evidence: When hearing the words "greenhouse gas," most people think immediately of carbon dioxide. This is indeed the greenhouse gas that is currently producing the greatest impact on the Earth’s rapidly changing climate. But it is far from the only one making its mark, and for mitigating climate change it’s important to be able to compare the effects of the various gases that contribute to warming the planet. [...] Gases are not the only contributors to the greenhouse effect: Black carbon, otherwise known as soot, as well as some other particulate matter can also play a role. But such materials have even shorter residence times, typically just days or weeks, as they tend to be flushed out of the air by the next rainfall. Which brings us to the biggest greenhouse gas of all: water vapor. There’s no doubt that water vapor is responsible for more greenhouse warming than any other atmospheric constituent. But water vapor’s behavior depends on the climate, so it is not a driver of climate change but rather an amplifying feedback, since the water cycle is a constant part of the atmospheric circulation. As the air gets warmer, it can hold more water vapor, so a warming climate leads to more vapor in the air, providing a feedback effect — and potentially leading to dramatic changes in rainfall patterns. But, water vapor only stays around until the next rainfall. "Water vapor is a slave to the climate system, it’s not a master," Solomon says.