Humidity holds the heat

Water vapour will remain the planet's chief warming agent, even if atmospheric CO₂ doubles.

Credit: iStockphoto/Thaddeus Robertson

When it comes to the greenhouse effect, humidity, it seems, is the main source of misery. A study that estimates the relative contributions of Earth's atmospheric constituents suggests that half of natural planetary warming is caused by water vapour.

Researchers led by Gavin Schmidt of NASA's Goddard Institute for Space Studies in New York evaluated the contributions of clouds and greenhouse gases to global heat-trapping at the present time and with doubled atmospheric CO₂ (ref 1). They found that CO₂ — the most recognized contributor to climate change — accounts for just 19 percent of the heat trapped by the atmosphere's constituents. Clouds contribute about 25 percent and other heat absorbers such as soot and trace gases together account for around 7 percent or so.

If atmospheric concentrations of CO₂ were to double, the balance of greenhouse contributions wouldn't change much, the analyses show. So, adding even a small amount of man-made CO₂ would cause the amount of water vapour to increase as well, magnifying the effect of CO₂ on climate change.