On the Time-Varying Trend in Global-Mean Surface Temperature

The Earth has warmed at an unprecedented pace in recent decades. In assessing how much of this warming is natural and how much of it is human-induced it is useful to partition the global-mean surface temperature into the secular trend and the (oscillatory) multidecadal variability. Previously, we showed that the rapidity of the warming in recent decades was a result of concurrence of a secular warming trend and the warming phase of a multidecadal (~65-year period) oscillation and we estimated the contribution of the former to be about 0.08°C per decade since ~1980. Here we demonstrate the robustness of those results and focus on their physical interpretation, considering in particular the shape of the secular trend and the spatial patterns associated with the secular trend and the multidecadal variability. The shape of the secular trend and rather globally-uniform spatial pattern associated with it are both suggestive of a response to the buildup of well-mixed greenhouse gases. In contrast, the multidecadal variability tends to be concentrated over the extratropical Northern Hemisphere and particularly over the North Atlantic, suggestive of a possible link to low frequency variations in the strength of the thermohaline circulation. Depending upon the assumed importance of the contributions of ocean dynamics and the time-varying aerosol emissions to the observed trends in global-mean surface temperature, we estimate that up to half the late 20th century warming could have been a consequence of natural variability. In addition, we show that the long term global warming associated with the secular trend is not accelerating in recent decades.