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Future Atmosphere Changes in Greenhouse Gas and Aerosol Concentrations
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Near Term Scenarios | Long Term Scenarios
The extent and speed of future atmosphere changes will be driven by the level of greenhouse gas (and aerosol) emissions over time. Human activities are major sources of these emissions, which have increased in the past and are projected to continue increasing in the future, although the U.S. and other governments are taking steps to slow their growth.
To project changes in greenhouse gas and aerosol emissions, and the resulting changes in their concentration in the atmosphere, future scenarios are developed that include assumptions about global population, living standards, energy use and technology. These scenarios are uncertain because Earth will change in ways that are difficult to predict. Nonetheless, a diverse set of scenarios can offer insight into the range of possible future atmosphere conditions.
Near Term Scenarios
Near-term scenarios (through 2020 or 2030) of future greenhouse gas emissions have been developed by EPA and the Department of Energy (DOE):
- EPA has developed non-carbon dioxide (non-CO2) greenhouse gas emissions projections for both the U.S. and internationally through 2020. Global non-CO2 greenhouse gas emissions are projected to grow 44% to 13 billion metric tons (CO2 equivalent) by 2020 relative to 1990 levels (about 9 billion tons CO2 equivalent) under “business as usual” conditions.
- In its Annual Energy Outlooks for both the U.S. and the world, DOE presents a forecast and analysis of energy supply, demand and prices through 2030 that includes CO2 emissions projections. Global emissions of CO2 are predicted to increase from 25 billion metric tons in 2003 to 43-44 billion metric tons in 2030, or about 74% based on changes in energy supply and demand and prices.
- Developing countries such as China will be the primary source of new emissions (see Figure 1).
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| Reference: (1) SGM Energy Modeling Forum EMF-21 Projections, Energy Journal Special Issue, in press, reference case CO2 projections. (2) Non-CO2 emissions are from EPA's Global Anthropogenic Emissions of Non-CO2 Greenhouse Gases 1990-2020. |
Long Term Scenarios
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| Figure 2: Long Term Scenarios for Greenhouse Gas Concentrations, based on data provided by IPCC WG1. |
Long term scenarios (through 2100) developed by the Intergovernmental Panel on Climate Change (IPCC), which cover a wide range of possible future characteristics, project changes in greenhouse gas and aerosol concentrations (based on projected changes in emissions) as follows:
- Carbon dioxide concentrations (see Figure 2) in the atmosphere will increase throughout the 21st century according to all IPCC scenarios. The scenarios project CO2 concentrations ranging from 535 to 983 parts per million (ppm) by 2100, which is 41 to 158 percent higher than current levels (IPCC, 2007).
- Methane concentrations (see Figure 2) in the atmosphere are projected to range from 1.46 ppm to 3.39 ppm by 2100, or about 18 percent lower to 91 percent higher than the current concentration (IPCC, 2007). For more information on methane projections, see EPA’s Methane Site.
- Nitrous Oxide concentrations (see Figure 2) are projected to be 0.36 to 0.46 ppm in 2100, values that are 11 to 45 percent higher than current concentrations (IPCC, 2007). For more information on nitrous oxide projections, see EPA’s Nitrous Oxide Site.
- Fluorinated gases, such as HFCs, PFCs and SF6 (also known as high global warming potential gases), are expected to increase significantly in part because some of these gases are substitutes for chlorofluorocarbons, which are being phased out through the Montreal Protocol. For more information on fluorinated gas projections, see EPA’s High Global Warming Potential Site.
- Tropospheric ozone concentrations are projected to increase 40-60% under high emissions scenarios (IPCC, 2007). For a range of low, medium and high emissions scenarios, projections ranged from a 12% decrease to a 62% increase by 2100 (IPCC, 2001).
- Sulfate aerosols are generally projected to decrease whereas projections for black carbon (soot) are uncertain.
The U.S. Climate Change Science Program has constructed a new set of 15 long term scenarios as part the development of Synthesis and Assessment Product 2.1: “Scenarios of Greenhouse Gas Emissions and Atmospheric Concentrations and Review of Integrated Scenario Development and Application”. These scenarios were produced by three different modeling groups and contain reference scenarios (that assume no climate policies are implemented beyond the set of policies currently in place in the U.S. and elsewhere) and stabilization scenarios (where carbon dioxide levels are stabilized at roughly 450 ppm, 550 ppm, 650 ppm, and 750 ppm).
In addition, long term scenarios
have been developed by Stanford University’s
Energy Modeling
Forum 
, an international forum for sharing and facilitating
discussions on energy policy and global climate issues.
References
- IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning (eds.)].
- IPCC, 2001: Climate Change 2001: The Scientific Basis.
Contribution of Working Group I to the Third Assessment Report of the Intergovernmental
Panel on Climate Change [Houghton, J.T., Y. Ding, D.J. Griggs, M. Noguer,
P.J. van der Linden, X. Dai, K. Maskell, and C.A. Johnson (eds.)]. Cambridge
University Press, Cambridge, United Kingdom and New York, NY, USA, 881pp.