SRM Cannot Address Ocean Acidification

CO₂ emissions, the main driver of human-caused climate change, have also acidified the ocean since the start of the industrial revolution. Ocean acidification is the result of chemical reactions between CO₂ in the air and water that make key shell-building compounds less available for marine organisms, weakening their skeletons and shells. This would threaten the foundation of marine food webs, impacting commercial and indigenous fisheries,¹ the livelihoods of 300 million people, and a major source of protein for more than half of the world’s population.² 

Today, the ocean has an average pH³ of around 8.05. While seawater has a pH above 7 and is therefore alkaline, it is more acidic than it has been in the past 2 million years and is at least 25% more acidic now than before the industrial revolution.

Sunlight reflection methods (SRM) like stratospheric aerosol injection (SAI) cannot directly address the ocean acidification problem. Instead SRM could enhance the ability of some ecosystems on land to retain carbon, decreasing CO₂ in the atmosphere.⁴ However, CO₂ dissolves more readily in colder water. This enhanced CO₂ solubility could cancel out any benefit of SRM on the carbon cycle and may lead to increased acidity at greater ocean depths.⁵  

The chemistry of ocean acidification 

The ocean absorbs CO₂ at its surface, taking in about 25% of human-caused CO₂ emissions per year.⁶ The ocean’s capacity to absorb CO₂ makes it the second largest carbon reservoir after the solid earth, partially mitigating the warming effects of fossil fuel combustion.⁷ However, the accumulation of CO₂ in the ocean since the start of the industrial revolution has affected ocean water chemistry and marine life.

When atmospheric CO₂ dissolves in water, it reacts to form carbonic acid (H₂CO₃) – the substance that gives sparkling water its sharp or acidic taste. As CO₂ emissions increase, carbonic acid in the ocean also increases, making the ocean more acidic.⁸

Once CO₂ reacts with water, forming carbonic acid, a series of chemical reactions takes place. These chemical reactions ultimately decrease the relative abundance of carbonate ions (CO₃²⁻), which shell-forming organisms use to build their shells.⁹

Models predict ocean acidity will continue to increase. By the end of the 21^st century, models predict pH will decrease by around 0.01 if emissions are significantly cut (scenario SSP1-1.9) or around 0.39 in an extreme high-emissions scenario (scenario SSP5-8.5).¹⁰ Increasingly acidic water could lead to extensive die-offs of coral reefs and exacerbate harm to other marine ecosystems.⁷

The costly environmental impacts of ocean acidification 

Ocean acidification, through its effects on shell-forming organisms, ultimately impacts the people and industries that rely on the ocean. Increased ocean acidification makes it more difficult for organisms, including shellfish and tiny organisms that form the basis of the marine food web, to build and maintain shells.¹¹ This causes shells to become thinner or dissolve.⁹

Impacts on shell-builders affect other animals that rely on them for food, including humans. Globally, economic losses in the shellfish industry alone are estimated to be $6 billion to $100 billion per year by 2100.¹ Loss of shellfish will also have major impacts on indigenous and rural groups​​.¹

Ocean acidification has additional impacts on the concentrations of harmful pollutants that fish accumulate, which ultimately pose threats to human health. For example, fish in acidified waters accumulate more mercury, aluminium, iron, zinc, copper, and lead in their tissues, posing a poisoning hazard to people who rely on fish for protein.²

Coral reefs are also at risk from warming and ocean acidification. Around the world, an estimated 500 million people rely on coral reefs for food, coastal protection, and income.¹ Though it is difficult to assign a cost to these valuable “ecosystem services”, estimates range from $29.8 billion to $376 billion per year.¹

SRM cannot directly address ocean acidification 

The ocean’s capacity to store carbon is changing as the climate changes. Increased acidification, warmer ocean temperatures, and changes in wind and storm patterns alter the amount of CO₂ the ocean can absorb and store.^12,13 Without interventions, the ocean’s ability to store carbon is likely to weaken as the climate continues to change.¹⁴ This would lead to more CO₂ in the atmosphere and increased warming. 

Suggested sunlight reflection methods (SRM) like stratospheric aerosol injection (SAI) are not expected to have a direct impact on ocean acidification but could have indirect effects through the global carbon cycle, i.e., the flow of carbon atoms between Earth’s oceans, atmosphere, and land.⁵ SRM could decrease temperatures, enhancing the ability of non-marine carbon sinks to retain carbon or remove CO₂ from the air. For example, by cooling the Arctic, SAI could slow permafrost melting and the associated carbon loss to the atmosphere.⁴ Further, SAI, by scattering light, could enhance the ability of plants to take CO₂ from the atmosphere.⁵ 

SRM could change the distribution of acidity in the ocean relative to a climate without SRM deployment. Without SRM, ocean currents that transport CO₂ to the deep ocean are expected to weaken. With SRM, these currents are expected to stay strong.¹⁵ Therefore, the net effect of SRM in a high-CO₂ world would likely be little change in acidity at the ocean surface but increased acidity at greater depths.⁵ 

To tackle rising ocean acidification, CO₂ emissions would need to be eliminated, and to reverse it would require large-scale deployment of carbon dioxide removal methods to safely store CO₂ away from the atmosphere and ocean.⁵