The decision on whether to increase the ambition of climate change mitigation efforts to stabilise temperatures at 1.5°C rather than 2°C above pre-industrial is arguably one of the most momentous to be made in the coming decade, and is currently poorly served by the paucity of scientific analysis of the relative risks associated with these two outcomes (James et al, 2016), particularly regarding the role of extreme weather. The Conference of the Parties of the United Nations Framework Convention on Climate Change (UNFCCC), in its Paris Agreement of 2015, invited the Intergovernmental Panel on Climate Change (IPCC) to prepare a Special Report in 2018 “on the impacts of global warming of 1.5°C above pre-industrial levels and related greenhouse gas emission pathways.” To inform such an assessment, research will need to be undertaken immediately, over the period 2016 to 2017.

Planned coupled model integrations under CMIP6 may not be ready in time to inform the IPCC 2018 Assessment. Furthermore, relatively small ensembles of coupled model integrations, as requested by CMIP6, are primarily suited to the assessment of expected changes in mean climate, not weather extremes. Among existing CMIP5 experiments, the RCP2.6 scenario ensemble mean is approximately 1.6°C above pre-industrial by 2100 (SST patterns are given in Fig. 1), while the RCP4.5 mean is about 2.4°C above pre-industrial (1.0°C and 1.8°C above the 1986-2005 baseline respectively), but in both cases with a substantial spread, so neither ensemble can be regarded as truly representative of a 1.5°C or 2°C world. CESM have performed similar scenario simulations that reach 1.2C by 2100, and these will aid the experimental design (Rogelj et al, 2015). The UNFCCC calls specifically for information on the impacts of a warming of 1.5°C and the emission pathways that might be required to achieve this, not information on the response to an emission scenario that might, at some level of confidence, be consistent with a warming of 1.5°C. The scenario-driven experiments of CMIP5 and CMIP6 are not designed to provide this information.

It is likely that the impacts of a global warming of 1.5°C, and the impacts avoided by stabilising temperatures at 1.5 instead of 2°C, will be dominated, in many regions, by changing risks of extreme weather events. To quantify these changes, both relatively high atmospheric resolution and large initial-condition ensembles are required. Climatological biases in many coupled models remain of the order of 0.5°C globally, compromising their use in comparing risks associated with these levels of warming. Similar-magnitude regional biases emerge if transient simulations are used as a proxy for equilibrated conditions. If additional research is not undertaken as a matter of urgency, there is a danger, under the UNFCCC/IPCC timetable, that the evidence available by the end of 2017 will be insufficient to clearly distinguish between impacts at 1.5°C and 2°C of warming, even if very different levels of risk are associated with these two outcomes in reality. The HAPPI project calls on climate modelling groups to undertake a simple series of experiments specifically designed to quantify the relative risks associated with 1.5°C and 2°C of warming, drawing directly on the “Climate of the 20th Century” experiments that already focus on extreme weather and the relative risks of low-probability extreme weather events.