What was done
In the words of the scientists who conducted the work, "monthly global temperature anomaly time series for the surface, for two tropospheric layers, and for two stratospheric layers were modeled using four simple statistical models incorporating linear slopes and instantaneous step changes," where "breakpoints were selected to be as few as possible" and where "their exact timing was determined using an objective statistical method," after which "the Schwarz Bayesian Information Criterion was used to determine which models provided the best fit to the observations."

What was learned
In six of the ten cases examined, the researchers found that the best fitting models were not simple linear fits but the ones that involved breakpoints. They also determined that "best fit models yield more surface warming, less tropospheric warming, and generally less stratospheric cooling than simple linear fits." Of particular interest to us, in this regard, Seidel and Lanzante report that "the choice of the sloped steps model for the tropospheric data suggests it is reasonable to consider most of the warming during 1958-2001 to have occurred at the time of the climate 'regime shift,' modeled here at the start of 1977."

What it means
The two researchers say that "for the troposphere the nature of the modeled 1977 upward shift in temperature is unclear, and we suggest that attempts to attribute the warming over the past half century to natural or anthropogenic effects should consider the sloped or flat steps model as well as the traditional simple linear model to describe the change," and if this course is taken - which seems to us to be the most logical one to pursue - it further suggests (to us at least) that the temperature history of the past half century was primarily orchestrated by natural, as opposed to anthropogenic, forces, because periodic climatic regime shifts are naturally-occurring phenomena that have been around far longer than the anthropogenic forcing of global temperature change by way of enhanced greenhouse gas emissions, as demonstrated, for example, by the study of D'Arrigo et al. (2005).

Reference
D'Arrigo, R., Wilson, R., Deser, C., Wiles, G., Cook, E., Villalba, R., Tudhope, A., Cole, J. and Linsley, B. 2005. Tropical-North Pacific climate linkages over the past four centuries. Journal of Climate 18: 5253-5265.