While there has been a great deal of discussion in other locations about possible urban heat island effect, there has been relatively little discussion about SST (sea surface temperature) adjustments and NMAT (night marine air temperature) adjustments. This is too bad. I’m not going to get into this, but there are some handy sources which I’ll direct interested people to and make a few short comments. 19th century temperatures are used to "verify" proxy models, but there are some curious inconsistencies between 19th century temperatures and proxy behavior, which I’ll not get into here, but made me go back to look at some of the 19th century adjustments. Generally, it worries me when "adjustments" become as large as the effect. One of the biggest changes in later IPCC reports resulted from new assumptions on 19th century use of wooden versus canvas buckets!. Parker, Folland and Jackson  reported:
Overall global warming in SST between the 1860s and the 1970s is about 0.3 °C greater in the present analysis than in Folland et al. (1984), mainly owing to reduced early corrections applicable under the assumption of the predominant use of wooden buckets (Section 3).
You may recognize Folland as a major IPCC author (Folland et al.  is sometimes the citation). SST sampling is not homogeneous – it changed from buckets to engine inlets – engine inlet temperature ran a little hotter. Both canvas and wooden buckets appear to have been used. There are millions of SST measurements and how the measurement was done is not known for most measurements (as far as I can tell.) There are two main adjustments in Folland et al. The first is a one-time adjustment for from buckets to engine inlets in December 1941. This is premised on a comparison between the "corrected" NMAT temperature [ I haven't checked what these "corrections" are] and the uncorrected SST temperatures. Folland et al. express this as follows:
The abrupt change in SST in December 1941 coincides with the entry of the USA into World War II and is likely to have resulted from a realization of the dangers of hauling sea buckets onto deck in wartime conditions when a light would have been needed for both hauling and reading the thermometer at night.
This is exemplified in the follow graph:
Original Caption: Folland and Parker  Figure 3. Annual anomalies from a 1951-80 average of uncorrected SST (solid) and corrected NMAT (dashed) for (a) northern (b) southern hemisphere, 1856-1992. Only collocated 5 deg. x 5 deg. SST and NMAT values were used.
The idea of an abrupt changeover seems a little weird to me. I’ve also seen reminisinces of an oceanographer taking about taking measurements in a research ship with steel buckets in the 1950s, so I’m not sure how realistic this assumption is. If the changeover were phased in, it would presumably have a material impact on the SST history. It seems like an important enough issue that it shouldn’t be glossed over. The big change in the 1995 Folland work is the conjecture that there was a changeover from wooden buckets to canvas buckets in the late 19th century, which caused a gradual "cooling" of measurements. This enabled them to depress 19th century temperatures a further 0.3 degree C and obtain a more compelling degree of temperature increase. The following graphic shows the difference:
Original Caption: Parker et al. [Clim Chg 1995] Fig. 14. Global anomalies of SST, after Folland et al. (1984) (light continuous smoothed curve), after Bottomley et al. (1990) (dashed smoothed curve) and using MOHSST6 with Folland and Parker (1995) corrections (heavy continuous smoothed curve and annual bars). The dash-dot smoothed curve is NMAT, from the present analysis. Smoothing as Figure 9. Reference period is 1951-80. See here.)
I don’t plan to wade through this material but someone should. I haven’t looked yet to see what the NMAT correction is, but these things are always worth looking at. If Folland’s 1984 version or even a lesser adjustment were applied, then it makes a difference to calibration of the mutiproxy studies. An interesting collection of papers is here.
Reference: D. E. Parker, C. K. Folland and M. Jackson, 1995, MARINE SURFACE TEMPERATURE: OBSERVED VARIATIONS AND DATA REQUIREMENTS, Climatic Change 31: 559-600 here
Folland, C. K., D. E. Parker, and F. E. Kates. 1984. Worldwide marine temperature fluctuations 1856–1981. Nature 310, no. 5979: 670-673.
C. K. Folland and D. E. Parker, 1995, CORRECTION OF INSTRUMENTAL BIASES IN HISTORICAL SEA SURFACE TEMPERATURE DATA, Q.J.R. Meteorolol. Soc. 121, 319-367 here