Limiting Global Warming to 2 deg C and Beyond
Abstract
This presentation addresses the question of how feasible is it to limit global warming to a specific temperature rise, whether 1.5, 2 or 3 deg C. Inherent in the idea of limiting global warming to a specific temperature level is the notion that future GHG emissions will be subject to a top-down international agreement. In the post-Copenhagen era, however, such an agreement is unlikely, and a bottoms-up approach of national pledges will likely have to serve as a surrogate for achieving emissions reduction. In this case, an additional question is what temperature targets are realistic under scenarios that are bounded by achievable national pledges as opposed to binding mandates. The question of feasibility depends largely on future emission pathways of CO2, other GHGs, black carbon and aerosols. Those pathways depend on many societal, technological and economic factors, but it is likely that the ultimate limiting factor is the maximum possible rate of absolute emission reduction. That rate is limited by how rapidly energy infrastructure can be turned over. Most studies suggest that an absolute emission reduction rate of 3.5% is the highest rate achievable. Climate sensitivity and the current cooling effect of aerosols and earth system responses such as the rate of ocean heat uptake and carbon cycle feedbacks determine how a specific emissions pathway translates into probable climate change. A useful framework for CO2 alone is provided by the newly emerging paradigm of cumulative emissions, which holds that peak temperature can be largely predicted by the total amount of carbon emitted, regardless of pathway. Most studies suggest that 1 Tt of cumulative carbon is equivalent to ~2 deg of peak warming. A consideration of these factors suggests that limiting warming to 1.5 deg C is no longer possible under any feasible economic scenario. For one, currently emitted GHGs are equivalent to a ~1.3 deg C warming commitment. This leaves very little room for future emissions and makes a 1.5 deg C target far more difficult to achieve than a 2 deg C target. For example, one credible 1.5 deg C scenario requires average total emission reductions of ~15% per year, well above the rate that is considered feasible. Limiting warming to 2 deg C requires an immediate start to mitigation, with emissions peaking in c. 2020 and absolute emission reduction rates of ~3% per year. Such a reduction is considered economically feasible, although historical absolute emission reduction rates from 1990 - 2010, which reflect the net of carbon intensity decline and economic growth, have only reached ~1% per year in a few EU countries such as Denmark and Germany. Delaying the start of mitigation rapidly increases the required absolute emission reduction rates and moves the 2 deg C goal beyond the realm of economic feasibility. A later start of mitigation or lower absolute emission reduction rates would lead to warming of 3 to 4 deg C. For example, a 3 deg goal could be achieved with emission reductions of 0.7% per year and emissions peaking in c. 2030. Current national pledges, as analyzed by both UNEP and IEA, still allow for growth in emissions of >1% per year and therefore do not meet the requirements of any of the temperature targets.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMGC51C0980L
- Keywords:
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- 1600 GLOBAL CHANGE