Coral reefs are vibrant underwater ecosystems formed by colonies of tiny animals called coral polyps. Built from calcium carbonate, these intricate structures support an unparalleled diversity of marine life as biodiversity hotspots. They provide habitat and food for over a quarter of all marine fish species, along with countless other organisms. Ocean temperature profoundly influences their existence and health.
The Optimal Temperature Range for Coral Reefs
Most reef-building corals thrive within a narrow temperature range, preferring water temperatures between 23°C and 29°C (73°F and 84°F). While some species can tolerate brief periods outside this range, prolonged deviations cause stress. This preference is linked to the symbiotic relationship between corals and microscopic algae, zooxanthellae, which live within coral tissues.
Zooxanthellae are sensitive to temperature changes, with their optimal range typically between 18°C (64°F) and 30°C (86°F). These algae perform photosynthesis, providing the coral with up to 90% of its energy. If water temperatures exceed this threshold, even by 1-2°C for several weeks, this partnership can break down, harming the coral.
Coral Bleaching: The Primary Threat from Temperature Increases
When ocean temperatures rise above the corals’ thermal tolerance, they experience stress, causing them to expel their symbiotic zooxanthellae. This expulsion results in the coral losing its color and turning white, a phenomenon known as coral bleaching. The white appearance is due to the transparent coral tissue revealing the underlying white skeleton.
While corals can sometimes recover from mild, short-term bleaching events if temperatures return to normal, severe or prolonged bleaching often leads to coral death. Mass coral bleaching events have increased in frequency and intensity since the 1980s, driven by rising sea temperatures. For instance, a fourth global bleaching event, which began in early 2023, has impacted coral reefs in over 70 countries and territories across all major oceans, with more than 74% of global coral reefs experiencing extreme heat stress.
The Great Barrier Reef, for example, has experienced multiple mass bleaching events, including in 2020, 2022, and March 2024. The 2022 event was notable as it occurred during La Niña conditions, which typically bring cooler summer temperatures. These recurring events highlight the increasing vulnerability of coral reefs, as corals require several years to recover, making frequent events unsustainable.
Broader Impacts of Ocean Warming on Reef Ecosystems
Beyond coral bleaching, rising ocean temperatures have broader impacts on the entire reef ecosystem. Elevated temperatures can directly inhibit the ability of corals to calcify, the process of building their calcium carbonate skeletons. This reduced calcification weakens the structural integrity of reefs, making them more susceptible to erosion and physical damage.
Ocean warming also negatively affects coral reproduction. Temperature stress can reduce the reproductive success of corals, hindering their ability to replenish populations. This decreased reproductive output, combined with increased mortality from bleaching, slows down the natural recovery processes of reefs.
The effects of warming extend to other inhabitants of the reef ecosystem. Fish, invertebrates, and other marine life that depend on coral reefs for shelter, breeding, and food face declines as reef health deteriorates. Changes in ocean temperature can alter species distribution, impacting the availability of food sources for various organisms within the marine food web. The decline in biodiversity and disruption of food web dynamics can have cascading effects throughout the marine environment.