Is the world 1.3°C or 1.5°C warmer? Historical ship logs hold answers

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https://www.science.org/content/article/world-1-3%C2%B0c-or-1-5%C2%B0c-warmer-historical-ship-logs-hold-answers

By PAUL VOOSEN, Science. 

Excerpt: Last month’s announcement that 2023 was the hottest year in history was no surprise. But it came with one: No one knows exactly how much the world has warmed. One group of climate scientists found the planet has warmed 1.34°C over the 1850–1900 average, whereas another found temperatures had risen 1.54°C. ...the current disagreement is not over present temperatures, but rather the past. The warmth of the ocean in the late 19th century is a key part of the baseline against which the warming of the planet is measured—and figures are at odds. ...No estimate of global temperature is possible without including the oceans, which cover 70% of the planet’s surface. ...But ocean temperature records in the 19th century were few and far between. A global record began in the 1850s thanks to a controversial figure, Matthew Fontaine Maury, a superintendent at the U.S. Naval Observatory who avidly supported slavery and would go on to serve the Confederacy. ...he encouraged merchant sailors to collect weather observations, including measurements of water temperature from buckets heaved to the deck; if captains shared the data with the government, they would receive naval charts in return. ...Today, two organizations maintain these historical sea surface temperature records: the U.S. National Oceanic and Atmospheric Administration (NOAA) and the U.K.’s Met Office. They both catalog the same underlying data, but differ in how they approach a key question. “How to correct the bucket temperature?” .... NOAA does so by cross-checking the bucket temperatures with air temperatures taken at the same place and time, whereas the Met Office relies on a “bucket model” to estimate the water’s temperature before it was scooped up. ...after the 1930s, temperature measurements from Japanese ships tended to be 0.35°C colder than those from other countries. This wasn’t because of any oddity in Japanese data collecting. Rather, when the U.S. Air Force was digitizing these records after World War II, putting them on punch cards, it dropped the decimal to save space. “They floored everything to the whole degree,” Chan says. A staggering number of logbooks have yet to be digitized, says Ed Hawkins, a climate scientist at the University of Reading. The U.K.’s National Archives has 6 million pages that are so far untouched, for example. “We could at least double the quantity of data we have available,” Hawkins says....

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