Why reversing ocean acidification is so difficult: the scale problem
The fundamental challenge is that ocean acidification is driven by the sheer volume of CO₂ already in the atmosphere. The ocean has absorbed about 30% of human-caused CO₂ emissions [1], and this process has already lowered the average pH of surface seawater by 0.1 units since the Industrial Revolution. To reverse this, you would need to remove CO₂ from the atmosphere or directly counteract its chemical effects in the ocean on a massive, global scale.
Even a dramatic, temporary reduction in emissions—like the 8.8% drop in global CO₂ during the first half of 2020 due to COVID-19 lockdowns—had a measurable but very limited effect on the ocean. While coastal waters saw a slight cooling (0.5°C) and some temporary improvements in water quality, the overall impact on ocean acidification was minimal and short-lived [1]. This shows that reversing acidification requires not just a temporary dip, but a sustained, deep cut in emissions over decades.
Can we engineer our way out? The promise and limits of ocean alkalinity enhancement
One proposed intervention is Ocean Alkalinity Enhancement (OAE), which involves adding alkaline minerals (like crushed olivine or limestone) to the ocean to neutralize acidity. A detailed modeling study on the Great Barrier Reef found that continuously releasing 90,000 tons of alkalinity every three days for a full year along a major shipping route would increase the aragonite saturation state (a key measure of coral health) by 0.05 across all 3,860 reefs. This would offset only about 4.2 years of ongoing acidification [3]. The effect was even smaller for most marine life: a separate study on 27 calcifying species (like shellfish and pteropods) found that a realistic OAE addition of 50 μmol/kg would only reverse a small fraction of the calcification losses already suffered—on average, these species have lost 22% of their calcification ability since preindustrial times [2].
Furthermore, OAE has major practical drawbacks. The Great Barrier Reef study noted that the intervention would be "extremely expensive," carry unquantified ecological risks, and would need to continue indefinitely—likely for centuries—until atmospheric CO₂ returns to preindustrial levels [3]. Once the alkalinity injection stops, the ocean chemistry returns to its acidified state within just six months [3]. Enhanced weathering on land, which uses soils to slowly release alkalinity into rivers and then the ocean, could help replenish the ocean's buffering capacity, but research on its downstream impacts is still very limited [4].
Can marine life adapt? The potential of natural and assisted evolution
While large-scale geoengineering faces huge hurdles, some research explores whether marine organisms can adapt to more acidic waters. One promising avenue is selective breeding or hybridization. A study on the noble scallop (Chlamys nobilis) found that intraspecific hybridization (crossing different populations of the same species) produced offspring that grew faster and survived better under acidified conditions compared to inbred scallops. Under acidified water, the hybrid scallops showed higher expression of genes related to stress response and DNA repair [6]. This suggests that assisted evolution could help some commercially important species cope with acidification, but it is not a solution for the entire ecosystem.
However, this approach has clear limits. It cannot help the vast majority of wild species, nor does it address the root cause of the problem. As one review of ocean acidification impacts concluded, the changes to marine food webs and ecosystem resilience are so profound that "effective mitigation strategies and informed policy decisions" are urgently needed [5]. Adaptation strategies like hybridization are best seen as a temporary buffer, not a reversal.
Sources used in this answer
Global impacts of COVID-19 on sustainable ocean development
COVID-19 lockdowns caused an 8.8% drop in global CO₂ emissions in early 2020, leading to a 0.5°C cooling of some coastal waters and temporary improvements in water quality, but the effect on ocean acidification was minimal and short-lived.
Substantial Limitations of Ocean Alkalinity Enhancement in Mitigating the Negative Impacts of Ocean Acidification on Marine Calcifiers.
A realistic addition of 50 μmol/kg of alkalinity to the ocean would reverse only a small fraction of the calcification losses already suffered by 27 marine calcifier species, which have declined by an average of 22% since preindustrial times.
Reversing ocean acidification along the Great Barrier Reef using alkalinity injection
Adding 90,000 tons of alkalinity every three days for a year along the Great Barrier Reef would offset only about 4.2 years of ocean acidification, and the effect would disappear within six months after stopping the injection.
The role of soils in the regulation of ocean acidification
Enhanced weathering of minerals in soils could increase alkalinity in drainage waters and help replenish the ocean's buffering capacity, but research on its downstream and oceanic impacts is very limited.
The Impacts of Ocean Acidification on Marine Ecosystems
A meta-analysis confirms that ocean acidification decreases calcification rates in corals and shellfish, alters species composition, and disrupts marine food webs, threatening ecosystem resilience and fisheries.
Intraspecific hybridization as a mitigation strategy of ocean acidification in marine bivalve noble scallop Chlamys nobilis.
Intraspecific hybridization in noble scallops produced offspring with higher survival and growth rates under acidified conditions, linked to enhanced expression of genes involved in stress response and DNA repair.