um of all CO2 emissions that matters. In practice, substantial reductions in global emissions have to begin soon, before 2020. If we wait any longer, the required phase-out of carbon emissions will involve tremendous economic costs and technological challenges - miles beyond what can be considered politically feasible today. The longer we wait, the more likely our path will lead us into dangerous territory,” concludes Malte Meinshausen.
The authors of both papers have collaborated on a Commentary article (7) focussing on their long-term policy implications, published today in Nature Reports Climate Change. See as well the News & Views piece in Nature today (8).
Two possible futures: One in which no climate policies are implemented (red), and one with strong action to mitigate emissions (blue). Shown are fossil CO2 emissions (top panel) and corresponding global warming (bottom panel). The shown mitigation pathway limits fossil and land-use related CO2 emissions to 1000 billion tonnes CO2 over the first half of the 21st century with near-zero net emissions thereafter. Greenhouse gas emissions of this pathway in year 2050 are ~70% below 1990 levels. Without climate policies, global warming will cross 2°C by the middle of the century. Strong mitigation actions according to the blue route would limit the risk of exceeding 2°C to 25%. For more details, see Figure 2 in Meinshausen et al. (2009). Credit: M. Meinshausen et al. (2009)
(1) Meinshausen, M., Meinshausen, N., Hare, W., Raper, S. C. B., Frieler, K., Knutti, R., Frame, D. J. & Allen, M. Greenhouse gas emission targets for limiting global warming to 2°C. Nature, doi: 10.1038/nature08017 (2009).
(2) Note to Editors: For dangerous climate change to be avoided, 109 of the 192 signing countries of the United Nations Framework Convention on Climate Change are calling for warming to be limited to two degrees Celsius or lower relative to pre-industrial levels. Some of the most vulnerable countries as Small Island States and the least developed countries even consider 1.5 degrees Celsius as the maximally acceptable warming level.
(3) The study was led by the two brothers Malte and Nicolai Meinshausen from the Potsdam Institute for Climate Impact Research and Oxford University’s Statistics Department, respectively.
(4) More than 300 billion tonnes of carbon dioxide have been emitted since year 2000 due to burning of fossil fuels and deforestation. Thus, the emission budget of 1,000 billion tonnes (GtCO2) for the years 2000 up to 2050 is reduced to less than 700 billion tonnes for the remaining period since 2009. This remaining budget is hence less than a quarter of our estimate for known economically recoverable fossil fuel reserves (2800 GtCO2). See Meinshausen et al. (2009) for further details.
(5) IPCC Fourth Assessment Report, Intergovernmental Panel on Climate Change, http://www.ipcc.ch/ipccreports/assessments-reports.htm
(6) Companion study: Allen, M. R., Frame, D. J., Huntingford, C., Jones, C. D., Lowe, J. A., Meinshausen, M. & Meinshausen, N. Warming caused by cumulative carbon emissions towards the trillionth tonne. Nature, doi:10.1038/nature08019 (2009).
(7) Associated Commentary: Allen, M. R., Frame, D. J., Frieler, K., Hare, W., Huntingford, C., Jones, C., Knutti, R., Lowe, J., Meinshausen, M., Meinshausen, N. & Raper, S. The exit strategy: Emission targets must be placed in the context of a cumulative carbon budget if we are to avoid dangerous climate change. Nature Reports Climate Change, doi:10.1038/climate.2009.38 (2009).
(8) News & Views piece in Nature by Schmidt, G. & Archer, D. Too much of a bad thing. Nature (2009).