Recently, a paper by Craig Loehle and Nicola Scafetta has come to my attention. The simplicity of the math makes it attainable for the technical reader who doesn’t want to spend a month learning climate jargon. This isn’t a Mannian stew. The paper received a strong critique at WUWT, much of which was undeserved in my opinion. What the authors did was recognize the widely discussed 60ish year signal in climate data and project it onto temperature records to determine the percent of recent trends are due to human contribution. The problem is that measured temperatures are 150 years long so our recently invented thermometers (think about that sometimes folks) have only recorded two of the suspected 60 year cycles and those barely visible cycles are further masked by a warming trend. So the question becomes, are the 60 year climate cycles regular or are they not? We don’t have proof of anything but if they are real, there is some additional realistic evidence of lower climate sensitivity to CO2. A smaller amplitude high frequency component was also used in the work.
The abstract is a good place to start, and may have caused much of the problem in interpretation.
Abstract: The climate change attribution problem is addressed using empirical decomposition. Cycles in solar motion and
activity of 60 and 20 years were used to develop an empirical model of Earth temperature variations. The model was fit to
the Hadley global temperature data up to 1950 (time period before anthropogenic emissions became the dominant forcing
mechanism), and then extrapolated from 1951 to 2010. The residuals showed an approximate linear upward trend of about
0.66°C/century from 1942 to 2010. Herein we assume that this residual upward warming has been mostly induced by
anthropogenic emissions, urbanization and land use change. The warming observed before 1942 is relatively small and is
assumed to have been mostly naturally induced. The resulting full natural plus anthropogenic model fits the entire 160
year record very well. Residual analysis does not provide any evidence for a substantial cooling effect due to sulfate
aerosols from 1940 to 1970. The cooling observed during that period may be due to a natural 60-year cycle, which is
visible in the global temperature since 1850 and has been observed also in numerous multisecular climatic records. New
solar activity proxy models are developed that suggest a mechanism for both the 60-year climate cycle and a portion of
the long-term warming trend. Our results suggest that because current models underestimate the strength of natural
multidecadal cycles in the temperature records, the anthropogenic contribution to climate change since 1850 should be
less than half of that previously claimed by the IPCC. About 60% of the warming observed from 1970 to 2000 was very
likely caused by the above natural 60-year climatic cycle during its warming phase. A 21st Century forecast suggests that
climate may remain approximately steady until 2030-2040, and may at most warm 0.5-1.0°C by 2100 at the estimated
0.66°C/century anthropogenic warming rate, which is about 3.5 times smaller than the average 2.3°C/century
anthropogenic warming rate projected by the IPCC up to the first decades of the 21st century. However, additional
multisecular natural cycles may cool the climate further.
So if we have a global temperature record which looks like this:
The top pane is hadley temperature with an overlayed model, the bottom is the residual from a full length model fit showing a cyclic plus trend method. Since there is long term signal still visible (kind of a U shape in the bottom data), the authors determined the model is absolutely misspecified. The thick black line over the top temperature pane doesn’t seem entirely unreasonable though. It is quite possible that there is a cyclic nature to the signal, but without more information, little can be concluded with certainty.
This is the comment Leif Svalgard left at WUWT:
The paper is cyclomania at its worst
A comment which I believe is way over the top. Perhaps the conclusions are too strongly worded but the concept behind the paper is not bad, it actually could be perfectly correct. Willis made more complete arguments with other fits having different timescales. Nicola Scafetta replied with this comment which will sum up my own understanding.
Dear Willis Eschenbach,
I am sorry but you continue to say no-sense.
1) you cannot disprove our research by simply using a model slightly different from our, for example using a 62-year cycle instead of a 60-year cycle, and claim we are wrong because you get similar results! Why don’t you use a 100-year cycle instead of a 62-year cycle?
2) the existence of major quasi 60-year and quasi 20-year cycles are easily detected by power spectrum analysis, see figure 3, 10 & 11 in N. Scafetta, “Empirical evidence for a celestial origin of the climate oscillations and its implications”. Journal of Atmospheric and Solar-Terrestrial Physics 72, 951–970 (2010), doi:10.1016/j.jastp.2010.04.015
The bold is mine. The paper by Loehle and Scafetta isn’t disproven by the mere mention that different cycles can be fit to the same data. If the cyclomania is even close to a real process – say some cycles are 40 and some are 70 years, the fits address a substantial fraction of the climate warming we’ve seen. Solar cycles experience some of the same imperfect variance as Leif will attest to himself. Where the critics went really wrong was to fail to recognize the proxy data shown in the same paper, having a longer timeframe and similar cycles.
See, the 60 year cycle may not be only real since 1850. It may not be a perfect cycle, but like sunspots, ice ages, and many other climate phenomena, there is potentially a generally repeated nature to the variance. All that is required is for the cycle to exist and Loehle and Scafetta have a reasonable way to estimate the percentage of warming caused by CO2.
The last paragraph of the conclusion may be where they have gotten themselves in trouble.
It is also worth noting that by starting in a cycle trough in
1900 and ending on a cycle peak in 2000, graphical
depictions of warming in IPCC and other reports give an
exaggerated depiction of the rate of warming, a point noted
by Karlén  and Soon et al. . By further failing to
mention solar effects and warm bias in the instrumental
record, it is implied that almost all the warming of about
0.8°C over this period is human caused, when really no more
than 0.4°C is likely to be. Reporting such a lower number
would probably cause much less alarm than what was
recently claimed by Rockström et al. . Moreover,
because the 60-year natural cycle will be in its cooling phase
for the next 20 years, global temperatures will probably not
increase for the next few decades in spite of the important
role of human emissions (see Fig. 5A), as predicted by
multiple studies (reviewed in Loehle ; Scafetta ).
While the methods seem fine to me, they are far from definitive. The paper did achieve an excellent fit to the data based on reasonable assumptions of observed natural cycles, like predicting the next sunspots, the total variation in the natural cycles is basically an unknown. I would have prefered a less definitive conclusion on the order of, “if the previously observed cycles are an accurate component of recently observed temperatures and continue for the near future then we can make the following conclusions:” Although similar statements are made in the paper, without more of that, a zealous reader can easily go too far critiquing the work. The proxies are a longer record and do seem to support the cycles.
I don’t think that anything in the work is unreasonable though and the critiques of it in the WUWT thread were too severe IMO. It is yet another paper which lends support to the ever more likely fact that climate models are running too hot and the potential that AGW is a far less severe effect than the IPCC likes to state.