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 a priori assumptions regarding the number of breakpoints, their position in the sequence or the interval between them. Breakpoints are revealed as abrupt jumps in the NOAA/NCDC surface temperature analysis at the years 1902, 1945 and 1963 and possible causes are suggested. A statistical model of surface temperature change whereby periods of quasi-linear change are interrupted by abrupt shifts (i.e., “sloped steps”) requires a larger number of degrees of freedom than previously suggested piecewise linear models without step-changes. However, a comparison of the Schwarz Bayesian Information Criterion suggests that sloped step models fit the sequence of average global surface temperature values better than piecewise linear models. Moreover, a more objective resolution of breakpoints that allows step changes may force a reinterpretation of the low frequency behavior of global surface temperature in the instrumental period. In this new interpretation, the apparent mid-twentieth century neutral-to-slightly negative trend is, at least in part, a consequence of confounding step changes and trend changes.