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The Globa l Picture
The Fifth Assessment Report (AR5) of the IPCC provides a very comprehensive analysis of the physical science basis and the
observed and projected impacts of climate change as well as of the economics of climate change mitigation. The following chapter
should be read as an addition to the AR5, with emphasis on topics that are of particular relevance for the focus regions of this
report and where scientific studies published after the literature cutoff dates of the AR5 led to an update of the findings from IPCC
for specific issues.
The IPCC projections always draw on the largest model ensemble
available (which differs significantly in size ranging from less than
10 to more than 30 climate models). This report restricts most of
the projections presented in this chapter and throughout to five
state-of-the-art CMIP5 models that are bias-corrected and used in
the ISI-MIP framework (see Appendix). Although the robustness
of projections based on smaller ensembles is generally lower, this
approach allows for direct comparison between different impacts
in an impact cascade (e.g., of changes in precipitation patterns,
river discharge, and agricultural impacts).
2.1 How Likely is a 4°C World?
The previous Turn Down the Heat reports estimated that current
emissions reductions pledges by countries worldwide, if fully
implemented, would lead to warming exceeding 3°C before 2100.
New assessments of business-as-usual emissions in the absence
of strong climate mitigation policies, as well as recent reevaluations
of the likely emissions consequences of pledges and targets
adopted by countries, point to a considerable likelihood of warming
reaching 4°C above pre-industrial levels within this century.
• Assessments of recent trends and “current policies” in the
world’s energy system analyzed 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. Another assessment, by Climate Action Tracker, of
these trends and policies leads to a warming of 3.7°C, about
0.6°C higher than the median estimate of the effect of the
Copenhagen pledges in this assessment. 12 As a consequence,
it is projected on the basis of these assessments that under
recent trends and current policies there is about a 40 percent
chance of exceeding 4°C by 2100 and a 10 percent risk of
exceeding 5°C.
• Based on a sample of 114 energy-economic model scenarios
estimating emissions in the absence of further substantial policy
action (baseline scenarios), climate-model projections reach
a warming of 4.0–5.2°C above pre-industrial levels by 2100
(95 percent range of scenarios—grey shaded area Figure 2.1)
(Blanford et al. 2014; Kriegler et al. 2013; Kriegler, Tavoni et al.
2014; Kriegler, Weyant et al. 2014; Luderer et al. 2013; Riahi
et al. 2013; Tavoni et al. 2014). Only under extreme assumptions
regarding autonomous improvements leading to large
decreases in energy intensity and an energy demand by 2100
(e.g., 35–40 percent lower than under default assumptions)
(Blanford et al. 2014; Kriegler, Weyant et al. 2014) does the
lower end of the baselines’ 95 percent range decrease to below
4°C (3.5°C) in 2100.
• The IPCC Working Group III Fifth Assessment Report assessment
of baseline scenarios leads to a warming of 3.7–4.8°C
by 2100 (80 percent range of scenarios; 2.5–7.8°C including
climate-system uncertainty), which includes the scenarios with
assumptions on autonomous improvements in energy intensity.
12
http://climateactiontracker.org/news/151/In-talks-for-a-new-climate-treaty-a-raceto-the-bottom.html.
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