Ocean Acidification

The Chemistry Beneath the Surface

CO₂, Carbonic Acid, and the pH Decline

When atmospheric CO₂ dissolves in seawater, it reacts with water molecules to form carbonic acid (H₂CO₃). Carbonic acid rapidly dissociates into bicarbonate (HCO₃⁻) and hydrogen ions (H⁺), then further into carbonate (CO₃²⁻) and additional H⁺. The net effect of elevated atmospheric CO₂ is twofold: hydrogen ion concentration increases (lowering pH) and carbonate ion concentration decreases. Feely et al. [1] mapped this process globally using ocean survey data from the World Ocean Circulation Experiment and found that the aragonite saturation horizon — the depth below which seawater is corrosive to aragonite — has shoaled measurably in both the Atlantic and Pacific since pre-industrial times.

Aragonite Saturation: The Key Metric for Reef Divers

Aragonite saturation state (Ωarag) is defined as the ratio of the ion product [Ca²⁺][CO₃²⁻] in seawater to the solubility product of aragonite. When Ωarag > 1, seawater is supersaturated and corals can build skeletons. When Ωarag < 1, aragonite dissolves spontaneously. Pre-industrial tropical surface waters had Ωarag values of approximately 3.5; current values average around 2.8 in tropical reef regions and are declining at approximately 0.1–0.2 units per decade under current emissions trajectories [2]. Ricke et al. [4] modelled future aragonite saturation states using Earth system model projections and found that under RCP8.5, the majority of coral reef locations will experience Ωarag < 2 before mid-century — a threshold at which calcification rates in many coral species decline precipitously.

Calcifiers in the Crossfire

Reef-Building Corals

Hermatypic (zooxanthellate) corals are the architects of tropical reefs, and they are among the most pH-sensitive of all marine calcifiers. Laboratory and mesocosm studies have consistently shown that coral calcification rates decline by 10–40% per 0.1-unit pH decrease, depending on species and thermal history. Hoegh-Guldberg et al. [3] synthesised multiple lines of evidence to conclude that at atmospheric CO₂ concentrations above 500 ppm, reef erosion rates would exceed accretion across most of the tropics, fundamentally shifting reefs from net calcium carbonate producers to net losers. For divers, this manifests as reefs with reduced topographic complexity, fewer overhang structures, and decreased coral cover — a degradation of the very architectural features that make reef diving visually extraordinary.

Echinoderms, Molluscs, and Pteropods

Ocean acidification affects far more than corals. Sea urchins (critical reef grazers that control algal overgrowth) show skeletal deformities and reduced righting response at pH values projected within decades. Pteropods — free-swimming sea snails often called sea butterflies, which form dense swarms consumed by salmon, herring, whales, and seabirds — show shell dissolution at pH values already observed seasonally in parts of the Southern Ocean and upwelling zones off the US Pacific coast [2]. Orr et al. [2] projected that Southern Ocean surface waters could become undersaturated for aragonite by 2050 under business-as-usual emissions, directly threatening the pteropod populations that underpin polar and sub-polar food webs. For divers in temperate and cold-water destinations, this predicts restructured food webs: fewer of the intermediate-level species that attract pelagic megafauna.

Cold-Water and Deep-Sea Corals

Cold-water corals form spectacular three-dimensional reef structures at depths from 200 to 2,000 m, providing essential habitat for a disproportionate diversity of deep-sea species. Because cold water holds more CO₂, cold-water coral habitats are acidifying faster than their tropical counterparts. Orr et al. [2] showed that waters corrosive to aragonite are already present at the depth ranges of many Lophelia pertusa reefs in the North Atlantic, and that by mid-century these undersaturated waters will shoal into the depth zones where cold-water coral frameworks currently thrive. Sweetman et al. [6] calculated that up to 70% of deep cold-water coral habitats globally could be exposed to corrosive conditions by 2100 under high-emissions scenarios.

If atmospheric CO₂ is stabilised at 500 ppm, coral reefs will be in a marginal position; at 600 ppm, conditions will be lethal for most reef-building corals.

— Hoegh-Guldberg et al., Science, 2007 [3]

IPCC AR6: The Projections Divers Cannot Ignore

The IPCC Sixth Assessment Report Working Group I (2021) synthesised projections from 18 CMIP6 Earth system models [5]. Under the intermediate SSP2-4.5 scenario, surface ocean pH is projected to decline by approximately 0.2 units by 2100, reaching values last seen in the Pliocene epoch, some 3 million years ago. Under the high-emissions SSP5-8.5 pathway, pH declines of 0.3–0.4 units are projected — ocean conditions without analogue in the history of modern reef-building coral evolution. Kwiatkowski et al. [7] demonstrated using CMIP6 models that tropical surface pH will decline below 7.95 under SSP5-8.5, a level at which most tropical reef systems cease net calcification. Critically, these projections do not account for the synergistic interaction with ocean warming: thermal bleaching and acidification act simultaneously on the same organisms, and their combined effect is more damaging than either stressor alone [3].

Biological Winners and Losers Under Acidification

• Losers — aragonite builders: Acropora, Porites, Orbicella corals; pteropods; sea urchins; bivalve larvae; juvenile fish with otolith (ear stone) disruption.
• Losers — calcite builders (modestly impacted): Coralline algae, encrusting bryozoans — still affected but more resistant than aragonite builders.
• Apparent winners — non-calcifiers: Some fleshy macroalgae and cyanobacteria may expand as coral cover declines, accelerating phase shifts to algae-dominated reefs.
• Context-dependent: Some fish species show behavioural disruption (reduced predator avoidance, olfactory changes) at elevated CO₂ concentrations projected for end-century.

What Divers Observe: The Slow-Motion Reef Collapse

Ocean acidification does not produce the dramatic overnight spectacle of mass bleaching. Its signature is subtler and cumulative: reduced structural complexity as carbonate framework dissolves faster than it is replenished; rubble fields where once-massive coral bommies stood; recruitment gaps where coral larvae fail to settle or suffer post-settlement dissolution before growing large enough to contribute to the framework. Divers with decades of experience on the same reefs often report these changes viscerally — the gradual lowering of the reef crest, the loss of the overhanging structures that once sheltered sharks and groupers, the encroachment of turf algae across areas of bare substrate. Hughes et al. [8] documented a pan-tropical decline in coral cover of approximately 50% over the past 50 years, with acidification and thermal bleaching acting as co-conspirators.

Monitoring Acidification: Tools for the Dive Community

The Global Ocean Acidification Observing Network (GOA-ON) maintains a global array of pH and pCO₂ sensors. NOAA's Ocean Acidification Program publishes annual data accessible to the public. Several citizen science programs — including CoralWatch and Reef Life Survey — allow divers to contribute bleaching and coral cover data that complement remote monitoring. While divers cannot directly measure pH on a dive, the Coral Triangle Initiative's monitoring protocols and NOAA's National Coral Reef Monitoring Program include carbonate chemistry measurements at sentinel stations across key dive destinations.