A century and half of reconstructed ocean warming offers clues for the future08 January 2019
Scientists at the University of Reading have helped reconstruct ocean temperature change from 1871 to 2017, where previously these estimates only started in the 1950s.
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 team of scientists, led by the University of Oxford, estimate that the world's oceans have warmed the equivalent of around 1,000 times the annual global human primary energy consumption since 1871. They also show that comparable warming happened over the periods 1920-1945 and 1990-2015.
"Understanding the way heat is transported around the oceans is important to give us confidence in predicting sea level rises that could be caused by climate change in the future" - Professor Jonathan Gregory, University of Reading
The estimates, published in the journal PNAS, calculate global warming of the oceans at 436 x 10²¹ Joules. This supports evidence that the oceans are absorbing most of the excess energy in the climate system arising from greenhouse gases emitted by human activities.
Professor Jonathan Gregory, co-author at the University of Reading, said: "This new method agrees with others for the period during which observations of ocean temperatures below the surface are available, namely the last few decades. Secondly, the new method allows us to do things which can't be done with existing ocean temperature observations alone. It allows us to estimate heat uptake for periods before these records began, and to show for the first time that changes in ocean transport have had an important effect.
"Understanding the way heat is transported around the oceans is important to give us confidence in predicting sea level rises that could be caused by climate change in the future."
Professor Laure Zanna (Physics), who led the international team of researchers said: "Our reconstruction is in line with other direct estimates and provides evidence for ocean warming before the 1950s."
A 200-year-old idea
The researchers’ technique to reconstruct ocean warming is based on a mathematical approach originally developed by Professor Samar Khatiwala (Earth Sciences) to reconstruct manmade CO2 uptake by the ocean. Professor Khatiwala said: "Our approach is akin to 'painting' different bits of the ocean surface with dyes of different colors and monitoring how they spread into the interior over time. We can then apply that information to anything else - manmade carbon, heat anomalies, etc - that is transported by ocean circulation.
"The idea goes back nearly 200 years to the English mathematician George Green. The bottom line is that if you know what the sea surface temperature anomaly was in 1870 in the North Atlantic ocean we can figure out how much it contributes to the warming in, say, the deep Indian Ocean in 2018."
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 are 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. Professor Zanna said: "Strictly speaking, the technique is only applicable to tracers like manmade carbon that are passively transported by ocean circulation. However, heat does not behave in this manner as it affects circulation by changing the density of seawater.
"We were pleasantly surprised by how well the approach works. It opens up an exciting new way to study ocean warming in addition to using direct measurements."
This work offers an answer to an important gap in knowledge of ocean warming, but is only a first step. It is important to understand the cause of the ocean circulation changes to help predict future patterns of warming and sea level rise.
Full paper reference:
Zanna, L., Khatiwala, S., Gregory, J., Ison, J. and Heimbach, P. (2019); 'Global reconstruction of historical ocean heat storage and transport'; Proceedings of the National Academy of Sciences of the United States of America (PNAS); doi/10.1073/pnas.1808838115