RCP4.5

RCP4.5
Name	RCP4.5
Type	Climate scenario
Year	2000–2100
Radiative forcing	~4.5 W/m2 by 2100
Developed by	Integrated Assessment Model intercomparison

RCP4.5 is a stabilized greenhouse gas concentration pathway used in climate modeling and assessment frameworks by entities such as the Intergovernmental Panel on Climate Change, NASA, NOAA, European Space Agency, and the Met Office. It serves as a mid-range scenario informing reports like the AR5 and analyses by research centers including the National Center for Atmospheric Research, Princeton University, Stanford University, Lawrence Berkeley National Laboratory, and the Potsdam Institute for Climate Impact Research. RCP4.5 underpins impact studies relevant to agencies such as the World Bank, United Nations Environment Programme, International Energy Agency, OECD, and the World Health Organization.

Overview and Definition

RCP4.5 represents a pathway in which radiative forcing reaches approximately 4.5 watts per square meter by 2100 and then stabilizes, a construct employed by modeling groups at institutions such as MIT, University of California, Berkeley, Imperial College London, Columbia University, and the University of Oxford. It is one of the four Representative Concentration Pathways developed for the Climate Modeling Intercomparison Project and was used across assessments coordinated by the Intergovernmental Panel on Climate Change and modeling centers including the National Oceanic and Atmospheric Administration, German Climate Computing Center, Centre National de Recherches Météorologiques, and Japan Meteorological Agency. The scenario links trajectories of atmospheric concentrations to socioeconomic narratives drawn from sources like the Shared Socioeconomic Pathways analyses led by researchers at IIASA, Harvard University, Yale University, University of Maryland, and Columbia Business School.

Emission Trajectory and Assumptions

The emissions pathway assumes progressive implementation of mitigation technologies and policies consistent with studies from McKinsey & Company, International Energy Agency, BloombergNEF, Rocky Mountain Institute, and Energy Information Administration, with declining global CO2 emissions mid-century informed by carbon pricing and deployment of renewables such as those analyzed by Siemens Gamesa, Vestas, GE Renewable Energy, First Solar, and Orsted. Land-use change, afforestation, and bioenergy with carbon capture and storage are parameterized in models from IIASA, Potsdam Institute for Climate Impact Research, Stockholm Environment Institute, International Institute for Applied Systems Analysis, and CSIRO. Assumptions include technological diffusion patterns studied at Carnegie Mellon University, Argonne National Laboratory, Lawrence Livermore National Laboratory, National Renewable Energy Laboratory, and KTH Royal Institute of Technology that influence methane, nitrous oxide, and fluorinated gas projections used by groups such as Scripps Institution of Oceanography and Woods Hole Oceanographic Institution.

Climate Projections and Impacts

Climate model ensembles using RCP4.5 run on platforms from ECMWF, NOAA Geophysical Fluid Dynamics Laboratory, Met Office Hadley Centre, NASA GISS, and MPI-M project global mean surface temperature increases by 2–3 °C by 2100 relative to preindustrial levels, with regional outcomes assessed in studies by IPCC Working Group I, IPCC Working Group II, IPCC Working Group III, World Meteorological Organization, United Nations Framework Convention on Climate Change, and Global Carbon Project. Projected impacts include sea-level rise synthesized by Intergovernmental Panel on Climate Change, altered precipitation patterns studied by National Center for Atmospheric Research, enhanced extreme temperature frequency analyzed by NOAA, and ecosystem responses evaluated by International Union for Conservation of Nature, Conservation International, NatureServe, The Nature Conservancy, and World Wildlife Fund. Agricultural yield responses under RCP4.5 are modeled in analyses from FAO, CGIAR, International Rice Research Institute, CIMMYT, and USDA, while health impact assessments draw on work by the World Health Organization, Centers for Disease Control and Prevention, London School of Hygiene & Tropical Medicine, Johns Hopkins Bloomberg School of Public Health, and Harvard T.H. Chan School of Public Health.

Mitigation Pathways and Policy Implications

Transitioning to trajectories consistent with RCP4.5 implies policy mixes examined by UNFCCC negotiators, market mechanisms such as those implemented under the European Union Emissions Trading System, carbon pricing research from World Bank, and national strategies evaluated by agencies like the US EPA, Ministry of Environment of Japan, Environment and Climate Change Canada, Department of Environment, Food and Rural Affairs, and Australian Department of the Environment and Energy. Technology portfolios emphasized include deployment of renewables studied by International Renewable Energy Agency, electrification trends in reports from Bloomberg New Energy Finance, energy efficiency measures analyzed by IEA, and negative emissions options compared in literature from Rockström, Hansen, Fuss, Smith, and Dooley. Policy-relevant tradeoffs related to equity, finance, and development are discussed by United Nations Development Programme, International Monetary Fund, World Bank Group, Asian Development Bank, and African Development Bank.

Comparison with Other RCPs

RCP4.5 sits between lower forcing pathways such as those used in studies by Rogelj and van Vuuren that motivated scenarios like the one informing RCP2.6 and higher forcing pathways paralleling narratives explored by Moss and Edmonds associated with RCP8.5. Comparisons across model intercomparisons coordinated by CMIP5, CMIP6, Scenario Model Intercomparison Project, Climate Model Intercomparison Project, and institutions like IPCC and NCAR highlight differences in peak warming, mitigation effort, and socioeconomic assumptions in analyses by IIASA, PBL Netherlands Environmental Assessment Agency, Tyndall Centre, CICERO, and Energy Modeling Forum.

Limitations and Uncertainties

Limitations of RCP4.5 include sensitivity to assumptions about technological change reviewed by Nordhaus, Stern, Nordhaus 2013, Weitzman, and Stiglitz, uncertainties in carbon cycle feedbacks examined by Friedlingstein, Cox, Schneider, Friedlingstein et al., and Gregory, and scenario representation issues critiqued in evaluations by van Vuuren, Moss, Rogelj, Calvin, and Kriegler. Model structural uncertainties persist across centers such as GFDL, Hadley Centre, MPI-M, NCAR, and CSIRO and are compounded by socioeconomic unpredictability highlighted by IPCC, World Bank, OECD, UNEP, and IIASA. These sources underscore limits in projecting regional extremes, tipping points discussed in work by Lenton, Scheffer, Rockström, and Steffen, and the efficacy of unproven negative emissions technologies analyzed by Keith, Fuss, Smith, and McLaren.

Category:Climate change scenarios