By Zeke Hausfather
In recent weeks we’ve seen a political controversy over NOAA’s adjustments to temperature records, with accusations from some in congress that records are being changed to eliminate a recent slowdown in warming and to lend support to Obama administration climate policies. This makes it sound like the NOAA record is something of an outlier, while other surface temperature records show more of a slowdown in warming. This is not true; all of the major surface temperature records largely agree on temperatures in recent years. This includes independent groups like Berkeley Earth that receive no government funding. A record warm 2014 and 2015 (to date) has largely eliminated any slowdown in temperatures, whether data is adjusted or not.
Figure 1, below, shows a 12-month running average of temperatures for each of the five series. All are quite similar in trajectory, with only small differences between each series. In all five the last 12 months have been the warmest on record, and 2015 will almost certainly be the warmest year on record in all five.
It’s worth noting that while each group has a different methodology, much of the underlying data is the same. NOAA, the UK’s Hadley Centre, and independent researchers Cowtan and Way (C&W) use effectively the same land data (the Global Historical Climatological Network, or GHCN for short). NASA also uses GHCN data, but adds additional stations in the U.S. and in the Arctic and Antarctic. Berkeley uses a much larger set of stations (about 36,000 stations, compared to around 7,000 for the other records), though NOAA will be switching to a similarly large database of stations soon. Berkeley, Hadley, and C&W all use a sea surface temperature series called HadSST3 produced by the Hadley Centre. NOAA and NASA use a sea surface temperature series called ERSST (version 4) produced by NOAA.
Automated adjustments to land data to remove detected problems like station moves or instrument changes are used by NOAA, NASA, and Berkeley Earth; Hadley and C&W do relatively little adjustments to land data outside of quality control. NOAA and Hadley only calculate temperatures in areas with nearby stations, while NASA, Berkeley, and C&W fill in areas without stations based on statistical techniques using the nearest available stations.
No matter which groups’ record you use, you end up with a pretty similar global temperature record. Figure 2, below, shows the trend in temperatures for three periods: 1950-present, 1970-present, and the nominal “slowdown” period of 1998-present. It shows that while the 1998-present period is warming a tad slower than the 1970-present period on average, the uncertainties are large and the warming rate over the post-1998 period is pretty much the same as the longer 1950-present period.
The relatively lower 1998-present trends are also a result of cherry-picking the 1998 El Nino as a start date (since temperature were anomalously high at that point, the trend thereafter will be lower than starting before or after the El Nino event). For example, calculated trends from 1996 or 2000-present are more similar to the 1970-present trends.
The actual adjustments that NOAA does to the record have a relatively small impact on temperatures in recent years, though small changes can have outsized impacts when calculating short-term trends. The larger impacts of NOAA adjustments by far are in the early part of the record, where they raise temperatures compared to the unadjusted series. Contrary to what most folks assume, the net effect of adjustments is to reduce, not increase, the amount of warming that we’ve experienced over the past century.
The fact that independent groups like Berkeley Earth find results nearly identical to NOAA should help put to rest and lingering concerns that some nefarious scheme has been hatched among scientists to cook the proverbial book. Rather, temperature data is complex and inhomogeneous, coming from multiple different sources and instruments over the past 250 years. Some adjustments are needed when switching from buckets to ship engine intake valves to buoys, as each will read temperatures a bit differently. The overall effect of these adjustments is small on a global level, however, and they have relatively little bearing on our understanding of modern warming.
