Every national climate strategy and net-zero pledge is based on the carbon budget, which is predicated on the rarely expressed premise that the ocean will continue to behave as it has. year after year, absorbing about 25% of humanity’s annual carbon dioxide emissions, acting as a buffer that has significantly reduced the buildup of CO2 in the atmosphere. What happens to that assumption under high-emission scenarios is examined in the latest modeling work from Columbia University’s Lamont-Doherty Earth Observatory, and the results are unsettling. According to the findings, if emissions remain on a high trajectory, the ocean’s carbon sink may significantly decrease or perhaps reverse its function this century.
The principles behind this are extensions of well-known ocean physics into higher-temperature futures, not conjecture. There is less dissolved gas in warmer water. This is fundamental physical chemistry, which also explains why carbonated drinks fall flat more quickly at room temperature than in a refrigerator. Seawater’s ability to absorb CO2 from the atmosphere decreases when ocean surface temperatures rise. The warming signal in the top ocean layers is measured and documented, and the ocean is already warmer than it was at the beginning of industrialization. How much additional warmth alters the absorption rate and how rapidly it changes under various emission pathways are the questions that the Columbia simulation attempts to answer.
A second mechanism that functions over longer durations and may be of greater size is added by the circulation dimension. In addition to absorbing carbon at the surface, the ocean also transfers it to deeper levels, where it remains isolated from the atmosphere for decades or even centuries. Mixing mechanisms that push surface water downhill and carry dissolved carbon with it are necessary for this vertical transfer. Density differences between water layers—warm, fresh water stays close to the top, while cold, salty water sinks—are partly responsible for these mixing processes.
The density gradient weakens, mixing slows, and less carbon reaches depth as the surface ocean warms and freshwater from melting ice dilutes salinity in critical areas. As the biological communities in warmer, more stratified water vary in composition and productivity, so does the biological pump, which is the rain of organic particles that carry carbon in a changing form from the surface to the floor.
These dynamics come together at AMOC to form the scenario that is most discussed when it comes to climate risk. Numerous study teams using various data sources have shown that the Atlantic circulation system, which transports cold deep water southward and warm surface water northward, is deteriorating. The North Atlantic, which is currently one of the most important regional carbon sinks, would lose the circulation-driven mechanism that transports absorbed carbon to depth if AMOC significantly decreases or experiences the kind of threshold transition that some models predict under ongoing warming. In addition to decreasing ocean carbon uptake in the North Atlantic, a weakened or disrupted AMOC may lead the region to change from being a net carbon absorber to a net carbon releaser as stored carbon from depths returns to the surface.
Although the policy implications of a declining ocean carbon sink are unsettling, the reasoning behind them is simple. The amount of CO2 that the world can still emit before reaching specific temperature thresholds is known as the current carbon budget, and it is computed under the assumption that the ocean will continue to absorb carbon at a rate that is comparable to its current pace. The same emission trajectory results in increased atmospheric concentrations if that absorption rate decreases. The budget for carbon decreases. Compared to current pledges, the emission reductions needed to keep temperatures below 1.5°C or 2°C must be more drastic, quicker, and comprehensive. And this occurs due to the internal dynamics of a climate system that was already shifting at the time of the commitments, rather than any fresh external disturbance.
A more precise measurement of the sink’s weakening under various emission paths is what the Columbia modeling adds. The high-emission scenario, also referred to as “business as usual” and roughly comparable to the IPCC’s SSP5-8.5 trajectory, exhibits the most disruption. The inherent justification for quick decarbonization is that lower-emission scenarios exhibit less sink weakening: the more quickly emissions are reduced, the more of the ocean’s buffering capacity is maintained. Although this argument isn’t new, it has a more tangible shape than the generic claim that ocean feedbacks must be taken into consideration because it is based on particular model projections of sink behavior.
The exact timing and threshold dynamics are still genuinely unknown. Different models provide different answers to the question of whether AMOC reaches a critical point in 2070 or 2090 or doesn’t pass one at all under specific emission scenarios, and the variation in their predictions is significant enough to be taken into consideration for planning purposes. It is easier to determine the risk’s direction than its precise size. It is challenging to convey this combination—confident about direction, unclear about scale—in policy contexts where decisions are based on precise statistics. However, it accurately depicts the current state of ocean carbon sink research.
