Warmer, more acidic oceans threaten global fisheries.
The sea is feeling the heat. Over the past half-century, the ocean has absorbed about four-fifths of the warmth added to the Earth’s climate. Arctic sea ice is retreating at an unprecedented pace. Ice sheets are melting into the ocean at a faster and faster clip. During the 1990s, global mean sea level—and sea level along the Atlantic coast—rose more quickly than at any other time during the twentieth century, contributing to erosion along vulnerable shorelines.
These trends are troubling. But, perhaps most alarming, fisheries around the globe are threatened by a one-two punch of rising temperatures and increasing acidity of seawater.
Ocean acidity has been called “climate change’s other problem.” Global warming doesn’t cause ocean acidity. Instead, escalating emissions of carbon dioxide, which mostly drive global warming, have also altered the ocean’s chemical balance.
By 2050, increasing ocean acidity is expected to disrupt the growth of marine creatures that build shells, including oysters, clams, lobsters, scallops, whelks, blue crabs, and many others. These animals could become increasingly smaller and malformed. Blue crabs in the Atlantic estuaries, for instance, would probably become runts by mid-century.
“Acidification would directly affect anything in the ocean that has a shell—about half of the total value of U.S. fisheries,” says Scott Doney, a senior scientist with the Woods Hole Oceanographic Institution.
Meanwhile, coral reefs around the world are dying. By 2050, scientists predict, coral reefs could disappear from the world’s oceans.
Shallow, sunlit waters, sparkling with brilliantly colored fish and coral species—we’ve all seen spectacular images of coral reefs. Over the past two decades, though, ocean warming has overheated many reefs, seriously threatening one of the ocean’s richest repositories of biological wealth. Corals reefs, moreover, would erode in more acidic waters, diminishing catches of grouper, snapper, and other reef fish.
“We are undertaking a massive experiment on coral reefs and on the ocean in general,” says Nancy Knowlton, a coral-reef biologist with the Smithsonian National Museum of Natural History.
Climate change’s most immediate threat in the ocean is to coral reefs, which provide habitat for one-fourth of all marine fish species, protect coasts from waves and storms, contain potential pharmaceuticals, and support tourism and fishing industries worth billions of dollars. About 2.6 billion people worldwide rely to some degree on seafood for animal protein, according to the United Nations Food and Agriculture Organization. Without coral ecosystems, many coastal economies, particularly those in poor countries, would suffer.
In early 2008, food riots followed rapidly escalating grain prices in nations from Mexico to Pakistan. Seafood prices would similarly skyrocket if fish catches decline and if aquaculture couldn’t maintain its aggressive pace of expansion. Over-harvesting, pollution, coastal development, and invasive species are increasingly damaging fishing grounds—and wild catches have been stagnant for years.
Climate change is adding a further burden to marine ecosystems. “We are now observing what may become, in the absence of policy changes, a collapsing (ocean) ecosystem with climate the final coup d’grace,” asserts Achim Steiner, executive director of the United Nations Environment Programme, in a February 2008 report.
In tropical reefs, ocean warming is breaking down the crucial symbiotic relationship between coral animals and algae called zooxanthellae, which live in healthy coral tissue. Zooxanthellae provide nutrients to coral animals through photosynthesis and help make the spectacular colors for which corals are known. But they are surprisingly sensitive to small increases in average temperatures.
When summer ocean temperatures increase one to two degrees Celsius (1.8 to 3.6 degrees Fahrenheit) above normal, zooxanthellae die or flee the coral. The corals bleach (lose their color), starve, get sick, and often die. During the summer of 1998, about 80 percent of coral reefs bleached in the Indian Ocean, and about 20 percent subsequently died.
“What I’ve seen in the past few decades is beyond strange,” says Knowlton. “This is an environmental catastrophe. The question has been whether coral reefs are the canary in the coalmine for the oceans. The answer to that is clearly yes. But the canary has already passed out on the floor of the cage. The question now is whether we can revive the canary.”
