For the month of April 2012, the National Oceanic and Atmospheric Administration (NOAA) reported the first-ever average concentration of atmospheric carbon dioxide (CO2) of 400 parts per million at a remote location. The air samples were collected at NOAA’s Barrow, Alaska Observatory, located at the northernmost point of the United States. This is not the global average, and atmospheric scientists expect that it will take about four more years for the global average to reach 400 ppm. To minimize global warming, the upper safe limit of atmospheric CO2 for maintaining life on Earth as we know it is believed to be 300–350 ppm. See also: Global climate change
Atmospheric carbon dioxide, a greenhouse gas, is increasing in concentration at a rate of about 2 ppm per year. The bulk of this CO2 comes from burning fossil fuels. Of all the CO2 emitted worldwide each year, about 50% remains in the atmosphere and the remainder is absorbed and stored, about equally, in plants on land and in the surface water of the ocean. See also: Greenhouse effect
In addition to increasing the global temperature, CO2 emissions are having two observable effects on the ocean: sea-level rise and ocean acidification. Sea-level rise is a long-term problem that cannot be stopped, even if the atmospheric CO2 concentration were to remain at its present level.
Sea-level rise results from the melting of land ice and thermal expansion of a warming ocean. For the period 2003–2010, NASA reported land-ice melting from the Earth's glaciers and ice caps totaling about 4.3 trillion tons (3.9 metric tons) and representing about 0.5 inches (12 millimeters) of sea-level rise. This is troubling news for regions of the world where sea-level rise is predicted to rise faster, such as the Atlantic Coast of North America. See also: East Coast faces faster sea level rise
Ocean acidification occurs when CO2 dissolves in seawater through a chemical reaction that produces hydrogen ions (acid), reducing pH. Ocean acidification also reduces the availability of carbonate ions, making it more difficult for some species, such as corals and shellfish, to produce their calcium-carbonate shells or skeletons. In the Arctic, very small free-swimming planktonic mollusks, called pteropods, may already be experiencing such problems. Pteropods are a food source for juvenile salmon, and their decline could lead to changes in the ecosystem. See also: Carbonate minerals; Corals and ocean acidification; Marine ecology; New insights on ocean acidification; pH
On a positive note, for the first quarter of 2012, the U.S. Energy Information Administration reported the lowest U.S. CO2 emissions in 20 years. This was due in part to switching from coal to natural-gas power plants. Although it is unknown whether lower CO2 emissions will continue in the United States, this result does demonstrate the benefits of cleaner energy sources.