IPCC AR5 weakens the case for AGW
by Judith Curry
Evidence reported by the IPCC AR5 weakens the case for anthropogenic factors dominating climate change in the 20th and early 21st centuries.
The IPCC assessment reports have expressed increasingly confident consensus views of the importance of anthropogenic influence on the global climate over the past 60 years, as reflected by these statements from the Summary for Policy Makers (SPM):
◾AR4 (2007): “Most of the observed increase in global average temperatures since the mid-20th century is very likely (>90% confidence) due to the observed increase in anthropogenic greenhouse gases.” (SPM AR4)
◾AR5 (2013) SPM: “It is extremely likely (>95% confidence) that human influence has been the dominant cause of the observed warming since the mid-20th century .” (SPM AR5)
This increase in confidence in the main conclusions in the AR5 SPM seems unwarranted based on the text, figures and analyses in the main WG1 Report, and also in comparison with the conclusions from the AR4. Several key elements of the report point to a weakening of the case for attributing the warming of human influences:
◾Lack of warming since 1998 and growing discrepancies with climate model projections
Recent hiatus in surface warming and discrepancies with climate models
The IPCC AR5 notes the lack of warming since 1998:
[T]he rate of warming over the past 15 years (1998–2012) [is] 0.05 [–0.05 to +0.15] °C per decade)which is smaller than the rate calculated since 1951 (1951–2012) [of] 0.12 [0.08 to 0.14] °C per decade.
The significance of this hiatus in warming since 1998 is in context of comparison with climate model projections. The IPCC AR4 stated:
For the next two decades, a warming of about 0.2°C per decade is projected for a range of SRES emission scenarios. (AR4 SPM)
For the IPCC AR5, CMIP5 has produced a multi-model dataset that includes long-term simulations of twentieth-century climate and projections for the twenty-first century and beyond. The IPCC summarizes near-term projections of global mean surface temperature anomalies in Figure 1. Figure 1 shows that observations particularly since 2005 are on the low end of the envelope that contains 90% of the climate model simulations. Observations in 2011-2012 are below the 5-95% envelope of the CMIP5 simulations. The trend in the model simulations is substantially larger than the observed trend over the past 15 years.
-After expecting an increase of 0.2oC per decade in the early decades of the 21st century from the AR4 statements, the rate of warming over the past 15 years is only ~0.05C.
- The IPCC AR5 bases its projection for the period 2016-2036 of 0.10 to 0.23oC per decade on expert judgment, rather than on the climate model results
- The IPCC does not have a convincing or confident explanation for the hiatus in warming.
◾Evidence of decreased climate sensitivity to increases in CO2
◾Evidence that sea level rise in 1920-1950 is of the same magnitude as in 1993-2012
◾Increasing Antarctic sea ice extent
◾Low confidence in attributing extreme weather events to anthropogenic global warming
If you read the fine print (not just the SPM) and compare the AR5 with statements made in the AR5, the IPCC AR5 WGI Report makes a weaker case for AGW than did the AR4. Of course there is nothing in the AR5 SPM that directly suggests weakening.