Climate change mitigation scenarios
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Climate change mitigation scenarios are possible futures in which global warming is reduced by deliberate actions, such as a comprehensive switch to energy sources other than fossil fuels. A typical mitigation scenario is constructed by selecting a long-range target, such as a desired atmospheric concentration of carbon dioxide (CO2), and then fitting the actions to the target, for example by placing a cap on net global and national emissions of greenhouse gases.
An increase of global temperature by more than 2°C has come to be the majority definition of what would constitute intolerably dangerous climate change, but some climate scientists are increasingly of the opinion that the goal should be a complete restoration of the atmosphere's preindustrial condition, on the grounds that too protracted a deviation from those conditions will produce irreversible changes.
Target levels of CO2 
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Contributions to climate change, whether they cool or warm the Earth, are often described in terms of the radiative forcing or imbalance they introduce to the planet's energy budget. Now and in the future, anthropogenic carbon dioxide is believed to be the major component of this forcing, and the contribution of other components is often quantified in terms of "parts-per-million CO2-equivalent" (ppm CO2e), or the increment/decrement in carbon dioxide concentrations which would create a radiative forcing of the same magnitude.
At present, non-CO2 contributions to climate change, positive and negative, are believed to roughly cancel out, so that the net radiative forcing being experienced at present, expressed in ppm CO2-e, is more or less the same as the actual current level of carbon dioxide (393 ppm CO2, as of April 2011). To some extent this legitimates the statement of targets just in terms of ppm CO2, as is usually the case. However, the positive and negative non-CO2 will not necessarily balance in future, and so a target stated in terms of CO2e is less ambiguous.
Stabilization wedges 
A stabilization wedge (or simply "wedge") is an action which incrementally reduces projected emissions. (The name derives from the triangular shape of the gap between reduced and unreduced emissions trajectories, when graphed over time.) For example, a reduction in electricity demand due to increased efficiency means that less electricity needs to be generated and thus fewer emissions need to be produced. The term originates in the Stabilization Wedge Game.
350 ppm 
This is the target level advocated in a 2008 paper by climate scientist James E. Hansen and others such as: Rajendra Pachauri, the chairperson of the U.N.'s Intergovernmental Panel on Climate Change (IPCC), the Director of the University of Minnesota's Institute on the Environment, Jonathan Foley , President of the Pacific Institute Peter H. Gleick, and the Policy Director of the Brookings Institution's Climate and Energy Economics Project Adele C. Morris. This maximum level is advocated by the 350.org campaign, along with other organizations such as the Tällberg Foundation. A strategy proposed is (1) no further oil and gas exploration (so that only already-known reserves will be consumed), (2) the elimination of all uncaptured burning of coal by 2030, and (3) an intensive program of reforestation and negative emissions with technologies such as BECCS, biochar agriculture and/or direct air capture.
A mitigation scenario modeled by Malte Meinshausen et al. suggests that to stabilize CO2 at 350 ppm, we will need to reduce emissions by slightly more than 5% per year. The amount of CO2 that has been released to date is believed to be too much to be able to be absorbed by conventional sinks such as trees and soil in order to reach low emission targets, which implies that negative emissions with technologies such as BECCS are needed to reach the 350 ppm target.
Per "We can afford to save the planet" October 23, 2009 Eban Goodstein, Frank Ackerman, Kristen Sheeran of the Economics for Equity and the Environment Network (E3), and Lester R. Brown per "We Only Have Months, Not Years, To Save Civilization From Climate Change", November 2009, are supporters of the 350 upper limit.
In her speech "The World's Tipping Point", Bianca Jagger states “the safe upper limit for atmospheric CO2 is no more than 350 ppm." and quotes the report "The Economics of 350: The Benefits and Costs of Climate Stabilization" by Stephen J. DeCanio, Eban Goodstein, Richard B. Howarth, Richard B. Norgaard and Catherine S. Norman, stressing " the need for immediate, direct intervention".
Scientific American August 2010 ocean acidification article Threatening Oceans from the Inside Out: How Acidification Affects Marine Life Blue Ocean Institute authors Marah J. Hardt and Carl Safina support 350 ppm as the maximum upper limit of carbon dioxide in the atmosphere for marine life health, see page 72 in print.
Less conservative targets 
450 ppm 
The BLUE scenarios in the IEA's Energy Technology Perspectives publication of 2008 describe pathways to a long-range concentration of 450 ppm. Joseph Romm has sketched how to achieve this target through the application of 14 wedges.
