Mitigating climate change through reductions in greenhouse gas emissions: the science and economics of future paths for global annual emissions

Headline issue

The 15th Conference of the Parties (COP15) to the United Nations Framework Convention on Climate Change (UNFCCC) in Copenhagen in December 2009 seeks to agree to an international framework on climate change policy to take effect after the expiry of the Kyoto Protocol in 2012. It is predicted to bring the UNFCCC process into mainstream discussion.

In the run up to COP15 and in order to inform the negotiations, this policy brief examines the extent to which global greenhouse gas emissions will have to fall from present levels to create a reasonable chance (i.e. a 50 per cent probability) of avoiding a rise in global average temperature of more than 2°C above its pre-industrial levels. It also explores the economics of achieving this target within the context of an international agreement on climate change policy.

Key findings

• Developments in the base of scientific evidence since the publication of the 2006 Stern Review lead us to conclude that its proposal to set an upper limit of 550 parts per million (ppm) of carbon-dioxide-equivalent (CO2e) for long-term stabilisation is too high.
• There is no agreed overall target for stabilising greenhouse gas concentrations in the atmosphere, and as a result, several different targets have been proposed. No emissions path that is currently regarded as feasible offers a 100 per cent probability of avoiding a temperature rise of more than 2°C. Our analysis suggests that we may not be able to ensure more than a 50-50 chance of limiting warming to 2°C or less.
• We conclude that the goal of limiting to 50-50 the chance of global average temperature rising by more than 2°C above pre-industrial levels is demanding. But with well-designed policies applied consistently across countries, industries and greenhouse gases, the available evidence suggests that it is feasible.
• We conclude that, to limit the probability of exceeding warming of 2°C to 50 per cent, policy-makers should aim for annual global emissions to be between 40 and 48 Gt CO2e by 2020, and peaking no later than 2020 to provide a reasonable chance of limiting warming to no more than 2°C above pre-industrial levels. The later the peak, the more rapidly emissions must be reduced subsequently. By 2050, global annual emissions would need to fall to no more than about 14-17 Gt CO2e.
• If action is delayed, with a less ambitious emissions target in 2020, reductions will have to be much more rapid up to 2050 and beyond to have a 50 per cent chance of avoiding warming of more than 2°C. A more ambitious path in the short term would reduce the economic costs of action.
• An ambitious long-term goal for climate change policies makes it imperative that a range of well-known market failures, particularly in relation to research, development and early deployment, are overcome. Targets for annual emissions set at regular intervals and observations of carbon prices can be used to monitor and provide incentives to policy-makers to achieve the long-term climate goal that they adopt.
• Defining an envelope of paths for annual global emissions that all lead to a reasonable, 50 per cent, chance of avoiding a rise in global average temperature of more than 2°C above pre-industrial levels, and to a chance of less than 5 per cent of exceeding 4°C, is not a problem that can be solved exactly; the uncertainties involved are too great.
• Given the uncertainties in both the science and the economics, it is essential that any policy framework for climate change mitigation incorporates, from the outset, mechanisms to update the long-term goal, in a transparent fashion, in response to new developments in the science or economics, while holding policy-makers accountable for their actions.

Alex Bowen and Nicola Ranger