Plain old H2O: the most abundant and powerful greenhouse gas in the Earth’s atmosphere. Water vapour accounts for around half the present-day greenhouse effect and without it our planet would probably be frozen and lifeless. But should we worry about adding more, given that temperatures are rising? A new study reveals that as long as our water vapour emissions remain close to Earth’s surface, there’s no need to fret.
Most water vapour ends up in the atmosphere naturally, via evaporation from the oceans, but human activities such as irrigation, power plant cooling and flying contribute too. Anthropogenic emissions of water vapour are small compared to ocean evaporation; they’re generally assumed not to be a significant climate forcing agent. Anthropogenic water vapour emissions do, however, make up a sizeable portion of our greenhouse gas emissions. Could they be responsible for more climate change than we thought?
To find out, Steven Sherwood from the University of New South Wales, Australia, used the CAM5 global atmospheric model to estimate the global warming potential and radiative forcing associated with water vapour emissions.
The largest source of anthropogenic water vapour emissions is currently irrigation. Assuming that this source remains fairly constant over the next century, Sherwood and colleagues show that its greenhouse warming potential is between –0.001 and +0.0005 and its effective radiative forcing is between –0.1 and +0.05 W/sq. m.
“This makes emitted water, at best, a thousand times less effective per kilogram at altering the heat budget of the Earth than emitted carbon dioxide,” write the scientists in Environmental Research Letters (ERL).
The model also showed top-of-atmosphere cooling, rather than warming, mostly because the added water vapour rained out before reaching altitudes where it could contribute significantly to the greenhouse effect. The researchers found that if anything, because water vapour is emitted at low altitudes by irrigation, it was more likely to increase low-level cloud cover, which tends to have a cooling effect.
But these water vapour emissions can’t combat global warming to any great extent. “We found it was only enough to offset a few percent of the warming effect by carbon dioxide,” says Sherwood.
Sherwood and the team stress that their results are very sensitive to the altitude at which water vapour is emitted, and don’t apply to aircraft water vapour emissions – these have a much higher global warming potential.
“If all our irrigation water went into the altitudes where aircraft fly, it would probably have a pretty big warming effect, but the actual amounts from aircraft would not be enough to have much effect unless air travel increased by an order of magnitude or more,” says Sherwood.
For now, we can rest easy about water vapour emissions from irrigation. “It is interesting that water vapour emissions appear to have slightly cooled the planet even though the vapour is a greenhouse gas,” says Sherwood. Even if our emissions at low altitude increase significantly we can still expect the impact on warming to be negligible, the researchers say.