It is complicated

Water vapour could be behind warming slowdown : Nature News

This article talks about a new suggestion that a drop in stratospheric water vapour might account for a (relative) levelling out of global temperatures in the last decade:

...a team led by researchers at the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado, report that a mysterious 10% drop in water vapour in the stratosphere — the atmospheric layer that sits 10–50 kilometres above Earth's surface — since 2000 could have offset the expected warming due to greenhouse gases by roughly 25%. Just as intriguingly, their model suggests that an increase in stratospheric water vapour might have boosted earlier warming by about 30% in the 1980s and 1990s. The team's work is published online by Science today¹.

The effect on temperature is dominated by water vapour in the lower part of the stratosphere, which absorbs and radiates heat in much the same way as water molecules and other greenhouse gases do in the lower atmosphere. The drop in water vapour doesn't explain the entire decrease in the rate of warming, but it could contribute to it, says Susan Solomon, first author of the study

It all seems a very tentative idea though:

Other researchers see different factors at play in the recent temperature trends. A study published last year³ hones in on the solar cycle and the El Niño Southern Oscillation, an upwelling of warm surface waters in the tropical Pacific. Both have been in their negative phases for most of the decade so temperatures may rise as they move into their positive phases.

"I think it's exciting that this [transition] is happening, because we are going to learn a lot," says Judith Lean, a solar physicist at the Naval Research Laboratory in Washington DC, who co-authored last year's study³ with David Rind, a climate modeller at NASA's Goddard Institute for Space Studies in New York.

The article seems to indicate that no one knows what water vapour in the stratosphere will do in the future.

Other researchers think current models account for the occasional decade long stall in increasing temperatures and it's not a good idea to worry too much about the issue anyway.

You can bet, however, that skeptics will seize on this paper, with their attitude that if something is not completely understood, you don't do anything about it. Which is, by most scientists reckoning, a good way to gamble on potential long term disaster.