "It explains a phenomenon that's a fingerprint of climate change, has been known to occur for decades, and has not been understood," says Robert Pincus, a research professor of ocean and climate physics at Lamont-Doherty Earth Observatory, which is part of the Columbia Climate School, and co-author of the study published in Nature Geoscience .…