Study: Very Long Term Strategy Needed for Reducing Greenhouse Gas Emissions

Jose Michael

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Implications of the G8 50% policy. Top panel shows historical and future emissions. The middle and bottom panels show the resulting changes in concentrations and temperature respectively. Click to enlarge. Credit: IOP

A team of scientists in the UK have combined the outcomes of proposed policies by the G8 countries and the UK Government’s Stern Review for greenhouse gas emissions reductions with the latest knowledge of climate change feedbacks relating to the carbon cycle (the way carbon moves between the oceans, atmosphere and land).

Their findings, published in the journal Environmental Research Letters, show that short-term cuts alone will not solve the problem and that policy makers need to plan for hundreds of years into the future.

To be able to predict the climatic impact of various levels of emissions we need to know, and account for, what happens to the greenhouse gases once they enter the atmosphere. Gases such as methane or nitrous oxide only remain in the atmosphere for a few years or decades. Carbon dioxide is a different matter as a portion of emitted gas stays in the atmosphere for thousands of years.

Furthermore, as the climate changes, a larger proportion of the carbon dioxide will remain in the atmosphere. Carbon dioxide is taken up by land and ocean sinks, which become less effective as the climate warms, leading to even greater warming for a given level of emissions—this is known as climate feedback. Our calculations demonstrate the level of emissions reduction we need to achieve to limit climate change to below what is considered ‘dangerous’.

—Jo House, University of Bristol (UK)

Working alongside colleagues from the UK’s Natural Environment Research Council (NERC) Centre for Ecology and Hydrology, the Met Office Hadley Centre and the University of Exeter, House ran computer models to see what would happen under the G8 plans to cut global emissions by 50% by 2050. The models show that under this scenario, unless emissions cuts continue beyond 2050, atmospheric carbon dioxide concentrations will continue to rise rapidly.

The models suggest that by 2100, carbon dioxide concentrations could be as high as 590 parts per million (ppm)—more than double the level of 280 ppm that persisted for thousands of years before the industrial revolution, and significantly higher than today’s level, caused by the burning of fossil fuels and deforestation, of 386 ppm. By 2300, the worst-case scenario shows that carbon dioxide levels could be 980 ppm with an accompanying rise in global temperature of 5.7° C. (The European Union has taken the stance in international climate negotiations that climate change should be limited to 2°C to avoid “dangerous impacts”.)

Using the Stern Review proposal of cutting emissions by 25% by 2050 and continuing to make cuts down to 80% towards the end of the century, the models show a future with lower concentrations. In this case, the carbon dioxide levels would become almost stable, at levels of between 500 and 600 ppm by 2100, although they would creep up further into the future if greater cuts were not made. In this case the temperature by 2100 ranges between 1.4 and 3.4 ° C depending on the model used, and by 2300 it is also almost stable with a maximum of 4.2 °C.

The Stern Review concluded that, to avoid the worst impacts of climate change, the concentrations of all greenhouse gases should be limited to what is equivalent to between 450 and 550 ppm of carbon dioxide concentration.

House and her colleagues say that making cuts in other greenhouse gases is no good if the longer term problem of atmospheric carbon dioxide is ignored.

On the timescale of decades to a century (the more common domain of climate-based policy making), Stern’s proposal of 80% emissions cuts remains an effective near-term target on the pathway to achieve stabilization of CO2 concentrations or climate. This conclusion is robust despite the large uncertainty in the climate–carbon cycle feedback. Ultimately, however, climate stabilization at any level can only be achieved if net global CO2 emissions decline over centuries to the level of persistent natural sinks («1 Pg C yr-1, or just a few % of today’s emissions). On the timescale of centuries to millennia, over which the impacts of today’s CO2 emissions are still being felt, stabilization in the presence of a non-trivial anthropogenic source of CO2 is only possible if this source is balanced by an artificial sink. The long-term impact of today’s emissions brings this planetary timescale into contemporary policy relevance.

—House et al. (2008)

The research was supported by the Natural Environment Research Council through the QUEST Programme and the Centre for Ecology & Hydrology, and by the Joint Defra/ MoD Integrated Climate Change Programme.

The Natural Environment Research Council’s QUEST programme (Quantifying and Understanding the Earth System) assimilates scientific knowledge of the Earth as an integrated system. It aims to substantially improve predictions of global environmental change. It intends to accelerate development of the next generation of environmental-change models, and will provide a focal point for UK work, forging collaborations and synergies between worldwide experts in Earth System research and modelling. QUEST is hosted by the University of Bristol.

Separately, an international group of scientists, led by Dr. James Hansen, director of NASA Goddard Institute for Space Studies, The Earth Institute at Columbia University, will publish a paper this week in Open Atmospheric Science Journal suggesting that global policies should have an initial target for atmospheric CO2 of 350 ppm. They note that the optimum CO2 level for maintaining a planet similar to that on which civilization developed is likely to be less than 350 ppm, but a 350 ppm target already reveals that dramatic policy changes are needed urgently. They argue for a prompt moratorium on new coal use that does not capture CO2 and phase-out of existing coal emissions by 2030.

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