Olive Heffernan
17 November 2009
www.nature.com/news/2009/091117/full/news.2009.1092.html
Carbon dioxide emissions will have to be all but eliminated by the end of this century if the world is to avoid a temperature rise of more than 2 ºC, scientists warned yesterday. And it might even be necessary to start sucking greenhouse gases out of the atmosphere.
The findings are the culmination of five years work by Ensembles, a major European research consortium led by the UK Met Office Hadley Centre and involving 65 other research institutes worldwide. In the first study of its kind, scientists in the project used a variety of the latest global climate models to determine the reductions needed to stabilize levels of greenhouse gases, termed CO2 equivalents, at 450 parts per million. That level, which offers a reasonable chance of keeping the temperature rise under 2ºC, is the goal of European climate policy.
The results suggest that to achieve that target, emissions would have to drop to near zero by 2100. One of Ensemble's models predicted that by 2050, it might also be necessary to introduce new techniques that can actually pull CO2 out of the atmosphere.
Ensemble is a radically different approach from the gold-standard climate projections, which are run by Intergovernmental Panel on Climate Change (IPCC), the United Nation's climate body. The IPCC runs models for a range of 'what if' scenarios that make various assumptions about the future, such as the level of emissions, technological and economic development. None of these scenarios account for the impact of policy on climate change.Latest model
The Ensembles project works the other way around. Its scenario, named E1, assumes that atmospheric levels of CO2 equivalents cannot rise above 450 parts per million, then looks at the mitigation that policy-makers would need to pursue to achieve that. Currently, the level of atmospheric CO2 sits at 390 parts per million, about a third higher than that of pre-industrial times.
The results suggest that simply switching to renewable sources of energy may not be enough to stabilize emissions. "It's clear that if we continue our current emissions trajectory and we want to stay at 450 parts per million, we'll need to pull CO2 out of the atmosphere," says atmospheric scientist Ken Caldeira, who works at the Carnegie Institution for Science's Department of Global Ecology in Stanford, California. That could mean deploying new techniques for capturing carbon, such as biochar, reforestation or air filtering, on a massive scale.
Caldeira adds that action now could be a better option. If people stop building new CO2-emitting devices within the next decade, they could achieve the same result at a lower cost.
The results should send a strong message to climate negotiators, who are expected to delay signing a global climate treaty beyond the December deadline set by the United Nations. "Policy-makers need to think very seriously about what kind of world they can point us towards," says Paul van der Linden, director of the Ensembles project.
"We need to reverse the trend of increasing emissions and for that we will need strong policy," adds Jean-François Royer of the National Meteorological Research Centre in Toulouse, France, who headed up the work on the new scenario. "If there is no international agreement to reduce emissions, no one will do it by themselves".
E1 is a taste of things to come. The IPCC will employ the same approach in its next report, due out in 2013.
