Emission scenarios and climate change

cracked earth

Work Package A: Emission scenarios and climate change

Understanding the link between emissions pathway and climate response for a range of temperature limits.

Lead researchers:
Jason Lowe
Met Office
Dan Bernie
Met Office
Rachel Warren
Tyndall Centre

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.


Research area description and outputs

A1. Future temperature responses based on IPCC and other existing emissions scenarios
Projections of the future temperature response will be made for a range of existing emissions pathways, with emphasis on re-evaluating the emissions from IPCC WG3. It will take account of the latest understanding on the link between emissions, concentrations and climate response.
View report 1a Report 1b due Jun 2016
A2. Future temperature responses based on new AVOID 2 emissions scenarios
The AVOID programme will use a number of integrated assessment models in Work Package C to produce new emissions pathways, with varying assumptions about technical feasibility. This project will help to specify the scenarios and then later calculate the temperature response and the probability of crossing various temperature thresholds for these scenarios.
Jun 2014 – Mar 2016
A3. Pathways that temporarily overshoot concentration, forcing or temperature goals
Many emissions pathways that limit warming to below 2°C at the end of the 21st century involve the atmospheric concentrations of greenhouse gases temporarily overshooting their long-term levels. There is evidence this may lead to lower costs, some trajectories even involve the temperature temporarily overshooting the target level. The first part of this project will review the current literature on overshooting. The second part will carry out additional climate modelling to better understand the potential for different amounts and time periods of overshoot.
View report
A4. Regional responses to climate change
While the focus in IPCC WG3 and in parts of AVOID is on estimates of the global average climate response, this project will use more complex models to examine regional climate change information. This will be a useful check of the global average calculations. It will also provide a richer picture of regional changes and changes in extremes that will be useful for communications. The output will be a report and spatial information.
Jul 2015
A5. An updated view of tipping points and the relevance for long-term climate goals
A literature review exploring the importance of new information on tipping points to future climate pathways and the setting of the long-term climate goal.
View report

Policy card: What are climate ‘tipping points’ and how likely are they?

A6. The prospect of earlier detection of the benefits to climate of mitigation
It is a generally accepted view that the benefits of mitigation on climate will not really be felt until the second half of the 21st century. This project will challenge this view by using a new technique to identify the impact of mitigation on climate extremes earlier in the century.
Nov 2015
A7. IPCC WG3 emissions and temperature response tool
A web tool using the IPCC WG3 IAM data to determine expected temperature responses based on user defined emissions constraints over the 2020-2030 period, and a cumulative emissions constraint. It will also allow users to determine which emissions pathways are compatible with a given user defined temperature limit.
Feb 2016

Peer reviewed publications

Any peer reviewed publications associated with this research area will appear here.