By Doug L. Hoffman, The Resilient Earth
While climate skeptics have gleefully pointed to the past decade’s lack of temperature rise as proof that global warming is not happening as predicted, climate change activists have claimed that this is just “cherry picking” the data. They point to their complex and error prone general circulation models that, after significant re-factoring, are now predicting a stretch of stable temperatures followed by a resurgent global warming onslaught. In a recent paper, a new type of model, based on a test for structural breaks in surface temperature time series, is used to investigate two common claims about global warming. This statistical model predicts no temperature rise until 2050 but the more interesting prediction is what happens between 2050 and 2100.
David R.B. Stockwell and Anthony Cox, in a paper submitted to the International Journal of Forecasting entitled “Structural break models of climatic regime-shifts: claims and forecasts,” have applied advanced statistical analysis to both Australian temperature and rainfall trends and global temperature records from the Hadley Center’s HadCRU3GL dataset. The technique they used is called the Chow test, invented by economist Gregory Chow in the early 1960s. The Chow test is a statistical test of whether the coefficients in two linear regressions on different data sets are equal. In econometrics, the Chow test is commonly used in time series analysis to test for the presence of a structural break.
A structural break appears when an unexpected shift in a time series occurs. Such sudden jumps in a series of measurements can lead to huge forecasting errors and unreliability of a model in general. Stockwell and Cox are the first researchers I know of to apply this econometric technique to temperature and rainfall data (a description of computing the Chow test statistic is available here). They explain their approach in the paper’s abstract:
A Chow test for structural breaks in the surface temperature series is used to investigate two common claims about global warming. Quirk (2009) proposed that the increase in Australian temperature from 1910 to the present was largely confined to a regime-shift in the Pacific Decadal Oscillation (PDO) between 1976 and 1979. The test finds a step change in both Australian and global temperature trends in 1978 (HadCRU3GL), and in Australian rainfall in 1982 with flat temperatures before and after. Easterling & Wehner (2009) claimed that singling out the apparent flatness in global temperature since 1997 is ‘cherry picking’ to reinforce an arbitrary point of view. On the contrary, we find evidence for a significant change in the temperature series around 1997, corroborated with evidence of a coincident oceanographic regime-shift. We use the trends between these significant change points to generate a forecast of future global temperature under specific assumptions.
The climatic effects of fluctuations in oceanic regimes are most often studied using singular spectrum analysis (SSA) or variations on principle components analysis (PCA). In other words, by decomposing rainfall and temperature into periodic components. Such approaches can capture short period phenomena like the effects of El Nino , and the potential impact of longer term phenomena such as the Pacific Decadal Oscillation (PDO) on variations in global temperature. These phenomena take place over a period of years or decades. For finding and testing less frequent regime-shifts different techniques are called for. According to the authors: “An F-statistic known as the Chow test (Chow, 1960) based on the reduction in the residual sum of squares through adoption of a structural break, relative to an unbroken simple linear regression, is a straightforward approach to modeling regime-shifts with structural breaks.” All the statistical details aside, the point here is that a sequence of data that contains sudden shifts or jumps is hard to model accurately using standard methods.
The Chow test since 1978 finds another significant breakpoint in 1997, when an increasing trend up to 1997 (0.13 plus/minus 0.02C per decade) changes to a practically flat trend thereafter (-0.02 plus or minus 0.05C per decade). Contrary to claims that the 10 year trend since 1998 is arbitrary, structural change methods indicate that 1997 was a statistically defensible beginning of a new, and apparently stable climate regime. Again, according to the authors: “The significance of the dates around 1978 and 1997 to climatic regimeshifts is not in dispute, as they are associated with a range of oceanic, atmospheric and climatic events, whereby thermocline depth anomalies associated with PDO phase shift and ENSO were transmitted globally via ocean currents, winds, Rossby and Kelvin waves .”
Perhaps most interesting is the application of this analysis to the prediction of future climate change, something GCM climate modelers have been attempting for the past 30 years with little success. Figure 3 from the paper illustrates the prediction for temperatures to 2100 following from our structural break model, the assumptions of continuous underlying warming, regime-shift from 1978 to 1997, and no additional major regime-shift. The projections formed by the presumed global warming trend to 1978 and the trend in the current regime predicts constant temperatures for fifty years to around 2050. This is similar to the period of flat temperatures from 1930-80.
Icecap Note: A good analysis. Actually, properly adjusted, the trends from the1940 to late 1970s and since 2001 were down. How far down we go between now and 2050 depends on whether we will be like the PDO- phase from 1947 to 1977 or the early 1800s due to a Dalton like Minimum. A Maunder like Minimum seems unlikely unless the projections of Livingston and Penn for the disappearance of spots after 2014 occurs. The following chart (enlarged here) from Don Easterbrook captures these scenarios.
