Ocean cold period in the early 20th century less pronounced than thought
The oceans were less cold in the early 20th century (1900–1930) than previously thought. During this period the ocean appears too cold due to the way some measurements were taken. However, the new findings do not affect the quantification of the global warming relative to 1850-1900 and the human contribution to that warming, stress climate researchers around Sebastian Sippel (Leipzig University) and Reto Knutti from ETH Zurich.
Global temperature estimates often combine ocean and land surface data. However, interpreting early 1900s data can be challenging due to advancements in measurement technologies over the years. According to climate researchers, this makes global ocean surface temperature measurements during this period inconsistent with both land air temperatures and palaeoclimatic data and the differences between land and ocean are larger than shown in climate models.
Correcting the cold period
The newly published discovery has far-reaching implications for our understanding of past climate variability and future climate change. However, the new findings do not affect the quantification of the global warming relative to 1850-1900 and the human contribution to that warming, emphasizes Sebastian Sippel, Junior Professor for Climate Attribution at Leipzig University. The land and ocean temperatures of the 19th century (1850–1900), before the onset of the cold period, provide a physically very consistent picture of temperature changes up to the present day. Nevertheless, correcting this cold period could increase confidence in the amount of observed warming, changing what we know about historical climate variability and improve the quality of future climate models.
An eye on measurement methods
Understanding global temperature trends is crucial for climate research. However, the temperature measurements in the ocean from the time before the Second World War are subject to uncertainties in the range of one to two tenths of a degree due to changing measurement methods. Sebastian Sippel worked with international scientists to reconstruct the global mean temperature from historical climate data like a jigsaw puzzle – including historical land and ocean measurements and palaeoclimatic analyses. When comparing land and ocean, the researcher noticed a systematic deviation: at the beginning of the 20th century, ocean temperatures were lower than in previous decades, while over land air temperatures remained relatively constant. This outcome is not consistent with physical theory and climate models.
“Our new understanding confirms the climate models and shows even more clearly the human impact since pre-industrial times.”Reto Knutti, Professor for Climate Physics at ETH Zurich
Revised data confirms climate models
Using many different lines of evidence, the new study shows that reconstructions of the global mean temperature from ocean surface data for this period are too cold: on average about 0.26 degrees Celsius colder than seen in land-based reconstructions. This discrepancy is greater than what would be possible under natural climate variability. "Our latest findings do not change the long-term warming since 1850. However, we can now better understand historical climate change and climate variability," says Sippel. For example, the reasons for the early 20th century warming period between 1900 and 1950 have never been fully understood. If the ocean temperatures are corrected, the warming trend of the early 20th century is weaker. "The discrepancies between the climate models and the observed temperature trend at the beginning of the 20th century are mainly due to an incomplete understanding of the observations, rather than incomplete climate models or natural climate variability. There are well-established approaches to account for the effects of changing measurement methods on ocean surface temperature measurements. The new research shows that in the early 20th century these methods don’t properly account for very rapidly changing differences in the way the observations were made. Our new understanding confirms the climate models and shows even more clearly the human impact since pre-industrial times," says Reto Knutti, Professor for Climate Physics at ETH Zurich.
Why historical climate data is relevant
The study itself offers indications that the cause of the ocean cold anomaly could lie in insufficiently documented information about the measurement techniques used at that time. Before the Second World War, ocean temperatures were mainly measured with buckets on ships, but the method of measurement and the composition of ship fleets changed from decade to decade, making it much more difficult to correct for systematic measurement errors. The authors of the study therefore recommend a variety of approaches to data processing and analysis: "Our methodological approach emphasises the need to continuously rescue and digitise historical climate data and compare it with independent data. At the same time, very different assumptions regarding systematic adjustments of early climate data should be tested, as the observational data are of central importance as a basis for climate understanding and modelling," says Sippel.
Reference
Sippel S, Kent EC, Meinshausen N et al. Early-20th-century cold bias in ocean surface temperature observations. Nature 635, 618–624 (2024). doi: external page 10.1038/s41586-024-08230-1