Abstract
Observed surface temperature trends over the period 1998–2012/2014 have attracted a great deal of interest because of an apparent slowdown in the rate of global warming, and contrasts between climate model simulations and observations of such trends. Many studies have addressed the statistical significance of these relatively short-trends, whether they indicate a possible bias in the model values and the implications for global warming generally. Here we re-examine these issues, but as they relate to changes over much longer-term changes. We find that on multi-decadal time scales there is little evidence for any change in the observed global warming rate, but some evidence for a recent temporary slowdown in the warming rate in the Pacific. This multi-decadal slowdown can be partly explained by a cool phase of the Interdecadal Pacific Oscillation and a short-term excess of La Niña events. We also analyse historical and projected changes in 38 CMIP climate models. All of the model simulations examined simulate multi-decadal warming in the Pacific over the past half-century that exceeds observed values. This difference cannot be fully explained by observed internal multi-decadal climate variability, even if allowance is made for an apparent tendency for models to underestimate internal multi-decadal variability in the Pacific. Models which simulate the greatest global warming over the past half-century also project warming that is among the highest of all models by the end of the twenty-first century, under both low and high greenhouse gas emission scenarios. Given that the same models are poorest in representing observed multi-decadal temperature change, confidence in the highest projections is reduced.
Highlights
The behaviour of globally average surface temperatures over the period 1998 to 2012/2014 has attracted a great deal of attention, with questions being raised as to whether the observed values represented a serious hiatus in global warming or whether they corresponded to a decadal-scale fluctuation associated with internal variability of the climate system (Lewandowsky et al 2015, 2016)
We consider the role of the Interdecadal Pacific Oscillation (IPO), the frequency of El Niño-Southern Oscillation (ENSO) events, and internal climate variability more broadly in accounting for the differences between observed and simulated multi-decadal changes in the Pacific and global temperatures
We refer to Pacific Ocean averages calculated for the box region 10 °S to 25 °N and 160 °E to 110 °W. This region has been chosen since it encompasses a node of the Interdecadal Pacific Oscillation, a pattern of sea surface temperatures (SSTs) variability at decadal time scales centred on the Pacific Ocean (England et al 2014; Kirtman et al 2013; Power et al 1999; Folland et al 1999; Meehl and Arblaster 2012; Salinger et al 2001; Henley et al 2015)
Summary
The behaviour of globally average surface temperatures over the period 1998 to 2012/2014 has attracted a great deal of attention, with questions being raised as to whether the observed values represented a serious hiatus (or pause) in global warming or whether they corresponded to a decadal-scale fluctuation associated with internal variability of the climate system (Lewandowsky et al 2015, 2016). They concluded, that “most simulations of the historical period do not reproduce the observed reduction in global mean surface warming trend over the last 10–15 years” They concluded that these differences were due, to a substantial degree, to internal variability in the real world with possible contributions from forcing errors and, for some climate models, an overestimation of the response to increasing greenhouse gases (GHGs) and other anthropogenic forcings including sulphate aerosols We consider the role of the Interdecadal Pacific Oscillation (IPO), the frequency of El Niño-Southern Oscillation (ENSO) events, and internal climate variability more broadly in accounting for the differences between observed and simulated multi-decadal changes in the Pacific and global temperatures This is an important extension of previous work to longer time-scales.
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