This study might also be useful for calculating the likelihood that unconventional actions like SRM and CDR will be necessary to stabilize global temp, and when. - G
Nature | News & Views Climate change: All in the timing * Steve Hatfield-Dodds<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html?WT.ec_id=NATURE-20130103#auth-1> Nature 493, 35–36 (03 January 2013) doi:10.1038/493035a Published online 02 January 2013 How influential are the various factors involved in curbing global warming? A study finds that the timing of emissions reduction has the largest impact on the probability of limiting temperature increases to 2 °C. See Letter p.79<http://www.nature.com/nature/journal/v493/n7430/full/nature11787.html> Subject terms: * Climate sciences<http://www.nature.com/nature/archive/subject.html?code=106> * Economics<http://www.nature.com/nature/archive/subject.html?code=159> Climate science sometimes seems to have overtaken economics as the most dismal science. But a study by Rogelj et al. on page 79<http://www.nature.com/nature/journal/v493/n7430/full/nature11787.html> of this issue1<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref1> might just change that. The authors quantify the importance of five 'uncertainties' that are thought to influence the chance of limiting global temperatures to different levels, using a suite of models to generate around 500 scenario variations. They find that the timing of international action to limit emissions has by far the largest impact. Furthermore, the models show that the impact of timing is highly nonlinear, and that delaying emissions limits by only five years, from 2020 to 2025, would dramatically cut the likelihood of limiting warming to 2 °C. The findings should help to make risks and consequences more transparent, and thereby support better-informed economic and political decisions. The five major uncertainties assessed by Rogelj and colleagues were the following: the responsiveness of the physical climate system to cumulative emissions; the deployment of energy- and land-based emission-reduction technologies; the global demand for energy (which includes combined uncertainties about population, income growth and energy efficiency); the global carbon price that the international community is willing to impose; and the timing of substantive action to limit emissions (phased in from 2010). The analysis covers limiting the temperature in 2100 to 1.5, 2, 2.5 and 3 °C above pre-industrial levels, with a focus on 2 °C. These scenario comparisons revealed timing of global action to be the uncertainty with the greatest effect. For example, the authors find that bringing forward global action on emissions from 2020 to 2015 would improve the chance of limiting temperatures to 2 °C from 56% to 60%, all else being equal. To put this another way, achieving the same 60% chance of success with action starting in 2020 would require a 2020 carbon price of around US$150 per tonne of carbon dioxide equivalent (CO2e) — more than double the $60 per tonne CO2e required if action begins in 2015. However, delaying emissions limits from 2020 to 2025 would bring the chance of success down to 34%, and the authors found no scenario in which a feasible increase in carbon price or improvements in energy technology could make up for these five years of delay. Geophysical uncertainties are the next most significant factor, followed by societal factors influencing energy demand, and then uncertainties surrounding mitigation technologies. This information brings clarity to the relative contributions of the timing of global action; the role of energy efficiency; potential mitigation technologies; and required carbon prices. And it will help to assess trade-offs that might be made between these factors. Rogelj and colleagues' analysis also suggests that higher carbon prices will drive more-rapid deployment of low-emission technologies and improve the chance of success, but that imposing a carbon price above $150 would make very little difference to temperature outcomes, because this carbon-price path is sufficient to drive the uptake of all the modelled reduction technologies. The study, and another recent analysis by some of the same authors2<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref2>, also find that the use of different technologies affects both the chance and cost of success. Here, the key results are that the development and deployment of carbon capture and storage technology for fossil fuels and, subsequently, bioenergy are essential for limiting temperature increases to 1.5 °C, and that failure to deploy such technology would reduce the chance of limiting temperature increases to 2 °C by 14–16 percentage points (with a 2020 carbon price of $60–150). Rogelj and colleagues' assessment complements existing published economic analysis of the costs and benefits of emissions reductions. That literature is characterized by two underappreciated points of consensus. First, the key protagonists in the debate over the British government's 2006 Stern Review3<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref3> on the economics of climate change now all agree that action to limit temperature rise to 2 °C (or emissions to 450 parts per million CO2e) would provide net benefits4<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref4>, 5<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref5>; this represents a quiet reversal6<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref6>, 7<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref7>in the position of economist William Nordhaus, who was initially critical of the review. Second, economic analysis shows that ambitious global action to limit emissions is fully consistent with strong economic growth and improvements in living standards8<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref8>. A recent multi-model review9<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref9> finds, for example, that average global income is projected to roughly double over the four decades to 2050 across all scenarios targeting 450 parts per million CO2e, and that the average income growth in emerging economies and other developing countries would be above the global average. Perhaps the only significant limitation of Rogelj and colleagues' analysis is that the exploration of geophysical uncertainties does not fully incorporate known positive feedbacks on climate change, particularly the effect of CO2 and methane releases from warming permafrost10<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref10>, 11<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref11>. (This is a limitation of climate modelling generally.) Although the timing and magnitude of these releases are highly uncertain, on current temperature trends the cumulative emissions from permafrost are considered likely to be more than 30 gigatonnes CO2e by 2040, with significant emissions continuing for more than two centuries11<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref11>, 12<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref12>. This has the potential to dwarf reductions in anthropogenic emissions. However, this consideration simply strengthens the case for early and decisive action to limit anthropogenic emissions, because it would reduce the temperature increases that could trigger such feedbacks. This suggests that it would be valuable for economic studies to explore the 'insurance value' of reducing the risk of climate feedbacks. Minor limitations of the study are that it uses a single modelling suite for analysis and is based on a single, somewhat optimistic view of underlying population and economic trends13<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref13>, 14<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref14>. This implies that the authors' estimated probabilities of achieving particular temperature limits may also be somewhat optimistic, and that it would be good to replicate this work using other models and a wider range of underlying scenarios. Rogelj et al. have provided a new benchmark for assessing the relative contributions of several major uncertainties in the quest to limit climate change. The study's key message reinforces previous findings14<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref14>, 15<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref15>, 16<http://www.nature.com/nature/journal/v493/n7430/full/493035a.html#ref16> that urgent and more ambitious global action is required to maintain any chance of limiting global warming to 2 °C. The clear finding that the world would be better off acting from 2015 rather than 2020 also raises sharp and serious questions about the trade-offs implicit in the current pace of global negotiations and action. The window for effective action on climate change is closing quickly, and Rogelj et al. have put a price tag on each year of delay. 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