An international research team, led by the University of Liverpool, sought to explore whether there are differences in how the Southern Ocean contributes to the global uptake of heat and carbon.
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 the global ocean uptake of heat compared to the global uptake of carbon.
Their analysis of state-of-the-art models 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 ocean carbon uptake.
Further analysis using single-radiative forcing experiments indicated that this contrast between hemispheres in heat uptake was due to aerosol forcing which is predominantly found over northern hemisphere and caused a reduction in the uptake of heat by northern oceans.
Aerosol forcing – or aerosol particles such as dust, haze or smoke – acts as a barrier to reflect the sunlight and counters the effect of the extra heating from rising atmospheric carbon which then impacts the uptake of heat by northern oceans.
However, the study revealed that the historical hemispheric contrast does not apply for the carbon cycle, with atmospheric carbon increasing in a broadly similar manner over both hemispheres.
Looking to the future, the study projects that where greenhouse gases increasingly dominate radiative forcing the northern ocean’s heat uptake will increase more in line with the northern ocean carbon uptake. Thus the Southern Ocean contributions to global heat and carbon uptake 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 primary reason for the enhanced Southern Ocean uptake of heat relative to carbon are differences in their atmospheric sources.
“Over the historical period, there are strong contrasts in radiative forcing between the southern and northern hemispheres with aerosols forcing providing a cooling over much of the northern hemisphere.
“What is really interesting is that when you look to the future, we see a different response: the greenhouse gas forcing is expected to dominate more than the aerosol forcing, so that there is less hemispheric contrast in the radiative forcing. Hence, the future response may be different to the past.”
Dr Andrew Meijers, leader of the British Antarctic Survey Polar Oceans group, and coauthor of the study, said: “This study shows how aerosol pollution has been shielding the northern oceans from warming over the last century or so. It also demonstrates the worrying potential for dramatically increased ocean heating, particularly in the North Atlantic, in the coming decades as aerosol emissions are reduced, and CO2 warming begins to dominate.”
The study involves British Antarctic Survey (BAS), Imperial College London, the National Oceanography Centre (NOC) , ETH Zurich with support from UKRI NERC.
Below are two figures illustrating the processes at work in this study: air-sea exchange over the Southern Ocean and the effect of dust over the northern hemisphere.
Figure 1.
An example of air-sea exchange over the Southern Ocean. Sea smoke in the Weddell Sea, taken from the RRS James Clark Ross in 2014. This image shows the process of air-sea heat exchange from the ocean where (relatively) warmer water encounters extremely cold, dry air: The water warms up a small layer of very damp air immediately above it, which then rises up. As the air rises, the air mixes with the very cold, very dry air above, and the water condenses out making a thick mist. By Dr Andrew Meijers, BAS.
Figure 2.
Aerosol plume over Eastern US and North Atlantic (NASA SeaWiFs), marked by extent of white/grey plume at mid latitudes. The aerosols represent haze and smoke that reduces incoming sunlight, as well as altering cloud brightness. The presence of aerosols over the northern hemisphere has historically reduced the northern hemisphere uptake of heat. Reproduced from https://www.gfdl.noaa.gov/aerosols-and-climate/