The new version of the HadSST4 has reassigned ship based measurements from before WW II into the early 1990s. Bias adjustments depend on the fraction of measurements using wooden or canvas buckets and engine room intakes (ERI), which are partly defined by the metadata in ICOADS based on ship logs. The assignment of each measurement to the type of bucket or ERI is sometimes uncertain. HadSST4 now use instead the diurnal temperature dependence of the measurements (time of day) to identify which measurement type was used by each ship. The overall bias adjustment to SST will change if this procedure changes the fraction of data falling into into each category since each adjustments is different.
They claim that 75% of measurements could be classified in this way, and that buckets were still in use in US ships into the early 1990s. Since then measurements are based on floating buoys and Argo buoys and these recent temperatures measurements are unaffected. However that doesn’t matter because the crucial 1961-1990 normalisation period certainly is affected and HadSST4 only publishes temperature anomalies – not absolute temperatures. So the net effect of the new assignments is to to lower the zero line (normals) from which anomalies are calculated, and as a result all recent anomalies have indeed increased in ‘temperature’. Hey presto the oceans have now warmed by an extra 0.1C.
We saw in the last post how moving from V3 to V4C had boosted warming when global temperatures are calculated in 3D using HadSST3. So what happens if instead we now use HadSST4 instead of HadSST3?
Recent temperatures get a boost increasing the apparent 2016 temperature by 0.25C compared to the latest HadCRUT4. The record 2016 now stands at 1.05C or 1.45C above the pre-industrial era. Temperature anomalies are wonderful things. Changes to the past can affect the future. So expect to see alarmist headlines in the press soon once HadSST4 gets integrated into Berkeley Earth or into a future HadCRUT5?
V4/HADSST4 also kind of gazumps the Paris Agreement since the 1.5C target seems to have been almost been reached already in 2016.