Dessler Demolishes Three Crucial 'Skeptic' Myths

Andrew Dessler's new paper, which we first examined in a post yesterday, has some very far-reaching implications in terms of refuting climate "skeptic" myths.  In fact, its results are relevant to three seperate myths in the Skeptical Science databaseAs a result, we have incorporated the findings of Dessler (2011) into the three myth rebuttals as follows.

It's not internal variability

Roy Spencer is the driving force behind the "internal variability" hypothesis, which posits that some unknown and undefined mechanism is causing cloud cover to change, which, by changing the overall reflectivity of the Earth, is the driving force behind the current global warming.

In attempting to substantiate this internal variability hypothesis, Spencer & Braswell (2011) assumed that the change in top of the atmosphere (TOA) energy flux due to cloud cover changes from 2000 to 2010 was twice as large as the heating of the climate system through ocean circulation.  Dessler (2011) used observational data (such as surface temperature measurements and ARGO ocean temperature) to estimate and corroborate these values, and found that the heating of the climate system through ocean heat transport was 20 times larger than TOA energy flux changes due to cloud cover over the period in question. 

This empirical finding contradicts Spencer's hypothesis that cloud cover changes are driving global warming.  However, it is consistent with our current understanding of the climate: ocean heat is exchanged with the atmosphere, which causes surface warming, which alters atmospheric circulation, which alters cloud cover, which impacts surface temperature.  While Spencer hypothesizes that the changes in cloud cover are the main driver behind global warming, Dessler concludes that they're only responsible for a small percentage of the changes in surface temperature from 2000 to 2010.  Spencer's internal variability hypothesis is contradicted by the observational data.

Spencer and Braswell (2011) is Contradicted by Observational Data

A highly-touted (and exaggerated in the media) claim in Spencer & Braswell (2011) was that their results suggested that climate sensitivity is low because climate scientists are misinterpreting climate feedbacks as climate forcings.  In their paper, Spencer and Braswell analyzed 14 models, but they only plotted the 3 with highest and 3 with lowest equilibrium climate sensitivities.  In the process, Spencer and Braswell excluded the three of the climate model runs which best matched the observational data, and also cherrypicked the data set furthest from the model runs (HadCRUT) (Figure 1). 

dessler 2011

Figure 1: Dessler (2011) reconstruction of Spencer & Braswell's Figure 3, showing relationship between top-of-atmosphere (TOA) net flux and surface temperature, as a function of lag between them.  The blue line is the observational data chosen by Spencer and Braswell (HadCRUT).  The red lines show other available observational data.  The black lines show climate model results.  The black lines with crosses show the climate model runs chosen by Spencer and Braswell in their paper.

Dessler found that these three model runs excluded by Spencer which best matched the data are also among those which best simulate El Niño and La Niña, which is not surprising, given that much of the temperature change over 2000-2010 was due to the El Niño Southern Oscillation (ENSO).  Thus Dessler concludes that

"since most of the climate variations over this period were due to ENSO, this suggests that the ability to reproduce ENSO is what's being tested here, not anything directly related to equilibrium climate sensitivity."

Spencer's claim of low sensitivity and negative feedbacks is based on this test, which is actually a test of models' ability to reproduce ENSO, and based on his internal variability hypothesis, which as noted above, Dessler's paper has also put to rest.  Thus Spencer's claim of low sensitivity and negative feedbacks is not supported by the empirical observational data.

Lindzen and Choi (2011) is Fundamentally Flawed

Lindzen and Choi (2009), slightly revised as Lindzen & Choi (2011), used measurements of sea surface temperature in the tropics and satellite measurements of outgoing radiation from 2000 to 2010 in an attempt to determine climate sensitivity, ultimately concluding that sensitivity is less than 1°C for doubled atmospheric CO2.

Lindzen and Choi plot a time regression of change in TOA energy flux due to cloud cover changes vs. sea surface temperature changes.  They find larger negative slopes in their regression when cloud changes happen before surface temperature changes, vs. positive slopes when temperature changes happen first, and thus conclude that clouds must be causing global warming.

However, Dessler also plots climate model results and finds that they also simulate negative time regression slopes when cloud changes lead temperature changes.  Crucially, sea surface temperatures are specified by the models.  This means that in these models, clouds respond to sea surface temperature changes, but not vice-versa.  This suggests that the lagged result first found by Lindzen and Choi is actually a result of variations in atmospheric circulation driven by changes in sea surface temperature, and contrary to Lindzen's claims, is not evidence that clouds are causing climate change, because in the models which successfully replicate the cloud-temperature lag, temperatures cannot be driven by cloud changes.

Major Myth Mashing

It's difficult to exaggerate the impact of Dessler's findings, because these are three of the most crucial arguments for climate "skeptics."  In order for the man-made global warming theory to be incorrect, climate sensitivity must be low (see Climate Sensitivity: The Skeptic Endgame).  Since all previous studies using many different lines of evidence point to the same answer, that climate sensitivity is not low, climate "skeptics" had to rely on Spencer & Braswell and Lindzen & Choi as the only game in town arguing otherwise.  In one fell swoop, Dessler has demonstrated that the only two modern papers arguing for low climate sensitivity are both fundamentally flawed, and their assumptions are contradicted by observational data.  In short, there's absolutely no reason to believe the IPCC's equilibrium climate sensitivity range of 2 to 4.5°C for doubled CO2 is incorrect.

Additionally, climate "skeptics" have yet to put forth a plausible, coherent, internally consistent alternative to challenge the robust man-made global warming theory.  Despite its fundamental problems, Spencer's internal variability hypothesis was probably the best alternative presented to this point, and Dessler drove another nail into its coffin by demonstrating what a small effect clouds have had on global temperature changes over the past decade.



Posted by dana1981 on Thursday, 8 September, 2011

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