The following conclusion of AR5 undoubtably had the most political impact.
“It is extremely likely that human activities caused more than half of the observed increase in GMST from 1951 to 2010.”
Humans cause global warming with 95% confidence thereby confirming that tough action is needed to enforce carbon targets in Europe and elsewhere. Skeptics have been proved wrong etc.
Pretty convincing eh!
Well now just 4 months later we have a new claim that the 17 year hiatus in warming has instead been caused by an increase in trade winds over the western pacific – part of the Pacific Decadel Oscillation (PDO). A Met Office report on the storms and flooding now affecting parts of the UK says that perturbations to the Jet stream has been driven in part by “persistent rainfall over Indonesia and the tropical West Pacific”.
What they don’t point out though is that this explanation is totally at odds with the explanation given in the AR5 chapter 10 which defined the above attribution statement. Peter Stott of the Met Office was the lead author of this chapter. If explanations for the pause in warming can change so fast so that now PDO is the dominant effect then surely also the attribution statement needs itself to be revised.
The observed recent warming hiatus, defined as the reduction in GMST trend during 1998–2012 as compared to the trend during 1951–2012, is attributable in roughly equal measure to a cooling contribution from internal variability and a reduced trend in external forcing (expert judgement, medium confidence).The forcing trend reduction is primarily due to a negative forcing trend from both volcanic eruptions and the downward phase of the solar cycle. However, there is low confidence in quantifying the role of forcing trend in causing the hiatus because of uncertainty in the magnitude of the volcanic forcing trends and low confidence in the aerosol forcing trend. Many factors, in addition to GHGs, including changes in tropospheric and stratospheric aerosols, stratospheric water vapour, and solar output, as well as internal modes of variability, contribute to the year-to-year and decade- to-decade variability of GMST.
The Atlantic Multi-decadal Oscillation (AMO) could be a confounding influence but studies that find a significant role for the AMO show that this does not project strongly onto 1951–2010 temperature trends.
They go on to write
Zhou and Tung (2013a) show that GMST are consistent with a linear anthropogenic trend, enhanced variability due to an approximately 70-year Atlantic Meridional Oscillation (AMO) and shorter-term variability. If, however, there are physical grounds to expect a nonlinear anthropogenic trend (see Box 10.1 Figure 1a), the assumption of a linear trend can itself enhance the variance assigned to a low-frequency oscillation. The fact that the AMO index is estimated from detrended historical temperature observations further increases the risk that its variance may be overestimated, because regressors and regressands are not independent. Folland et al. (2013), using a physically based estimate of the anthropogenic trend, find a smaller role for the AMO in recent warming. To summarize, recent studies using spatial features of observed temperature variations to separate AMO variability from externally forced changes find that detection of external influence on global temperatures is not compromised by accounting for AMO-congruent variability (high confidence). There remains some uncertainty about how much decadal variability of GMST that is attributed to AMO in some studies is actually related to forcing, notably from aerosols. There is agreement among studies that the contribution of the AMO to global warming since 1951 is very small (considerably less than 0.1°C; see also Figure 10.6) and given that observed warming since 1951 is very large compared to climate model estimates of internal variability (Section 10.3.1.1.2), which are assessed to be adequate at global scale (Section 184.108.40.206), we conclude that it is virtually certain that internal variability alone cannot account for the observed global warming since 1951.</blockquote>
There is no mention at all of the PDO in this chapter. But suddenly just a few months later it is now the PDO that is responsible for the global warming hiatus and seemingly the Met Office is confirming this. If that is the case then the whole basis behind Fig 10.5 where both natural forcing and natural variation were reduced to just 0.0± 0.1C disappears.
In addition the logic behind the SPM attribution statement must also need revisiting.