Heat

21st Century Projections
technologies to satisfy demand (Masui et al. 2011; Thomson et al.
2011; Vuuren et al. 2011; Rao and Riahi 2006).
The purpose of the RCP exercise was to derive a wide range
of plausible pathways through 2100 (and beyond) to be used to
drive the climate and climate impact models, the results of which
would be summarized in the IPCC.
The highest RCP scenario, RCP8.5 (Riahi, Rao, et al. 2011), is
the only nonmitigation pathway within this AR5 scenario group
and is comparable to the highest AR4 SRES scenario (SRES A1FI).
It projects warming by 2100 of close to 5°C. However, RCP6,
one of the RCP mitigation scenarios that assumes only a limited
degree of climate policy intervention, already projects warming
exceeding 4°C by 2100 with a probability of more than 15 percent.
As illustrated in Figure 20, the range of changes in temperature
for the RCP scenarios is wider than for the AR4 SRES scenarios.
The main reason for this is that the RCPs span a greater range of
plausible emissions scenarios, including both scenarios assuming
no mitigation efforts (RCP8.5) and scenarios that assume relatively
ambitious mitigation efforts (RCP3PD). This wide variety of the
RCP pathway range is further illustrated in Figure 21. The median
estimate of warming in 2100 under the nonmitigation RCP8.5
pathway is close to 5°C and still steeply rising, while under the
much lower RCP3PD pathway temperatures have already peak
and slowly transition to a downward trajectory before the end
of this century.
Figure 21: Probabilistic temperature estimates for new (RCP) IPCC
scenarios, based on the synthesized carbon-cycle and climate system
understanding of the IPCC AR4. Grey ranges show 66 percent ranges,
yellow lines are the medians. Under a scenario without climate policy
intervention (RCP8.5), median warming could exceed 4°C before the
last decade of this century. In addition, RCP6 (limited climate policy)
shows a more than 15 percent chance to exceed 4°C by 2100.
How Likely is a 4°C World?
The emission pledges made at the climate conventions in Copenhagen
and Cancun, if fully met, place the world on a trajectory for
a global mean warming of well over 3°C. Even if these pledges
are fully implemented there is still about a 20 percent chance of
exceeding 4°C in 2100. 10 If these pledges are not met then there
is a much higher likelihood—more than 40 percent—of warming
exceeding 4°C by 2100, and a 10 percent possibility of this
occurring already by the 2070s, assuming emissions follow the
medium business-as-usual reference pathway. On a higher fossil
fuel intensive business-as-usual pathway, such as the IPCC
SRESA1FI, warming exceeds 4°C earlier in the 21st century. It is
important to note, however, that such a level of warming can
still be avoided. There are technically and economically feasible
emission pathways that could still limit warming to 2°C or below
in the 21st century.
To illustrate a possible pathway to warming of 4°C or more,
Figure 22 uses the highest SRES scenario, SRESA1FI, and compares
it to other, lower scenarios. SRESA1FI is a fossil-fuel intensive, high
economic growth scenario that would very likely cause mean the
global temperature to exceed a 4°C increase above preindustrial
temperatures.
Source: Rogelj, Meinshausen et al. 2012
Most striking in Figure 22 is the large gap between the projections
by 2100 of current emissions reduction pledges and the
(lower) emissions scenarios needed to limit warming to 1.5–2°C
above pre-industrial levels. This large range in the climate change
implications of the emission scenarios by 2100 is important in its
10 Probabilities of warming projections are based on the approach of (Meinshausen
et al. 2011), which involves running a climate model ensemble of 600 realizations
for each emissions scenario. In the simulations each ensemble member is driven by
a different set of climate-model parameters that define the climate-system response,
including parameters determining climate sensitivity, carbon cycle characteristics, and
many others. Randomly drawn parameter sets that do not allow the climate model to
reproduce a set of observed climate variables over the past centuries (within certain
tolerable “accuracy” levels) are filtered out and not used for the projections, leaving
the 600 realizations that are assumed to have adequate predictive skill.
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