An Impossible Task?

editorial An impossible task?

The Paris Agreement on climate change has shifted international focus to more stringent mitigation, and asked the scientific community to work out what that means on a tight timeline. The challenge is steep, but well worth a go.

Not for the first time, a meeting of As a result, projections for key policymakers has thrown the climate research climate variables, such as temperature community into some turmoil. The Paris and precipitation, do not show obvious, Agreement1 affirms the canonical goal of societally important differences in their limiting warming to no more than 2 °C above global mean amplitudes or patterns pre-industrial levels, but it also states the aim between a 1.5 and 2 °C world2. Indeed, it to pursue a warming limit as low as 1.5 °C. has been argued6 that we may not know With this addition, a new question entered enough to clearly distinguish differences centre stage: how much of a difference does between the two target warming levels — the extra half a degree of mitigation effort after uncertainties are taken into account — make, both in terms of permissable emissions by the time of the requested 2018 and in terms of potential impacts? This IPCC assessment. question was referred back to the scientific However, the task may not be community in the form of an invitation to impossible. In some regions, such as the the Intergovernmental Panel on Climate Mediterranean, the contiguous United Change (IPCC) to provide a Special Report States and central Brazil, warming is greater in 2018. Climate researchers seem keen to than global mean surface temperature rise, help: at Nature Geoscience we are already and the signal is therefore amplified. For seeing heightened interest in exploring a example, relative to 1861–1880, we would 1.5 °C world. A Commentary on page 187 of expect annual maximum temperatures to this issue outlines some of the key points that rise by 2.2 °C in the Mediterranean region need the attention of geoscientists. in a 1.5 °C world, compared to more than The original 2 °C target had evolved 3 °C in a 2 °C world7. Another avenue for through the Copenhagen Accord in 2009 as quantifying the gains from more stringent a political compromise between the desirable mitigation could be to shift our focus and the achievable, and caused extensive towards discussing risks avoided rather discussions among climate scientists — for than trying to pin down benefits we are example, whether global mean surface confident about. The risks for most impacts temperatures are an adequate measure of will be discernibly higher in a 2 °C world climate change — that are just beginning to than at half a degree less. settle2. But now the Paris Agreement has set Obviously, climate researchers should the sights of governments on a 1.5 °C world. not abandon their other pursuits, including The task of teasing out differences curiosity-driven research, in order to between these two targets is by no means lend their services to policymakers. But easy. The emissions side of the question is it is worth diverting some attention to not straightforward, even though cumulative the tasks mapped out in Paris. A wealth budgets of allowable carbon dioxide of studies and climate model simulations emissions can be quantified with some already exists that can be re-examined confidence for any given temperature target3. with a new focus on understanding the full Any realistic pathway to a 1.5 °C world implications of a 1.5 °C world. They deserve

will probably require an initial overshoot PHOTO STOCK / ALAMY © NEWZULU to benefit from all the ingenuity that the beyond the temperature target and the climate science community can muster. ❐ subsequent removal of significant amounts On the impacts side, the picture is not of carbon dioxide from the atmosphere4, clear either — even without considering References 1. Adoption of the Paris Agreement FCCC/CP/2015/L.9/Rev.1 which may turn out to be unfeasible. The potential irreversible impacts resulting from (UNFCCC, 2015). 5 ambition to limit anthropogenic warming an overshoot of the 1.5 °C target . To be clear, 2. Knutti, R., Rogelj, J., Sedláček, J. & Fischer, E. M. Nature Geosci. is also complicated by the warming effects a 1.5 °C world will almost certainly look very 9, 13–18 (2016). of greenhouse gases other than CO , of different from a world at 2 °C. The problem 3. IPCC Climate Change 2013: The Physical Science Basis 2 (eds Stocker, T. F. et al.) 3–29 (Cambridge Univ. Press, 2013). aerosols and of albedo changes, which act on is not that there is no difference, but that 4. Anderson, K. Nature Geosci. 8, 898–900 (2015). a variety of time horizons and spatial scales internal variability and differences between 5. Ricke, K. L., Moreno-Cruz, J. B., Schewe, J., Levermann, A. and are less well studied. These contributions model projections make the uncertainties & Caldeira, K. Nature Geosci. 9, 5–6 (2016). 6. Hulme, M. Nature Clim. Change http://doi.org/bcgj (2016). to warming will need to be factored into a so large that it is hard to say with confidence 7. Seneviratne, S. I., Donat, M. G., Pitman, A. J., Knutti, R. temperature-based target. what exactly these differences will look like. & Wilby, R. L. Nature 529, 477–483 (2016).

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