A recent study has revealed that the oceans were not as cold during the early 20th century (1900-1930) as previously believed. The perceived chilliness of the ocean during this time can be attributed to the methods used for certain measurements. This discrepancy leads to an inconsistency in the global ocean surface temperature data from that era when compared to land air temperatures and paleoclimatic records, indicating that the differences between land and ocean temperatures are greater than what climate models suggest.
A recent study published in the journal Nature indicates that the oceans were warmer during the early 20th century (1900-1930) than has been assumed until now. This misjudgment stems from how certain temperature measurements were conducted, resulting in global ocean surface temperatures for this period being at odds with land air temperatures and paleoclimate data. The disparities between terrestrial and ocean temperatures are more pronounced than depicted in climate models.
This finding is significant as it may reshape our understanding of historical climate fluctuations and future climate trends. However, Dr. Sebastian Sippel, the lead author and a junior professor at Leipzig University, emphasizes that this new information does not alter the documented global warming trends relative to the period of 1850-1900 or the human impact on that warming: the temperature data for land and ocean from the 19th century (1850-1900) presents a consistent overview of temperature variations up to the present day. Despite this, rectifying the perceptions of this cold period could bolster confidence in the observed warming levels, adjust our understanding of historical climate shifts, and enhance future climate model accuracy.
Grasping global temperature trends is essential for climate science. Dr. Sippel, who specializes in Climate Attribution at Leipzig University, collaborated with international researchers to piece together the global mean temperature from a wealth of historical climate data, akin to assembling a jigsaw puzzle. This included analyzing historical land and ocean measurements alongside paleoclimatic studies. In his comparisons, Sippel identified a notable systematic deviation: early 20th century ocean temperatures were lower than those of preceding decades, while surface air temperatures over land showed stability. This observation does not align with existing physical theories and climate models.
Fresh insights into historical events
With various lines of evidence, this new study indicates that reconstructions estimating the global mean temperature from ocean surface data during this timeframe are lower than they should be—approximately 0.26 degrees Celsius colder than land-based reconstructions. This gap exceeds what could be attributed to natural climate variability. “While our recent discoveries do not modify the long-term warming trend since 1850, they allow for a deeper understanding of past climate shifts and variability,” states Dr. Sippel. The factors influencing the warming trends from 1900 to 1950 remain unclear, but if ocean temperatures are adjusted, the early 20th-century warming trend appears less pronounced. “The differences noted between climate models and the temperature patterns observed at the start of the 20th century primarily stem from an inadequate grasp of the measurements, rather than shortcomings in climate models or natural variability. Well-established methods exist to address the effects of evolving measurement techniques on ocean surface temperature data. Our findings indicate that during the early 20th century, these methods failed to account fully for the dramatically changing observation techniques in use. This new clarity supports climate models and further illustrates the human influence since pre-industrial times,” explains co-author Professor Reto Knutti, a Professor of Climate Physics at ETH Zurich.
A comprehensive approach
The study indicates that the origins of the ocean cold anomaly may reside in insufficiently documented measurement techniques from that period. Before World War II, ocean temperatures were primarily obtained using buckets on boats; however, the measurement techniques and fleet compositions shifted from decade to decade, complicating efforts to correct for persistent measurement errors. Consequently, the authors propose diverse strategies for data processing and analysis, stating: “Our methodological approach highlights the necessity of continuously safeguarding and digitizing historical climate records while aligning them with independent data sources. Concurrently, various assumptions regarding systematic adjustments to early climatic data should be thoroughly investigated, as these observational records are essential for understanding and modeling climate,” remarks Sippel.
