Coral reefs are sensitive ecosystems. Somewhat of a “canary in the coal mine,” they serve as an indicator of the pressures we are placing on global ecosystems. They provide a key example of the effect of increasing greenhouse gas concentrations in the atmosphere (and oceans) and provide additional incentives for far-reaching action to reduce our emissions.

This work provides projections of future global threats to coral reefs driven by increasing concentrations of carbon dioxide (CO₂) and other greenhouse gases in the atmosphere and in the oceans. We look in particular at present and future ocean warming and acidification in the 2010s, 2030s and 2050s, and how these ‘global threats’ compound local threats to coral reefs.

Despite their value to humans for food, livelihood, recreation and coastal protection, coral reefs are severely threatened by both local and global pressures. Over 60% of the world’s coral reefs are endangered by local activities such as overfishing, destructive fishing, coastal development and pollution from land and sea. Threats to coral reefs will increase in the coming decades due to continued greenhouse gas emissions driving ocean warming and acidification, but the degree of threat will be greatly influenced by our greenhouse gas emission trajectories. Shifting from the emissions trajectory of business as usual (RCP 8.5) to a two degree increase stabilization level (RCP 4.5) would make a tremendous difference in the intensity of threats to coral reefs by the 2050s. Such a shift reduces the percentage of coral reefs in the highest three threat categories (very high, critical and extreme) by one third, significantly improving the chance of those reefs surviving through the 2050s.

Despite sensitivities, coral reefs are resilient and can often recover if threats are reduced. A growing body of evidence has shown that by reducing local threats (including overfishing and nutrient and sediment pollution), reefs may recover more quickly from coral bleaching. Resilience can be enhanced in critical areas – such as coral reefs which are an important source of larvae – by reducing local threats. In addition, active management to reduce local threats in areas which have recently bleached can aid in coral recovery.

Local threats. Reefs at Risk Revisited (WRI, 2011) identifies 61% of the world’s coral reefs as threatened (medium or higher) by local human activity, with over 27% at high or very high threat.

Threat from Warming Seas. The warming data from the climate models suggest relatively low thermal stress on coral reefs for now, increasing in the 2030s and increasing very significantly by 2050. Depending on the emissions scenario, between 40% (RCP 4.5) and 99% (RCP 8.5) of reefs will be in areas at medium or high threat from thermal stress.

Threat from Acidification. Data on aragonite saturation state from an ensemble of climate models suggests that about one-quarter of the world’s reefs are threatened (medium or high threat) from acidification during the current decade (under the RCP 8.5 scenario). By the 2030s, between 65% (RCP 4.5) and nearly 80% (RCP 8.5) of reefs will be in danger. By the 2050s, the overall percentage of reefs threatened by acidification will have risen to over 80% in both scenarios. In the RCP 8.5 scenario, coral growth would be severely hampered.

Integrated local, warming and acidification threat. Integration of local and global threats to coral reefs allows comparison of the change in projected threat to coral reefs over time; differences in projected threat depending on greenhouse gas emissions; and the proportion of threat in a given decade / scenario attributed to global versus local sources.

Contribution of global vs local sources. We estimate the proportion of integrated threat coming from global sources (warming and acidification) to be 20% in the current decade, 40-45% in the 2030s and between 55 and 65 percent in the 2050s.

This analysis method reflects a simplification of human activities and complex natural processes. The model relies on available data and predicted relationships, but cannot capture all aspects of the dynamic interactions between people, climate, and coral reefs.

The threat indicators gauge current and potential risks associated with human activities, warming seas and ocean acidification. A strength of the analysis lies in its use of globally consistent data sets to develop globally consistent indicators of human pressure on coral reefs. A conservative approach has deliberately been used in the modeling, where thresholds for threat grades are set at reasonably high levels to counter any data limitations and avoid exaggerating the estimated threats.