The Greenhouse Effect
The physical mechanisms that cause greenhouse gases to warm the planet, commonly known as the 'greenhouse effect', are well understood and were scientifically demonstrated beginning in the mid-1800s (Tyndal 1861). Of the solar energy that is directed toward Earth, about 30% is reflected back to space by clouds, dust, and haze (Ramanathan & Feng 2009). The remaining 70% is absorbed by the atmosphere and the Earth's surface. The Earth's warmed surface releases some of that absorbed energy as infrared radiation, a form of light, but invisible to human eyes. Greenhouse gases in the atmosphere including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water vapor, absorb this infrared radiation and keep it from passing out into space. This energy is then reradiated in all directions, and the energy that is directed back toward the Earth warms the planet.
Human Influence on Greenhouse Gases
Without the natural presence of energy-absorbing greenhouse gases in the Earth's atmosphere, the average temperature at Earth's surface would be below the freezing point of water (IPCC 2007a Ch.1). However human activities have led directly to increases in greenhouse gas concentrations and therefore an enhanced greenhouse effect.
The 2007 United Nations Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) provides the most substantive and authoritative evaluation of human-caused global climate change to date. According to this report, "Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely [> 90% probability] due to the observed increase in anthropogenic greenhouse gas concentrations." (IPCC 2007b). Independent studies using a variety of methods strongly corroborate this conclusion (e.g. Lean 2010, Huber & Knutti 2011). Examinations and model simulations of many possible explanations of global warming show that we can only explain the strong temperature increases of the past 120 years if we account for human influences (e.g. Figure 5).
Human activity has had the most notable impact on carbon dioxide concentrations, which as noted earlier, have increased dramatically (Figure 2), mainly through fossil fuel burning, cement production, and deforestation. Methane, another potent greenhouse gas, is emitted by activities such as rice and livestock agriculture and biomass burning, and is currently at its highest concentrations of the past 650,000 years (IPCC 2007a Ch.2). Nitrous oxides have increased due in part to agricultural fertilization and fossil fuel burning; other gases emitted from industrial processes, such as halocarbons, also play a role in warming (Figure 5). Many of these greenhouse gases are likely to reside in the atmosphere for decades to centuries (CDIAC 2011). The most abundant greenhouse gas is water vapor, but water vapor is short lived in the atmosphere (on the order of days) and is dependent on temperature. So, human activities have little direct influence on water vapor, although human-caused warming can increase water vapor concentrations and amplify the warming effect (Held and Soden 2000).
Need more information?
See the following primers and resources for more introductory information on climate change.
Climate Change Resource Center:
United States Global Change Research Program:
Global Climate Change Impacts in the United States
The US EPA
Climate Change Science Facts
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