Cooperative Institute for Research in Environmental Sciences

Water vapor in the upper atmosphere amplifies global warming, says new study

Water vapor in the upper atmosphere amplifies global warming, says new study

A new study shows that water vapor high in the sky and the temperature at the Earth‘s surface are linked in a “feedback loop” that further warms our climate. Published today, this study gives the first estimate of the size of the feedback‘s effect, which may help researchers improve modeling to better understand climate change.

“Water vapor in the stratosphere increases in tandem with increases in the Earth‘s surface temperature,” said coauthor Sean Davis, a scientist with the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder, who works at the NOAA Earth System Research Laboratory. “Because water vapor is a greenhouse gas, this generates additional warming. We show that this feedback loop could be about 10% of the climate warming from all greenhouse gases.”

The new study, published online on September 30 in the prestigious journal Proceedings of the National Academy of Sciences, quantifies the magnitude of the stratospheric water vapor feedback for the first time, making use of satellite observations and a climate model.

“While it‘s not really surprising that this process is going on, we were surprised at how important the process is for our climate system,” said Andrew Dessler, an atmospheric sciences professor at Texas A&M University who was lead author of the paper. Dessler was a CIRES Visiting Fellow this year, working with Davis and other colleagues on this paper.

For well over 100 years it has been known that increased emissions of greenhouse gases such as carbon dioxide will warm the planet. As the lowest layer of the atmosphere, called the troposphere (surface to ~7 miles), is warmed, the air becomes more humid because warmer air holds more water vapor. This “tropospheric water vapor feedback” approximately doubles the initial warming caused by carbon dioxide.

The new study shows that in addition to the well-understood tropospheric water vapor feedback on climate change, there is also a significant amplifying feedback associated with water vapor in the stratosphere, the layer of the atmosphere above the troposphere that extends to ~30 miles above Earth‘s surface. This “stratospheric water vapor feedback,” although hypothesized by previous studies, has remained elusive to quantification.

The new results suggest that the stratospheric water vapor feedback may be an important component of our climate system. The researchers estimated that at a minimum this feedback adds another ~5-10% to the climate warming from the addition of greenhouse gases, and is possibly substantially more than this amount.

Most climate models contain a representation of stratospheric water vapor, so this feedback is already operating in the models to some extent. Thus, this new finding does not necessarily mean that models have underestimated future global warming. However, since the importance of this feedback has not been previously recognized, it is possible that the stratospheric water vapor feedback may help to explain some of the spread among future projections of climate change from different models. Indeed, of the ~20 models participating in the 5th Assessment report of the Intergovernmental Panel on Climate Change (IPCC), the authors found substantial differences among the models‘ future simulation of stratospheric water vapor.

Though the study has moved understanding an important step forward, many questions remain about the role of stratospheric water vapor in climate.

“The stratospheric water vapor feedback effect could be even larger than the 5-10% we found in our study,” said Davis. “Our analysis suggests that the pathways for water vapor to reach the stratosphere are not completely understood, so we view our numbers as a minimum estimate of the effect of this feedback.”

The authors of the study are Andrew Dessler (lead author) and Tao Wang (Texas A&M University); Mark Schoeberl (Science and Technology Corporation); Sean Davis (CIRES and NOAA-ESRL); and Karen Rosenlof (NOAA-ESRL).

CIRES is a joint institute of the National Oceanic and Atmospheric Administration (NOAA) and the University of Colorado Boulder.

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