More bad news for coral reefs: If nothing is done to reduce carbon emissions, there may be nowhere left in the world's ocean where they can survive, new research suggests.
"Under the current emissions trajectory, well before the end of the century there will be no water left in the ocean with the kind of chemistry that has supported coral growth in the past," said Ken Caldeira, a researcher at the Carnegie Institution for Science at Stanford University.
Besides trapping heat in the atmosphere via the greenhouse effect, carbon dioxide also dissolves into the water, leading to ocean acidification. Since the beginning of industrialization, the oceans have become more acidic, absorbing more than 40 percent of the carbon that humans emit each year.
This is a problem for many shellfish and coral reefs, whose shells and skeletons are made of calcium carbonate and which are more difficult for these animals to form and maintain in an acidic environment. If it gets acidic enough, this material can dissolve entirely, said Kate Ricke, a postdoctoral researcher in Caldeira's lab.
Specifically, coral is made of aragonite, a type of calcium carbonate. For the millions of years coral has existed, most varieties haven't been found in ocean waters with concentrations of aragonite below a saturation level of three and a half.
It's safe to guess then that coral prefers these concentrations. But saturation levels have already dipped below three and a half in some areas.
In a study published on July 3, 2013 in the journal Environmental Research Letters, Caldeira, Ricke and co-authors looked at how concentrations of this mineral are likely to fare in the future. (Carbon dioxide reduces levels of aragonite when dissolved in sea water, causing more of it to turn into bicarbonate, which is less available to corals.)
They looked at computer models to predict what the future ocean's chemical makeup might look like given certain levels of emissions. If nothing is done to reduce emissions, there will be nowhere left in the ocean above an aragonite saturation level of 3, at which coral isn't expected to survive, at least as far as we know to date, Caldeira said.
However, if aggressive emissions cuts are made—meaning emissions drops to zero before mid-century—a majority of the ocean's water could still remain hospitable for corals, Ricke said.
The big unknown is how adaptable corals are under these circumstances, and exactly what is the tipping point at which corals cannot survive.
Complicating the math is the influence of other factors vital for coral survival, such as levels of light and water temperature. Water temperatures in many areas are rising, occasionally leading to coral bleaching (in which corals expel their symbiotic bacteria), which can lead to coral death.
"This question of adaptability is something about which rational well-informed people can differ," Caldeira said. "My own view is that if coral reefs have not adapted to low saturation conditions in the past tens of millions of years, they are unlikely to do so in the next decades."
The different computer models the group used, employed by the Intergovernmental Panel on Climate Change, were all in close agreement about their conclusions.
The long and short of it is that if serious cuts aren't made, coral reefs are in serious trouble, Caldeira suggested.