A mysterious drop in water vapor in Earth’s stratosphere has likely contributed to the recent slowdown in increasing global surface temperatures, according to new research in Science.
Water vapor is a potent greenhouse gas—it absorbs sunlight and re-emits heat into Earth’s atmosphere. The findings reveal that stratospheric water vapor has been an important driver of global climate change over the past decade.
“Stratospheric water vapor has played a significant role in the ups and downs in the rate of global warming of the past few decades,” said Susan Solomon, Senior Scientist at the National Oceanic and Atmospheric Administration and lead author of the study. “It’s amazing how narrow the layer is that is doing this.”
Using a combination of data and models, Solomon and colleagues show that a decline in the concentration of water vapor in the stratosphere around the year 2000 had an effect on global average surface temperatures roughly between the years 2000 and 2009.
In particular, lower stratospheric water vapor has probably been a significant factor in the flattening of global average temperatures since 2000, acting to slow the rate of warming by about 25%, the researchers report.
Moreover, the authors show that the amount of water vapor in the stratosphere probably increased between 1980 and 2000, a period of more rapid global warming. Although it remains unclear why water vapor levels have recently decreased, the results of the study signal how important the concentration of stratospheric water vapor might be to Earth’s climate.
“This doesn’t change the view that the world has warmed over the last 100 years or so, but it does help us understand why it’s not warming quite as fast in the past decade as it did in the decade before that,” said Solomon.
One of the next steps in research on stratospheric water vapor will include collecting more observations and targeting key regions.
“It’s an exciting issue for modeling too—climate modeling has been focused on the need for high horizontal resolution, to get the climate rate not just averaged over a continent but in a particular spot,” Solomon said. “This finding tells us that we also have to worry about resolution as the atmosphere goes up.”