This is my comment in reply to a post by Dr Tamsin Edwards on her blog ‘All Models Are Wrong’ entitled Nine Lessons and Carols in Communicating Climate Uncertainty. I have submitted this comment on Tamsin’s blog.
Thanks, Tamsin, for this article. It is a pity, though, that what the IPCC said about the attribution of global warming in AR4 and AR5 is not reported correctly.
“In 2007, the IPCC said the likelihood that most of global warming since the mid-20th century was caused by greenhouse gas emissions was assessed to be greater than 90%. This year they made a similar statement but the likelihood was 95% or greater.”
In fact, in AR5 the IPCC did not change the attribution confidence level on the role of greenhouse gases from that in AR4.
AR4 (2007) Chapter 9: “Greenhouse gas forcing has very likely caused most of the observed global warming over the last 50 years.”
AR5 (2013) Chapter 10: “More than half of the observed increase in global mean surface temperature (GMST) from 1951 to 2010 is very likely due to the observed anthropogenic increase in greenhouse gas concentrations.”
And in the AR5 Technical Summary the comparison with AR4 was made explicit: “Consistent with AR4, it is assessed that more than half of the observed increase in global average surface temperature from 1951 to 2010 is very likely due to the observed anthropogenic increase in greenhouse gas concentrations”.
The IPCC definition of very likely has not changed, (AR4 Chapter 1: Very likely 90% probability; AR5 Chapter 1: Very likely 90–100% probability).
The ‘extremely likely’ (95–100% probability) statement in AR5 refers to ‘human activities’ (chapter 10) or ‘anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together’ (SPM) and I don’t think that there is an exact parallel statement in AR4 (though please let me know if I am wrong on that). It may not be possible to make this comparison, as the IPCC itself cautions:
“The AR5 Guidance Note refines the guidance provided to support the IPCC Third and Fourth Assessment Reports. Direct comparisons between assessment of uncertainties in findings in this report and those in the IPCC Fourth Assessment Report and the IPCC Special Report on Managing the Risk of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX) are difficult, because of the application of the revised guidance note on uncertainties, as well as the availability of new information, improved scientific understanding, continued analyses of data and models, and specific differences in methodologies applied in the assessed studies. For some climate variables, different aspects have been assessed and therefore a direct comparison would be inappropriate.” (Technical Summary of AR5 (TS-4))
I am sure that the IPCC authors were very careful indeed with the language that they used in these crucial attribution statements, so I don’t think I’m just being pedantic pointing this out. In an article that expects accuracy and statistical insights from journalists, I would like to see equal precision in reporting the scientific statements.
Update: Jonathan Jones’ comment on Tamsin’s post is well worth reading! Jonathan adds nine carols to Tamsin’s lessons.