Since the first industrial revolution, the ocean absorbed about 525 billion tonnes of CO2, and presently takes up around 22 million tonnes per day
Human activities such as the burning of fossil fuels, deforestation for change of land use, and burning coal for generating electricity have pushed up carbon dioxide (CO2) concentration in the atmosphere dramatically above 411 parts per million. About 30-40 per cent of CO2 is absorbed by oceans, while the excess that resides in the atmosphere absorbs heat and re-radiates it back to the Earth (greenhouse effect), contributing to global warming.
Since the first industrial revolution (1760 -1830), the ocean absorbed about 525 billion tonnes of CO2, and presently takes up around 22 million tonnes per day. Once dissolved, CO2 undergoes chemical reactions with seawater; excess CO2 intake makes the water acidic and its pH (a measure of how acidic or alkaline the ocean is) declines. Today, the ocean surface waters have become acidic by 0.1 pH or 25 percent from pre-industrial levels (8−8.1). As the surface layers slowly mix with deep water, the entire ocean is affected, and signs of its impact appear even in the deep oceans.
Although marine researchers have been tracking ocean pH for more than 30 years, biological studies started only in 2003, when the drop in the pH drew their attention and the term ocean acidification was first coined. If we continue to release CO2 at current rates, according to US National Oceanic and Atmospheric Administration, seawater pH may drop to 7.8 or 7.7, i.e. by 150 percent by the end of this century, creating an ocean more acidic than seen for the past 20 million years or more.
The so-called evil twin of global warming, acidic seawater dissolves calcium carbonate − the main compound that is used by invertebrate communities to synthesize shells and skeletons (corals and mollusks). Under a reduced pH scenario, it takes longer for skeletons to form; leaving them stressed and prone to attack from invasive predators. A new study in Nature found that larval urchins have trouble digesting their food under raised acidity. Jellyfish compete with fish and other predators for food and consume young fish. If jellyfish thrive under more acidic and warm conditions, it is going to dominate the flora and fauna - a problem already seen in parts of the globe, including Goa’s coastal belt.
Reef-building corals craft their own homes from calcium carbonate, forming complex reefs that house the coral animals and provide a home for many other organisms. Acidification may curtail the coral growth by corroding pre-existing coral skeletons and slowing the growth of new and weaker reefs that are more vulnerable to erosion from storm waves, and also from organisms that drill into or eat coral. A study published in Science predicts that roughly by 2080 seawater will be so acidic that erosion of even otherwise healthy coral reefs will overtake their rebuilding. This change is also likely to affect in unpredictable ways many other organisms that coexist in the coral habitat, including fish.
In humans, a drop in blood pH by 0.2-0.3 from the normal range of 7.35- 7.45 can cause seizures, comas, and even death. Likewise, a small change in the pH of seawater can have harmful effects on marine life, impacting their physiology − reproduction, growth, and communication. Mussels and oysters grow less shell by 25 percent and 10 percent respectively by the end of the century. Acidic waters affect fish behaviour by disrupting a specific receptor in the nervous system, called GABAA, which is present in most marine organisms with a nervous system. So their decision-making is impaired and recognition/response to predator threats becomes weak. Furthermore, juvenile fish become more active when reared at CO2 levels, and exhibit a riskier behaviour, such as venturing further from the shelter. These behavioural changes can increase mortality from predation.
Two major types of zooplankton – foraminifera and pteropods are particularly important in the marine food chain since all larger marine organisms feed on them directly or indirectly. These are vital to the global carbon cycle, which describes how carbon moves between air, land, and sea. The Foraminifera is so sensitive to increased acidity that it causes their shells to dissolve, and some species from tropical waters may be extinct by the end of this century.
Since ocean acidification is a global problem, there will be losers and winners, and some fish species that rely on specific resources and environmental conditions during their life stages could disappear. Some species such as purple sea urchins have already adapted to higher acidity or have the ability to do so. Acidification effects on marine life may be exacerbated when combined with other stressors, such as increased temperature, depleted oxygen, and pollution. This would lead to the composition of fish communities changing soon with severe consequences for marine flora and fauna and the quality of food they provide to humans.
The amplitude of future acidification levels will depend on actual and future CO2 emissions. By 2100 various projections indicate that seawater pH will drop by 0.3-0.4, which is more than many organisms like corals can withstand. This will create conditions not seen on Earth for at least 40 million years. Scientists say “besides directly impacting on the fishing industry and its contribution to the human food supply at a time when global food demand is doubling, a major die-off in the oceans would affect birds and many land species and change the biology of Earth as a whole profoundly.”
Many countries depend on the sea as their main source of food and the loss of species is highly detrimental to them. Coral reefs serve as a valuable tourist attraction and a natural fortification against natural hazards such as tsunamis. Together with climate change, acidification poses a major challenge to the oceans as a human habitat. Immediate actions are needed to reduce greenhouse gasses, particularly CO2, by adopting renewable sources of energy to limit the negative ecological and socioeconomic effects. The government should increase carbon sinks by growing more mangroves, increasing seagrass beds, and salt marshes. If we act now, over hundreds of years from now, CO2 in the atmosphere and ocean would return to normal.
(The writer is a scientist and researches global climate change issues)
