1369 / 2024-09-25 18:24:35

Complexity in the Southern Ocean meridional overturning circulation and carbon uptake changes

Meridional overturning circulation, Antarctic bottom water, ocean carbon pump, climate change

The ocean has absorbed a considerable portion of anthropogenic carbon dioxide (CO2) since the Industrial Revolution, which effectively buffered the greenhouse warming effect on the globe’s climate. Among others, the Southern Ocean surrounding Antarctica, defined broadly as the oceanic areas south of 35°S, has taken up ~40% of the anthropogenic CO2 via its deep-reaching overturning circulation. The formation of Antarctic bottom water (AABW) drives the lower cell of the Southern Ocean overturning, carrying the anthropogenic CO2 to the abyssal ocean, playing an essential role in the global ocean carbon pump. In 2023, observation-based studies revealed the volume decrease of the coldest component of the AABW, which was subsequently reported by Science using the title of ‘Earth’s carbon pump is slowing’. This report extends the implications of the AABW volume reduction on the ocean carbon pump. It was indicated that the global meridional overturning circulation (GMOC) has been weakening, along with reduced ability of capturing anthropogenic CO2, which will act to further amplify the climate change. However, the logical chain of AABW reduction – GMOC slowdown – carbon pumping weakening involves many complicated oceanic and climatic processes, over which the scientific communities have not reached consensus views. There remain major gaps in understanding these issues through observational, modeling and theoretical efforts. To complement the perspective provided by this report, the scientific basis should be clarified to better inform the decision-makers and the public. This article provides reviews on four relevant scientific issues: 1) the AABW formation, 2) changes in wind-driven Southern Ocean circulation, 3) Southern Ocean climate change, and 4) the relationship between ocean carbon pump and ocean circulation. In addition to an updated review of established notions, uncertainties in observations and model simulations are also discussed.