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Why has global warming paused? Water vapor may be in the answer.

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Still, he and others agree, the new results indicate that stratospheric water vapor, especially in the lowest regions of the stratosphere, can have a significant impact on global average temperature trends when viewed in decade-long time frames.

Water vapor is the most abundant greenhouse gas in the atmosphere. By some estimates, it accounts for anywhere from 36 percent to 85 percent of the atmosphere's greenhouse effect, depending on whether clouds are included.

But the vast majority of that water vapor resides in the troposphere, which is the layer below the stratosphere. This is where the most of the day-to-day weather – and over long periods, climate – activity takes place.

Despite claims in some circles that global warming is over, the past decade was the warmest on record globally, according to records compiled by the GISS. Four of the 10 years were in a statistical tie for second place for the distinction of warmest year on record.

Researchers have shown that even in the context of global warming, the climate will still display natural variations that can lead to decades sprinkled throughout a century that display little or no warming, or even cooling.

Scientists have also been aware that water vapor in the stratosphere, particularly in its lowest reaches, can affect temperatures up there as well as at Earth’s surface.

But the team's work, which appears in this week's Science Express, an online publication affiliated with the journal Science, appears to be the first time anyone has looked at that mechanism in the context of the past decade's slowdown in warming, says Sam Levis, a scientist with the National Center for Atmospheric Research in Boulder, Colo.

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