Articles | Volume 10, issue 2
Geosci. Model Dev., 10, 571–583, 2017
Geosci. Model Dev., 10, 571–583, 2017

Methods for assessment of models 08 Feb 2017

Methods for assessment of models | 08 Feb 2017

Half a degree additional warming, prognosis and projected impacts (HAPPI): background and experimental design

Daniel Mitchell1,a, Krishna AchutaRao2, Myles Allen1,3, Ingo Bethke4, Urs Beyerle5, Andrew Ciavarella6, Piers M. Forster7, Jan Fuglestvedt8, Nathan Gillett9, Karsten Haustein1, William Ingram3,6, Trond Iversen10, Viatcheslav Kharin9, Nicholas Klingaman11, Neil Massey1, Erich Fischer5, Carl-Friedrich Schleussner12,13, John Scinocca9, Øyvind Seland10, Hideo Shiogama14, Emily Shuckburgh15, Sarah Sparrow16, Dáithí Stone17, Peter Uhe16,1, David Wallom16, Michael Wehner17, and Rashyd Zaaboul18 Daniel Mitchell et al.
  • 1Environmental Change Institute, School of Geography and the Environment, Oxford University, Oxford, UK
  • 2Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, New Delhi 110016, India
  • 3Atmospheric, Oceanic and Planetary Physics (AOPP), Oxford University, Oxford, UK
  • 4Uni Research Climate, Bjerknes Centre for Climate Research, Bergen, Norway
  • 5ETH Zurich, Institute for Atmospheric and Climate Science, Zurich, Switzerland
  • 6Met Office Hadley Centre for Climate Science and Services, Exeter, UK
  • 7School of Earth and Environment, University of Leeds, Leeds, UK
  • 8Center for International Climate and Environmental Research – Oslo (CICERO), PO Box 1129 Blindern, 0318 Oslo, Norway
  • 9Canadian Centre for Climate Modelling and Analysis, Environment and Climate Change Canada, University of Victoria, Victoria, V8W 2Y2, Canada
  • 10Norwegian Meteorological Institute, Oslo, Norway
  • 11National Centre for Atmospheric Science – Climate, Department of Meteorology, University of Reading, Reading, UK
  • 12Climate Analytics, Berlin, Germany
  • 13Potsdam Institute for Climate Impact Research, Potsdam, Germany
  • 14Center for Global Environmental Research, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan
  • 15British Antarctic Survey (BAS), High Cross, Madingley Road, Cambridge, UK
  • 16Oxford e-Research Centre (OeRC), University of Oxford, Oxford, UK
  • 17Lawrence Berkeley National Laboratory, Berkeley, CA, USA
  • 18International Center for Biosaline Agriculture, P.O. Box 14660 Dubai, UAE
  • anow at: School of Geographical Sciences, University of Bristol, Bristol, UK

Abstract. The Intergovernmental Panel on Climate Change (IPCC) has accepted the invitation from the UNFCCC to provide a special report on the impacts of global warming of 1.5 °C above pre-industrial levels and on related global greenhouse-gas emission pathways. Many current experiments in, for example, the Coupled Model Inter-comparison Project (CMIP), are not specifically designed for informing this report. Here, we document the design of the half a degree additional warming, projections, prognosis and impacts (HAPPI) experiment. HAPPI provides a framework for the generation of climate data describing how the climate, and in particular extreme weather, might differ from the present day in worlds that are 1.5 and 2.0 °C warmer than pre-industrial conditions. Output from participating climate models includes variables frequently used by a range of impact models. The key challenge is to separate the impact of an additional approximately half degree of warming from uncertainty in climate model responses and internal climate variability that dominate CMIP-style experiments under low-emission scenarios.

Large ensembles of simulations (>  50 members) of atmosphere-only models for three time slices are proposed, each a decade in length: the first being the most recent observed 10-year period (2006–2015), the second two being estimates of a similar decade but under 1.5 and 2 °C conditions a century in the future. We use the representative concentration pathway 2.6 (RCP2.6) to provide the model boundary conditions for the 1.5 °C scenario, and a weighted combination of RCP2.6 and RCP4.5 for the 2 °C scenario.

Short summary
This paper provides an experimental design to assess impacts of a world that is 1.5 °C warmer than at pre-industrial levels. The design is a new way to approach impacts from the climate community, and aims to answer questions related to the recent Paris Agreement. In particular the paper provides a method for studying extreme events under relatively high mitigation scenarios.