San José, Costa Rica, since 1956

Ocean acidification worries scientists

From the print edition

Carbon dioxide emissions are causing the world’s oceans to acidify at rates unseen in recorded history, scientists from Columbia University reported.

That means critical ocean species, particularly coral, could be at risk.

 “It has been known for a while that we are increasing the carbon dioxide levels in the atmosphere because we’re burning fossil fuels, and as we’re doing that we have increased the carbon dioxide [in the atmosphere] from about 280 parts per million to over 390 ppm just over the past 100 years,” said Bärbel Hönisch, the paleoceanographer who led the study. 

As that happens, more carbon dioxide dissolves into ocean surface water, decreasing sea-surface pH levels, and that affects coral reefs.

Coral reefs, besides providing habit for all manner of marine life, protect coastal areas from tsunamis and provide residents of those areas with sources of revenue from fishing and tourism -– industries that are dependent on a reef’s healthiness.

When absorbed in seawater, carbon dioxide forms carbonic acid that causes pH levels of the water to drop, or become more acidic. Sea-surface pH has dropped by about 0.1 pH unit since the 1950s, Hönisch said, and the rate of acidification continues to accelerate. 

“When the seawater becomes more acidic, and particularly when it becomes more acidic relatively quickly, it reduces the carbonate ion concentration in the water,” said Hönisch.

In order to grow, corals need access to carbonate ions in the water, particularly one called aragonite, which the coral uses to calcify and form the skeletons that make reefs. Microscopic algae live on these calcified skeletons, giving corals their bright colors.

The last time ocean pH levels are known to have dropped as rapidly as they are today was about 56 million years ago during what is known as the Paleocene-Eocene Thermal Maximum. At that time, massive amounts of carbon dioxide entered Earth’s atmosphere, Hönisch said, possibly from the release of large amounts of frozen methane previously trapped in the ocean floor. Temperatures worldwide surged an average of 5 to 9 degrees Celsius, and a significant amount of carbon dioxide was absorbed back into the world’s oceans.

“The largest change in pH [during the Paleocene-Eocene Thermal Maximum] was about 0.4 pH units,” said Hönisch. “So that’s four times greater than what we’ve seen already and that sounds pretty large, but this change happened over a period of at least 5,000 years.”

Today, by contrast, there has already been a negative 0.1 pH unit change in the last 100 years or less. Hönisch said that by the end of the century the world’s oceans could become more acidic by another 0.2 or 0.3 pH units.

The Paleocene-Eocene Thermal Maxim-um – when the oceans acidified at a rate of negative 0.008 pH units per hundred years – was characterized by the extinction of nearly 50 percent of ocean-borne single-cell organisms and the loss of many coral species. 

“The rate of acidification that we’re seeing today is about 10 times greater than what we’ve seen at this Paleocene-Eocene Thermal Maximum,” Hönisch said. “And that is really what the organisms care about, the rate of change.”

Laboratory experiments have shown that coral does not grow as quickly or form as strong skeletons in waters with low pH as they do in normal conditions. 

Ocean acidification is far from the only threat to corals globally, and here in Costa Rica, silt from rivers, pollution from human activities and overfishing threaten reefs, said Lauretta Burke, senior associate at the World Resources Institute (WRI) and leader of the institute’s “Reefs at Risk” series of map-based analyses of threats to reefs.

That analysis, released in 2011, concluded that 75 percent of the world’s reefs are currently threatened, and that by 2050 nearly every reef in the world will be threatened by both global and local pressures.

“On a global basis,” said Burke, “overfishing of reefs is the most widespread problem.”

Overfishing removes herbivorous fish that keep algae on coral from growing out of control and killing the reef. Silt from nearby rivers or construction can smother that same algae by blocking out sunlight and preventing photosynthesis, which kills algae and bleaches coral. Pollution and physical damage from anchors or careless swimmers are also major threats.

WRI’s website indicates that all of Costa Rica’s Caribbean coral reefs are affected by these types of threats, but does not provide data for the Pacific coast.

These are local threats that can be dealt with by enforcing fishing regulations and taking actions to reduce pollution and silt. 

“Coral reefs are increasingly being affected by global threats,” said Burke. “One that is already affecting many of the world’s reefs is climate change. … Elevated sea-surface temperature, when it’s high enough or when it’s for an especially long period, can prompt coral bleaching, the loss of symbiotic algae. Sometimes reefs recover from that and sometimes they don’t.”

Costa Rican reefs – particularly on the Caribbean – saw large amounts of bleaching as recently as 2010 and 2004. 

Burke said she is most apprehensive about the future of reefs in the Caribbean and in Southeast Asia, where overfishing and stress from large populations is taking a toll.

“It is important to remember that reefs are resilient. Reefs can recover, reefs do bounce back,” she said. “It’s largely an issue of reducing local pressures to create an environment where they can recover.”

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