Not to use an overly technical term here, but there’s a neat paper in this week’s Science that explains clearly why carbon dioxide (CO2) is the main agent behind changes in the Earth’s climate—now and in the geologic past. First a bit of background: one argument you might hear from skeptics of manmade climate change is that CO2 is much less important as an atmospheric warming agent than water vapor. Here’s how historical climatologist and skeptic Tim Ball summarizes the case for water vapor:

Water vapour is the most important greenhouse gas. If you get a fall evening and the sky is clear, heat will escape, the temperature will drop. If there’s cloud cover, the heat is trapped by water vapour and the temperature stays warm. If you go to In Salah in southern Algeria, they recorded at noon 52°C. By midnight, it’s -3.6°C. It’s caused because there is very little water vapour in the atmosphere and is a demonstration of water vapour as the most important greenhouse gas.

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.

Lacis and his colleagues note that water vapor, while much more common in the atmosphere than CO2, has a short atmospheric lifespan, eventually condensing and falling as precipitation as it responds to changes in temperature and air pressure. CO2, however, is a well-mixed gas that just builds up in the atmosphere over time, which is part of the reason why carbon dioxide emitted today can have a warming effect that lingers for hundreds of years. The amount of water vapor in the atmosphere is a function of temperature—the warmer it gets, the more water will evaporate and the more water the atmosphere can hold. And water vapor does have a warming effect, and it can act as a positive feedback—the warmer it gets, the more water vapor there is in the atmosphere, which makes it warmer, and so on.

But as the Science authors point out, it’s CO2 that provides the stable temperature structure for the climate, the skeleton on which climate is built. Here’s how they know: Lacis and his colleagues created a simple climate experiment where they removed all CO2, aerosols and other greenhouse gases from the model atmosphere, but left in the water vapor. They let the climate model run forward in time, and the results were startling. In just one year without any carbon, global annual mean temperature fell by 4.6 C. After 50 years, the global average temperature had fallen to -21 C, 34.8 C less than it is today. As the average global temperature fell, so did the water vapor in the atmosphere, while global sea ice cover increased from 4.6% to 46.7%, further increasing the planetary albedo effect and freezing the planet further. Without carbon and other greenhouse gases, we’d be living on Planet Hoth.

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:

The anthropogenic radiative forcings that fuel the growing terrestrial greenhouse effect continue unabated. The continuing high rate of atmospheric CO2 increase is particularly worrisome, because the present CO2 level of 390 ppm is far in excess of the 280 ppm that is more typical for the interglacial maximum, and still the atmospheric CO2 control knob is now being turned faster than at any time in the geological record (20). The concern is that we are well past even the 300- to 350-ppm target level for atmospheric CO2, beyond which dangerous anthropogenic interference in the climate system would exceed the 25% risk tolerance for impending degradation of land and ocean ecosystems, sea-level rise, and inevitable disruption of socioeconomic and foodproducing infrastructure (21, 22). Furthermore, the atmospheric residence time of CO2 is exceedingly long, being measured in thousands of years (23). This makes the reduction and control of atmospheric CO2 a serious and pressing issue, worthy of real-time attention.

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.