Climate change mitigation scenarios

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. These are actions that minimize emissions so atmospheric greenhouse gas concentrations are stabilized at levels that restrict the adverse consequences of climate change. Using these scenarios, the examination of the impacts of different carbon prices on an economy is enabled within the framework of different levels of global aspirations.[1]

Scenarios of global greenhouse gas emissions. If all countries achieve their current Paris Agreement pledges, average warming by 2100 will go far beyond the target of the Paris Agreement to keep warming "well below 2°C".

A typical mitigation scenario is constructed by selecting a long-range target, such as a desired atmospheric concentration of carbon dioxide (CO
2), 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 with efforts to limit the temperature increase to 1.5 °C above pre-industrial levels per the Paris Agreement. 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.

Stabilization wedges

A stabilization wedge (or simply "wedge") is an action which incrementally reduces projected emissions. The name is derived 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. As a reference unit, a stabilization wedge is equal to the following examples of mitigation initiatives: deployment of two hundred thousand 10 MW wind turbines; completely halting the deforestation and planting of 300 million hectares of trees; the increase in the average energy efficiency of all the world's buildings by 25 percent; or the installation of carbon capture and storage facilities in 800 large coal-fired power plants.[2] Pacala and Socolow proposed in their work, Stabilization Wedges, that seven wedges are required to be delivered by 2050 - at current technologies - to make a significant impact on the mitigation of climate change.[3] There are, however, sources that estimate the need for 14 wedges because Pacala and Socolow's proposal would only stabilize carbon dioxide emissions at current levels but not the atmospheric concentration, which is increasing by more than 2 ppm/year.[2] In 2011, Socolow revised their earlier estimate to nine.[4]

Target levels of CO
2

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 carbon dioxide equivalent" (ppm CO2e), or the increment/decrement in carbon dioxide concentrations which would create a radiative forcing of the same magnitude.

At present, non-CO
2 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 (406.75 ppm CO
2, as of December 2017). To some extent this legitimates the statement of targets just in terms of ppm CO
2, as is usually the case. However, the positive and negative non-CO
2 will not necessarily balance in future, and so a target stated in terms of CO2e is less ambiguous.

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.[5]

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 CO
2 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".[6]

550 ppm

This is the target advocated (as an upper bound) in the Stern Review. As approximately a doubling of CO
2 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 CO
2 levels below 550 ppm.[7]

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.[8]

Other greenhouse gases

Greenhouse gas concentrations are aggregated in terms of carbon dioxide equivalent. Some multi-gas mitigation scenarios have been modeled by Meinshausen et al.[9]

As a short-term focus

In a 2000 paper,[10] Hansen argued that the 0.75 °C 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 CO
2 had been offset by cooling due to aerosols, implying the viability of a strategy initially based around reducing emissions of non-CO
2 greenhouse gases and of black carbon, focusing on CO
2 only in the longer run.[11]

This was also argued by Veerabhadran Ramanathan and Jessica Seddon Wallack in the September/October 2009 Foreign Affairs.[12]

See also

References
  1. Commonwealth of Australia, “Climate Change Mitigation Scenarios: Modeling report provided to the Climate Change Authority in support of its Caps and Targets Review,” 2013. Retrieved 13 December 2018 from https://www.environment.gov.au/system/files/resources/a28424ae-cce9-48c9-aad2-56b3db0920a5/files/climate-change-mitigation-scenarios.pdf
  2. Dawson, Brian; Spannagle, Matt (2008). The Complete Guide to Climate Change. Oxon: Routledge. pp. 283. ISBN 978-0415477895.
  3. Pacala, S.; Socolow, R. (2004-08-13). "Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies". Science. 305 (5686): 968–972. Bibcode:2004Sci...305..968P. CiteSeerX 10.1.1.642.8472. doi:10.1126/science.1100103. ISSN 0036-8075. PMID 15310891. S2CID 2203046.
  4. Socolow, Robert (September 27, 2011). "Wedges reaffirmed - Bulletin of the Atomic Scientists". Bulletin of the Atomic Scientists. Retrieved 2018-08-27.
  5. Is 450 ppm (or less) politically possible? Part 2: The Solution
  6. "OECD Environmental Outlook to 2050, Climate Change Chapter, pre-release version" (PDF). OECD. 2011. Retrieved 2012-01-16.
  7. Pacala, S.; Socolow, R. (13 August 2004). "Stabilization Wedges: Solving the Climate Problem for the Next 50 Years with Current Technologies". Science. 305 (5686): 968–972. Bibcode:2004Sci...305..968P. CiteSeerX 10.1.1.642.8472. doi:10.1126/science.1100103. PMID 15310891. S2CID 2203046.
  8. http://www.iea.org/weo/docs/weo2008/fact_sheets_08.pdf Archived 2008-11-17 at the Wayback Machine World Energy Outlook 2008 Fact Sheet
  9. Meinshausen, M.; Hare, B.; Wigley, T. M. M.; Vuuren, D.; Elzen, M. G. J.; Swart, R. (2006). "Multi-gas Emissions Pathways to Meet Climate Targets" (PDF). Climatic Change. 75 (1–2): 151. Bibcode:2006ClCh...75..151M. doi:10.1007/s10584-005-9013-2. hdl:20.500.11850/36894. S2CID 55462579.
  10. Hansen, James; Sato, Makiko; Ruedy, Reto; Lacis, Andrew; Oinas, Valdar (29 August 2000). "Global warming in the twenty-first century: An alternative scenario". Proceedings of the National Academy of Sciences. 97 (18): 9875–9880. Bibcode:2000PNAS...97.9875H. doi:10.1073/pnas.170278997. PMC 27611. PMID 10944197.
  11. Review of Hansen et al.: Global Warming in the Twenty-First Century: An Alternative Scenario
  12. Why Black Carbon and Ozone Also Matter
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