For the past two centuries, and at a faster rate in recent years, the basic composition of the Earth's atmosphere has been changed by the burning of fossil fuels. Human-induced warming of Earth's climate ("infrared forcing" to scientists) is emerging as one of the major scientific, social, and economic issues of the twenty-first century, as the effects of climate change become evident in everyday life in locations as varied as the small island nations of the Pacific Ocean and the shores of the Arctic Ocean.
Carbon dioxide, methane, and other greenhouse gases hold heat in the atmosphere. As levels of these gases have risen, so have global temperatures, with most of the warmest years in the world's temperature record (instrumental records exist for about a century and a half) having occurred within the last decade. By 2007, 12 of the warmest years globally had occurred within the previous 13 years. This is true of individual locations as well. In August 2007, Atlanta, Georgia, saw 5 of its 10
1880 1900 1920 1940 1960 1980 2000
1880 1900 1920 1940 1960 1980 2000
Global average temperatures, 1880 to the present (Jeff Dixon)
hottest days on record during a single heat wave. Also, in 2007, Phoenix registered 32 days at 110oF or higher.
Today, due mainly to the combustion of fossil fuels, the amount of heat-retaining gases in the atmosphere is increasing rapidly. In a century and a half of rapid worldwide industrialization, after about 1850, the proportion of carbon dioxide has risen from roughly 280 to about 380 ppm. By the year 2006, scientists had estimated the composition of the atmosphere to roughly 60 million years in the past. The level of carbon dioxide today is believed, according to such measurements, to be as high now as it has been in at least 20 million years. During the half-century since Charles Keeling and colleagues first devised ways to measure carbon dioxide levels in the atmosphere precisely, the figure has risen from about 315 ppm to today's 380 ppm.
Carbon dioxide levels in the atmosphere jumped suddenly between 2000 and 2004, according to calculations published in the Proceedings of the National Academy of Sciences in 2007. The rate of increase nearly tripled over the average rate in the 1990s. Instead of rising by 1.1 percent a year, as in the previous decade, emissions grew by an average of 3.1 percent a year from 2000 to 2004. "Despite the scientific consensus that carbon emissions are affecting the world's climate, we are not seeing evidence of progress in managing those emissions," said Chris Field, the director of the Carnegie Institution's Department of Global Ecology in Stanford, California, a coauthor of the report. The growth rate of emissions exceeds even the most pessimistic of six options presented in the Intergovernmental Panel on Climate Change's assessments (Boyd, 2007, A-8; Raupach et al., 2007, 10,288).
Carbon dioxide emissions were 35 percent higher in 2006 than in 1990, a much faster growth rate than anticipated, researchers led by Josep G. Canadell of Australia's Commonwealth Scientific and Industrial Research Organization reported in the October 23, 2007, edition of the Proceedings of the National Academy of Sciences. Much of the increase is being traced to the reduction of the oceans' ability to remove additional carbon dioxide from the air as water temperatures increase. According to the new study, carbon released from burning fossil fuel and making cement rose from 7.0 billion metric tons per year in 2000 to 8.4 billion metric tons in 2006. A metric ton is 2,205 pounds. Methane emissions have declined, however, and so greenhouse gases as a whole are not increasing as much as carbon dioxide alone (Carbon Dioxide, 2007).
Atmospheric carbon dioxide levels measured at Mauna Loa, Hawaii (Jeff Dixon)
The Mauna Loa record indicated an 18.8 percent increase in the mean annual concentration, from 315.98 ppm by volume of dry air in 1959 to 375.64 ppm in 2003. The El Nino-aided 1997-1998 increase of 2.87 ppm was the largest single yearlyjump since the Mauna Loa record began in 1958 (Keeling and Whorf, 2004).
The jump in the emission rate is alarming because it indicates a reversal of a long-term trend toward greater energy efficiency and away from carbon-based fuels, the report's authors said. (It may also indicate increasing feedback emissions from such things as melting permafrost.) Carbon dioxide levels recorded during March 2004 at Hawaii measured 379 ppm. If emissions increase 3.1 percent a year, CO2 concentration in the atmosphere would rise from the present 380 ppm to 560 ppm in 2050 and 1,390 ppm in 2100, according to Michael Raupach, an atmospheric scientist at the Center for Marine and Atmospheric Research in Canberra, Australia. "A CO2 future like this would spell major climate-change disaster in the latter part of the 21st century," Raupach said (Boyd, 2007, A-8).
The rapid increases in carbon dioxide levels have raised speculation among scientists that atmospheric carbon dioxide may be increasing in a runaway fashion (Keeling and Whorf, 2004). Before 2002, a back-to-back reading of 2.0 ppm or more had never been recorded, and the only other years with increases of 2.0 or more (1973, 1988, 1994, and 1998) had involved El Nino conditions, when warming of ocean waters of the Pacific near the equator prompt added heat worldwide. No El Nino was present in 2003 or 2004.
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