Roy Spencer's recently published study is incorrect

Dr. Roy Spencer argues that previous studies have overestimated the Earth’s climate sensitivity. He presents two main pieces of evidence to support this argument: 1) Lag regression analysis of satellite data shows that net radiative gain precedes, and radiative loss follows temperature maxima. A simple forcing-feedback model appears to show that this is the behavior expected from radiatively forced temperature changes. However, only in the case of non-radiative forcing can an accurate diagnosis of the feedback parameter be made. 2) “There is a rather large discrepancy in the time-lagged regression coefficients between the radiative signatures displayed by the real climate system in satellite data versus the climate models.” I will demonstrate that both of these arguments are likely incorrect, and Spencer’s resulting conclusions are physically unrealistic. Regarding Spencer’s first argument: I agree that Spencer’s lag regression analysis “supports the interpretation that net radiative gain precedes, and radiative loss follows temperature maxima”. However, this does not necessarily imply that the temperature variations during the relative time period were radiatively forced. The simple forcing-feedback model Spencer used to draw this conclusion was much too simple. For instance, his model did not have the ability to simulate ENSO or the hydrologic cycle. In addition, the model was likely tuned to give the result it gave. Spencer used an ocean mixed-layer depth of only 25 meters. However, it appears that mixed-layer depths of 100–200 meters are more appropriate for simple climate models. [1] Spencer also used a lambda value of 3, much greater than the consensus value of about 1.4. [2] Notably, Spencer made no attempt to find out how sensitive his model fits were to these different parameter values. In fact, he never presented any error bars or uncertainties in his study. As Dr. Andrew Dessler stated, “The argument made in these papers...is extremely weak. What they do is show some data, then they show a very simple model with some free parameters that they tweak until they fit the data. They then conclude that their model is right. However, if the underlying model is wrong, then the agreement between the model and data proves nothing.” [3] Regarding Spencer’s second argument: Spencer claims to have demonstrated that, “There is a rather large discrepancy in the time-lagged regression coefficients between the radiative signatures displayed by the real climate system in satellite data versus the climate models.” However, this argument does not withstand the slightest amount of scrutiny. As other scientists have pointed out, Spencer failed to account for the full range of model outputs. He compared the satellite data to only six climate models – the three most sensitive and the three least sensitive. While these models did not match the observations well, there certainly are some that do. As Dr. Kevin Trenberth found, models which accurately simulate ENSO tend to match the observations reasonably well. [4] This suggests that Spencer should have considered a larger number of climate models and stratified these models by their ability to simulate ENSO. In addition, Spencer failed to account for decadal variability in the model simulations he did consider. Clearly, climate model simulations for one decade will never precisely match model simulations for another decade. This is due to factors such as random fluctuations in ocean circulation within the models. When scientists account for the full range of model outputs as well as decadal variability, they find that Spencer's claims cannot be correct. As Dr. Trenberth concluded, “The net result is that the models agree within reasonable bounds with the observations.” [4] The implications of Spencer’s paper are physically unrealistic: As Spencer stated himself, “Since much of the temperature variability during 2000–2010 was due to ENSO, we conclude that ENSO-related temperature variations are partly radiatively forced. We hypothesize that changes in the coupled ocean-atmosphere circulation during the El Niño and La Niña phases of ENSO cause differing changes in cloud cover.” However, energy budgets of the surface show that ENSO related temperature variations are not radiatively forced. [5] Therefore, Spencer’s conclusions cannot be correct. Moreover, Spencer’s results imply that climate sensitivity is considerably lower than estimates from current climate models. However, numerous paleoclimate reconstructions do not support this. [6] Therefore, past climate changes couldn’t have occurred if climate sensitivity were as low as Dr. Roy Spencer alleges. References: http://tinyurl.com... http://ipcc.ch... http://tinyurl.com... http://tinyurl.com... http://tinyurl.com... http://www.iac.ethz.ch...