Mapping the climate change challenge Stephane Hallegatte1, Joeri Rogelj2,3, Myles Allen4,5, Leon Clarke6, Ottmar Edenhofer7,8,9, Christopher B. Field10,11, Pierre Friedlingstein12, Line van Kesteren13, Reto Knutti3, Katharine J. Mach11, Michael Mastrandrea11, Adrien Michel14,15, Jan Minx7,9,16, Michael Oppenheimer17, Gian-Kasper Plattner14,22, Keywan Riahi2,18, Michiel Schaeffer19,20, Thomas F. Stocker14,15, Detlef P. van Vuuren13,21 1 The World Bank, Climate Change Group, 1818 H Street NW, Washington, DC 20433, USA 2 Energy Program, International Institute for Applied Systems Analysis (IIASA), Schlossplatz 1, A-2361 Laxenburg, Austria 3 Institute for Atmospheric and Climate Science, ETH Zurich; Universitätstrasse 16, 8092 Zürich, Switzerland 4 Department of Physics, University of Oxford, Oxford, UK 5 Environmental Change Institute, University of Oxford, Oxford, UK 6 Pacific Northwest National Laboratories, Joint Global Change Research Institute, College Park, MD, USA 7 Potsdam Institute for Climate Impact Research, Potsdam, Germany 8 Technische Universität Berlin, Berlin, Germany 9 Mercator Research Institute on Global Commons and Climate Change, Berlin, Germany 10 Department of Earth System Science, Stanford University, Stanford, California 94305, United States 11 Department of Global Ecology, Carnegie Institution for Science, 260 Panama St, Stanford, California 94305, United States 12 College of Engineering Mathematics and Physical Sciences, University of Exeter, Exeter, EX4 4QF, United Kingdom 13 PBL Netherlands Environmental Assessment Agency, PO Box 303, 3720 AH Bilthoven, The Netherlands 14 Climate and Environmental Physics, Physics Institute, University of Bern, Switzerland 15 Oeschger Center for Climate Change Research, University of Bern, Switzerland 16 Hertie School of Governance, Friedrichstr. 189, 10117 Berlin, Germany 17 Department of Geosciences and the Woodrow Wilson School, Princeton University, Princeton, NJ 08544, USA 18 Graz University of Technology, Inffeldgasse, A-8010 Graz, Austria 19 Climate Analytics GmbH, Friedrichstr 231 - Haus B, 10969 Berlin, Germany 20 Environmental Systems Analysis Group, Wageningen University and Research Centre, PO Box 47, 6700 AA Wageningen, The Netherlands. 21 Utrecht University, Copernicus Institute for Sustainable Development, Utrecht, The Netherlands 22 Swiss Federal Research Institute WSL, CH-8903 Birmensdorf, Switzerland Abstract The recent negotiations on climate change in Paris have left partly open the discussion on a long-term global goal, in the form of an upper limit to warming. Agreement on such a limit is a political decision that need to be informed by scientific knowledge through a mapping that systematically explores the consequences of different policy choices. Such a multi-disciplinary effort, based on the analysis of a set of scenarios, helped structure the IPCC AR5 Synthesis Report. This Perspective summarizes this approach and its results, reviews its strengths and limitations, and discusses how decision-makers can use these results in practice. It also identifies research needs to facilitate integrated analysis of the climate change problem and to help better inform policy-makers and the public. 1 A key component of international negotiations on climate change has been the agreement on a long- term global goal, in the form of an upper limit to warming, to “prevent dangerous anthropogenic interference with the climate system” (Article 2 of the United Nations Framework Convention on Climate Change). The Paris Agreement now quantifies for the first time this goal as the objective to hold “the increase in the global average temperature to well below 2°C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels, recognizing that this would significantly reduce the risks and impacts of climate change.” The choice of this objective is the result of a long and complex political process that has been informed by the science of climate change, notably as summarized by the Fifth Assessment Report (AR5) Synthesis Report of the Intergovernmental Panel on Climate Change (IPCC), published in November 2014. Similarly, the eventual decision on making the additional effort needed to hold the temperature increase below 1.5°C will be informed by existing and future IPCC assessments. The mandate of the IPCC is to inform decision-making at all levels, including the international negotiations, and the IPCC has responded positively to the invitation by the UNFCCC to “provide a special report in 2018 on the impacts of global warming of 1.5 °C above pre-industrial levels and related global greenhouse gas emission pathways.” As the IPCC engages with the scientific community in scoping its report, it is essential to reaffirm that scientific