Copenhagen: 100 nations unite over climate cuts (Image: Anders Sune Berg)

There is no alternative to ending human emissions of carbon dioxide, if we want to prevent dangerous climate change. Suggestions that cutting a range of non-CO 2 gases might do at least part of the job are based on faulty accounting, according to a new study.

This finding reinforces the hard-headed conclusions of the latest report from the Intergovernmental Panel on Climate Change (IPCC), which on Sunday called for an end to all CO 2 emissions by 2100 to prevent what it called “severe, widespread, and irreversible” impacts on people and nature.

The latest report is a synthesis of three assessments on the science, impact and mitigation of climate change, completed over the past 14 months. It contains no new science. But it toughens up recommendations to governments that are working towards a new deal to halt climate change, due to be signed in Paris late next year.


The conclusions are the result of a week-long meeting in Copenhagen, Denmark, attended by more than a hundred governments, all of whom signed off on the final text. “The intergovernmental nature of IPCC is essential to its success,” says Jean-Pascal van Ypersele, vice-chair of the IPCC. “All governments have contributed.”

Avoiding 2 degrees

The new 100-page synthesis underlines that CO 2 emissions must start falling soon and will have to be “phased out almost entirely by 2100”. The science assessment, published in 2013, calculated that CO 2 emissions must be capped at 2900 billion tonnes to give a 66 per cent chance of avoiding warming above 2 degrees Cs – widely regarded as a threshold for dangerous climate change.

But some 2000 billion tonnes had already been emitted by the end of 2011. The only alternative to cutting emissions fast and soon is developing technology to capture billions of tonnes of CO 2 emissions and bury them out of harm’s way – known as carbon capture and storage (CCS).

“We are currently not on track to meet the agreed 2 degrees goal,” said Ottmar Edenhofer, chief economist of the Potsdam Institute for Climate Impact Research, Germany, in a statement on Sunday. “And the more we delay the respective mitigation efforts, the greater the risks facing us will be.”

No third way

Some scientists and governments have in the past argued that there is a third way to curb warming. Since CO 2 is not the only pollutant warming the atmosphere, action to curb other types of greenhouse emissions might be cheaper and quicker, buying time for later action on CO 2 . The US administration and the UN Environment Programme in particular have promoted cutting emissions of methane gas and soot as an effective emergency response to global warming.

But a new study published independently of the IPCC report by Joeri Rogelj of the International Institute for Applied Systems Analysis in Laxenberg, Austria, says that the potential for preventing 2 degrees of warming by controlling these pollutants has been “strongly and consistently overestimated”.

The problem is twofold. Because these gases have much shorter atmospheric lifetimes – days to decades – they do not accumulate in the atmosphere the way that CO 2 does, lessening their contribution to warming.

Secondly, in many cases these other gases have the same sources as CO 2 emissions, such as burning fossil fuels. This means that some of the impact cutting other greenhouse gases would have on curbing climate change has already been included in CO 2 emissions’ calculations, so those gases have in effect been “double-counted”.

Many studies have not considered this, says Rogelj.

“Science has spoken,” UN Secretary-General Ban Ki-moon said at the launch of the report in Copenhagen. “There is no ambiguity in their message. Leaders must act. Time is not on our side.”

Journal reference: PNAS, DOI: 10.1073/pnas.1415631111