Does a new study lend support to Richard Lindzen's "iris effect" hypothesis?
The fact that carbon dioxide (CO2) acts as a greenhouse gas to warm the atmosphere is a hard-and-fast scientific fact borne of 100-plus years of research, discovery, and confirmation. How much the planet will warm from a given increase in CO2 is not as well established. It depends on something referred to as the climate sensitivity -- the number of degrees the atmosphere will warm from a doubling of CO2. A wide variety of analyses, including studies of past climate changes, indicate that the climate sensitivity lies somewhere between 3.6 and 8 degrees Fahrenheit (or 2 and 4.5 degrees Celsius).
If the climate sensitivity ends up falling at the high end, we would have major problems -- even modest increases in CO2 would lead to large temperature increases. And that would mean we have little (or maybe even no) time to begin to lower greenhouse gases to prevent disruptive climate change.
But if we're lucky, and the climate sensitivity is at the low end, temperature increases in the coming decades would be far more modest, and we'd therefore have more time to get our act together to lower greenhouse gas emissions. Note: This doesn't mean we can go on emitting CO2 forever; it just means the atmosphere can withstand more CO2 before things get out of hand.
(For more discussion of the relationship between climate sensitivity and the time we have to act on emissions, you might check out the National Academies report on America's Climate Choices summed up in this post.)
Feedbacks Feed Uncertainty
So which is it? A high sensitivity or a low one? Despite a plethora of published investigations using a variety of methods to estimate the climate sensitivity, there is not yet a definitive answer. The reason: we don't yet have a good enough handle on feedbacks -- processes that act to either enhance the initial warming from CO2 (a positive feedback) or dampen (or oppose) the warming (a negative feedback).
- An increase in CO2 increases temperature;
- An increase in temperature increases water vapor (which, like CO2, is a greenhouse gas);
- An increase in water vapor increases temperature;
- Go back to #2.
Eyes on the Iris Effect
Dick Lindzen, from the Massachusetts Institute of Technology, has been arguing that the climate sensitivity is low because of a negative feedback he calls the iris effect. Why iris? Because the feedback works like an eye's iris that expands and contracts (opening and closing the pupil) in response to changes in the amount of light. In Lindzen's climate iris effect, sea-surface temperatures take the place of light, and high-altitude cirrus clouds are like the iris. It works something like this:
- As sea-surface temperatures increase (from global warming), vertical motion in the atmosphere is strengthened and this causes the high-altitude clouds in the tropics (and more specifically anvil cirrus) to dissipate.
- The decrease in cirrus clouds allows more infrared radiation to escape to space, cooling the atmosphere.
The net result is a negative feedback that dampens the warming.
A recent paper appearing in the journal Geophysical Research Letters by Roger Davies and Matthew Molloy of the University of Auckland, New Zealand, seems to provide support for the first step of a feedback akin to Lindzen's iris effect. The authors analyzed 10 years of data from 2000 to 2010 from the Multi-angle Imaging SpectroRadiometer (MISR) instrument on NASA's Terra satellite, and found that the average global cloud height declined by about 30 to 40 meters over the decade. They argue that the changes in cloud height are "affected more by changes in high cloud fraction than by equivalent changes in low cloud fraction" -- the implication being that the observed decline is mostly due to a loss of high-altitude clouds as in Lindzen's iris effect.*
Intriguing to be sure, but before we conclude that Lindzen is indeed right, there are issues.
- Lindzen's negative feedback requires that the observed decrease in cloud height be a response to increases in sea-surface temperatures. However, this is not at all clear. The existence of strong La Niña conditions during the 10-year period appears to be a significant factor in the cloud-height trend. Was there another factor such as sea-surface temperatures at play as well? Davies and Molloy argue that the 10 years of observation are simply too short to tell, although they do say they may be indicative of a "measure of long-term [negative] cloud feedback." But the data that Davies and Molloy present seem to indicate just the opposite. In their Figure 3 they show a fairly consistent positive correlation between surface temperatures and cloud-height anomalies -- exactly the opposite of what is posited for the iris effect. This raises the distinct possibility that the trend Davies and Molloy observed has nothing to do with a negative feedback in the climate system.
- The analysis does not address the second step of the feedback process: that significant cooling resulting from the loss of high-altitude cirrus. And there are reasons to question that contention. Qiang Fu, Marcia Baker and Dennis L. Hartmann (all of the University of Washington) have argued [pdf] that while reductions in cirrus clouds lead to cooling, it is much weaker than estimated by Lindzen. And Bing Lin (of NASA's Langley Research Center) et al argue that decreases in high-altitude cirrus will actually lead to a warming rather than a cooling -- making the process a positive feedback instead of a negative one.
Good News On Our Doorstep?
Davies and Molloy could be the bearers of good news -- MISR is set to gather measurements through 2020. If the trend is sustained and can be shown to be driven by the effect of rising surface temperatures, and, if decreasing high cirrus clouds cause cooling (and do note the "ifs"), there is an increased chance that climate sensitivity to rising CO2 is on the low end of the uncertainty range and that we have more time to reverse course and bring down global emissions of greenhouse gases.
Some people will no doubt use this new result to argue that we can now just forget about the problem entirely. Unfortunately, that is not an appropriate inference. For one, we shouldn't count our falling clouds before they are confirmed. And, even if confirmed, more CO2 will still lead to more warming, and it will also make ocean acidification more severe. This is a problem that simply will not go away. The clouds may be falling, but there are still some big global-warming storm clouds building on the horizon. They're headed our way, we just don't know how fast.
* Davies and Molloy note that "the main limitations of the technique are: a sampling time at only 10:30 am local time; the omission of thin clouds (cirrus with optical depth less than ≈0.3); and the omission of very homogeneous cloud (some anvil cirrus)." Since Lindzen's iris hypothesis is based on the response of high anvil cirrus clouds, it would appear that their results technically do not address Lindzen's specific iris hypothesis. But they are applicable to testing for a more general feedback between clouds and temperature. It is for this reason that we refer to the work here as being related to a "feedback akin to Lindzen's iris effect."
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