A century and half of reconstructed ocean warming offers clues for the future

January 07, 2019

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 have 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.

Prof 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.'

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.

Prof 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 - for example manmade carbon or heat anomalies - that is transported by ocean circulation. If we 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 idea goes back nearly 200 years to the English mathematician George Green.'

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.

Prof 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 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.
-end-
For more information, please contact the University of Oxford press office at ruth.abrahams@admin.ox.ac.uk or Prof Laure Zanna on: laure.zanna@physics.ox.ac.uk

Full paper title: 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

Notes to editors

Oxford University has been placed number 1 in the Times Higher Education World University Rankings for the third year running, and at the heart of this success is our ground-breaking research and innovation.

Oxford is world-famous for research excellence and home to some of the most talented people from across the globe. Our work helps the lives of millions, solving real-world problems through a huge network of partnerships and collaborations. The breadth and interdisciplinary nature of our research sparks imaginative and inventive insights and solutions.

Through its research commercialisation arm, Oxford University Innovation, Oxford is the highest university patent filer in the UK and is ranked first in the UK for university spinouts, having created more than 170 new companies since 1988. Over a third of these companies have been created in the past three years.

University of Oxford

Related Sea Level Rise Articles from Brightsurf:

Sea-level rise will have complex consequences
Rising sea levels will affect coasts and human societies in complex and unpredictable ways, according to a new study that examined 12,000 years in which a large island became a cluster of smaller ones.

UM researcher proposes sea-level rise global observing system
University of Miami (UM) Rosenstiel School of Marine and Atmospheric Science researcher Shane Elipot proposes a new approach to monitoring global sea-level rise.

Sea-level rise projections can improve with state-of-the-art model
Projections of potentially dramatic sea-level rise from ice-sheet melting in Antarctica have been wide-ranging, but a Rutgers-led team has created a model that enables improved projections and could help better address climate change threats.

How much will polar ice sheets add to sea level rise?
Over 99% of terrestrial ice is bound up in the ice sheets covering Antarctic and Greenland.

Sea-level rise could make rivers more likely to jump course
A new study shows that sea level rise will cause rivers to change course more frequently.

Sea level could rise by more than 1 meter by 2100 if emission targets are not met
An international study led by Nanyang Technological University, Singapore (NTU Singapore) scientists found that the global mean sea-level rise could exceed 1 meter by 2100 and 5 meters by 2300 if global targets on emissions are not achieved.

UCF study: Sea level rise impacts to Canaveral sea turtle nests will be substantial
The study examined loggerhead and green sea turtle nests to predict beach habitat loss at four national seashores by the year 2100.

Wetlands will keep up with sea level rise to offset climate change
Sediment accrual rates in coastal wetlands will outpace sea level rise, enabling wetlands to increase their capacity to sequester carbon, a study from the Marine Biological Laboratory, Woods Hole, shows.

Scientists discover evidence for past high-level sea rise
An international team of scientists, studying evidence preserved in speleothems in a coastal cave, illustrate that more than three million years ago -- a time in which the Earth was two to three degrees Celsius warmer than the pre-industrial era -- sea level was as much as 16 meters higher than the present day.

Corals in Singapore likely to survive sea-level rise: NUS study
Marine scientists from the National University of Singapore found that coral species in Singapore's sedimented and turbid waters are unlikely to be impacted by accelerating sea-level rise

Read More: Sea Level Rise News and Sea Level Rise Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.