The reconstruction, created by a team of Oxford scientists let by Wadham’s Professor Laure Zanna is published in the leading journal PNAS. (Proceedings of the National Academy of Sciences of the United States of America).
Due to a scarcity of data, most global estimates of ocean warming start only in the 1950s. Zanna's University of Oxford team has now succeeded in reconstructing ocean temperature change from 1871 to 2017.
Over the past century, increased greenhouse gas emissions have given rise to an excess of energy in the Earth system. More than 90% of this excess energy has been absorbed by the ocean, leading to increased ocean temperatures and associated sea level rise, while moderating surface warming.
The multi-disciplinary Oxford team has published estimates in PNAS, that global warming of the oceans of 436 x 1021 Joules has occurred from 1871 to present (roughly 1000 times annual worldwide human primary energy consumption) and that comparable warming happened over the periods 1920-1945 and 1990-2015.
The estimates support evidence that the oceans are absorbing most of the excess energy in the climate system arising from greenhouse gases emitted by human activities.
Professor Zanna said: "Our reconstruction is in line with other direct estimates and provides evidence for ocean warming before the 1950s."
The researchers’ technique to reconstruct ocean warming is based on a mathematical approach originally developed by Prof Samar Khatiwala (Earth Sciences) to reconstruct manmade CO2 uptake by the ocean.
The new estimate suggests that in the last 60 years up to half the observed warming and associated sea level rise in low- and mid- latitudes of the Atlantic Ocean is due to changes in ocean circulation. During this period, more heat has accumulated at lower latitudes than would have if circulation were not changing. While a change in ocean circulation is identified, the researchers cannot attribute it solely to human-induced changes.
Much work remains to be done to validate the method and provide a better uncertainty estimate, particularly in the earlier part of the reconstruction. However the consistency of the new estimate with direct temperature measurements gives the team confidence in their approach.