This report provides a review of the most relevant recent literature in the areas of climate science, impacts of climate change and climate change economics, with a focus on post IPCC Fourth Assessment report work. It also provides information on the key research gaps which might have a major impact on mitigation decisions.
• Climate science
The AR4 provided more definite statements of human influence on the climate system, and subsequent work has strengthened that further. The AR4 also highlighted greater complexity in many of the physical systems that climate models attempt to simulate. A large amount of post-AR4 research has focussed on better understanding of physical processes, including highlighting where model improvements are needed and where they are most robust. Risk assessments can be made using the information presented to-date but many of these are likely to be refined with improved models during AR5. This may impact on mitigation targets and impact assessments.
There have been many papers published since the AR4 on water, agriculture and health impacts. Many of these have added case studies from areas previously poorly-represented – specifically Africa and Central and South America – but few offer new insights into the effects of climate policy on these impacts. For agriculture, the tone of the post-AR4 literature on agricultural impacts is more pessimistic than that of the AR4, largely due to an increased understanding of the role of pests, extreme events, and changes in ozone concentrations, on either increasing the adverse effects of climate change or offsetting the positive effects. In the field of health impacts there has not been a great deal of progress in looking at the combined effects of climate change and local air quality issues. A number of local assessments of future sea level, including changes in extremes, have been published since the AR4 and these have tried to deal with the current uncertainty in the climate projections of sea level rise.
Since AR4 the number of projections of extinction risks which would result from future climate change has significantly increased. However, climate change induced extinctions would be significantly reduced by mitigation, and the first quantifications of these reductions are being made. Coral reefs, mountain and polar ecosystems and Mediterranean climate systems are the most vulnerable to climate change.
The SRES scenarios are still broadly representative of plausible future ranges of future emissions without specific mitigation policy, but new scenarios are being developed. Assessments of the economic potential for and macroeconomic costs of mitigation of global greenhouse gas emissions have changed little since AR4.
New studies cite costs of around 1% GDP, or even macroeconomic benefits, for stabilization at 445-710 ppm CO2eq, or -2.3 to +2.5% GDP change for stabilization at 450-550 ppm. The carbon price necessary to achieve a particular stabilisation target remains very uncertain. There are large cost savings from including reductions in non-CO2 gases in mitigation strategies, and these may be essential to reach the lower targets. Energy efficiency has a key role in mitigation though its efficacy is substantially offset by the rebound effect. The costs of inaction continue to be highly uncertain but literature now more strongly emphasizes potentially high costs including that due to extreme weather.
Early assessments show that the EU ETS is working to reduce European carbon emissions, but its effectiveness would be improved by auctioning the emissions allowances.