A dummy’s guide to oceanic symptoms of global warming.
Thursday, November 13, 2008
-- Global warming causes less mixing of nutrient-rich deep waters and oxygen-rich shallow waters.
-- Sea level rises as glaciers melt and waters expand.
-- Some marine animals are shifting their ranges toward colder waters. Off the coast of California, researchers have found an increase in tropical and sub-tropical species and a decrease of cooler-water species.
-- The California Current System has warmed significantly since industrialization.
-- Global warming may be making storms stronger. Scientists debate whether El Niño events, a natural phenomenon that causes storms in some places and droughts in others, are happening more frequently than in the past.
-- As atmospheric CO2 dissolves in water, chemical reactions create carbonic acid. As a result, the ocean’s acidity increases.
-- On the logarithmic pH scale, 7 is neutral, less than 7 is acidic and more than 7 is alkaline. As the ocean acidifies, naturally alkaline seawater – averaging just over 8 units at the surface – becomes less alkaline, and pH decreases.
-- Some marine animals need carbonate to make shells in a process called calcification. Acidification reduces the amount of carbonate available to those animals. As a result, calcification rates are projected to decrease 60 percent by 2100.
-- As pH declines, the “saturation horizon” – the deep-sea water level below which calcite and aragonite shells dissolve – rises. That forces deep-sea shell-making animals toward the surface.
-- Calcifying animals vulnerable to acidification include coral reefs, clams, sea stars, urchins, crabs, shrimp and microscopic coccolithophores, foraminifera and pteropods, which are major marine food sources.
-- The deep sea is naturally more acidic than the surface, on average. As the ocean absorbs atmospheric CO2, pH declines fastest at the surface, but deep sea animals are likely to have the most trouble adapting.
-- The average surface ocean pH has already declined about 0.1 units, from 8.2 to 8.1, which represents roughly one-fourth more acidity than before industrialization. At current CO2 emission rates, it’s projected to fall another 0.3 units – enough to dissolve chalk – as early as 2050.
-- By 2300, the ocean’s pH is projected to decline by an average 0.7 units at the surface, 0.4 units at 1,000 meters and 0.2 units at 2,000 meters. That’s the biggest oceanic pH change in 300 million years, excepting rare catastrophic events.
-- Researchers have found fossil fuel “fingerprints,” in the form of isotopes, as deep as 2,000 meters below the ocean’s surface – proving that fossil fuel burning, and not a natural process, is causing acidification.
-- A more acidic ocean is believed to cause respiratory and metabolic stress in certain animals, impairing their ability to grow and reproduce.
-- Acidification is expected to decrease biodiversity and impact animals at all levels of the food chain. That’s likely to hurt commercial fisheries.
EXPANSION OF LOW-OXYGEN LAYER
-- The level of oxygen dissolved in the tropical ocean has decreased over the past 50 years, impacting oxygen-breathing marine life beneath 300 meters.
-- The natural oxygen minimum zone (OMZ) has gotten thicker and more intense in the tropical north and south Pacific Ocean.
-- Oxygen levels in the southern California Current System have declined sharply in the past 20 years, and the OMZ has expanded by more than 90 meters.
-- Off the coast of central Oregon, large areas with low or no oxygen began appearing in 2002, killing sea-floor animals and confounding scientists.
-- The OMZ expansion could impact sea-floor and open-ocean ecosystems, compressing habitats and reorganizing marine communities. It could particularly impact deeper-dwelling species such as rockfish and hake.
-- Models predict that global warming will impair the ocean’s ability to mix and breathe, resulting in less oxygen availability at mid-water depths.
-- In a study of the waters off California, the deep sea has lost the highest percentage of oxygen. Near the surface, the biggest oxygen decline occurred along the continental slope.
-- Oxygen loss can benefit certain species, such as myctophid fish and crustaceans, that can out-compete species less tolerant of low-oxygen waters.
-- Jumbo squid have been invading the waters offshore North America from Mexico to Alaska and South America from Peru to Chile, perhaps in conjunction with the expanding OMZs. The fact that this is occurring in both hemispheres could implicate global warming.
SOURCES: Jacqueline Ruttimann, “Sick Seas,” Nature, Aug. 31, 2006. World Ocean Circulation Experiment data. “Anthropogenic Carbon and Ocean pH.”Ken Caldiera and Michael Wickett, Nature, Sept. 25, 2003. Scott Doney, “The Dangers of Ocean Acidification,” Scientific American, March 2006. Ocean Update, June, July and August 2008. Lothar Stramma et al, “Expanding Oxygen-Minimum Zones in the Tropical Oceans,” Science, May 2, 2008. Steven Bograd et al, “Oxygen Declines and the Shoaling of the Hypoxic Boundary in the California Current,” Geophysical Research Letters, June 2008. Reports by the Intergovernmental Panel on Climate Change and U.S. National Oceanic and Atmospheric Administration. Interviews with marine scientists including Hopkins Marine Station Director George Somero and Monterey Bay Aquarium Research Institute scientists Francisco Chavez, David Field and Jim Barry.
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