Abstract
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.
Highlights
The latest two assessment reports of the Intergovernmental Panel on Climate Change (IPCC) have stated that warming of the climate system is unequivocal, citing among other evidence the increases in global average surface air and ocean temperatures inferred from observations (IPCC, 2007, 2013)
MERRA-2 does not benefit from an analysis of synoptic surface air temperature observations such as is used in ERA-Interim and JRA-55, but this screen-level analysis adds relatively little for these two reanalyses, as their background fields are close to the analysis fields
HadCRUT4 gives smaller values, 3.0 ◦C and 3.4 ◦C, but its lower spatial resolution has to be taken into account: ERA-Interim anomalies are reduced to 2.9 ◦C and 3.4 ◦C respectively if an equivalent of the 5◦ × 5◦ HadCRUT4 dataset is first constructed from ERA-Interim, and averaged over European land areas following what was done for HadCRUT4
Summary
The latest two assessment reports of the Intergovernmental Panel on Climate Change (IPCC) have stated that warming of the climate system is unequivocal, citing among other evidence the increases in global average surface air and ocean temperatures inferred from observations (IPCC, 2007, 2013). The Paris Agreement includes an undertaking to take stock periodically of progress towards achieving its purpose and long-term goals, starting in 2023 and continuing at intervals of 5 years unless subsequently decided otherwise This points to a continuing need to reduce uncertainties in estimates of surface temperature changes and to improve the interpretation of the sub-decadal variations in a temperature record that combines interacting effects of anthropogenic and natural external forcings and internal variability of the climate system. Comprehensive reanalyses that use fixed modern data assimilation systems to synthesize states of the atmosphere and interacting components of the climate system from past and present observations have the potential to make a substantial contribution to satisfying these requirements They provide globally complete estimates of many of the key variables with a frequency and resolution that are becoming increasingly high in newer products, and provide global and regional averages that can be competitive with those from conventional monthly temperature and humidity products for identifying trends and low-frequency variability over recent decades.
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