Local View: Climate change puts oceans at risk

Midwesterners usually pay scant attention to saltwater. Nevertheless, ocean plankton are responsible for over half the oxygen we and other animals breathe. A growing concern is that human emissions of carbon dioxide are causing significant changes in the oceans.The oceans absorb at least one quarter - and possibly one third - of the carbon dioxide released when fossil fuels are burned. As a result, in the past 200 years, seawater has become 30 percent more acidic. This drop from pH 8.2 to 8.1 is the fastest known change in ocean chemistry in the past 50 million years.Because pH is measured on a logarithmic scale, small changes in pH have big consequences for living things. For instance, a small drop in the pH of human blood may cause seizures, comas or death.By the end of the century, assuming current rates of fossil-fuel use, seawater may become another 120 percent more acidic, with pH dropping as low as 7.7. The oceans would then be more acidic than they have been in 20 million years. The last great ocean acidification event occurred 55 million years ago and was associated with mass extinctions.Marine animals such as corals and shellfish that have shells of calcium carbonate are particularly sensitive to ocean acidity because hydrogen ions bind to carbonate, making it unavailable for building shells.Coral reefs are important to humans partly because they provide habitat for thousands of marine plants and animals in different life stages. In many locations, people depend on coral reefs for food. Where natural seeps of carbon dioxide occur and increase seawater acidity locally, the diversity of corals and their associated animals is markedly reduced. The branching corals that provide nurseries and habitat for many small animals are especially vulnerable, possibly because their thin branches are easily dissolved.Coral reefs are also threatened by warming seas that cause bleaching. The massive bleaching of corals that has been observed in recent years occurs when stressed corals expel their associated symbiotic algae. These algae provide more than 90 percent of the corals’ nutrition. Ninety-three percent of Australia’s Great Barrier Reef exhibits some bleaching.Aquaculture, too, is threatened by acidification. In the Pacific Northwest, oyster larvae fail to begin growing their shells because seawater eats away at them before they can form. In oyster growing tanks, large quantities of lime are required to neutralize acid. And mussels have a harder time clinging to substrates in more acidic seawater.Some tiny drifting plants and animals that build shells of calcium carbonate are declining for similar reasons. These animals are important in the food web, but they are also important for carbon sequestration. Sequestration occurs when the organisms die and fall to the ocean floor taking their calcium carbonate with them. This carbon is stored as rock or sediment for the foreseeable future, thereby slowing the greenhouse effect and global temperature increases.Curiously, lobsters, crabs and shrimp grow stronger shells in more acidic water. Seagrasses also benefit. Other species may adapt to rising ocean acidity. One concern is that jellyfish that compete with young fish may dominate warmer, more acidic oceans.Fish are sensitive to pH in several ways. Because the pH of their blood is lowered, they must burn more energy to secrete acid, and their growth may slow. Fish behavior is altered by acidification because of effects on hearing, smell and changes in brain chemistry. Changes in dominant fish species could have major impacts on human fisheries.The long-term consequences of lowered ocean pH are unknown but extremely worrisome. We should act now to reduce carbon emissions. David Gerhart of Duluth has a Ph.D. in aquatic ecology from Cornell University and has published and reviewed manuscripts for scientific journals in the fields of ecology and biochemistry. This commentary was reviewed and edited by Byron Steinman, a climatologist and assistant professor at the Large Lakes Observatory at the University of Minnesota Duluth, before it was submitted to and edited by the News Tribune.
Midwesterners usually pay scant attention to saltwater. Nevertheless, ocean plankton are responsible for over half the oxygen we and other animals breathe. A growing concern is that human emissions of carbon dioxide are causing significant changes in the oceans.The oceans absorb at least one quarter - and possibly one third - of the carbon dioxide released when fossil fuels are burned. As a result, in the past 200 years, seawater has become 30 percent more acidic. This drop from pH 8.2 to 8.1 is the fastest known change in ocean chemistry in the past 50 million years.Because pH is measured on a logarithmic scale, small changes in pH have big consequences for living things. For instance, a small drop in the pH of human blood may cause seizures, comas or death.By the end of the century, assuming current rates of fossil-fuel use, seawater may become another 120 percent more acidic, with pH dropping as low as 7.7. The oceans would then be more acidic than they have been in 20 million years. The last great ocean acidification event occurred 55 million years ago and was associated with mass extinctions.Marine animals such as corals and shellfish that have shells of calcium carbonate are particularly sensitive to ocean acidity because hydrogen ions bind to carbonate, making it unavailable for building shells.Coral reefs are important to humans partly because they provide habitat for thousands of marine plants and animals in different life stages. In many locations, people depend on coral reefs for food. Where natural seeps of carbon dioxide occur and increase seawater acidity locally, the diversity of corals and their associated animals is markedly reduced. The branching corals that provide nurseries and habitat for many small animals are especially vulnerable, possibly because their thin branches are easily dissolved.Coral reefs are also threatened by warming seas that cause bleaching. The massive bleaching of corals that has been observed in recent years occurs when stressed corals expel their associated symbiotic algae. These algae provide more than 90 percent of the corals’ nutrition. Ninety-three percent of Australia’s Great Barrier Reef exhibits some bleaching.Aquaculture, too, is threatened by acidification. In the Pacific Northwest, oyster larvae fail to begin growing their shells because seawater eats away at them before they can form. In oyster growing tanks, large quantities of lime are required to neutralize acid. And mussels have a harder time clinging to substrates in more acidic seawater.Some tiny drifting plants and animals that build shells of calcium carbonate are declining for similar reasons. These animals are important in the food web, but they are also important for carbon sequestration. Sequestration occurs when the organisms die and fall to the ocean floor taking their calcium carbonate with them. This carbon is stored as rock or sediment for the foreseeable future, thereby slowing the greenhouse effect and global temperature increases.Curiously, lobsters, crabs and shrimp grow stronger shells in more acidic water. Seagrasses also benefit. Other species may adapt to rising ocean acidity. One concern is that jellyfish that compete with young fish may dominate warmer, more acidic oceans.Fish are sensitive to pH in several ways. Because the pH of their blood is lowered, they must burn more energy to secrete acid, and their growth may slow. Fish behavior is altered by acidification because of effects on hearing, smell and changes in brain chemistry. Changes in dominant fish species could have major impacts on human fisheries.The long-term consequences of lowered ocean pH are unknown but extremely worrisome. We should act now to reduce carbon emissions.David Gerhart of Duluth has a Ph.D. in aquatic ecology from Cornell University and has published and reviewed manuscripts for scientific journals in the fields of ecology and biochemistry. This commentary was reviewed and edited by Byron Steinman, a climatologist and assistant professor at the Large Lakes Observatory at the University of Minnesota Duluth, before it was submitted to and edited by the News Tribune.