One piece of good news is that U.S. policymakers have started to acknowledge the seriousness of global warming because of a “drumbeat of new climate science and experience,” says John P. Holdren, a professor of environmental policy at Harvard University.
Climate science, he says, shows that global warming is occurring at a pace and scale that is “faster, bigger, and more dangerous than anyone thought possible before.”
Americans are experiencing symptoms of climate change in their own communities and reading news reports of warming trends around the world. Major corporations, religious groups, labor organizations, national-security enterprises, and other interested parties have recently urged policymakers to take action on climate change. The Intergovernmental Panel on Climate Change (IPCC), which shared the Nobel Peace Prize with Al Gore in 2007, raised global awareness of the problem.
State and local governments, meanwhile, have begun addressing climate change. South Carolina is one of 25 states that have completed or are working on climate-action plans.
In June 2005, Charleston Mayor Joseph Riley, Jr., signed the U.S. Mayors Climate Protection Agreement, setting goals for the city to reduce its carbon dioxide emissions to seven percent below 1990 levels by the year 2012. The mayors of Rock Hill, Columbia, Greenville, and Sumter also signed the agreement. Charleston created a citizen Green Committee, which is forming a comprehensive plan to meet specific emissions targets.
Even so, state and local actions, while important, won’t stem acceleration of U.S. greenhouse-gas emissions and provide global leadership on the problem, Holdren says. That’s why federal action is urgently needed. “We are near a political tipping point” regarding a U.S. policy to manage climate change, which “needs to be national, mandatory, stiff, and soon.”
A threshold is approaching
There’s a number that climate scientists watch with special concern: the atmospheric concentration of carbon dioxide (CO2).
Since the 1750s, industrializing societies have drilled or mined for fossil fuels (coal, oil, natural gas) and burned them to run vehicles, power plants, and factories. The resulting waste—carbon dioxide—rises into the atmosphere where it captures a portion of the sun’s radiant energy and, in turn, raises the temperature of the Earth. Numerous studies have confirmed links between rising atmospheric CO2 concentrations and warmer global temperatures over geological timeframes.
Since the beginning of the Industrial Revolution, the atmospheric concentration of carbon dioxide has gone up from 280 parts per million (ppm) to 385 today, the highest it’s been in more than a half-million years. (The most important greenhouse gas is carbon dioxide; others include water vapor, methane, and nitrous oxide.)
From 1750 to 2007, the atmospheric CO2 concentration rose at an average annual rate of 0.40 ppm. During the last few decades of that period, however, this rise accelerated. From 1970 to 2000, the concentration increased about 1.5 ppm each year, as human activities sent more of the gas into the atmosphere. That’s almost four times faster than the historical rate since the beginning of the Industrial Revolution.
From 2001 to 2007, the concentration accelerated even further, by an average of 2.1 ppm annually, according to the National Oceanic and Atmospheric Administration (NOAA). That’s more than five-fold above the average since 1750. From 2006 to 2007, it rose 2.6 ppm, more than a six-fold increase.
Because of our past and current actions, the concentration of atmospheric CO2 will continue to accelerate—unless leading industrial nations find ways to reduce greenhouse-gas emissions drastically.
The core of the problem is that the global economy depends on burning fossil fuels for its energy needs. Today, about 80 percent of worldwide energy use—and 88 percent of U.S. energy use—is derived from burning fossil fuels.
What does all this mean for marine ecosystems, particularly coral reefs? Scientists say there is a tipping point of CO2 beyond which coral reefs worldwide would bleach and disappear. That tipping point is probably about 500 ppm of atmospheric CO2, though it could be as low as 450 ppm, says Ove Hoegh-Guldberg, a coral biologist with the University of Queensland in Australia. (Remember, it’s now 385 ppm.)
If the atmosphere reached 500 ppm, the Earth would eventually warm about three degrees Celsius (4.8 degrees Fahrenheit) above pre-industrial levels —which might not sound like much. Even so, scientists say, it would burn up coral reefs around the world and cause more extreme droughts, storms, and floods with catastrophic effects on agriculture and water supplies.
