The Intergovernmental Panel on Climate Change (IPCC) and its scientific community assert in the IPCC Fourth Assessment Report that warming of the climate system is unequivocal and that, since the mid-twentieth century, most of the increases in global temperature were very likely caused by an increase in greenhouse gas concentrations derived from anthropogenic sources.
Greenhouse gases occur naturally in the atmosphere and are fundamental to regulating Earth’s liveable temperature. However, high concentrations of important greenhouse gases may threaten the planet’s equilibrium. Carbon dioxide, often considered the most important of the greenhouse gases due to its high level of atmospheric concentration and integral role in absorbing infrared radiation, has increased by 35% over pre-industrial concentrations. The increase in carbon dioxide from pre-industrial levels of 280 parts per million (ppm) to 379 ppm in 2005 is largely attributed to human activities such as deforestation, land degradation, and fossil fuel combustion from both stationary and mobile sources. Methane and nitrous oxide, which have increased by 148% and 18% from pre-industrial levels, respectively, are equally important as carbon dioxide on a molecular level but are considered somewhat less important in the overall progression of climate change because of lower atmospheric concentration levels. The IPCC observes that increases in greenhouse gas concentrations have enhanced the natural greenhouse effect, causing eleven of the last twelve years (1995 – 2006) to rank among the warmest twelve years recorded since 1850.