by Judith Curry at judithcurry.com
The Intergovernmental Panel on Climate Change (IPCC) has issued a new Synthesis Report, with fanfare from the UN Secretary General Antonio Guterres:
“The climate time-bomb is ticking but the latest IPCC report shows that we have the knowledge & resources to tackle the climate crisis. We need to act now to ensure a livable planet in the future.”
The new IPCC Report is a synthesis of the three reports that constitute the Sixth Assessment Report, plus three special reports. This Sythesis Report does not introduce any new information or findings. While the IPCC Reports include some good material, the Summary for Policy Makers for the Synthesis Report emphasizes weakly justified findings on climate impacts driven by extreme emission scenarios, and politicized policy recommendations on emissions reductions.
The most important finding of the past 5 years is that the extreme emissions scenarios RCP8.5 and SSP5-8.5, commonly referred to as “business-as-usual” scenarios, are now widely recognized as implausible. These extreme scenarios have been dropped by UN Conference of the Parties to the UN Climate Agreement. However, the new Synthesis Report continues to emphasize these extreme scenarios, while this important finding is buried in a footnote:
“Very high emission scenarios have become less likely but cannot be ruled out.”
The extreme emissions scenarios are associated with alarming projections of 4-5oC of warming by 2100. The most recent Conference of the Parties (COP27) is working from a baseline temperature projection based on a medium emissions scenario of 2.5oC by 2100. Since 1.2oC of warming has already occurred from the baseline period in the late 19th century, the amount of warming projected for the remainder of the 21st century under the medium emissions scenario is only about one third of the warming projections under the extreme emissions scenario.
The Synthesis Report emphasizes “loss and damage” as a central reason why action is needed. It is therefore difficult to overstate the importance of the shift in expectations for future extreme weather events and sea level rise, that is associated with rejection of the extreme emissions scenarios. Rejecting these extreme scenarios has rendered obsolete much of the climate impacts literature and assessments of the past decade, that have focused on these scenarios. In particular, the extreme emissions scenario dominates the impacts that are featured prominently in the new Synthesis Report.