If the chosen global mean temperature limit is to remain feasibly within reach, a decision must be reached urgently. Given this urgency and the recent approval of the IPCC to accept the invitation to write the report, there is no time to waste – we must join forces to provide the most robust possible understanding of the impacts of 1.5°C and 2°C global warming.

Mike Hulme translates the request for a 1.5°C Special Report into two questions: "what are the impacts relative to today of another 0.5°C of warming?" and what are "the damages avoided by limiting warming to 1.5°C relative to 2°C?" He argues that these questions are ill-posed because the differences in impact between 1.5 and 2°C are blurred by interactions of complex social and physical systems.

However, simulation experiments provide a framework for quantifying the additional impacts due to climatic forcing, given other human-induced changes. Even without representing a full disentanglement of the forces of change, impact models at least offer a basis for quantifying the additional climate-induced effect. So, we believe the questions are actually very well posed, but an important third one is missing: what are the damages caused by the full 1.5°C warming above pre-industrial levels, including those we already see today?

Addressing this question will motivate the comparison of simulated impacts against historical observations and pre-industrial control runs, leading to model improvement and a better attribution of impacts. Current impact simulations use present-day reference levels, while we will need pre-industrial control runs for impacts to address our third question. However, there is still time to provide them.

There are certainly fewer studies considering the impacts from 1.5°C of warming than from 3°C or 4°C. However, it is possible to quantify the differences between 1.5°C and 2°C of global warming globally (1–4) and regionally (5). What’s more, the freely available output of the Inter-Sectoral Impact Model Intercomparison Project (6) (ISI-MIP) contains simulations of a wide range of impact models driven by a consistent set of global climate models, some with scenarios that stabilize at about 1.5°C.

The number of potential 1.5°C pathways will force the development of "reduced complexity impacts models", which allow scaling of complex impact-model simulations to chosen carbon dioxide and global mean temperature pathways, without the need for adding computationally-expensive runs (7). These tools will be invaluable to adequately represent damages in integrated assessment models, replacing outdated damage functions and facilitating assessments of aggregated impacts. With the Sixth Assessment Report of the IPCC on the horizon, these tasks will only become more relevant in the coming years.

We agree with Mike Hulme that researchers should be realistic about the information they can reliably provide to policy makers. However, ISIMIP and related projects show that the climate change impact modelling community has reached a level of maturity and organization capable of accepting the challenge.

References

(1) K Frieler et al 2015 Earth Syst. Dyn. 6 447–460
(2) D Gerten et al 2013 Environ. Res. Lett. 8 034032
(3) F Piontek et al 2014 Proc. Natl. Acad. Sci. 119 3233–3238
(4) C F Schleussner et al 2015 Earth Syst. Dyn. Discuss. 2447–2505
(5) S Adams et al 2014 Turn Down the Heat: Confronting the New Climate Normal. The World Bank, Washington, US
(6) L Warszawski et al 2014 Proc Natl. Acad. Sci. 111 3228–3232
(7) M Mengel 2016 Proc Natl. Acad. Sci. 113 2597–2602

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