Research reveals southern ocean heat, carbon uptake mismatch

The Southern Ocean plays a central role in the global sink of heat and carbon, which is widely thought to be due to its unique growth and circulation.

An international research team, led by the University of Liverpool, set out to explore whether there are differences in how the Southern Ocean contributes to global heat and carbon uptake.

Their findings, published in the journal Nature Climate Change, show that for the period from 1861 to 2005, the Southern Ocean contributed nearly twice as much to global ocean heat absorption compared to global carbon uptake.

Their analysis of recent patterns of carbon and heat uptake across global oceans covering this period revealed large differences in uptake, with the Southern Ocean accounting for 83% of global ocean heat uptake and 43% of global carbon uptake from the ocean.

Further analysis using single-radiation forcing experiments showed that this interhemispheric contrast in heat uptake was due to aerosol forcing found primarily in the Northern Hemisphere and caused a reduction in heat uptake from the northern oceans.

Aerosol forcing – or aerosol particles such as dust, fog or smoke – acts as a barrier to reflect sunlight and counteracts the additional warming effect from increased atmospheric carbon, which then affects the uptake of heat by the northern oceans.

However, the study found that the historical hemispheric contrast does not apply to the carbon cycle, with atmospheric carbon increasing in a similar way in both hemispheres.

Looking to the future, the study projects that where greenhouse gases increasingly dominate radiative forcing, the absorption of heat by the northern ocean will increase more in line with the uptake of carbon in the northern ocean. Thus, the Southern Ocean’s contributions to global warming and carbon sequestration will become more comparable in the future.

Professor Ric Williams, from the University of Liverpool and lead author of the paper, said: “While there are unique upwelling and overturning circulations occurring in the Southern Ocean, our analysis suggests that the main reason for increased heat uptake by the Southern Ocean in with carbon are changes in their atmospheric sources.

“Over the historical period, there are strong contrasts in radiative forcing between the Southern and Northern Hemispheres with aerosols providing a cooling over much of the Northern Hemisphere.

“What’s really interesting is that when you look into the future, we see a different response: the greenhouse gas forcing is expected to dominate more than the aerosol forcing, so there’s less hemispheric contrast in radiative forcing. Therefore, the future answer may be different from the past.”

Dr Andrew Meijers, leader of the British Antarctic Survey Polar Oceans group, and co-author of the study, said: “This study shows how aerosol pollution has protected the northern oceans from warming over the past century or so. It also shows the worrying potential for dramatic increase in ocean warming, particularly in the North Atlantic, in the coming decades as aerosol emissions are reduced and CO2 warming begins to dominate.”

The study involves the British Antarctic Survey (BAS), Imperial College London, National Oceanographic Center (NOC), ETH Zurich with support from UKRI NERC.

The paper “Asymmetries in the contribution of the Southern Ocean to global warming and carbon sequestration” (doi: 10.1038/s41558-024-02066-3) is published in Nature Climate Change.