2018 was the fourth warmest year in the global surface temperature record, according to year end updates from NASA, NOAA and the UK MetOffice, albeit somewhat cooler than the 2016 peak. This continues a string of record or near-record annual temperatures, with the top four occurring in the past four years.
As in my previous updates (see here and here), I’ll provide some additional context, highlighting the issue of coverage bias and ongoing Arctic warming. Series that use statistical methods to account for areas with few or no observations, especially in the Arctic, continue to diverge markedly from those with little or no interpolation. Recent observations also show the emergence of a seasonal pattern in Arctic warming, an important aspect of coverage bias that has been less explored thus far. In addition, the last decade or so has seen the emergence of ship-buoy SST bias, first identified in a key 2015 paper from NOAA and confirmed in our 2016 paper.
The recent synchronized update of all four of the most closely followed temperature series (along with the buoy-based SST series from our 2016 paper, Hausfather et al) has inspired a long overdue surface temperature update. First I’ll look at the four global temperature series over the last 35 years, and then examine 2017 in the context of recently identified biases in GMST records.
In retrospect, 2014 was an interesting year for climate watchers, and a turning point of sorts. That year, global mean surface temperature matched record highs for the instrumental period, without any assist from the El Nino weather pattern that usually accompanies such warm years. That turned out to be a prelude to a record-smashing 2015. And 2016 may well provide an unprecedented third surface temperature record in a row, as the influence of the current super El Nino will likely peak in the first half of this year.
But 2014 was also notable scientifically for the emergence of a previously under-examined scientific issue: namely coverage bias in observed surface temperature series, especially the HadCrut4 record issued by the UK Met Office.
This most widely cited temperature series does not account for missing areas, especially in high latitudes, likely leading to an underestimate of the overall rise in global temperature since the 19th century.
Not only that, but there is increasingly compelling evidence that the recent short-term slowdown in the surface temperature record was much less pronounced than previously estimated, if rapid Arctic warming is fully reflected, along with potential biases from the changing mix of sea surface temperature measurement sources in recent years. Thus the discrepancy between very short-term and multi-decadal trends in the observations appears to have been exaggerated in prior estimates, including IPCC AR5.