Only zero emissions can prevent a warmer planet
By Kate Ravilious Greenhouse gas emissions will have to be eliminated completely to stabilise the Earth’s climate and prevent temperatures from rising. That’s the conclusion of climatologists in the US who say that our current efforts to merely stabilise emissions will not be enough. Damon Matthews, from Concordia University in Canada, and Ken Caldeira, from the Carnegie Institution for Science, Stanford, USA, used a global climate model to study how greenhouse emissions would need to change in order to stabilise global temperatures over the next few hundred years. Previous studies have only looked at what happens when emissions are stabilised. Humans have been releasing carbon dioxide into the atmosphere in increasing quantities since the industrial revolution. But to simplify the simulation, Matthews and Caldeira injected a single pulse of carbon dioxide into a pre-industrial atmosphere. Pulse sizes of 50, 200, 500 and 2000 billion tonnes of carbon were used. The model was set to calculate global temperatures and atmospheric and ocean carbon dioxide levels over a simulated 500 years. At the end of that period, Matthews and Caldeira found that between 20% and 35% of the initial emission pulse remained in the atmosphere – even for the smallest emission pulse – with the remainder having been absorbed by land and ocean carbon sinks. The lingering carbon dioxide means that global warming persisted for the entire simulation. For the four different emission scenarios, global temperatures stabilised at 0.09, 0.34, 0.88 and 3.6 ºC above pre-industrial levels respectively. So far industrial emissions total around 450 billion tonnes. “Even if we eliminated carbon dioxide today we are still committed to a global temperature rise of around 0.8 ºC lasting at least 500 years,” says Caldeira. One of the reasons for the persistence is the slow response of oceans. “It takes a lot of energy to heat them up and then a long time for them to cool back down,” he explains. Roger Pielke, a climate policy expert at the University of Colorado in Boulder, agrees with the findings. “This research makes the case that simply stabilising concentrations is insufficient to stabilise temperatures. Their argument, if widely accepted, raises the bar on what it means to mitigate climate change,” he says. Matthews and Caldeira warn that current emissions targets for 2050 are insufficient to avoid substantial future warming. Instead they believe that we need to eliminate emissions, or find a way of actively removing carbon dioxide from the atmosphere. “It is technologically challenging, but not impossible. The biggest challenge will be to get political consensus,” says Caldeira. Potential tools to achieve zero emissions include renewable energy, electric cars and carbon capture and some countries such as Costa Rica are already aiming for zero emissions. Dave Reay, a climate scientist at the University of Edinburgh, thinks that it is a feasible long-term aim. “If used on a large enough scale then new technologies like carbon capture could get us to zero emissions.” Journal reference: Geophysical Research Letters (DOI: 10.1029/2007.GL032388) Climate Change – Want to know more about global warming: the science, impacts and political debate?