Abrupt global temperature change and the instrumental record

An objective evaluation of statistical breakpoints in the global surface instrumental temperature record is discussed. Contrary to previous evaluations, the possibility of both trend changes and abrupt jumps in temperature are considered, independent of any a priori assumptions regarding: the number of breakpoints; their position; or the interval between them. Abrupt jumps, widely acknowledged in climate theory, but not in the instrumental period, are revealed in the years 1902, 1945 and 1963 and possible causes are suggested. This alternative model of temperature changes requires a larger number of degrees of freedom since both step and trend changes are estimated at each breakpoint. Nevertheless, the new model was found to be a better fit than previous solutions using the Schwarz Bayesian Information Criterion. Moreover, the resolution of these abrupt shifts forces a reinterpretation of the conventional view of global surface temperature trends. In this alternative view, the apparent mid-twentieth century neutral to slight negative trend is shown to be, in part, a consequence of confounding step changes and trend changes. By implication, the present temperature trend begins in 1964 rather than in 1976.