ng of known economically recoverable fossil fuel reserves—oil, gas and coal—and found that these reserves are four times larger than the emission budget between now and 2050 (4). “To keep warming below 2°C, we cannot burn and emit the CO2 from more than a quarter of the economically recoverable fossil fuels up to 2050, and in the end only a small fraction of all known fossil fuel reserves,” concludes Bill Hare, co-author of the study.
The study used a single, efficient computer model which incorporated the effects of all greenhouse gases, aerosols and air pollutants, and the range of possible responses of the carbon cycle and earth’s climate system. This was combined with about a thousand emission pathways.
The study explicitly takes into account the uncertainties related to modelling climate change. Throughout the study, probability statements were used to summarize the current level of knowledge based on observational data. It also used a huge number of different simulation results from the latest assessment report of the Intergovernmental Panel on Climate Change (5). In taking this comprehensive approach the researchers went a step further than previous work.
The new results have direct relevance to the international negotiations now underway.
“Our study draws on a huge body of research reported in the numerous assessments of the Intergovernmental Panel on Climate Change. It clearly shows that the 2°C target which many countries have adopted will require quick action in order to follow the blue route rather than the red one in the figure,” says Sarah Raper, co-author from Manchester Metropolitan University, UK.
“With every year of delay, we consume a larger part of our emissions budget, losing room to manoeuvre and increasing the probabilities of dangerous consequences, “ adds Reto Knutti, co-author from the ETH Zurich.
A companion study (6), also published in Nature today by Myles Allen and colleagues, show the necessity to limit the total amount of carbon that humankind ever emits.
“In principle, it is the sum 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.