Most estimates of ocean warming only go back as far as the 1950s, owing to limited data. A new study published on 7 January in PLoS One presents a reconstruction of historical ocean warming from 1871 to 2017 covering the full ocean depth across the entire globe.
Oceans cover nearly 70 per cent of the Earth’s surface and play an important role in the Earth’s climate and global warming. Yet, remain poorly understood owing to the enormous difficulty of probing deep layers and the sparseness of data ― in particular, the paucity of observations in the tropical Atlantic and Indian oceans. Moreover, before the 1990s, most ocean temperature measurements were above 700 m; therefore, the data is not sufficient on its own to accurately estimate global ocean warming. According to the authors, near-global data coverage has been only achieved since 2006 and earlier observations were geographically more sparse and mainly restricted to shallower depths.
To achieve an accurate global estimate for the warming of the full ocean depth before the 1950s, the international team of researchers, led by Prof Laure Zanna from the University of Oxford, combined observed sea surface temperatures with a mathematical tool called Green’s function to represent the ocean transport processes. The mathematical approach was originally developed by Prof Samar Khatiwala, also from the University of Oxford, to reconstruct CO2 uptake by the ocean.
For 1955–2017, the estimates are comparable to direct estimates available for the same time period at the same depths. Furthermore, the increase in ocean warming during 1921–1946 was shown to be as big as the increase from 1990–2015. The findings also suggest that over the past 60 years, as much as half of the observed warming may have been caused by changes in ocean circulation resulting in sea level rises in the low- and mid-latitudes of the Atlantic Ocean. Changes in circulation have led to more heat accumulation at lower latitudes.
The ocean is driven by the atmosphere and vice versa. Moreover, ocean circulation is a response to atmospheric forcing, i.e., changes in wind stress, heat, and moisture. Thus, future changes in these processes cause by human-induced climate change could have severe consequences. Most of the excess energy stored in the climate system due to anthropogenic greenhouse gas emissions have been taken up by the oceans, leading to thermal expansion and sea-level rise. Between 1971 and 2010, the ocean absorbed more than 90 per cent of the additional heat resulting from global warming ― mostly held in the top 2000 meters (2).
More work is still needed to validate the method but so far, the new estimates support previous evidence that oceans are absorbing most of the excess energy in the climate system arising from greenhouse gases emitted by human activities. As the authors write, “Monitoring and understanding OHC [ocean heat content] change and the role of circulation in shaping the patterns of warming remain key to predicting global and regional climate change and sea-level rise.” In other words, understanding the cause of historical changes can help to predict ― and to better prepare for ― future warming patterns.
(1) Zanna, L. et al. Global reconstruction of historical ocean heat storage and transport. PNAS (2019) DOI: 10.1073/pnas.1808838115
(2) Cheng, L. et al. Improved estimates of ocean heat content from 1960 to 2015. Science Advances (2017). DOI: 10.1126/sciadv.1601545