Stabilising carbon dioxide levels gets tougher
29 October 2008
Long-term stabilisation of carbon dioxide in the atmosphere at 550 parts per million - it is now at 386ppm and climbing - will require cuts in emissions of 81-90 per cent by 2300.
Emissions have far-reaching effects on the global carbon cycle.
Beyond this date more cuts will still be necessary because up to one third of CO2 emitted by human activity remains in the atmosphere for 'thousands to tens of thousands of years'.
The 550 ppm figure is widely regarded as the upper bound beyond which the risk of facing the worst impacts of climate change, often called 'dangerous climate change', rises markedly.
The findings, published in the journal Environmental Research Letters, combine the latest proposals to cut emissions with the latest research on how the carbon cycle is responding to climate change.
The G8 group of leading industrialised nations have set a non-binding target 'to at least halve global emissions of CO2 by 2050'. But the new analysis says that if no further action is taken this 'leaves CO2 concentrations rising rapidly, approaching 1000 ppm by 2300'.
The Minister for Climate Change and Energy, Ed Miliband, recently announced large emissions cuts based on recommendations from the Committee on Climate Change. The new QUEST analysis was submitted to this committee.
The worst case scenario shows that carbon dioxide levels could be 980ppm with an accompanying rise in temperature of 5.7ºC.
But cutting emissions by 25 per cent by 2050, proposed in the UK's Stern Review on the economics of climate change in 2006, and, crucially, eventually dropping them to 80 per cent below current levels, will 'approach stabilisation', according to the paper. Under this scenario, CO2 begins to level out between 500 and 600 ppm and so global temperatures keep between 1.4 and 3.4ºC above pre-industrial levels.
Lead author, Jo House from the Natural Environment Research Council's QUEST programme (Quantifying and Understanding the Earth System) at the University of Bristol says, 'Our calculations demonstrate the level of emissions reductions needed to limit climate change below what is considered dangerous.'
The work examines recent research on the global carbon cycle - how carbon moves between the atmosphere, the oceans and land.
Vicious circle
Climate change can influence the carbon cycle in a number of ways.
As ocean waters become warmer, the solubility of carbon dioxide in water decreases, meaning that oceans draw down less CO2 from the atmosphere. What is more, when the water at the top of the ocean warms it makes it harder to mix with the layers below. This lack of mixing will make the upper layer more carbon rich, reducing further its ability to draw down carbon dioxide from the atmosphere.
On land warmer temperatures speed up the rate of decay of plants and soils, releasing CO2 back into the atmosphere at a faster rate than during cooler times. In addition changes in rainfall and temperature will affect plant growth, with more areas potentially stricken by drought. Some models predict large areas of vegetation die-back in, for example, the Amazon, towards the end of the century. This would reduce draw down by plants and increase carbon release from dead vegetation and exposed soils.
G8 emissions scenario. Top: Carbon emissions from human activity, measured in petagrams of carbon per year. 1 PgC = 1 Gigatonne of carbon. Middle: Various climate model predictions of atmospheric concentrations of carbon dioxide (parts per million). Bottom: Subsequent temperature predictions.
The paper's authors, which also include scientists from the Centre for Ecology & Hydrology, the Met Office Hadley Centre and the University of Exeter, are concerned that while scientists now have a much better understanding of the relationship between the changing climate and the carbon cycle, the policy implications of this understanding have yet to be assessed.
The paper states, 'The modelled effect of climate-carbon cycle feedbacks imply substantially greater impacts for a given emissions trajectory - or lower allowable emissions to meet a given concentration or temperature target.'
The paper points out that reducing other greenhouse gases, such as methane or nitrous oxides, cannot compensate for the long term effects of emitting CO2 because these gases remain in the atmosphere for just 12 years and 114 years respectively.
The findings make it plain that governments need to plan hundreds of years into the future to have any chance of keeping temperature rises to within 2ºC of pre-industrial levels.
Stern Review emissions scenario. Top: Carbon emissions from human activity, measured in petagrams of carbon per year. 1 PgC = 1 Gigatonne of carbon. Middle: Various climate model predictions of atmospheric concentrations of carbon dioxide (parts per million). Bottom: Subsequent temperature predictions.
House says, 'We applaud the government's new plans to cut emissions by 80 per cent by 2050. This is a realistic assessment of the scale of the problem.
'Our research confirms that bringing other countries on board to meet a global target of 80 per cent reductions towards the end of the century will virtually stabilise CO2 levels, but a much longer-term strategy is still needed to reduce future emissions even further.'
'What do recent advances in quantifying climate and carbon cycle uncertainties mean for climate policy?' published in the Institute of Physics journal Environmental Research Letters.
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