Dating Hiatuses: A Statistical Model of the Recent Slowdown in Global Warming – and the Next One
Much has been written about the so-called hiatus or pause in global warming, also known as the stasis period, the start of which is typically dated to 1998. HadCRUT4 global mean temperatures slightly decreased over 1998-2013, though a simple statistical model predicts that they should have grown by 0:016_C/yr, in proportion to the increases in concentrations of well-mixed greenhouse gases and ozone. We employ a statistical approach to assess the contributions of model forcings and natural variability to the hiatus. We find that none of the model forcings explain more than a fifth of the missing heat and that the El Nino Southern Oscillation (ENSO) explains at least half and possibly more than four fifths of the missing heat. Looking forward, the simple model also fails to explain the large increases since then (0:087_C/year over 2013-2016). This period coincides with another El Nino, but the ENSO fails to satisfactorily account for the increase. We propose instead a semiparametric cointegrating statistical model that augments an energy balance model with a novel multibasin measure of the oceans' multidecadal temperatures cycles. The model partially explains the recent slowdown and explains nearly all of the subsequent warming. The natural cycle suggests the possibility of a much longer hiatus over roughly 2023-2061, with rather important policy implications.