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. 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 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.
Target levels of CO2
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 (406.75 ppm CO2, as of December 2017). 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.
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 million 1 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. 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. 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. In 2011, Socolow revised their earlier estimate to nine.
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. suggested in 2006 that to stabilize CO2 at 350 ppm, we would have needed 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.
From an "energy technology and policy perspective", Joe Romm says a 350-ppm target will require eight wedges, each saving 1 gigaton of carbon per year, by 2030, and another ten by 2060.
Supporters of the 350 upper limit include Eban Goodstein, Frank Ackerman, Kristen Sheeran of the Economics for Equity and the Environment Network (E3), Lester R. Brown Worldchanging's Alex Steffen the Will Steger Foundation, Barbara Kingsolver, Ed Begley, and Bonnie Raitt
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".
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".
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
Greenhouse gas concentrations are aggregated in terms of carbon dioxide equivalent. Some multi-gas mitigation scenarios have been modeled by Meinshausen et al.
As a short-term focus
In a 2000 paper, 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 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.
- 4 Degrees and Beyond International Climate Conference
- Adaptation to global warming
- Bio-energy with carbon capture and storage (BECCS)
- Carbon dioxide removal (CDR)
- Carbon negative fuel
- Climate change mitigation
- Climate engineering (geoengineering)
- Emissions trading
- Energy modeling
- Environmental protection
- Virgin Earth Challenge
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- Why Black Carbon and Ozone Also Matter