Claim: Long term oscillations in the ocean/atmosphere - the Pacific Decadal Oscillation (PDO), and the Atlantic Multi-Decadal Oscillation (AMO), account for ~half of global warming; so the climate sensitivity of CO2 must be lower than we thought.

Why this claim is wrong: The latest incarnation of this is due to Tung and Zhou (2013). Tung and Zhou ignore that their AMO data is strictly limited to sea surface temperatures, which are strongly affected by a greenhouse warming atmosphere. Remember, 93% of the observed warming during the past ~60 years has been deposited into the ocean, heat transferring from the surface downward. Thus, Tung and Zhou's reasoning is circular. They assume the AMO is unaffected by GHG warming, then notice SST's are rising and assume it's the AMO driving global atmospheric temperatures. Tamino (2013 ) has shown that no matter how the AMO and atmospheric temperature correlations are analyzed (trended or de-trended, linear or not), that atmospheric temperatures lead the AMO SST's by months, suggesting the causation direction is atmospheric greenhouse warming leading to AMO ocean warming, as basic theory also supports. Here's a link to clarify.

The PDO and ENSO oscillations have also been claimed to explain a significant part of the post 1950 global warming, but even if you cherry pick the interval and use 1950 to 1998 (1997/98 having the strongest El Nino warming on record), PDO/ENSO accounts for only 12% of the cumulative global warming (Trenberth et al. 2002 , Foster et al. 2010) (more detail here). The longer term PDO and AMO indices show a mild shift away from the warm El Nino phase to the cooler La Nina phase, at least through 2013 (see here), and this no doubt has helped drain atmospheric heat into the oceans more efficiently and allow global surface temperatures to not rise as quickly, despite the increasing radiative forcing from constantly rising CO2 levels.

The PDO is clearly a cycle, which is on top of a secular trend which must have a different cause. While the PDO does have a correlation with the rate of global temperature change, the PDO is almost certainly a cycle which has been in existence for far longer than just the 20th century, yet global temperatures rose steeply only when CO2 levels began to rise strongly. The PDO is apparently an oscillation in the way heat is transferred from the atmosphere to the Pacific Ocean.

The oceans get their heat from the atmosphere and the sun. The atmosphere is warming, and therefore so are the oceans, albeit with relatively short term variations around the rising trend.

In Short: The reasoning is circular. AMO and global temperatures are clearly correlated and cause-effect might be in either direction before analysis is done. They ASSUME that AMO and PDO are not caused by global air temperatures, and therefore, unsurprisingly, find the causation goes the other way, having assumed away the reverse (and proper) direction. More careful analysis shows air temperatures lead the ocean AMO temperatures by months, indicating cause and effect is opposite to that claimed by Tung and Zhou. The PDO has been in a cooling phase during the past 16 years, yet global temperatures are still rising, just at a slightly reduced rate.



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