World Energy Outlook 2008, mentioned above, also describes a "450 Policy Scenario", in which extra energy investments to 2030 amount to $9.3 trillion over the Reference Scenario. The scenario also features, after 2020, the participation of major economies such as China and India in a global cap-and-trade scheme initially operating in OECD and European Union countries. Also the less conservative 450 ppm scenario calls for extensive deployment of negative emissions, i.e. the removal of CO2 from the atmosphere. According to the International Energy Agency (IEA) and OECD, "Achieving lower concentration targets (450 ppm) depends significantly on the use of BECCS".
550 ppm 
This is the target advocated (as an upper bound) in the Stern Review. As approximately a doubling of CO2 levels relative to preindustrial times, it implies a temperature increase of about three degrees, according to conventional estimates of climate sensitivity. Pacala and Socolow list 15 "wedges", any 7 of which in combination should suffice to keep CO2 levels below 550 ppm.
The International Energy Agency's World Energy Outlook report for 2008 describes a "Reference Scenario" for the world's energy future "which assumes no new government policies beyond those already adopted by mid-2008", and then a "550 Policy Scenario" in which further policies are adopted, a mixture of "cap-and-trade systems, sectoral agreements and national measures". In the Reference Scenario, between 2006 and 2030 the world invests $26.3 trillion in energy-supply infrastructure; in the 550 Policy Scenario, a further $4.1 trillion is spent in this period, mostly on efficiency increases which deliver fuel cost savings of over $7 trillion.
Other greenhouse gases 
As a short-term focus 
In a 2000 paper, Hansen argued that the 0.75°[clarification needed] rise in average global temperatures over the last 100 years has been driven mainly by greenhouse gases other than carbon dioxide, since warming due to CO2 had been offset by cooling due to aerosols, implying the viability of a strategy initially based around reducing emissions of non-CO2 greenhouse gases and of black carbon, focusing on CO2 only in the longer run.
See also 
- Adaptation to global warming
- Bio-energy with carbon capture and storage (BECCS)
- Carbon negative fuel
- Carbon dioxide removal (CDR)
- Climate change mitigation
- Environmental protection
- Virgin Earth Challenge
- Target atmospheric CO2: Where should humanity aim?
- "Pachauri's call for 350ppm is breakthrough moment for climate movement". London: guardian.co.uk. 2009-08-26. Retrieved 2009-09-15. Text " Bill McKibben " ignored (help)
- Johnson, Keith (2009-08-25). "Climate Debate: IPCC Head Pachauri Joins the 350 Club - Environmental Capital - WSJ". Wall Street Journal. Retrieved 2009-09-15.
- "AFP: Top UN climate scientist backs ambitious CO2 cuts". Agence France-Presse via Google.com. Retrieved 2009-09-15.
- http://www.scientificamerican.com/article.cfm?id=boundaries-for-a-healthy-planet March 19, 2010 Scientific American
- Meinshausen, M., Hare, W., Wigley, T. M. M., Van Vuuren, D., Den Elzen, M. G. J. and Swart, R. (2006) Multi-gas Emissions Pathways to Meet Climate Targets, Climatic Change 75: 151–194
- Hare, B., and Meinshausen, M.: (2006)"How much warming are we committed to and how much can be avoided?" Climatic Change, 75, pp 111-149.
- An open letter to James Hansen on the real truth about stabilizing at 350 ppm in Climate Progress
- Rockström J, Steffen W, Noone K, Persson Å, Chapin III FS, Lambin EF, Lenton TM, Scheffer M, Folke C, Schellnhuber HJ, Nykvist B, de Wit CA, Hughes T, van der Leeuw S, Rodhe H, Sörlin S, Snyder PK, Costanza R, Svedin U, Falkenmark M, Karlberg L, Corell RW, Fabry VJ, Hansen J, Walker B, Liverman D, Richardson K, Crutzen P and Foley JA (2009) "Planetary Boundaries: Exploring the Safe Operating Space for Humanity" Ecology and Society, 14(2): 32.
- "Climate change" pp. 34-45.
- http://climateprogress.org/2008/04/22/is-450-ppm-or-less-politically-possible-part-2-the-solution/ Is 450 ppm (or less) politically possible? Part 2: The Solution
- "OECD Environmental Outlook to 2050, Climate Change Chapter, pre-release version". OECD. 2011. Retrieved 2012-01-16.
- Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies in Science (journal)
- http://www.iea.org/weo/docs/weo2008/fact_sheets_08.pdf World Energy Outlook 2008 Fact Sheet
- Global warming in the twenty-first century: An alternative scenario, Proceedings of the National Academy of Sciences of the United States of America
- Review of Hansen et al.: Global Warming in the Twenty-First Century: An Alternative Scenario
- Why Black Carbon and Ozone Also Matter