Predicting Ocean Warming #ClimateChange

Widespread ocean warming and sea level rise, compared to the past, is suggested in a recent study by scientists at the University of Oxford.

The researchers at the Department of Physics have discovered that the influence of circulation changes on shaping ocean warming will diminish in the future. This is despite having been identified and modelled as a key factor over the past 60 years.

According to their modelling there will be increased warming near the Eastern edges of ocean basins leading to more sea level rise along the Western coastlines of continents in the North Atlantic and Pacific Oceans.

Map of ocean warming change and its decomposition estimated from observations

The scientists used climate models to suggest that ocean warming patterns will increasingly be influenced by simple uptake of atmospheric warming – making them easier to predict. This will hopefully lead to more accurate predictions.

Map of ocean warming change and its decomposition estimated from climate Models

Co-author of the study , Laure Zanna, Visiting Professor in Climate Physics at Oxford University and Professor in the Center of Atmosphere Ocean Science at NYU Courant, said:

‘In the future, the imprint of rising atmospheric temperatures on ocean warming will likely dominate that of changes in ocean circulation. Initially, we might think that as the climate warms more, changes in ocean currents and their impact on ocean warming patterns will become larger. However, we show that that this is not the case in several regions of the ocean.’

The study, published in Nature, shows that the global ocean heat ( the energy absorbed by the ocean, which is stored as internal energy) and carbon uptake go hand-in-hand, and the uptake rates are set by the present state of the ocean. This relationship is at the core of the method developed in this study.  

As humans change the ocean state by adding more heat and carbon, the ability of the ocean to take up both heat and carbon will be altered.

A possible implication could be that the later emissions are reduced, the slower the reductions in atmospheric surface temperature are likely to be, due to the coupling between heat and carbon uptake by the ocean.

The study shows that ocean carbon and heat, although separate systems, are deeply interconnected, via the capacity of the ocean to absorb these quantities. These results help explain why atmospheric warming depends linearly on cumulative carbon emissions.

Dr Ben Bronselae, Lead Author said:

‘During our research, we found a surprising relationship between ocean heat and carbon storage which appears to be unique. While there is a connection between these two quantities that is not yet fully understood, we think we have made significant progress towards uncovering it.’

 “Better prediction of warming patterns implies better prediction of regional sea level rise, which will help to mitigate climate impacts such as flooding on individual communities. Of course, we do need to understand predictions of ocean circulation better to solidify this result.”

Prof Laure Zanna explained that the present ocean state will regulate surface warming whether CO2 emissions continue to rise or decline. She said:

‘The rates of ocean warming over the past 60 years have been significantly altered by changes in ocean circulation, particularly in the North Atlantic and parts of the Pacific Ocean, where we can identify cooling over some decades.

However, in the future changes in ocean currents appear to play a smaller role on patterns of ocean warming, and the oceans will transport the excess anthropogenic heat in the ocean in a rather passive manner in these regions.’

In 2012 a study which looked at carbon uptake and where it was going stated that carbon dioxide emissions from cement and burning fossil fuels, for example, were estimated to have increased by nearly 30 per cent between 2000 and 2009.

It went on to explain the crucial role forests and oceans play as ‘sinks’ storing the carbon. It estimated that:

“global carbon uptake almost doubled between 1960 and 2010 – from roughly 2.4 billion tonnes of carbon per year to around 5.0 billion tonnes of carbon per year.”

Carbon Brief

Pieter Tans, a climate researcher with NOAA’s Earth System Research Laboratory, argued that the ‘sinks’ will not continue to absorb all the carbon created by human action indefinitely. He expected the uptake of carbon dioxide by the oceans and by ecosystems to slow down gradually.

The paper ‘Heat and carbon coupling reveals ocean warming due to circulation changes’ was published in Nature

The modelling relied on a set of creative simulations by The Geophysical Fluid Dynamics Laboratory (GFDL), and other published work. Using these simulations, the scientists were able to draw hypotheses on how the patterns of heat and carbon are related and how they differ. 

Reporter: Fiona Grahame

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