evidence: Makes sense—and in a sense, he’s right. Water vapor is the most important agent behind the greenhouse effect—it has more than twice the warming effect that CO2 does, and water vapor and clouds account for about 75% of the Earth’s greenhouse effect, with CO2 accounting for 20% and the other greenhouse gases and aerosol particulates accounting for the remaining 5%. But despite that, it is changes in CO2 concentrations in the atmosphere that is responsible for changing the climate—in other words, as the research team led by the physicist Andrew Lacis makes clear in the Science paper, CO2 is the principal "control knob" governing the Earth’s temperature. [...] As the Science authors point out, there’s also ample evidence in our own geologic history that changes in CO2 levels in atmosphere—chiefly due to volcanic eruptions—have been a main driver in changes in Earth’s climate. Water vapor levels may amplify the effect of CO2, but it’s CO2 that is the main control knob—and as Lacis and his colleagues write, we’re turning that knob up to 11 thanks to rising manmade carbon emissions: [...] It’s important to note that while CO2 may be climate’s control knob, the metaphor only goes so far. CO2 isn’t the only knob on the climate controls (alterations in land use and deforestation can change the climate as well), and our understanding of exactly how different levels of atmospheric CO2 concentrations will change the climate in the future is still developing. But CO2 is the main show.