A recent study by Johnstone and Mantua (2014) found a high correlation (r = 0.78) between sea surface temperatures since 1900 and changes in atmospheric pressure over the Northeastern Pacific, claiming that 80% of the variance in sea surface temperatures in the Northeastern Pacific was explained by changes in the North Pacific high.
WUWT, CATO, Breitbart.com, and various blogs). Unfortunately, most of those "sources" appear to have gotten their information mostly from newspaper articles (the LA Times article was cited frequently), with a few references to the abstract of the study.
Johnstone and Mantua found that temperatures in the Northeastern Pacific and coastal regions of the US Pacific Northwest have risen since 1900 due to a weakening of the North Pacific high and resultant changes in surface winds and ocean currents (see also summaries in the NY Times and Climate Central). They also noted that such changes were not predicted by current climate models. Their conclusions were that, for the Northeastern Pacific and coastal regions of the Pacific Northwest, changes in the North Pacific high and resultant changes in winds and currents accounted for 80% of the temperature rise since 1900 and that such changes were not explained by global warming.
Johnstone and Mantua were careful to note in interviews (i.e. Wines 2014) that their conclusions only apply to the coastal regions and the ocean, not areas further inland, finding that the relationship between between the North Pacific high and temperature decreased the further inland they went. Indeed, a study published earlier this year on those same inland areas found that the rise in greenhouse gases, not changes in natural factors, was the dominant factor in rising temperature in that area (Abatzoglou et al. 2014). Johnstone and Mantua also emphasized that the relationship they found was unique to the Northeastern Pacific region and was unlikely to apply to other regions, as the Northeastern Pacific had several unique characteristics that combined to make wind the dominant factor in that region.
Critics have noted three main weaknesses in Johnstone and Mantua's study. The first is that their results depend solely on a correlation between the North Pacific high and surface temperatures, without any concurrent work done to actually show that changes in the North Pacific high can cause the observed changes in surface temperatures. As the mantra states, correlation does not imply causation.
The second is that they have no evidence showing that the changes in the Northeastern Pacific winds are natural. Johnstone and Mantua based that conclusion on the fact that none of the global climate models they examined showed any similar changes to the Northest Pacific winds. However, as they themselves admit, current climate models do not show such regional changes very well. So they are basing their conclusion that the change is natural on an absence of evidence in current models while also noting that current models do not have the ability to show the evidence they seek.
The third is that their results depend on the sea level pressure data set they picked to represent the North Pacific high. As Abatzoglou et al. (2014) noted in a comment on Johnstone and Mantua's paper, there are several data sets available with highly divergent results prior to 1940. Johnstone and Mantua picked the one that showed the highest correlation with sea surface temperatures whereas different data sets would not show any such relationship.
I have one nit to add of my own. Johnstone and Mantua claim that they showed that up to 80% of the temperature rise in the Northeastern Pacific is natural. That appears to be based on the correlation coefficient (r = 0.78) that they found between observed and predicted sea surface temperatures. There are several problems with that conclusion. The correlation coefficient does not show how much of the variation in temperatures was explained by the natural variables. It doesn't even show how much of the variation in observed sea surface temperatures is explained by predicted sea surface temperatures. To get that, you must calculate the R2 value. In the case of Johnstone and Mantua's study, the R2 value is 0.61, meaning that only 61% of the variation in observed temperatures could be explained by the predicted temperatures, not the 80% claimed.
A correlation with an R2 of 61% is still exceptional. However, that degree of correlation between observed and predicted temperatures should not surprise anyone. Johnstone and Mantua used observed sea surface temperatures to construct their statistical model. We should expect that there would be a high correlation between observed and predicted sea surface temperatures. That does not mean that 61% of the temperature rise is due to changes in the North Pacific high, just that there is a high correlation between observed and predicted sea surface temperatures. To get at the relationship between sea surface temperatures and the North Pacific high, you have to go to the statistical model itself. When I downloaded their data and attempted to replicate their statistical model (lm(formula = SSTarc~lagged SLP1), I found an R2 value of 0.32 for the model itself, still highly statistically significant (p ≤ 2.2 x 10-16) but nowhere near the claimed 80% of variance.
So, what do I make the study? An interesting result—IF it holds up to scrutiny—but it does not say anything about whether or not the rise in Northeastern Pacific sea surface temperatures is ultimately due to global warming and most certainly does not disprove anything of what we understand about global warming.