Regional, national, and local scenarios, as well as decision-making processes involving values and difficult trade-offs are important for understanding the challenges of limiting GMST increase to 1.5☌ and are thus indispensable when assessing implementation.ĭifferent climate policies result in different temperature pathways, which result in different levels of climate risks and actual climate impacts with associated long-term implications. This report assesses the spectrum from global mitigation scenarios to local adaptation choices – complemented by a bottom-up assessment of individual mitigation and adaptation options, and their implementation (policies, finance, institutions, and governance, see Chapter 4). These include baseline scenarios that assume no climate policy scenarios that assume some kind of continuation of current climate policy trends and plans, many of which are used to assess the implications of the nationally determined contributions (NDCs) and scenarios holding warming below 2☌ above pre-industrial levels. Other scenarios are also assessed, primarily as benchmarks for comparison of mitigation, impacts, and/or adaptation requirements. The pace and scale of mitigation and adaptation are assessed in the context of historical evidence to determine where unprecedented change is required (see Chapter 4). This report focuses on pathways that could limit the increase of global mean surface temperature (GMST) to 1.5☌ above pre-industrial levels and pathways that align with the goals of sustainable development and poverty eradication. Because of their harmonised assumptions, scenarios developed with the SSPs facilitate the integrated analysis of future climate impacts, vulnerabilities, adaptation and mitigation. These scenarios offer an integrated perspective on socio-economic, energy-system (Bauer et al., 2017) 133, land use (Popp et al., 2017) 134, air pollution (Rao et al., 2017) 135 and, GHG emissions developments (Riahi et al., 2017) 136. The SSP-based 1.5☌ pathways are assessed in Chapter 2 of this report. SSP-based scenarios were developed for a range of climate forcing levels, including the end-of-century forcing levels of the RCPs (Riahi et al., 2017) 131 and a level below RCP2.6 to explore pathways limiting warming to 1.5☌ above pre-industrial levels (Rogelj et al., 2018) 132. The SSPs were developed to complement the RCPs with varying socio-economic challenges to adaptation and mitigation. Based on the CMIP5 ensemble, RCP2.6, provides a better than two-in-three chance of staying below 2☌ and a median warming of 1.6☌ relative to 1850–1900 in 2100 (Collins et al., 2013) 130.
They were used to develop climate projections in the Coupled Model Intercomparison Project Phase 5 (CMIP5 Taylor et al., 2012) 129 and were assessed in the IPCC Fifth Assessment Report (AR5). The RCPs comprise a set of four GHG concentration trajectories that jointly span a large range of plausible human-caused climate forcing ranging from 2.6 W m −2 (RCP2.6) to 8.5 W m −2 (RCP8.5) by the end of the 21st century (van Vuuren et al., 2011) 128. The SRES scenarios are superseded by a set of scenarios based on the Representative Concentration Pathways (RCPs) and Shared Socio-Economic Pathways (SSPs) (Riahi et al., 2017) 127. Subsequently, many policy scenarios have been developed based upon them (Morita et al., 2001) 126. The SRES scenarios (named after the IPCC Special Report on Emissions Scenarios published in 2000 IPCC, 2000) 125, consist of four scenarios that do not take into account any future measures to limit greenhouse gas (GHG) emissions. Climate change scenarios have been used in IPCC assessments since the First Assessment Report (Leggett et al., 1992) 124.
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