Ocean warming offers clues

29 Jan 2019
Ruth Abrahams

Due to a scarcity of data, most global estimates of ocean warming start only in the 1950s. However, a team of scientists at the University of Oxford 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 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.

 

 

 

 

All picture credits: Shutterstock

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

Comments

Ian Phillips
We now see innocent and ignorant children being led by the nose, to take on the AGW climate crisis, being fed fake and exaggerated "science" by teachers and presumably social media. This comes from the IPCC, a completely politicised organisation, and sources such as Al Gore or Michael Mann. Remember Gore said all the summer Arctic sea ice would be gone by 2013. And the lie of Polar Bears being at risk of extinction is still being peddled, despite their numbers being officially around 25000....plus plenty more Project "climate fear" false predictions. Can Oxford researchers, and their department heads, please rise above the need to satisfy the wishes of their financial sponsors, who wish to justify their climate/AGW generated business ambitions. Their logic is, presumably similar to that of the Spiritual healers who would agree to use their pendulum to determine the sex on an inborn child, years ago. The charge was £25 and if "occasionally" they were proved wrong you could have your money back! In other words, it doesn't matter if we exaggerate temperature rise, or sea level rise a little bit because it's all in a good cause, and if AGW is just a myth, then no harm is done.... .........Really? Is that so?
Rui Namorado Rosa
I am presently researching on solar radiation transfer balance in the atmosphere. I would find useful to get to know the approach and main findings of this "Global reconstruction of historical ocean heat storage and transport". I should be grateful if you send it to me.

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