Early-twentieth-century cold bias in ocean surface temperature observations
Nature volume 635, pages 618–624 (2024 )Cite this article
The observed temperature record, which combines sea surface temperatures with near-surface air temperatures over land, is crucial for understanding climate variability and change1,2,3,4. However, early records of global mean surface temperature are uncertain owing to changes in measurement technology and practice, partial documentation5,6,7,8, and incomplete spatial coverage9. Here we show that existing estimates of ocean temperatures in the early twentieth century (1900–1930) are too cold, based on independent statistical reconstructions of the global mean surface temperature from either ocean or land data. The ocean-based reconstruction is on average about 0.26 °C colder than the land-based one, despite very high agreement in all other periods. The ocean cold anomaly is unforced, and internal variability in climate models cannot explain the observed land–ocean discrepancy. Several lines of evidence based on attribution, timescale analysis, coastal grid cells and palaeoclimate data support the argument of a substantial cold bias in the observed global sea-surface-temperature record in the early twentieth century. Although estimates of global warming since the mid-nineteenth century are not affected, correcting the ocean cold bias would result in a more modest early-twentieth-century warming trend10, a lower estimate of decadal-scale variability inferred from the instrumental record3, and better agreement between simulated and observed warming than existing datasets suggest2.
Global mean surface temperature (GMST) is a crucial indicator of climate change and is essential for guiding climate policies such as the Paris Agreement11. It reflects key aspects of Earth’s global temperature variability, such as the response to external forcing and large-scale ocean–atmosphere variability1,12. Instrumental GMST datasets, which blend sea surface temperature (SST) with surface air temperature over land and sea ice (LSAT)13, agree broadly on long-term changes and variability4. However, assessing the accuracy and consistency of SSTs and LSATs in the early record is challenging, because of (1) observational uncertainties and biases, (2) incomplete coverage, and (3) different physical processes affecting the sea surface and land air temperatures. First, SSTs and LSATs are derived from different measurement techniques and protocols, introducing distinct biases and uncertainties. Early SST records, collected before the Second World War (1939–1945), primarily utilized ship-based bucket measurements. The transition within the early record from wooden to canvas buckets around the late nineteenth century14, combined with the shifting patterns of shipping routes and shipping fleets, complicates systematic bias adjustments and adds to their uncertainty. Furthermore, essential metadata are often incomplete or missing6,15. Engine-room intake measurements replaced buckets over time, and much work has focused on understanding the biases of buckets relative to this more modern measuring technique16,17.
