Over the past 200 years, the oceans have absorbed approximately a quarter of the CO2 produced from human activities. This CO2 would otherwise have accumulated in the atmosphere leading to greater climate change. However, the absorption of this CO2 has affected ocean chemistry and has caused the oceans (which are on average slightly alkaline) to become more acidic. The average pH of oceanic surface waters has been lowered by 0.1 units since the pre-industrial period. This represents a 30% increase in hydrogen ion activity. Hydrogen ions attack carbonate ions which are the building blocks needed by many marine organisms, such as corals and shellfish, to produce their skeletons, shells and other hard structures. This loss of carbonate ions produce lower saturation levels for the carbonate minerals, aragonite and calcite, which are used in many shells and skeletons. Carbonate ion concentrations are now lower than at any other time during the last 800 000 years.
Global atmospheric CO2 concentrations are now at 387 ppm. If current trends in CO2 emissions continue, model projections suggest that by mid-century CO2 concentrations will be more than double pre-industrial levels and the oceans will be more acidic than they have been for tens of millions of years. The current rate of change is much more rapid than during any event over the last 65 million years. These changes in ocean chemistry are irreversible for many thousands of years, and the biological consequences could last much longer.
Ocean acidification impacts on marine life will depend on the rate and magnitude of changes in ocean chemistry and biological responses. While the ocean chemistry changes are predictable with high certainty, our understanding of the impacts is still developing. Nevertheless, there is strong evidence emerging for a range of biological effects and changes in the marine biogeochemical processes that affect the carbon cycle. The long-term consequences of this are difficult to predict.
Impacts are already being observed in the polar and tropical regions. Coral calcification rates have declined in recent decades, although attributing causes for these impacts among multiple drivers (acidification, warming, pollution, etc.) is a challenge. Fundamental ecological ocean processes will be affected as many marine organisms depend directly or indirectly on calcium carbonate saturated waters and are adapted to current levels of seawater pH for physiological and metabolic processes such as calcification, growth and reproduction. The pH changes expected will exceed the seasonal and regional variations currently experienced naturally.
Ocean acidification is a global issue. However, changes in ocean chemistry will be regionally variable with some regions affected more rapidly than others. The high CO2 waters in polar and upwelling regions such as the eastern Pacific and Bering Sea for example, will experience low pH more rapidly than other regions. Tropical waters, such as those around the Great Barrier Reef will also experience rapid declines in the carbonate ions important for coral reef construction. According to recent model projections almost all tropical and sub-tropical coral reefs were surrounded by waters favourable to coral growth before the industrial revolution. If atmospheric CO2 is stabilized at 450 ppm, only a very small fraction (~8%) of existing tropical and subtropical coral reefs will be surrounded by such water, and at 550 ppm, coral reefs may be dissolving globally. Cold water corals are also vulnerable and are likely to be affected before they have even been fully explored. By 2100, it has been estimated that 70% will be in waters unfavourable for growth.
In the polar regions, model projections using current CO2 emission rates suggest that parts of the Southern Ocean will be undersaturated for aragonite by 2050. Aragonite undersaturation is projected for 10% of Arctic waters by around 2020, and by 2060, 80% of waters will be undersaturated for aragonite and calcite. This means the waters will be corrosive to Arctic calcifiers such as pteropods, and bivalves such as clams, which play a key role in Arctic food webs.
The ocean chemistry changes projected will exceed the range of natural variability, which is likely to be too rapid for many species to adapt to. Many coastal animals and groups of phytoplankton and zooplankton may be directly affected with implications for fish, marine mammals and the other groups that depend on them for food. Increased CO2 may be particularly stressful for organisms with high metabolic rates such as squid. The impacts of these changes on oceanic ecosystems and the services they provide, for example in fisheries, coastal protection, tourism, carbon sequestration and climate regulation, cannot yet be estimated accurately but they are potentially large.
Although some species may benefit, most are adapted to current conditions and the impacts on ocean biological diversity and ecosystem functioning will likely be severe. Analysis of past events in Earth’s geologic history suggests that chemical recovery will take tens of thousands of years – while the recovery of ecosystem function and biological diversity can take much longer.
Ocean acidification is irreversible during our lifetimes and those of many generations to come. To minimise the risk of these large-scale and long-term changes to the oceans the increase in atmospheric CO2 must be curbed by reducing emissions from human activities.
Recent scenario studies have estimated that stabilisation of atmospheric CO2 concentrations at 550 ppm will produce enough acidification to be disastrous for sensitive oceanic ecosystems in many parts of the world. Even at 450 ppm, more than 10% of the world’s oceans will be impacted including large parts of the Southern, North Pacific, and Arctic oceans.
Mitigation approaches such as adding chemicals to counter the effects of acidification are likely to be expensive, only partly effective and only at a very local scale, and may pose additional unanticipated risks to the marine environment. There has been very little research on the feasibility and impacts of these approaches. Substantial research is needed before these techniques could be applied.
Ocean acidification is a direct consequence of increasing atmospheric CO2 concentrations. To avoid substantial damage to ocean ecosystems, deep and rapid reductions of global CO2 emissions by at least 50% by 2050, and much more thereafter are needed.
We, the academies of science working through the InterAcademy Panel on International Issues (IAP), call on world leaders to:
The following academies have endorsed this statement.