Attribution of Global Warming

Human-Induced Climate Change

The dominant mechanisms to which recent global warming and climate change have been attributed all result from human activity, such as increasing atmospheric concentrations of greenhouse gases, or aerosols. These attributions are supported by observations from the Intergovernmental Panel on Climate Change (IPCC). The IPCC concluded that most of the observed increase in globally averaged temperatures since the mid-20th century is “very likely” due to the observed increase in anthropogenic greenhouse gas concentrations via the greenhouse effect. It also concluded that it is “extremely likely” that human activities have exerted a substantial net warming influence on climate since 1700. These conclusions have probabilities greater than 90 and 95 percent, respectively.

Since the 1700s, the CO2 content of the atmosphere has been increasing as a result of air pollution emitted by human activities. Pollution has also caused a significant increase in secondary greenhouse gases, namely methane from animal digestion and bacteria, ozone and nitrogen oxides from urban air pollution, and chlorofluorocarbons from spray cans and fugitive refrigerants. The increase in CO2 and the four secondary greenhouse gases is a global trend. Since 1760, the atmospheric concentrations of CO2 and methane have increased by 32 percent and 151 percent, respectively, compared to the pre-industry period. Therefore, the biggest root cause is worldwide population growth.

According to the IPCC, the projected increase of global surface temperatures is likely to be 2.1–11.7 degrees F (1.2–6.5 degrees C) between 1995 and 2100, based on the different scenarios of future greenhouse gas emissions and climate sensitivity. These conclusions have been endorsed by at least 30 scientific societies and academies of science, including all of the national academies of science of the major industrialized countries.

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