Model Projections Too Hot Over 55-Year Period – Study

In a new study published in the journal Environometrics, economists Ross McKitrick and Timothy Vogelsang compare climate models and observations over a 55-year span (1958-2012). Observations are from three sets of weather balloon measurements of tropical troposphere temperatures. Those are compared with 57 runs each of 23 CMIP3 models used by the IPCC in its 2007 Fourth Assessment Report (AR4).

In a lengthy post on the Drudge Report Climate Audit, McKitrick explains that the study focuses on the tropical troposphere because “that is where most solar energy enters the climate system, where there is a high concentration of water vapour, and where the strongest feedbacks operate.” The two economists used AR4 climate models because they began the study years ago before a “library” of CMIP5 models used in the IPCC’s Fifth Assessment Report (AR5) was available. (Note: McKitrick plans to update the study using the CMIP5 library.)

The graphic below shows how model projections compare with balloon data in the lower- and mid-troposphere over the observation period.

McKitrick and Vogelsang’s paper is 20 pages long and heavy on the math. Here is the bottom line as McKitrick presents it on Drudge Climate Audit:

Over the 55-years from 1958 to 2012, climate models not only significantly over-predict observed warming in the tropical troposphere, but they represent it in a fundamentally different way than is observed. Models represent the interval as a smooth upward trend with no step-change. The observations, however, assign all the warming to a single step-change in the late 1970s coinciding with a known event (the Pacific Climate Shift), and identify no significant trend before or after. In my opinion the simplest and most likely interpretation of these results is that climate models, on average, fail to replicate whatever process yielded the step-change in the late 1970s and they significantly overstate the overall atmospheric response to rising CO2 levels.

Click here to read McKitrick’s commentary as posted on Drudge Climate Audit. Click here to read columnist Andrew Bolt’s commentary posted Tuesday on the Drudge Report. Click here to read the full McKitrick-Vogelsang study.

(Corrected 9/5/2014)