Carbon budgets for limiting global temperature to 1.5 °C

2017

Detlef Van Vurren and colleagues at PBL Netherlands have written this Briefing Note to summarise the knowledge of emissions pathways and energy transitions in the IMAGE (Integrated Model to Assess the Global Environment) scenarios that assess the 1.5°C goal of the Paris Agreement. The Briefing Note is part of our work for the UK Government Department of Business, Energy and Industrial Strategy (BEIS). The complete note is here to download below are the Summary Findings.

Under the Paris Climate Agreement (December 2015), nearly all countries in the world, including the Netherlands, agreed to limit global temperature increase to well below 2 °C above pre-industrial levels, and to pursue efforts to limit this increase even further to 1.5 °C (UNFCCC, 2015). Scenario literature shows that achieving these objectives requires deep reductions in greenhouse gas emissions. However, the exact ambition level depends on several factors not specified in the agreement itself, notably the role of timing, probability to achieve the climate goals, risk thresholds, temporary overshoot, negative emissions, and the ability to make decisions in the context of these uncertainties and policy decisions. At the moment, the Paris Climate Agreement does not specify these dimensions. While leaving concepts somewhat ambiguous is often intentional in climate negotiations, it will be necessary to translate the overall objective of the Paris Climate Agreement into very concrete mitigation targets at all relevant scales to support effective negotiations. The scientific community can add value by providing the information necessary to identify, understand, interpret and, eventually, resolve these ambiguities. In this publication we briefly explore the implications of the 1.5 °C target according to different assumptions regarding the above uncertainties and decisions on carbon budgets, and on emission pathways and energy system implications.

Emission reduction targets and carbon budgets for meeting the 1.5 and 2 °C climate targets are still uncertain, influenced by both scientific uncertainty and policy choices. There are several important factors that influence the size of the carbon budgets or medium-term emission reduction targets that are consistent with the ambition ‘to limit global temperature increase to well below 2 °C above pre-industrial levels, and to pursue efforts to limit this increase even further to 1.5 °C’. Some of them are scientific uncertainties (e.g. limitations in the understanding of the climate system), but others are policy choices (the level of overshoot allowed in reaching the targets; the probability with which the target should be achieved).

Under all assumptions and policy choices, the Paris Climate Agreement requires very stringent emission reductions. Emission scenarios using global models show that very stringent emission reductions are needed if cumulative CO2 emissions need to be restricted to 1000 GtCO2, or significantly less, in the remainder of the century.

Scenarios show that, technology-wise, pathways that can reach the climate goals still exist. There are different pathways towards achieving the Paris Climate targets. These scenarios assume that it is possible to implement climate policies in most regions, leading to a peak in global emissions within the next decade, followed by rapid reductions.

Most 1.5 and 2 °C scenarios show the use of negative CO2 emission technologies. At the same time, in reality only relatively small investments are made in these technologies, and people have raised concerns regarding large-scale use. In order to compensate emission sources that are very difficult to mitigate, and to allow limited overshoot of the carbon budgets in the short term, model-based scenarios show extensive use of negative emission technologies. As, currently, the support for these technologies is low and experience in large-scale application is lacking, it is important to discuss the feasibility of these pathways, more explicitly.

The reliance of negative emissions from bio-energy can be reduced. However, broadening the portfolio of options that are considered and/or deeper reductions in other options are required. Lifestyle change, including changes in diet patterns and using less energy-intensive transport modes, can reduce emissions but are not often included in mitigation studies. Moreover, it is possible to consider more intensive use of other options such as deeper reduction of non-CO2 emissions, or promoting more reforestation.

For more information about how IMAGE works.

For more information about the IPCC Special Report Global Warming of 1.5C to which this short programme of research is contributing.