Coral reefs are growing more slowly now than they did hundreds of years ago, and we have now learned part of the blame lies with carbon dioxide emissions. The news demonstrates something scientists have long suspected, but struggled to prove, and suggests the future for coral reefs is dire.

Some of the carbon dioxide released from burning fossil fuels or cutting down forests is absorbed by the oceans. This has slowed temperature rise, but makes sea water more acidic (or, technically, less alkaline). Since corals can only produce the calcium carbonate they need to form reefs in alkaline conditions, sufficient carbon dioxide will end coral growth, and eventually dissolve reefs.

However, marine biologists have been unsure how high carbon dioxide concentrations have to get before coral reefs stop growing. According to a new study in Nature, the answer is that, while growth continues, it is already slower than it was before the Industrial Revolution started large-scale carbon emissions.

There is plenty of evidence that coral organisms are doing less calcifying today than in the past. Professor Ken Caldeira of Stanford University has shown a 40 percent decrease between the mid-70s and 2008 to 2009. However, with abundant other stress factors, including pollution, overfishing and global warming itself, past work couldn't pin the blame on acidification.

Now, however, Caldeira has contributed to a study that demonstrates acidification is already part of the problem. The researchers made use of the natural laboratory at One Tree Island, part of the Great Barrier Reef.

^{Part of the 25 meter by 30 meter (80 foot by 100 foot) reef at One Tree Island used for the study. Kennedy Wolfe}

University of Sydney Ph.D. student Kennedy Wolfe told IFLScience: “At low tide the lagoon forms a natural pool, where water trickles over the reef.” This gave team members the opportunity to control the pH level inside the pool, at least for a short time until the tide changed, bringing water from the open ocean.

Wolfe and Caldeira added sodium hydroxide and a colored dye to a 15,000-liter (4,000-gallon) tank and submerged the tank. Wolfe told IFLScience that, “We could watch the [dyed] water run over the reef.” The team then measured nutrient concentrations within the lagoon to determine rates of photosynthesis and calcification.

When the sodium hydroxide returned the water to pre-Industrial Revolution pH levels, calcification rates rose 7 percent compared to the same experiment conducted with unaltered sea water.

^{Team members surround the tank filled with dilute sodium hydroxide and dye. University of Sydney}

“Our work provides the first strong evidence from experiments on a natural ecosystem that ocean acidification is already slowing coral reef growth,” co-author Dr. Rebecca Albright from Stanford said in a statement.

“The only real, lasting way to protect coral reefs is to make deep cuts in our carbon dioxide emissions,” Caldeira added.

Coral reefs house 25 percent of marine species and are vitally important as a food source, for tourism and to protect the land from storms and tsunamis.

The work was published the day after a paper in Nature Communications indicated that the southern part of the Great Barrier Reef is even more vulnerable to acidification than previously anticipated.