From Karl et al 2015
First, several studies have examined the differences between buoy- and ship-based data, noting that the ship data are systematically warmer than the buoy data (15–17). This is particularly important, as much of the sea surface is now sampled by both observing systems, and surface-drifting and moored buoys have increased the overall global coverage by up to 15% (see supplemental material for details). These changes have resulted in a time-dependent bias in the global SST record, and various corrections have been developed to account for the bias (18). Recently, a new correction (13) was developed and applied in the Extended Reconstructed Sea Surface Temperature dataset version 4, which we use in our analysis. In essence, the bias correction involved calculating the average difference between collocated buoy and ship SSTs. The average difference globally was −0.12°C, a correction which is applied to the buoy SSTs at every grid cell in ERSST version 4. [Notably, IPCC (1) used a global analysis from the UK Met Office that found the same average shipbuoy difference globally, although the corrections in that analysis were constrained by differences observed within each ocean basin (18).] More generally, buoy data have been proven to be more accurate and reliable than ship data, with better known instrument characteristics and automated sampling (16). Therefore, ERSST version 4 also considers this smaller buoy uncertainty in the reconstruction (13)
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I don't wonder that you wonder. I'd guess that the buoy data was made to match the engine intake data in order to eliminate the "time-dependent bias" without having to alter data going back to the invention of the thermometer. But that's just a guess.
Do you agree that starting an analysis - as you folks love to do, with an enormous el Nino like 1998's will affect the observed trends?
Do you agree that coverage has been poor at the poles and that increased coverage there will show increased warming?