Earlier this year, Roy Spencer and William Braswell of the University of Alabama in Huntsville published a paper in Journal of Geophysical Research… On the diagnosis of radiative feedback in the presence of unknown radiative forcing. Using “phase space plots of variations in global average temperature versus radiative flux,” Spencer and Braswell indicated that the only clear feedback signal in recent years has been strongly negative… With cloud cover variations presumably providing that negative feedback. Their analysis pointed to a climate sensitivity of more more than 0.6°C to a doubling of pre-industrial CO2.
Andrew Dessler of Texas A&M University has just published a paper in Science which he claims contradicts Spencer & Braswell… A Determination of the Cloud Feedback from Climate Variations over the Past Decade. Dessler claims that his work, using the same data as Spencer & Braswell, shows that cloud cover yield a positive feedback.
Spencer says clouds will mute any effects of enhanced greenhouse warming (AKA AGW)… Dessler says clouds will amplify such effects.
Spencer and Dessler have been “discussing” their differences of opinion in the blogoshphere… Roy Spencer’s blog and Real Climate. It essentially boils down to Spencer attributing temperature changes to cloud cover variation and Dessler attributing cloud cover variation to temperature changes.
From Spencer’s blog…
How can two climate researchers, using the same dataset, come to opposite conclusions?
The answer lies in an issue that challenges researchers in most scientific disciplines – separating cause from effect.
Dessler’s claim (and the IPCC party line) is that cloud changes are caused by temperature changes, and not the other way around. Causation only occurs in one direction, not the other.
In their interpretation, if one observes a warmer year being accompanied by fewer clouds, then that is evidence of positive cloud feedback. Why? Because if warming causes fewer clouds, it lets in more sunlight, which then amplifies the warming. That is positive cloud feedback in a nutshell.
But what if the warming was caused by fewer clouds, rather than the fewer clouds being caused by warming? In other words, what if previous researchers have simply mixed up cause and effect when estimating cloud feedback?
To Dessler’s credit, he actually references our paper. But he then immediately discounts our interpretation of the satellite data.
Because, as he claims, (1) most of the climate variability during the satellite period of record (2000 to 2010) was due to El Nino and La Nina (which is largely true), and (2) no researcher has ever claimed that El Nino or La Nina are caused by clouds.
This simple, blanket claim was then intended to negate all of the evidence we published.
But this is not what we were claiming, nor is it a necessary condition for our interpretation to be correct. El Nino and La Nina represent a temporary change in the way the coupled atmospheric-ocean circulation system operates. And any change in the atmospheric circulation can cause a change in cloud cover, which can in turn cause a change in ocean temperatures. We even showed this behavior for the major La Nina cooling event of 2007-08 in our paper!
It doesn’t mean that “clouds cause El Nino”, as Dessler suggests we are claiming, which would be too simplistic and misleading of a statement. Clouds are complicated beasts, and climate researchers ignore that complexity at their peril.
Dessler’s comments from Real Climate…
In other words, Dr. Spencer is arguing that clouds are causing ENSO cycles, so the direction of causality in my analysis is incorrect and my conclusions are in error.
After reading this, I initiated a cordial and useful exchange of e-mails with Dr. Spencer (you can read the full e-mail exchange here). We ultimately agreed that the fundamental disagreement between us is over what causes ENSO. Short paraphrase:
Spencer: ENSO is caused by clouds. You cannot infer the response of clouds to surface temperature in such a situation.
Dessler: ENSO is not caused by clouds, but is driven by internal dynamics of the ocean-atmosphere system. Clouds may amplify the warming, and that’s the cloud feedback I’m trying to measure.
My position is the mainstream one, backed up by decades of research. This mainstream theory is quite successful at simulating almost all of the aspects of ENSO.
The crazy thing is that Dessler included a link to a pdf of their e-mail exchange. Spencer never said that “ENSO is caused by clouds” in the actual email exchange.
It’s odd that Dessler seems to almost have an OCD compulsion to falsely accuse Spencer of claiming that the ENSO is caused by clouds.
If clouds are a positive feedback over the last 10 years as Dessler claims… Why hasn’t there been any global warming over the last decade?
Another funny thing… Global Outgoing Long-wave Radiation (OLR) has been increasing over the last decade…
What could cause OLR to rise and temperatures to flatten out or decline a bit?
Negative feedback from increasing cloud cover could.