Water, Water Everywhere but a Little Less in the Stratosphere
by Bill Chameides | February 4th, 2010
posted by Erica Rowell (Editor)
Might thunderstorms have played a role in the temperature trend of the last decade? A new paper suggests yes. (NOAA
The long-term trend in global temperatures is pretty clear — it’s upward. With the exception of the 1950s, every decade of the last 100 years has been warmer, on average, than the preceding one. And despite lots of specious pronouncements that global warming stalled in the decade of the naughties (2000–2009) — which I’ll refer to as the ’00s or the 2000s from here on out — global average temperatures during that time period were about 0.3 degrees Fahrenheit warmer than they were in the ‘90s.
Still, temperature variations during the ’00s are puzzling. If you take a close look at the graphic, you will see that much of the temperature increase between the 2000s and the ‘90s occurred near the transition between the two decades. Temperatures during the 2000s were relatively flat despite the fact that greenhouse gases during the decade continued to increase. (It’s important to note that such behavior is not anomalous — compare the ‘80s to the ‘70s and the ‘90s to the ‘80s.)
The puzzle is this: If greenhouse gas concentrations increased each year during the decade, increasing the radiative forcing that drives global warming, why didn’t temperatures also increase each year?
Sussing Out Short-term Climate Variability from Longer Term Trends
And that brings us into the not particularly well-understood realm of short-term climate variability. The fact is that greenhouse gases are not the only drivers of climate change, and so we should not expect to see a perfect correlation between greenhouse gas concentrations and global temperatures from year to year.
Fine, you might say, but what are those other climate drivers, and was their behavior consistent with the temperature trend in the ’00s? Well, we have discussed two in past posts:
(i) solar variations and
(ii) the El Nino/Southern Oscillation (ENSO).
And it turns out that both of these acted during the ’00s in a way that would slow global warming and thus provide at least a partial explanation for the flat temperatures of the decade.
With regard to solar variations: The Sun reached the maximum of its 11-year cycle around 2000, and its activity was in decline for the rest of the decade.
With regard to ENSO: For much of the later part of the decade, the South Pacific Ocean was stuck in either a mild or negative (la Nina) phase of ENSO the later generally favoring cooler global temperatures.
New Paper Shows Water Vapor as a Significant Driver in Last Decade’s Climate
Last week in a paper published in the journal Science, Susan Solomon of the Earth System Research Laboratory at the National Oceanic and Atmospheric Administration and colleagues provided an additional explanation for the decade’s climate.
The authors noted that water vapor concentrations in the stratosphere were observed to have declined in the beginning of the decade, and because water vapor is a greenhouse gas, this would have had a cooling effect on the climate. (The decline in stratospheric water vapor should not be confused with the trend in water vapor in the lower atmosphere or troposphere, which has been positive because of the link between tropospheric water vapor and atmospheric temperatures.)
Solomon and her colleagues estimate that the decrease in stratospheric water vapor would have canceled out about 25 percent of the warming from the increase in greenhouse gases alone over the decade, thus providing a partial explanation for the flat temperatures of the 2000s.
Of course Solomon et al.’s work begs the question: what caused water vapor concentrations in the stratosphere to decrease? The short answer is that we are not sure.
Water vapor gets into the stratosphere from the breakdown of methane and by large (primarily tropical) storms that extend into the stratosphere. There are no mechanisms that could explain the water vapor change from methane, so more than likely the water vapor change is linked to a change in the intensity of storms.
Some Answers, Yes, but Still Lots of Questions
But what could have caused the change in storms? Was the change a response to climate change (and therefore a climate feedback)? Or was it unrelated to warming (and therefore a manifestation of internal climate variability)? These are good questions but not ones we are in a position to answer yet.
The Solomon et al. paper is an important contribution — it adds to our understanding of why global temperatures fluctuate (or not) from year to year. It also aptly illustrates that all of climate science is most definitely not “settled.” We have an awful lot to learn.
But nothing in the Solomon et al. paper or related works on temperature trends during the past decade undermines the fundamental fact that on decadal time scales global temperatures are on the rise nor does it undermine the multiple lines of evidence linking warming to emissions of greenhouse gases.filed under: climate change, faculty, global warming, temperatures
and: climate, research, solar cycle, solar maximum, solar minimum, solar variation, storms, sunspots, Susan Solomon, water vapor