It is also a fact that the concentrations of several naturally occurring gases have increased in the atmosphere beginning in the 19th century: carbon dioxide by 44 percent, methane (the dominant component of natural gas) by 150 percent, and nitrous oxide (the laughing gas your dentist uses to ease your anxiety) by 21 percent. These are called greenhouse gases because, unlike the nitrogen and oxygen comprising most of the atmosphere, they absorb and emit infrared radiation from the sun, keeping that energy from returning to space and thus warming the lower atmosphere. Scientists first discovered this greenhouse effect in 1824 and by 1896 had predicted the planet would warm as emissions of carbon dioxide grew. So, the global warming we have observed is totally unsurprising. Carbon dioxide concentrations now exceed 400 parts per million, the highest levels in four million years, and the added carbon has the chemical fingerprint of long-buried fossil fuels.