Between pollution, disease, and habitat destruction, coral reefs have it rough. But greenhouse gas emissions may be the ecosystems’ deadliest stressor yet. While scientists have worried about sea temperature rise for some time, only recently have they focused on another consequence of excess carbon dioxide: ocean acidification. The process may lead to the death of most coral reefs by the end of this century, according to a study published in the January issue of Science. Often called the rainforests of the sea, coral reefs are the ocean’s most biologically diverse ecosystems, supporting roughly 25 percent of marine life and more than 4,000 species of fish. The United Nations Environment Programme estimates that one square kilometer of coral reef holds a value of up to $600,000 per year by drawing tourists, supporting fisheries and helping mitigate beach erosion by breaking waves. Coral reef destruction represents a huge economic and biodiversity loss.  

“Coral reefs are always in a balance,” says Mark Eakin, a coauthor of the study and National Oceanic and Atmospheric Administration reef expert. “Acidification makes it harder for the building forces to build and easier for the eroding forces to erode.”

The ocean naturally absorbs about one-third the total amount of CO2 pumped into the atmosphere; as concentrations of the gas increase in the air, so does the amount taken up by the sea. When CO2 dissolves in water, carbonic acid forms — the same mild acid found in your Blue Sky soda. The compound naturally releases hydrogen protons, making the ocean more acidic or lowering its pH (see illustration on next page). The sea’s pH has dropped by 0.1 units over the past century. 

Carbonic acid dissolves the shells of mollusks and other animals, and also inhibits shells from forming. Additionally, the excess hydrogen protons draw calcium carbonate, or chalk, from the ocean, harming organisms as small as plankton and as vast as coral reefs, which absorb the compound and turn it into shell.

“There’s actually less of the material needed to build, so it slows or leads to malformed shells,” says Scott Doney, a geochemist at Woods Hole Oceanographic Institution.   

Climate change is also turning up the  temperature of our oceans. It has risen 0.74°C in the last 100 years. For reefs, warmer water stresses coral polyps, causing them to kick out the algae that live in their tissue and give them color — a process called“coral bleaching.” Scientists began reporting bleaching events around the world in the early 1980s. The most severe took place a decade ago in the western Indian Ocean when 46 percent of the region’s coral was impacted, igniting a decline of the whole ecosystem. 

To draw the world’s attention to these threats, the International Coral Reef Initiative—a partnership among governments, nonprofits and NGOs — declared 2008 the International Year of the Reef. The group aims to promote the importance of reefs, emphasizing that these diverse ecosystems are already on the brink of extinction. Ocean acidification is literally like pouring acid on the wound. 

“Even if we stop all CO2 emissions tomorrow, it will still impact coral reefs,” Eakin says. “The best we can do both for bleaching and ocean acidification is improve the resilience of coral reef communities by getting rid of other local stresses — make the reefs healthier so they can best survive while the warming takes place.”

Story by Victoria Schlesinger. This article originally appeared in Plenty in April 2008.

Copyright Environ Press 2008