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

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(a) 4 (b) peak CO 2-induced warming (ºC) 3 2 0.02 TtC 0.08 TtC 0.03 TtC 1 0 1 2 3 cumulative emissions from 1750 (TtC) (c) 4 (d) peak CO 2-induced warming (ºC) (e) 4 ( f ) peak CO 2-induced warming (ºC) 3 2 Cumulative carbon emissions 53 0.10 TtC 1 5 10 15 20 2020 emissions (GtC yr –1 ) 3 2 2.3 GtC yr –1 2.9 GtC yr –1 25 years 9 years 16 years 2.3 GtC yr –1 4.9 GtC yr –1 25 years 1 1980 2000 2020 2040 2060 2080 year of peak emissions (year) 0.08 TtC 0.20 TtC 0.16 TtC 0.22 TtC 0 0.2 0.4 0.6 0.8 cumulative emissions 2010–2050 (TtC) 9.4 GtC yr –1 0.1 GtC yr –1 1.9 GtC yr –1 0 5 10 15 20 25 2050 emissions (GtC yr –1 ) 6.2 GtCyr –1 2.2 GtCyr –1 4.5 GtCyr –1 10.1 GtC yr –1 9.0 GtCyr –1 5 10 15 20 25 peak emissions (GtC yr –1 ) Figure 2. Scatter plots showing the relationship between most likely peak CO2-induced warming and various global carbon emission metrics for 395 emission pathways. The x-axis for each panel shows: (a) emissions time-integrated between 1750 and 2500, (b) emissions time-integrated between 2010 and 2050, (c) emissions in the year 2020, (d) emissions in the year 2050, (e) the year in which emissions peak, and (f ) the peak or maximum in emissions. Black crosses indicate emission pathways in which the maximum rate of emissions decline is less than 4% yr −1 ; grey diamonds indicate the converse. The bars show the spreads of the metrics for pathways with a resultant peak warming of 2 ◦ Cor3 ◦ C. The black bars show the spread in pathways with peak rates of emissions decline less than 4%, while the grey bars show the spread in all emission pathways. We see that the strongest correlation is in (a), between peak warming and cumulative emissions between 1750 and 2500. Phil. Trans. R. Soc. A (2011) Downloaded from rsta.royalsocietypublishing.org on November 30, 2010
54 N. H. A. Bowerman et al. rates of decline between 0 and 4 per cent. For a given peak warming, and so for a given cumulative total, pathways with a faster rate of emissions decline will have relatively more of their cumulative total emitted sooner than for pathways with slower rates of decline. As a result, these rapidly declining pathways will have higher cumulative emissions between 2010 and 2050, higher 2020 emissions, higher peak emissions and a later year of peak emissions. This effect also holds for 2050 emissions above 5 GtC yr −1 . For emission pathways with cumulative emissions less than 1 TtC, corresponding to a peak warming of 2 ◦ C, 2050 emissions occur once the pathway has been declining exponentially for a considerable period, and so rapidly declining pathways will have relatively lower emissions in 2050. The correlation between peak warming and cumulative emissions between year 2010 and 2050, which is the emissions metric used by the German Advisory Council on Global Change [15], is plotted in figure 2b. We see that the correlation here is not as good as in figure 2a. Emissions before 2010 are not allowed to vary across emission pathways, so there can be no contribution to the spread in peak warming from this historical time period. The majority of the spread comes from the variation in post-2050 emissions, which will have a significant impact on peak temperatures, but which by definition are not included in 2010–2050 metrics. We see that, in cases where post-2050 emissions are small, the spread is much tighter, as shown by those pathways with cumulative emissions less than 0.3 TtC between 2010 and 2050. This is because the majority of future cumulative emissions in these pathways are emitted before 2050. This means that, up to roughly 1.8 ◦ C, the cumulative emissions between 2010 and 2050 has some skill in predicting peak CO2-induced warming, but this skill is reduced for higher temperatures. We also consider whether there is predictive skill in using the actual emission rates at a particular year; here we use year 2020 and year 2050. The former year is chosen because most of the Copenhagen Accord emission reduction pledges are quoted for this year [16]. The latter is chosen because several reduction targets for 2050 have also been presented [17,32]. Figure 2c,d shows 2020 and 2050 emissions against peak warming. As in Lowe et al. [33], we find that emissions in the year 2020 are not a good indicator of peak warming, because they are largely a function of current emissions, and are not a key determinant of cumulative emissions. For figure 2d, year 2050 emissions do seem to be a good indicator at lower rates of emission, particularly at values that cause roughly 2 ◦ C of warming or less. However, we found 2050 emissions to be a less good indicator of peak resultant warming at higher rates of emission. This is in part a consequence of the choice of pathways, as we have considered only smooth pathways with a single maximum and exponential tails (see §2a for more detail on how the emission pathways are chosen). A wider range of functional forms to describe emission pathways would be expected to reduce the strength of the relationship between 2050 emissions and peak warming. We also compare the peak emission rate and the year of peak emissions with the peak CO2-induced warming. As shown in figure 2e,f , the spread is very large for both of these metrics, and there is little correlation, except for the rapidly declining emission pathways (grey diamonds) appearing to the right of the slowly declining pathways (black crosses), as explained earlier. Under the assumption that society will work to avoid crossing a key temperature threshold, from figure 2a, the cumulative emission metric confirms that we have a choice of high emissions soon followed by rapid decarbonization, or Phil. Trans. R. Soc. A (2011) Downloaded from rsta.royalsocietypublishing.org on November 30, 2010
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ISSN 1364-503X volume 369 number 19
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Phil. Trans. R. Soc. A (2011) 369,
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Editorial David Garner Phil. Trans.
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Four degrees and beyond: the potent
Page 9 and 10: Preface 5 commitments might give us
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Water availability in +2 ◦ C and
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(ii) Water stress index Water avail
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(b) (c) (d) (a) Water availability
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(a) number of models, black line nu
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change in run-off (%) change in run
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Agriculture and food systems in sub
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118 P. K. Thornton et al. 1. Introd
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120 P. K. Thornton et al. increases
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122 P. K. Thornton et al. Table 1.
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124 P. K. Thornton et al. century.
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126 P. K. Thornton et al. and polit
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128 P. K. Thornton et al. — The r
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136 P. K. Thornton et al. 83 Challi
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Humid tropical forests on a warmer
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(a) Humid tropical forests on a war
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0 retraction risk 17 120 80 40 (a)
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0 retraction risk 17 120 80 40 0
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Sea-level rise and its possible imp
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162 R. J. Nicholls et al. expected.
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164 R. J. Nicholls et al. sea-level
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166 R. J. Nicholls et al. mean temp
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168 R. J. Nicholls et al. interglac
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170 R. J. Nicholls et al. discussed
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172 R. J. Nicholls et al. land loss
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174 R. J. Nicholls et al. global ad
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176 R. J. Nicholls et al. Hence, th
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178 R. J. Nicholls et al. to climat
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180 R. J. Nicholls et al. 43 Pardae
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REVIEW Phil. Trans. R. Soc. A (2011
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Review. Interactions in a 4 ◦ C w
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spread in mosquitoborne disease may
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[89,94,95] further acidification by
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Editorial Editorial 3 D. Garner Pre
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