Assessment of greenhouse gas emission pathways for climate change mitigation.
Dr Sarah C.B. Raper, Tom M.L. Wigley
The project is centred round the development and maintenance of anup-to-date model for assessing the global warming implications of green-house-gas and aerosol emissions over the coming decades and centuries. The model is called MAGICC (Model for the Assessment of Greenhouse-gas induced Climate Change). A user-friendly version can be downloaded and run on a lap-top computer and is available at http://www.cgd.ucar.edu/cas/wigley/magicc/. This project, which has been running for three decades, was initiated by Tom M.L. Wigley and Sarah C.B. Raper when they were both at the Climatic Research Unit, at the University of East Anglia (http://www.nature.com/nature/journal/v357/n6376/abs/357293a0.html ). The latest version MAGICC6 has been recoded and further developed by Malte Meinshausen (Meinshausen et al. 2011a http://www.atmos-chem-phys.net/11/1417/2011/acp-11-1417-2011.html ,b http://www.atmos-chem-phys.net/11/1457/2011/acp-11-1457-2011.html ). MAGICC has been used in all four of the WG1 Intergovernmental Panel on Climate Change (IPCC) assessment reports http://www.ipcc.ch/publications_and_data/publications_and_data_reports.shtml.
Studying policy relevant questions: A study by the team and published in the prestigious journal Nature (Meinshausen et al. 2009 http://www.nature.com/nature/journal/v458/n7242/full/nature08017.html ) shows that greenhouse gas emissions must be cut more than 50 % by 2050, if the risk of exceeding 2°C warming relative to pre-industrial levels is to be limited to 25 percent. Such a scenario is illustrated in the simplified figure 1.
Figure 1.0: GHG emissions and consequent global warming for a business-as-usual scenario (red) and for a mitigation scenario in which emissions are limited to 1000 GtCO2 between 2000 and 2050 (blue).
The study draws on a huge body of research reported in the numerous assessments of the Intergovernmental Panel on Climate Change. It shows that if we are to have a good chance of achieving the 2°C target, action is needed now in order to follow the blue route rather than the red one in the figure. Note that nearly half the allowed warming has already occurred since the globe is now about 0.8 °C warmer than in pre-industrial times.
The study concentrates on the 2°C target which is relevant because the UNFCC (United Nations Framework Convention on Climate Change) draft decision -/CP.16
‘recognizes that deep cuts in global greenhouse gas emissions are required according to science, and as documented in the Fourth Assessment Report of the Inter-governmental Panel on Climate Change, with a view to reducing global greenhouse gas emissions so as to hold the increase in global average temperature below 2 above pre-industrial levels, and that Parties should take urgent action to meet this long-term goal, consistent with science and on the basis of equity; Also recognizes the need to consider, in the context of the first review, as referred to in paragraph 138 below, strengthening the long-term global goal on the basis of the best available scientific knowledge, including in relation to a global average temperature rise of 1.5’ http://unfccc.int/files/meetings/cop_16/application/pdf/cop16_lca.pdf
Interpretation of the UNFCC directive in terms of emissions of greenhouse-gases and aerosols is complicated by scientific uncertainty so that statements about limiting warming to some temperature target must, in practice, be made in probabilistic terms.
With regard to strengthening the long-term goal to limiting warming to 1.5°C, on-going work indicates that by the time of the review, not scheduled to be completed until 2015, it is likely to be too late to achieve the necessary emission cuts (future link to paper). This work is part of the AVOID project to which Manchester Metropolitan University is contributing.
AVOID is a DECC/Defra funded research programme led by the Met Office – providing key advice to the UK Government on avoiding dangerous climate change (http://ensembles-eu.metoffice.com/avoid/ ). We have contributed to AVOID reports on questions such as ‘Will the Copenhagen Accord avoid more than 2°C of global warming?’http://ensembles-eu.metoffice.com/avoid/meetings/AVOID_Copenhagen_Accord_260310.pdf
We are currently leading an AVOID project ‘Reducing uncertainty in simple model projections’ that is putting the AVOID projections in the context of more information of the effect of climate sensitivity uncertainty, and the uncertainty arising from aerosol forcing (future link to report).
In the forthcoming 5th assessment report of the IPCC due to be published in 2014, new mitigation pathways will be assessed by WG1. The pathways are defined in terms of atmospheric concentrations of carbon dioxide. The global temperature changes and implied emissions have been preliminarily assessed with the MAGICC6 model (Meinshausen et al 2011b http://www.atmos-chem-phys.net/11/1457/2011/acp-11-1457-2011.html). Probabilistic results are generated spanning the parameter uncertainties rather quickly with a highly parameterized model such as MAGICC6. But for the important regional details months of time on super computers and subsequent analysis is required and such results will be published by the IPCC in 2014. A preliminary look at the results in figures 2 and 3 shows that the IPCC is considering scenarios that imply emissions ranging from a very high one that would result in catastrophic climate change to a strong mitigation scenario with implied negative emissions (ie CO2 must be removed from the atmosphere) later in the 21st century and a 90% of the results have a peak warming of between 1.3 and 2.0 °C. It is noteworthy that all the scenarios imply that green-house gas emissions will eventually fall to very low levels, a necessary condition for temperatures to stabilize at whatever level. So the all important question is the timing of the emissions reductions. For the strongest mitigation scenario, peak emissions will have to be achieved already by 2016 so reductions in emissions need to kick in now.