The Southern Ocean is one of the most important and poorly understood components of the global carbon cycle that profoundly shapes Earth’s climate. It is the primary hotspot for the oceanic sink of anthropogenic CO2, having captured half of all human-related carbon that has entered the ocean to date.
This vast anthropogenic perturbation to the Southern Ocean carbon system is activating a range of complex climate feedbacks (for example, a rising atmospheric CO2-induced acceleration of the westerly winds over the Southern Ocean, which may in turn be driving increased outgassing and raising atmospheric CO2 concentrations further) that are likely to exert a decisive control on the evolution of oceanic carbon uptake, atmospheric CO2 and global climate over the 21st century.
Many of these climate feedbacks are poorly understood and quantified, yet evidence from our planet’s past global climatic transitions (for example, the end of the last glacial period) suggests that they can induce changes in atmospheric CO2 as large as those caused by human activities since the industrial revolution.
As the focus of major knowledge gaps and a central player in global carbon and climate dynamics, the Southern Ocean carbon system is regularly singled out as the Achilles’ heel of the Earth system models on which humankind relies to understand contemporary climate change, predict its future evolution, and define international climate policy.
Inadequate understanding and unconstrained model representation of the Southern Ocean carbon system constitute a critical uncertainty in climate projections for the 21st century. Investigations of the present magnitude of the Southern Ocean carbon sink and its recent decadal variability yield wildly different estimates, yet unanimously show that the regional budget is the sum of large, uncertain outgassing and uptake terms that are both comparable in magnitude to the CO2 emissions from the world’s most polluting economies and highly sensitive to climate change.
In the recent Paris Agreement, 195 nations committed to undertaking rapid reductions in their greenhouse gas emissions to hold the global-mean temperature to well below 2°C above pre-industrial levels, recognising that this would significantly reduce the risks and impacts of climate change.
The effectiveness of this agreement is, however, acutely vulnerable to future changes in the Southern Ocean carbon system that are not captured by rudimentary representations in the present generation of Earth system models. Even modest variations in the Southern Ocean’s outgassing or uptake terms may lead to a significant shift in the reduction of global anthropogenic CO2 emissions required to meet long-term temperature goals, and produce a geopolitically challenging imbalance between the integral of national sources and the desired evolution of atmospheric carbon.
The evolution of the Southern Ocean carbon sink over the 21st century has been the subject of a persistent, heated debate in the scientific literature. This is reflected in the Southern Ocean accounting for most of the uncertainty in Earth system model predictions of global oceanic carbon uptake, which differ by up to approximately two billion metric tonnes of carbon per year (that is, the present net global oceanic carbon uptake) by 2050. The debate is fuelled by a paucity of essential carbon system observations in the Southern Ocean, which makes the region the most glaring biogeochemical ‘data desert’ in the world ocean; and by a damaging lack of understanding of the fundamental processes controlling the size of the Southern Ocean carbon sink, which raises large uncertainties in the nature and magnitude of associated climate feedbacks – particularly as global emissions start to decline toward zero.
Now, the advent of novel autonomous robotic platforms and biogeochemical micro-sensor technologies, in which the UK has played a world-leading role, makes it possible to tackle these issues directly for the first time, and settle the debate once and for all.