On the way to phasing out emissions: More than 50% reductions needed by 2050 to respect 2°C climate target

The study has, for the first time, calculated how much greenhouse gas emissions we can pump into the atmosphere between now and 2050, to have a reasonable chance of keeping warming lower than 2°C (above pre-industrial levels) – a goal supported by more than 100 countries (2). We can only emit 1000 billion tonnes of carbon dioxide between the years 2000 and 2050. The world has already emitted one third of that in just nine years.

“If we continue burning fossil fuels as we do, we will have exhausted the carbon budget in merely 20 years, and global warming will go well beyond two degrees,” says Malte Meinshausen, lead author of the study and climate researcher at the Potsdam Institute for Climate Impact Research. The three-year research project involved scientists from Germany, the United Kingdom and Switzerland (3).

The study concluded that greenhouse gas emissions must be cut by more than 50 percent by 2050 relative to 1990 levels, if the risk of exceeding 2°C is to be limited to 25 percent.

“Only a fast switch away from fossil fuels will give us a reasonable chance to avoid considerable warming. We shouldn't forget that a 2°C global mean warming would take us far beyond the natural temperature variations that life on Earth has experienced since we humans have been around.” says Malte Meinshausen.

The study also compared the volume of CO2 emissions that could result from the burning 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.

Companion study
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.

Policy implications
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).
Notes:
(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). (Contact press@nature.com for an advance copy)
(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). (Contact press@nature.com for an advance copy)
(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). (Contact press@nature.com for an advance copy)

(8) News & Views piece in Nature by Schmidt, G. & Archer, D. Too much of a bad thing. Nature (2009). (Contact press@nature.com for an advance copy)

Contacts for press:
PIK Press office, Phone: +49 331 288 25 07, E-mail: press@pik-potsdam.de
Dr. Malte Meinshausen (Potsdam Institute for Climate Impact Research) +49 163 175 0084, malte.meinshausen@pik-potsdam.de
Dr. (h.c.) Bill Hare, Potsdam Institute for Climate Impact Research,+49 170 90 570 15, bill.hare@pik-potsdam.de
Prof. Reto Knutti, ETH Zurich, +41 44 632 35 40, reto.knutti@env.ethz.ch
Dr. Sarah Raper, Manchester Metropolitan University, +44 161 2471596, S.Raper@mmu.ac.uk

Dr. Myles Allen, Oxford University, +44 7776 306691, myles.allen@physics.ox.ac.uk