Since the Industrial Revolution, the global increases in carbon dioxide (CO2) concentrations are primarily due to burning fossil fuels (such as coal, petroleum, and natural gas) and deforestation, while increases in methane (CH4), and nitrous oxide (N2O), are primarily due to agriculture.1 Fossil fuels come from the fossilized remains of organic matter from plants and animals that, over millions of years, have been compressed, heated, and chemically altered by the Earth. Records of greenhouse-gas concentrations over the last thousand years using ice-core data and recent atmospheric measurements show that levels were nearly constant between the years 1000 and 1800, and then began to increase from 1850 (see Figure 1). The most important of these gases is carbon dioxide. Since 1850, CO2 concentrations have risen from about 280 parts per million (ppm) to around 380 ppm today.2 However, the rise has been most rapid in the last few decades. Considering the increase of 100 ppm of extra carbon dioxide in the atmosphere, it took 125 years for CO2 levels to increase the first 50 ppm (between 1850 and 1975), whereas it only took thirty years for CO2 levels to increase the last 50 ppm (between 1975 and 2005). Today's levels of carbon dioxide are likely higher than anytime in the last 650,000 years,3 and if steps to reduce emissions are not taken, levels of carbon dioxide are forecast to double (compared to preindustrial levels) by 2050.4
Figure 1. Atmospheric concentrations of important long lived greenhouse gases over the last 2,000 years. Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion air molecules, respectively, in an atmospheric sample.5
1. Deforestation has two impacts on climate. First, because trees take up carbon dioxide, their removal ultimately acts to increase CO2 levels in the atmosphere and this leads to warming of the planet. However, deforestation also makes the land surface more reflective to sunlight (when trees are present, the surface is green; after trees are cut, the surface is gray or white), and this by itself would cool the planet. Further details on deforestation and its impact on climate can be found in chapter 2.5 of the IPCC report. IPCC, Climate Change 2007: Impacts, Adaptation and Vulnerability: Contribution of Working Group II to the Fourth Assessment Report on Climate Change, M. L. Parry, et al., eds. ( Cambridge , Cambridge University Press, 2007)
2. So, if carbon dioxide concentrations are 280 parts per million (ppm), then out of a million air molecules, 280 would be carbon dioxide.
3. IPCC, Solomon, et al. (2007).
4. Using the IPCC Special Report on Emissions Scenarios, 2000, found at http://www.ipcc.ch/ipccreports/ sres/emission/index.htm, one can use the "business as usual" A1FI scenario to project how carbon dioxide will change over the coming century.
5. IPCC, Solomon, et al. (2007), adapted from Fig FAQ2.1.