T he G lobal P icture
Figure 2.3: Sea-level rise from observations (orange: tide gauges, red: satellites) and models (blue: projections from IPCC TAR starting in 1990, green: projections from IPCC AR4 starting in 2000)
warming would be either lower than 4.2°C or higher than 6.5°C by 2100.14 On average, the most recent business-as-usual scenarios lead to warming projections close to those of RCP8.5 and there is a medium chance that end-of-century temperature rise exceeds 4°C. Approximately 30 percent of the most recent business-as-usual scenarios reach a warming higher than that associated with RCP8.5 by 2100 (see Figure 2.4, right-hand panel).
Can Warming be Held Below 2°C?
Models do not include a sea-level decline due to dam building estimated for 1961–2003 that is part of the observed time series. Including this in the models would widen the gap with observations further, although this is likely fully compensated by increased groundwater extraction during the last 2 decades Source: Adapted from Rahmstorf et al. (2012).
by countries, point to a considerable likelihood of warming reaching 4°C above pre-industrial levels within this century. The latest research supports both of these findings (see Appendix 1): The most recent generation of energy-economic models estimates emissions in the absence of further substantial policy action (business as usual), with the median projections reaching a warming of 4.7°C above pre-industrial levels by 2100, with a 40 percent chance of exceeding 5°C (Schaeffer et al. 2013). Newly published assessments of the recent trends in the world’s energy system by the International Energy Agency in its World Energy Outlook 2012 indicate global-mean warming above pre-industrial levels would approach 3.8°C by 2100. In this assessment, there is a 40 percent chance of warming exceeding 4°C by 2100 and a 10 percent chance of it exceeding 5°C. In relation to the effects of pledges, the updated UNEP Emissions Gap Assessment 2012, found that present emission trends and pledges are consistent with emission pathways that reach warming in the range of 3 to 5°C by 2100, with global emissions estimated for 2020 closest to levels consistent with a pathway leading to 3.5–4°C warming by 2100.12 The high emissions scenario underlying novel assessments, RCP8.5, reaches a global-mean warming level of about 4°C above pre-industrial levels by the 2080s and gives a median warming of about 5°C by 2100.13 According to new analysis (see Appendix 1), there is a 66-percent likelihood that emissions consistent with RCP8.5 will lead to a warming of 4.2 to 6.5°C, and a remaining 33-percent chance that
State-of-the-art climate models show that, if emissions are reduced substantially, there is a high probability that global mean temperatures can be held to below 2°C relative to preindustrial levels. Climate policy has to date not succeeded in curbing global greenhouse gas emissions, and emissions are steadily rising (Peters et al. 2013). However, recent high emission trends do not imply high emissions forever (van Vuuren and Riahi 2008). Several studies show that effective climate policies can substantially influence the trend and bring emissions onto a feasible path in line with a high probability of limiting warming to below 2°C, even with limited emissions reductions in the short term (for example, OECD 2012; Rogelj et al. 2012a; UNEP 2012; van Vliet et al. 2012; Rogelj et al. 2013). The available scientific literature makes a strong case that achieving deep emissions reductions over the long term is feasible; reducing total global emissions to below 50 percent of 2000 levels by 2050 (Clarke et al. 2009; Fischedick et al. 2011; Riahi et al. 2012). Recent studies also show the possibility, together with the consequences of delaying action (den Elzen et al. 2010; OECD 2012; Rogelj et al. 2012a, 2013; van Vliet et al. 2012).
Patterns of Climate Change This report presents projections of global and regional temperature and precipitation conditions, as well as expected changes in aridity and in the frequency of severe heat extremes. These analyses are based on the ISI-MIP database (Warszawski et al., in preparation), consisting of a subset of the state-of-the-art climate model projections of the Coupled Model Intercomparison Project phase 5 (CMIP5; K. E. Taylor, Stouffer, and Meehl, 2011) that were bias-corrected against late twentieth century meteorological observations (Hempel, Frieler,
12 This applies for the “unconditional pledges, strict rules” case. 13 RCP refers to “Representative Concentrations Pathway,” which underlies the
IPCC´s Fifth Assessment Report. RCPs are consistent sets of projections for only the components of radiative forcing (the change in the balance between incoming and outgoing radiation to the atmosphere caused primarily by changes in atmospheric composition) that are meant to serve as inputs for climate modeling. See also Box 1, “What are Emission Scenarios?” on page 22 of the previous report. 14 A probability of >66 percent is labeled “likely” in IPCC’s uncertainty guidelines adopted here.
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