Summary
The main human influence on the climate comes from the emission of greenhouse gases (GHG) into the atmosphere, such as carbon dioxide, methane and nitrous oxide. The global debate on mitigation of climate change focuses on the level of future climate change that might be considered tolerable, the emission reductions from current business-as-usual emission trends that could achieve this, and the feasibility of these emissions reductions. This area of the AVOID 2 research programme focuses on the quantitative link between emissions and future global warming and enables estimates to be made of the probability of various amounts of future warming for given hypothetical emissions pathways.
What do we already know?
The relationship between emissions of greenhouse gases and future global average warming is uncertain and depends on key parameters such as a measure of climate sensitivity and the strength of climate-carbon cycle feedbacks. Working Group I of the most recent IPCC assessment established that the sensitivity of the climate to changes in radiative forcing is still a major uncertainty but reported several different ways to estimate the uncertainty, all giving somewhat different answers. For a given emissions pathway, these will lead to different estimates of the risk of warming exceeding levels such as 2°C, however the IPCC Working Group III assessment only focused on a single uncertainty estimate for climate sensitivity. Another development of IPCC Working Group I was to catalogue the current estimates of the strength of several additional earth system feedbacks, such as the effect of carbon released from thawing permafrost, but the impact of these on the emissions pathways compatible with a given temperature target was not considered. Therefore, although the IPCC fifth assessment provided an improvement in understanding of feedbacks it left open many key questions around the implications for mitigation.
What will this research achieve?
Firstly this work will reprocess the IPCC’s working group III database of emissions pathways to take account of key climate science uncertainties that were not adequately included. Secondly, the work will be extended to include new emissions pathways developed in AVOID work package C. Additional work will focus on giving regional rather than global projections using more sophisticated climate models, and providing a better understanding of the physical constraints on, so called, overshoot scenarios. These overshoot pathways seek to lower mitigation costs by allowing concentrations of greenhouse gases, and sometimes temperature, to temporarily exceed the eventual target level. A new novel approach will also look at early detection of the benefits of mitigation.
What is the policy relevance?
This work will contribute to the evidence around the choice of long-term climate goals, which is under discussion through the UNFCCC Subsidiary Body for Scientific and Technological Advice (SBSTA) as part of a structured expert dialogue. Additionally it will provide new quantification of the range of human driven greenhouse gas emissions compatible with different long-term temperature targets, which remains a central aspect of the UNFCCC climate negotiations.