A reassessment of temperature variations and trends from global reanalyses and monthly surface climatological datasets

The ERA‐Interim and JRA‐55 reanalyses of synoptic data and several conventional analyses of monthly climatological data provide similar estimates of global‐mean surface warming since 1979. They broadly agree on the character of interannual variability and the extremity of the 2015/2016 warm spell to which a strong El Niño and low Arctic sea‐ice cover contribute. Nevertheless global and regional averages differ on various time‐scales due to differences in data coverage and sea‐surface temperature analyses; averages from those conventional datasets that infill where they lack direct observations agree better with the averages from the reanalyses. The latest warm event is less extreme when viewed in terms of atmospheric energy, which gives more weight to variability in the Tropics, where the thermal signal has greater vertical penetration and latent energy is a larger factor. Surface warming from 1998 to 2012 is larger than indicated by earlier versions of the conventional datasets used to characterize what the Fifth Assessment Report of the Intergovernmental Panel on Climate Change termed a hiatus in global warming. None of the datasets exhibit net warming over the Antarctic since 1979. Centennial trends from the conventional datasets, HadCRUT4 on the one hand and GISTEMP and NOAAGlobalTemp on the other, differ mainly because sea‐surface temperatures differ. Infilling of values where direct observations are lacking is more questionable for the data‐sparse earlier decades. Change since the eighteenth century is inevitably more uncertain than change over and after a modern baseline period. The latter is arguably best estimated separately for taking stock of actions to limit climate change, exploiting reanalyses and using satellite data to refine the conventional approach. Nevertheless, early in 2016 the global temperature appears to have first touched or briefly breached a level 1.5 °C above that early in the Industrial Revolution, having touched the 1.0 °C level in 1998 during a previous El Niño. Global‐average surface‐air temperatures reached levels in 2015 and 2016 that are by a clear margin unprecedented in estimates based on measurements made since the 19th Century. Temperatures peaked about 1.5°C above typical values around the start of the industrial era. The paper assesses various estimates of the fluctuations and trends of global atmospheric temperature, and relates differences to incomplete data coverage and uncertainties in sea‐surface temperature.