Committee on Climate Change

No purely scientific analysis provides a definitive and obvious threshold for setting a climate target. But the CCC has drawn some general conclusions about the risks as temperatures rise, based on the IPCC assessments and other recent studies:

• Less than 2°C above pre-industrial levels would exacerbate impacts already being observed. These include increases in human mortality, loss of glaciers and increases in extreme events. Other impacts would be triggered such as reduced food security in many poorer regions. Some systems might benefit, such as global agricultural productivity
• 2-4°C would lead to worsening impacts at all scales, such as decreased global agricultural productivity and widespread biodiversity loss. If sustained, the ice sheets of Greenland and the West Antarctic could melt, leading to several metres of sea level rise over the course of the coming centuries.
• Beyond 4°C would see major increases in vulnerability across the world, with the adaptive capacity of many systems being exceeded.

The CCC’s climate target

Given these conclusions about climate risks, it is clear that global temperatures should ideally be kept to 2°C or even lower. But two other factors need to be considered: first, global GHG emissions continue to grow, even though current concentrations may already be enough to create an eventual 2°C warming. Second, there are uncertainties in our understanding of the climate system which make it difficult to aim precisely for a specific temperature outcome.

Weighing up observed emissions trends and conclusions from the science, the Committee took a risk-based approach to setting a climate target. Based on current levels of understanding, the world should aim to keep central (i.e. 50% probability) estimates of temperature increase by 2100 close to 2°C, and should limit the probability of a 4°C increase to very low levels (e.g. 1%).

The CCC’s global emissions target

To find the level of global emissions cuts required to meet the CCC’s climate target, the Met Office carried out research identifying probabilistic ranges of warming for different future emissions pathways. These ranges reflect the spread of outputs that would be produced by different world-leading climate models, because of the alternative ways by which they represent some climate processes (see technical annex from the Committee's 2008 report). 

Results show that to meet the CCC’s climate target, global emissions of Kyoto GHGs must peak before 2020, and then reduce at a rate of 3-4% per year throughout the century. On this pathway global emissions would be halved by 2050.

Stronger emissions reductions will result in a greater chance of staying within temperature limits. The Committee therefore recommended that the world should cut emissions in 2050 by at least 50% in order to stabilise global temperature rises at around 20C. The CCC also advised that the UK should aim to reduce emissions by at least 80% by 2050 to contribute to an overall global reduction in emissions of 50%.

Climate change is not just driven by the level of emissions in a given target year (e.g. 2020 or 2050) but by the accumulated total of long-lived GHGs over time. As a result, if global emissions peak in 2020 or later, or if they grow faster before peaking, further cuts will be required by 2050 in order to meet the climate target.

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