Ocean Acidification

Abstract

The ocean has absorbed 25 ± 2% of the total anthropogenic CO₂ emissions from the early 1960s to the late 2010s, with rates more than tripling over this period and with a mean uptake of –2.7 ± 0.3 Pg C year^–1 for the period 1990 through 2019. This growth of the ocean sink matches expectations based on the increase in atmospheric CO₂, but research has shown that the sink is more variable than long assumed. In this Review, we discuss trends and variations in the ocean carbon sink. The sink stagnated during the 1990s with rates hovering around –2 Pg C year^–1, but strengthened again after approximately 2000, taking up around –3 Pg C year^–1 for 2010–2019. The most conspicuous changes in uptake occurred in the high latitudes, especially the Southern Ocean. These variations are caused by changes in weather and climate, but a volcanic eruption-induced reduction in the atmospheric CO₂ growth rate and the associated global cooling contributed as well. Understanding the variability of the ocean carbon sink is crucial for policy making and projecting its future evolution, especially in the context of the UN Framework Convention on Climate Change stocktaking activities and the deployment of CO₂ removal methods. This goal will require a global-level effort to sustain and expand the current observational networks and to better integrate these observations with models.

Key points

• The long-term trend in the ocean carbon sink since the early 1960s was primarily driven by the increasing uptake of anthropogenic CO_2. Although the ocean is expected to have lost a few petagrams of natural CO₂ to the atmosphere in response to ocean warming, this loss cannot be quantified conclusively with observations.
• The oceanic uptake of anthropogenic CO₂ scaled proportionally with the increase in atmospheric CO₂ between the early 1960s and late 2010s, as expected given the quasi-exponential growth of atmospheric CO₂ during this period.
• The average ocean uptake rate of –2.7 ± 0.3 Pg C year^–1 for the period 1990 through 2019 is commensurate with a sensitivity β of 1.4 ± 0.1 Pg C per ppm atmospheric CO₂, suggesting a trend in the uptake of –0.4 ± 0.1 Pg C year^–1 per decade.
• The annual mean ocean carbon sink varies by about ±20% around this trend, primarily caused by changes in the sources and sinks of natural CO₂, with a lesser role for variations in atmospheric CO₂ growth rates impacting the uptake of anthropogenic CO₂.
• The net oceanic uptake rate of CO₂ will likely decrease in the future owing to several converging trends: reduced emissions of CO₂ leading to reduced atmospheric CO₂ growth rates in response to climate policy; reduced storage capacity owing to continuing ocean acidification; and enhanced outgassing of natural CO₂ owing to ocean warming and changes in ocean circulation and biology.

Gruber N., Bakker D. C. E., DeVries T., Gregor L., Hauck J., Landschützer P., McKinley G. A. & Müller J. D., 2023. Trends and variability in the ocean carbon sink. Nature Reviews Earth & Environment 4:119–134. doi: 10.1038/s43017-022-00381-x. Article.