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I have a piece in @nature today discussing how some early 20th century temperature variation in the North Atlantic and the North Pacific oceans are an artifact of changes in measurement approaches: nature.com/articles/d4158… 1/10
In the early 20th century, ocean temperature estimates were made using buckets that were thrown over the sides of ships, filled with water and hauled up. The temperature of the water in the buckets was then measured using a thermometer. 2/10
While the buckets were being hoisted up, evaporative cooling and exposure to ambient conditions would often reduce the temperature of the water by a few tenths of a degree Celsius. 3/10
Accounting for the cold bias in bucket measurements is the single largest adjustment to the ocean (and global) temperature record. Without the adjustment, the estimated rate of ocean warming from 1850 to the present would be about 30% higher. 4/10
However, how bucket measurements are done varies a lot across ships, and little ship-specific metadata has survived to the present day. As a result, researchers have often had to inaccurately treat many bucket measurements as having the same magnitude of bias. 5/10
Duo Chan and colleagues at Harvard found a clever way to tackle this problem. They looked at the difference between ships within 300 km and 2 days of one another, producing a data set of 6 million measurement pairs between 1908 and 1941. 6/10 nature.com/articles/s4158…
Their results suggest that scientists have been overestimating warming in the North Atlantic and substantially underestimating warming in the North Pacific during the early twentieth century because of not fully accounting for biases in bucket measurements. 7/10 
These findings bring the difference in estimated warming between the two regions in line with projections from climate models. However, there are still large differences between models and observations in the overall rate of global ocean warming during this period. 8/10
This study, and recent major updates to the SST record at the UK Met Office’s Hadley Centre by @micefearboggis et al, provide a useful reminder that large systematic biases might remain in our observational temperature records. 9/10
Improved quantification of these biases is still a key technical challenge for researchers, and will help to address questions about the performance of climate-model simulations of the past and the role of intrinsic climate variability in historical temperature change. 10/10
