Four degrees and beyond: the potential for a global ... - Amper

56 N. H. A. Bowerman et al.
(a)
peak CO 2-induced warming (ºC)
(c)
peak CO 2-induced warming (ºC)
4
3
2
1
0 1 2 3 4
cumulative emissions to 2500 (TtC)
4
3
2
1
0 1 2 3 4
cumulative emissions to 2100 (TtC)
(b)
(d)
0 1 2 3 4
cumulative emissions to peak temp. (TtC)
0 1 2 3 4
cumulative emissions to 2200 (TtC)
Figure 3. Most likely peak warming as a function of cumulative emissions for different emissions
floors. The type of cumulative emission metric varies between the plots: cumulative emissions to
(a) 2500, (b) the time of peak warming, (c) 2100, and (d) 2200. The panels in this figure are
as figure 2a, but with different floors preventing emissions from dropping below certain values at
certain times. The emissions floors used here are the same as those in figure 1, and use the same
colour code. The black squares represent pathways in which no floor is present, so emissions are
allowed to fall to zero. The yellow crosses and red diamonds indicate pathways in which a ‘hard’
floor is set at 1.5 or 3 GtC yr −1 ; in these pathways, emissions are unable to fall below the floor
and so remain at these values indefinitely. The blue crosses and green diamonds are pathways with
an exponentially decreasing emissions floor, which has a decay time of 200 years. The blue crosses
pass through 1.5 GtC yr −1 in the year 2050, while the green diamonds pass through 3 GtC yr −1
in that year. We observe the strongest correlation in (d), between peak warming and cumulative
emissions to 2200.
The observational constraints are much more effective in constraining the
short- and medium-term response of the climate–carbon-cycle system than they
are at constraining the multi-century response. Hence, it is inherently hard to
determine whether, after a significant injection of CO2 into the atmosphere,
Phil. Trans. R. Soc. A (2011)
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