Climatologist Dr. Judith Curry rips UN IPCC’s ‘expert judgement’ that humans are ‘extremely likely’ responsible for more than half of warming since 1951
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The IPCC’s AR5 attribution statement: 'It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together. The best estimate of the human induced contribution to warming is similar to the observed warming over this period.'
The reasoning process used by the IPCC in assessing confidence in its attribution statement is described by this statement from the AR4: “The approaches used in detection and attribution research described above cannot fully account for all uncertainties, and thus ultimately expert judgement is required to give a calibrated assessment of whether a specific cause is responsible for a given climate change.'
Curry: 'The attribution statement itself is at best imprecise and at worst ambiguous: what does “most” mean – 51% or 99%? Whether it is 51% or 99% would seem to make a rather big difference regarding the policy response.'
'The IPCC’s attribution statement does not seem logically consistent with the uncertainty in climate sensitivity.'
'I am arguing that climate models are not fit for the purpose of detection and attribution of climate change on decadal to multidecadal timescales.'
The IPCC does not have a convincing explanation for:
- warming from 1910-1940
- cooling from 1940-1975
- hiatus from 1998 to present
The IPCC purports to have a highly confident explanation for the warming since 1950, but it was only during the period 1976-2000 when the global surface temperatures actually increased.
The absence of convincing attribution of periods other than 1976-present to anthropogenic forcing leaves natural climate variability as the cause – some combination of solar (including solar indirect effects), uncertain volcanic forcing, natural internal (intrinsic variability) and possible unknown unknowns.
A key issue in attribution studies is to provide an answer to the question: When did anthropogenic global warming begin? As per the IPCC’s own analyses, significant warming didn’t begin until 1950.