I'm a graduate student and aspiring environmental economist at Yale F&ES. A number of us involved in climate science and economics have lately been debating what targets are really feasible, and whether there needs to be a bit of public reassessment that, even if we start as soon as possible with strong abatement measures, a mean 3 degrees C increase in global temperature relative to preindustrial temps is the lowest that may be achievable in practice. I've brought this up in passing with a number of climate scientists and economists over the last few months (including Gavin Schmidt last fall), and there seems to be some degree of agreement that 3 degrees may be the most realistic outcome of the best foreseeable realistic abatement efforts.
Hansen's alternate emissions scenario is often brought up to defend the position that stabilization under two degrees C above pre- industrial average temps (or at one degree over 1990 temps) is practical. However, looking over the equilibrium climate sensitivity in the AR4 (see Table 3.9 on the bottom of page 91 of chapter three of the WGIII report: http://www.mnp.nl/ipcc/pages_media/AR4-chapters.html ), a mean increase of 1 degree C over 1990 levels (1.6 over pre- industrial) would on average occur at 402 ppm CO2e, with current atmospheric concentration at ~430 CO2e. At 441 CO2e, a concentration that is for all intents and purposes inevitable at this point, we have an expected warming of 2 degrees C relative to preindustrial levels. Thus it appears that goals of limiting expected warming to a mean of 2 degrees C (such as the European proposal) would require immediately ceasing all emissions. If we structure our goal toward avoiding a particular level of warming, things look even more grim. To avoid more than 3 degrees warming over pre-industrial levels (e.g. having only a 17% chance of exceeding 3 degrees), we would also have to essentially cease all emissions immediately (see the figure reference above). Given the rapid economic development of China and India, and the manifest failure of the international community to take action over the past decade, it seems unlikely that global emissions will peak in the next ten years, as would be required to realistically limit warming to 2-2.4 degrees C. From a political economy standpoint, it would be very difficult to imagine peaking world emissions prior to 2020, which would put us roughly on course for a 550 ppm CO2e equilibrium concentration (accompanied by a mean expected warming of 3 degrees). The Stern Review contained a similar analysis, where it calculated the annual emission reductions associated with different stabilization scenarios. To meet a 450 ppm CO2e target, global emissions would have to be capped by 2010 with an annual decrease in emissions of 7% per year thereafter. To put this figure in perspective, the collapse of the economy of the former Soviet Union only decreased emissions in that region by roughly 5% annually. If an utter economic collapse cannot produce this level of reduction, especially considering the difficulty of rapidly replacing existing capital stock, and the rapidly growing Asian economies, it is difficult to see how such a stabilization level could be achieved. Likewise, a 500 ppm CO2e stabilization level would require capping emissions around 2020 (or possibly 2030 in an overshoot scenario), with subsequent reductions of 4-6% per year. Even a 550 ppm CO2e stabilization level will be difficult to achieve, but seems considerably more realistic to achieve given the political, economic, and infrastructural constraints that any climate change policy would face. Curiously, according to George Monbiot (http://www.monbiot.com/ archives/2007/05/01/1058/#more-1058), the EU target of 2 degrees C also uses a figure of 550 ppm CO2e (or 550 ppm CO2, in the case of the UK)! Even if they were (somewhat disingenuously) using a target of 2 degrees C max warming relative to current temperatures, instead of the more appropriate pre-industrial baseline, 550 ppm CO2e would still yield a mean warming of 2.4 degrees C according to the IPCC. This seems an odd disconnect, if Monbiot is correct. This discussion is not to detract from the urgency of the problem, or suggest that action can be postponed or that 3 degrees C would be an acceptable temperature. Much to the contrary, it suggests that we are in deeper trouble than is commonly perceived, and that immediate action is all the more necessary. We are already committed to roughly 2 degrees warming at equilibrium relative to pre-industrial levels, and will probably end up with at least three degrees. No matter what we do, climate change will have substantial impacts on our planet. If we start later rather than sooner, these impacts will be exponentially worse. -Zeke Hausfather MEM Candidate, '08 Yale School of Forestry and Environmental Studies --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups Global Change ("globalchange") newsgroup. Global Change is a public, moderated venue for discussion of science, technology, economics and policy dimensions of global environmental change. Posts will be admitted to the list if and only if any moderator finds the submission to be constructive and/or interesting, on topic, and not gratuitously rude. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/globalchange -~----------~----~----~----~------~----~------~--~---
