The U.N. IPCC has now released the detailed report for Working Group I: The Physical Basis of Climate Change. Based on a very quick initial reading of it, I’ll highlight a seemingly small, but I believe extremely significant, change from the prior draft of this report.
The original draft from several months ago said the following in Chapter 8 (Climate Models and Their Evaluation), Question 8.1: “As a consequence, models continue to display a substantial range of global temperature change in response to specified greenhouse gas forcing (refer Chapter 10), To date it has not been possible to quantify how errors in a model’s simulation of specific climate observations impact on errors in its future climate projections, but a few studies suggest this may be possible in the future. Despite such uncertainties,…”
In the final version that was just released this was changed to the following:
“Consequently, models continue to display a substantial range of global temperature change in response to specified greenhouse gas forcing (see Chapter 10). Despite such uncertainties,…”
The key phrase eliminated here is “it has not been possible to quantify how errors in a model’s simulation of specific climate observations impact on errors in its future climate projections.” In effect, the original draft said that we can’t quantitatively bound errors in climate forecasts generated by global climate models. That’s a pretty big deal if we are relying on these to make trillion-dollar decisions.
Why was this eliminated? Did some new research become available or better understood, or was this just editing out an inadvertent admission of scientific uncertainty that might undercut a political message?