A potential AGW buster, attributing decadal temperature variation largely to internal oceanic effects, ENSO and over the longer term the 1976 Great Pacific Climate Shift, as we did here, is a new paper by Australian John McLean, with New Zealander Chris de Freitas, and Australian ex-pat Kiwi Bob Carter.

That mean global tropospheric temperature has for the last 50 years fallen and risen in close accord with the SOI of 5–7 months earlier shows the potential of natural forcing mechanisms to account for most of the temperature variation.

While the bottom line of this paper is that the change in SOI accounts for 72% of the variance in global temperature for the 29-year-long MSU record and 68% of the variance in global temperature for the longer 50-year RATPAC record, I think the claim of a longer term temperature effect could have been better supported. They stated:

Lean and Rind [2008] stated that anthropogenic warming is more pronounced between 45°S and 50°N and that no natural process can account for the overall warming trend in global surface temperature. We have shown here that ENSO and the 1976 Great Pacific Climate Shift can account for a large part of the overall warming and the temperature variation in tropical regions.

However, the assertion comes down to Figure 4 where they identify that the mean of the SOI (and temperature) seems to change at 1976. This model is not identified rigorously with any analysis, but is stated as an observation in the text.

For the 30 years prior to the 1976 shift (i.e., 1946 – 1975) the SOI averaged +1.93 but in the 30 years after 1976 (i.e., 1977 – 2006) the average was -3.06, which represents a shift from a La Nina inclination to an El Nino inclination. The standard deviations for the two periods were 9.48 and 10.40 on monthly SOI averages, and 6.56 and 6.35 on calendar year averages, which indicates consistent variation about a new average value. … From 1959 to 1975 the RATPAC LTT averaged -0.191°C and from 1977 to 1993 it averaged +0.122°C. The standard deviations on the seasonal data were 0.193° and 0.163 C°, and on monthly data 0.162°C and 0.146°C. We have already illustrated the close relationship between SOI and GTTA, but this descrip- tion of the respective changes before and after the Great Pacific Climate Shift indicates a stepwise shift in the base values of each factor but otherwise relatively consistent ranges of variation.

Break tests on both SOI and global temperature indicating the date 1976, and finding of a 7 month lead in the position of the break for SOI over global temperature would have lent more support to this (crucial) claim. As it is, the finding of high frequency correlation between SOI and GT is not as important to the AGW debate, as is the finding of a low frequency (ie. step change) in SOI leading (and hence causing) the change in temperatures.

It can also be seen in Figure 4 that the slope of temperature is upward while the SOI is constant (but higher) after 1976. This would tend to support Bob Tisdale’s hypothesis that recent warming since 1976 can be attributed to a gradual accumulation of heat, due to higher frequency of El Nino events. He has dug up a long-recognized, but little studied mechanism of re-emergence to explain it.