Can we prevent the planet from passing the 500 ppm threshold? It’s possible, but difficult. National governments would have to collaborate very soon on a new global system of producing and consuming energy that releases scant or no carbon dioxide into the atmosphere.
What if the tipping point is 450 ppm? No one has come up with a politically and economically feasible plan to avoid crossing 450 ppm, a number that’s “right around the corner,” says Hoegh-Guldberg. The planet will breach 450 ppm within about three decades, he says. “We’re very, very close.”
The Acidifying Oceans
The ocean absorbs about a third of the carbon dioxide that industrial society puts into the atmosphere. The sea, in fact, takes in almost a million tons of CO2 per hour. That’s a good thing and a bad thing.
If the ocean at some point failed to dissolve atmospheric CO2, the planet would overheat very rapidly. But that won’t happen for some time, according to the IPCC. The ocean will continue dissolving CO2 for hundreds or thousands of years to come.
What keeps many marine scientists up at night, however, is that when the ocean absorbs increasing amounts of carbon dioxide, it turns acidic. The ocean is naturally slightly alkaline, but the ocean has become 30 percent more acidic since 1750.
Coral reefs will be among the earliest casualties of acidification. Corals use calcium compounds in seawater to form protective skeletons, or shells. A coral starts out as a very simple free-floating larva. When it attaches to a hard surface such as older coral, a rock, or a sunken ship, it becomes a polyp. Then the animal, scarcely more than a mouth and a gut, begins extracting calcium carbonate from seawater to build a chalky skeleton shaped like a cup.
Today, acidity is reducing the saturation levels of calcium carbonate in surface layers of the ocean. Corals, as a result, can’t build healthy structures as efficiently. In acidic water, a coral’s skeleton is weakened in a process similar to osteoporosis. Field experiments have shown that acidification has lowered growth rates—that is, calcification—of some corals on Australia’s Great Barrier Reef by 20 percent.
Perhaps the most troubling aspect of acidification is that “it’s essentially irreversible” once it’s started, says Scott Doney of the Woods Hole Oceanographic Institution. The only way to slow the acidification process is to prevent further CO2 emissions.
“We will have to do something very drastic in terms of reducing CO2 emissions or we will bring about the end of coral reefs as we know them,” says Knowlton of the Smithsonian.
To escape hotter temperatures, some tropical coral reefs might be able to migrate toward higher latitudes—from Florida and the Caribbean up into the Carolinas, for instance. But many reefs won’t relocate quickly enough and will succumb to heat stress. In any case, coral reefs that migrate pole-ward would almost certainly get hit by acidification and fade away by mid-century.
Says Knowlton, “Coral animals don’t necessarily die in highly acidic waters. Instead, they turn into little sea anemone-like creatures, attached to a hard substrate, but they stop building skeletons. Over time, storms and currents pound what’s left of the three-dimensional complexity of reefs that can be seen from space. The reef structures would eventually disappear.”
Ocean warming and acidification together would probably harm reefs faster than each impact could alone. “Oceans are going to change in a very complex fashion as they warm and become acidic at the same time,” says Gretchen Hofmann, a biologist at the University of California, Santa Barbara. “We’re facing double jeopardy, with potential synergistic influences between increasing acidity and higher seawater temperature.”
In response, the U.S. House of Representatives is considering the Federal Ocean Acidification Research and Monitoring Act (H.R. 4174). The act would authorize appropriations totaling $55 million from 2009 through 2012 to develop an interagency monitoring and research plan, which would be chaired by NOAA, and establish an ocean-acidification program at NOAA.
A similar Senate bill (S. 1581) would authorize appropriations totaling $100 million during 2009 to 2013 to establish a NOAA program to conduct research and public outreach on ocean acidification.
To read the rest of this article: Article.
John H. Tibbetts, Coastal Heritage Volume 23, Number 1, 2008. Article.
