The headline statement on anthropogenic in the Summary for Policy Makers taken from chapter 10 of WG1 reads as follows.
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. -
This statement is false because 1951-2010 includes some natural warming from the 60y AMO heat cycle reported by Tung and Zhou. Fig 10.5 in AR5 shows the natural internal component as being 0.0±0.05 C. This is wrong because the natural temperature cycle does not average to zero over this period. This is discussed in Natural versus Anthropogenic . My corrected Fig 10.5 is shown below.
The residual natural warming present in the 1950-2010 period is 0.2±0.05 C while the observed anthropogenic component is 0.45±0.05 as measured between 1940 to 2010. Both 1940 and 2010 are peaks of the AMO signal which subtract out when taking the difference. This is why the ‘observed’ global warming trend should be measured from 1940 to 2010 and not from 1950 to 2010.
CMIP5 values for both GHG and the net Anthropogenic signal are now about 50% higher then the observed warming. The correct attribution for the observed warming from 1951 to 2010 is therefore 75% anthropogenic and 25% natural variability. CMIP5 sensitivity to CO2 is very likely 50% too strong and overestimates the observed warming once the natural variation is properly included.
A more impartial version of the attribution statement would be
It is very 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, while about one third was due to natural variation. The best estimate of the human-induced contribution to warming is about 50% greater than the observed warming over this period.
The IPCC will live to regret their selective bias by analysing only the period 1950-2010 .