The best thing about a leaked climate report is that its dire forecasts make headlines twice. This is the case for the Fourth National Climate Assessment (NCA), which was leaked in August 2017 and published in final form last week by the U.S. Global Change Research Program. It bears repeating: most conscientious citizens have no wish to dispute the conclusions of climate scientists, and even fewer have the expertise to do so. But they are right to read the reports of the IPCC and its subsidiaries closely and critically, for they are political documents first and foremost.
As noted here in the summer, the draft NCA offered an intriguing juxtaposition, among its other findings: "The relationship between cumulative CO2 emissions and global temperature response is estimated to be nearly linear" (p. 34), yet "almost no increase [in temperature] occurred from the mid 1940s to the mid 1970s, which is not well understood" (p. 43). Why does this apparent contradiction matter? Because there are solid data to support the skeptics' claims that i) the global mean temperature dropped in the period 1940-1980, when CO2 emissions were soaring (see the NCA graph, above); and ii) there was an unexpected "hiatus" in global warming between 2000 and 2014. As the NCA itself concedes, such claims have been used as "informal evidence to cast doubt on the accuracy of climate projections from CMIP5 models" (p. 44). Both the August 2017 draft and last week's final Assessment contain a sidebar (Box 1.1) that seeks to explain these anomalies by invoking temperature variation that occurs over different time scales simultaneously: "For the 15 years following the 1997–1998 El Niño–Southern Oscillation (ENSO) event, the observed rate of temperature increase was smaller than the underlying long-term increasing trend on 30-year climate time scales."
Fair enough. But here's the interesting bit. The sentence"almost no increase occurred from the mid 1940s to the mid 1970s, which is not well understood" has been excised from the final NCA report—a critically important stroke of the delete key which has the effect of rehabilitating the authority of the CMIP5 models wholesale. In August this sentence was arguably one of the most important in the entire 669-page National Climate Assessment. Today, it has gone the way of the memory hole. Draw your own conclusions.