analysis cannot, by itself, identify a single best long-term goal. One major reason is that individuals and groups have distinct values and priorities. Because climate and climate policy have a wide range of impacts that differ across countries, communities and individuals, a temperature target also depends on preferences regarding equity and fairness. The need to aggregate the wide variety of impacts also involves value judgments, for example the relative importance ascribed to economic damages versus impacts that are harder to monetize, such as biodiversity loss. Another example is that the long-term goal depends on relative valuation of present versus future outcomes. As long as values and preferences vary widely, purely scientific methods cannot specify a unique and consensual upper limit for warming. A long-term goal for climate policy can be agreed upon only through a political process, both national and international 1. Science can inform such a process, however, through a mapping that systematically explores the consequences of different policy choices for climate change and the risks it creates 2. Such a multi- disciplinary effort, based on the analysis of a set of scenarios, helped structure the IPCC AR5 Synthesis Report (IPCC, 2014a). Here, we summarize this approach and – going beyond the AR5 – we review its strengths and limitations and discuss how decision-makers can use these results in practice, notably to think about future choices regarding the long-term objective of climate action. We also identify research needs to facilitate integrated analysis of the climate change problem and to help better inform policy- makers and the public. 2 Figure 1: The mapping of the relationship between risks from climate change, temperature change, cumulative CO2 emissions, and annual GHG emissions changes by 2050 and 2100. Limiting the additional risks due to climate change across five ‘Reasons For Concern’ (five shaded bars, panel a) implies a limit for future cumulative emissions of CO2 (panel b), which constrains annual emissions over the next few decades and until the end of the 21st century (panel c). See text for more information. (Figure based on IPCC, 2014a, with additional information.) THE FIGURE: LINKING RISKS AND EMISSIONS Figure 1 links long-term climate change risks to their implications for annual emission levels under a broad range of scenarios. In the top left panel of Figure 1, five “Reasons For Concern” summarize how additional risks for societies and ecosystems change with increasing global mean temperature, compared with pre-industrial levels, as assessed in the Working Group II (WGII) contribution to the IPCC AR5 3. Each Reason for Concern is anchored in different values and priorities, reflecting differences in the relative importance ascribed to the present versus the future, aggregate outcomes versus 3 inequalities across individuals, most-likely versus possible extreme scenarios, and different categories of impacts. The Reasons for Concern are associated with (1) unique and threatened systems, such as coral- reef ecosystems and the societies dependent on them; (2) extreme weather events, such as heat waves and coastal flooding; (3) the uneven distribution of impacts, especially for poor communities and most- affected ecosystems; (4) global aggregate impacts, for example for Earth’s biodiversity and the global economy; and (5) large-scale singular events, including, for example, ecosystem and ice-sheet tipping points. For each, the color indicates the additional risk due to climate change when a temperature level is reached and then sustained or exceeded, reflecting the judgments of AR5 authors based on the assessed literature. The top right panel describes the relationship between cumulative CO2 emissions and temperature change, from IPCC Working Group I (WGI) contribution to AR5 4. While warming also depends on concentrations of non-CO2 greenhouse gases (such as methane, nitrous oxide, hydrofluorocarbons, perfluorinated compounds, and sulphur hexafluoride) as well as short-lived air pollutants like soot and tropospheric ozone, the warming associated with CO2 emissions persists for many centuries, a result of the long-term persistence of some of the CO2 released to the atmosphere and the coupled dynamics of ocean-heat and CO2 uptake. The black solid line shows the consistent linear relationship between cumulative CO2 emissions since 1870 and CO2-induced warming. Comparing this to the grey solid line, representing the median relationship between total global mean temperature change and cumulative CO2 emissions, shows that net non-CO2 warming is expected to increase, from near zero at present to o over 0.5 C if cumulative CO2 emissions reach 4,000 GtCO2