|Title||Estimates of variations and trends of global surface temperature|
|Series/Collection||ERA Report Series|
|Authors||Simmons, A, Berrisford, P, Dee, D, Hersbach, H, Hirahara, S, Thépaut, J-N|
|Event Series/Collection||ERA Report|
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 recent warm spell to which a strong El Niño and low Arctic sea-ice cover contribute. Global and regional averages nevertheless differ on various timescales due to differences in data coverage and sea-surface temperature analyses; averages from those conventional datasets that infill where they lack direct observations show better agreement with the averages from the reanalyses. 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 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 the18th 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, however briefly, global temperature appears to have first reached or breached a level 1.5°C above that early in the Industrial Revolution, having touched the 1.0°C level briefly in 1998 during a previous El Niño.
Atmospheric energy is an alternative metric for tracking change. It gives more weight to tropical than high-latitude variability, due to the greater vertical penetration of the thermal signal and importance of latent energy in the tropics.