The 2°C goal for global warming is inadequate. Risks increase significantly between 1.5 and 2°C, a UN review of climate targets concludes.
In four sessions of the UN Climate Convention, which brought more than 70 experts in a face-to-face dialogue with policy makers, the Structured Expert Dialogue (SED) on the 2013–2015 Review assessed the state of the science relevant for an evaluation of the adequacy of the long-term 2°C global goal and the overall progress made towards it. This comprehensive assessment of different long-term global goals such as 2°C or 1.5°C, drawing upon the IPCC AR5 as well as more recent literature, makes the report an indispensable source of information for any assessment of the adequacy of the long-term global goal.
The report on the SED finds that the ‘guardrail’ concept, in which up to 2°C of warming is considered safe, is inadequate. In fact, the report confirms significant climate impacts are already occurring at the current level of global warming and additional magnitudes of warming will only increase the risk of severe, pervasive and irreversible impacts.
Consequently, the report suggests that a long-term goal of below 2°C is defined as a ‘defence line’, rather than a ‘guardrail’, confirming that warming of less than 2°C would be much more preferable and implying that a 1.5°C target would be more adequate. The report advises the pursuit of emission pathways in the immediate short term that are consistent with limiting warming to below 2°C and keeping the option of limiting warming to 1.5°C open. In addition to that, the defence line concept implies the need for high-probability below 2°C pathways that will be consistent with 1.5°C, indicating that it is only by aiming for 1.5°C that a target of below 2°C could be secured.
The outcome of SED should lead to increased recognition of the legitimacy and significance of the 1.5°C goal by all stakeholders in the climate negotiations.
1. Key Message: Impacts of Climate Change differ substantially between 1.5°C and 2°C
The SED report confirms that risks increase significantly between 1.5°C and 2°C: three out of five “Reasons for Concern” as identified by the IPCC are rated as transitioning from moderate to high risk between these warming levels. The report finds that limiting global warming to below 1.5°C would avoid or substantially reduce risks, including risks to food production or unique and threatened systems such as coral reefs or many parts of the cryosphere (glaciers, ice sheets of Greenland and Antarctica) and the risk of sea level rise.
In relation to the risks for the five “Reasons for Concern” identified by the IPCC, the SED finds for a warming of 2°C that:
Unique and threatened systems would be at high risk, in particular systems with limited or barely any adaptive capacity (e.g. Arctic sea ice and coral reefs).
Extreme weather events would pose a high risk for human health, urban housing and infrastructure in megacities, and in relation to the urban heat island effect, air pollution and differential vulnerabilities; displacement and permanent migration; livelihood struggles and conflict in resource-dependent livelihoods, such as agriculture and pastoralism; and high impacts on livelihood (trapped populations are more vulnerable to environmental change because of their inability to move).
The risks will be increasingly unevenly distributed, and are generally greater for disadvantaged people and communities in countries at all levels of development; populations that experience shifts from transient to chronic poverty and related social marginalization and food insecurity; and the elderly, children, the socially marginalized, and outdoor workers, who are disproportionally at risk from heat stress.
Global aggregated impacts show a moderate economic impact, but these aggregates may mask impacts across sectors and regions (evaluations are incomplete, in part because they do not take into account large-scale singular events affecting several sectors at once or other effects from disrupted interdependencies).
The risk of large-scale singular events, such as the disintegration of ice sheets in Greenland and Antarctica, would be moderate.
2. Key Message: The 2°C limit should be seen as a defence line, while less warming would avoid substantial impacts
The SED finds that the ‘guardrail’ concept, in which up to and including 2°C of warming is considered ‘safe’, is inadequate and would therefore be better seen as a defence line that needs to be stringently defended, while less warming would be preferable. Significant climate impacts are already occurring at the current level of global warming and additional magnitudes of warming will only increase the risk of severe, pervasive and irreversible impacts. Arguably this conclusion was already recognized in Copenhagen with insistence by many parties that the 2°C warming goal be qualified as limiting warming “below 2°C”. The SED findings confirm this policy judgment from 2009 and extend it by referencing the substantially reduced impacts and risks at 1.5°C.
The proposed defence line concept has substantial implications for the assessment of potential emission trajectories. For a defence line that needs to be stringently defended, while “less warming would be preferable”, an emission pathway that only has a likely chance (> 66%) of avoiding a 2°C increase, as specified in IPCC AR5 for example, may not provide the sufficient level of security. Consequently pathways with higher probability (85% or above) would appear far more consistent with the SED’s findings. While the SED does not provide information on the specific characteristics of such high-probability emission pathways, scientific results from the IPCC AR5 and the 2014 UNEP Emissions Gap report, and other recent scientific literature, provide guidance on this: Emission pathways that hold warming below 2°C throughout the 21st century with a high probability (above 85%) also limit warming below 1.5°C by 2100 with a 50% or greater probability.
3. Key Message: Limiting global warming below 2°C is still feasible and will bring about many co-benefits, but poses substantial technological, economic and institutional challenges.
While the world is not on track to achieve a long-term global goal of 2°C, the report confirms that limiting global warming to below 2°C is still feasible and will, while posing substantial technological, economic and institutional challenges, bring about many co-benefits. To hold warming below a 2°C target with a likely probability (>66% chance), the SED cites IPCC AR5 findings that a reduction of global greenhouse-gas emissions of 40–70 per cent by 2050 relative to 2010 levels is required. Cost-effective pathways are characterized in particular by immediate action. The costs are manageable, even without taking into account the co-benefits of mitigation, and various policy options could be deployed to manage the risks of the necessary mitigation action. The technologies required for the 1.5°C scenarios are the same as for the 2°C pathway, but need to be deployed faster, and energy demand needs to be reduced earlier, implying a higher direct mitigation cost than in the 2°C scenarios.
On the comparison of costs and avoided impacts between the 1.5°C and 2°C warming limits, the IPCC drew a distinction between mitigation costs and net benefits, noting that a simple cost-benefit analysis is inadequate to determine whether or not to pursue the 1.5°C warming limit.
Carl-Friedrich Schleussner, Michiel Schaeffer, Bill Hare