Below is quick review of some of the evidence and consequences of a 60 year climate cycle. According to Roy Spencer, the argument that increasing carbon dioxide concentrations alone are sufficient to explain global warming is reasoning in a circle. By ignoring natural variability, they end up claiming that natural variability is insufficient.
However, the recent paper by Craig Loehle finds only a very small linear warming trend is left (potentially attributable to AGW) after subtracting the 60â€“70 yr cycle. While cause of the 60yr cycles is unexplained at present, he claims the small trend disproves AGW because it is:
clearly inconsistent with climate model predictions because the linear trend begins too soon (before greenhouse gases were elevated) and does not accelerate as greenhouse gases continue to accumulate with no acceleration in recent decades.
That oscillations are persistent features of the climate has been known for a long time. Stoker and Mysak in 1992 reviewed ice cores, tree-ring index series, pollen records and sea-ice extents over the last 10,000 years, finding:
The traditional interpretation that decadal-to-century scale fluctuations in the climate system are externally forced, e.g. by variations in solar properties, is questioned. A different mechanism for these fluctuations is proposed on the basis of recent findings of numerical models of the ocean’s thermohaline circulation. The results indicate that this oceanic circulation exhibits natural variability on the century time scale which produces oscillations in the ocean-to-atmosphere heat flux. Although global in extent, these fluctuations are largest in the Atlantic Ocean.
Even a paper by Michael Mann in 2000 identifies the cycle:
Analyses of proxy based reconstructions of surface temperatures during the past 330 years show the existence of a distinct oscillatory mode of variability with an approximate time scale of 70 years.
As far back as 1995 Mann published a paper in Nature stating:
THE recognition of natural modes of climate variability is essential for a better understanding of the factors that govern climate change. Recent models suggest that interdecadal (roughly 15â€“35-year period) and century-scale (roughly 50â€“150-year period) climate variability may be intrinsic to the natural climate system.
The issue is: How large is the cycle relative to potential warming due to AGW?. Klyashtorin and Lyubushin (2003) demonstrated that a 50â€“60 year period temperature signal is dominant from about 1650 (the end of the Little Ice Age) in Greenland ice core records, in several very long tree ring records, and in sardine and anchovy records in marine sediment cores. This result was also reported by Biondi et al. (2001), who also made the pithy remark:
Anthropogenic greenhouse warming may be either manifested in or confounded by alterations of natural, large-scale modes of climate variability.
A wide range of phenomena move in sync with this cycle. Long-term changes of Atlantic spring-spawning herring and Northeast Arctic cod commercial stocks also show 50-70-year fluctuations: sufficient to predict the probable trends of basic climatic indices and populations of major commercial fish species for up to 20-30 years into the future.
Zhen-Shan and Xian (2007) found China temperature from 1881 can be completely decomposed into four quasi-periodic oscillations including an ENSO-like mode, a 6â€“8-year signal, a 20-year signal and also a prominent 60-year timescale oscillation of temperature variation. While they found CO2 concentration contributed a small trend, its influence weight on global temperature variation accounted for no more than 40.19% of the total increase.
Perhaps its all a coincidence. Or perhaps we have yet to see much global warming from CO2, and its all going to suddenly leap out and ambush us in 20 years time.
Maybe, but speculation is a mugs game. Just the facts please. The last 50 years coincides with an upswing in the 60 year cycle, and the recent flat global temperatures coincide with the peak and subsequent downturn.