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

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Cumulative carbon emissions 47 which has an atmospheric lifetime comparable to, albeit different from, that of CO2 and, crucially, longer than the response time of the physical climate system. Hence, it could be argued that nitrous oxide emissions should be considered in the same framework as CO2, and, throughout this paper, CO2 could be replaced by the combination of CO2 and N2O. Predicting long-term emissions of N2O, given current rapid developments in agricultural technology, is even more difficult than predicting CO2 emissions. Thus, for the sake of simplicity, we focus on CO2 alone, and do not consider the issue of what fraction of these long-term emissions might be made up of N2O, although we note that, in several other papers considering the impact of emissions over the very long term, this fraction is substantial. In this paper, we explore how cumulative emission targets relate to more widely known policy targets, such as limiting emission rates in 2020 or 2050. First, we analyse the relative skill of different emission measures to predict the resultant future peak warming, comparing cumulative emissions over a range of periods and actual emission rates at years 2020 and 2050. Second, we investigate whether the cumulative emissions metric still holds for a class of emission pathways that do not assume that all emissions can be mitigated over the coming centuries. It is often argued that it may not be technically, economically or politically feasible to eliminate emissions of all greenhouse gases while, for example, preserving global food security [24]. This limit has been referred to as an ‘emissions floor’ [25,26]. It is difficult to estimate a compelling emissions floor, either in terms of its size (in gigatonnes of carbon per year (GtC yr −1 )), or in terms of the extent to which it can reduce over time as new technologies become available. Nevertheless, it makes sense to consider the possibility that it may prove prohibitively expensive to reduce emissions beyond some positive level, particularly when the impact of N2O is also included. Thus, in this paper we examine the effects of emissions floors on the basic arguments behind cumulative emissions framings of climate change. We use the following conventions: if the emissions floor is constant, then we refer to it as a ‘hard floor’. If, on the other hand, society is able to continue to reduce residual CO2 emissions, eventually to the point where net emissions are zero, then we call this a ‘decaying floor’. Third, we recognize that mitigation alone will not avoid all potential impacts of climate change, even if global warming does remain below 2 ◦ C[27,28]. Since some adaptation will be required in the future, policy-makers also need information on the rates of future climate change. This will determine how quickly a response is needed. Neither the cumulative total metric nor 2 ◦ C warming targets provide information on short-term rates of change in global warming [29]. Here, we analyse correlations between rates of CO2-induced warming and short-term emission rates, noting that warming rates are also strongly influenced by non-CO2 climate-forcing agents. 2. Methods Our method consists of deriving a range of idealized CO2 emission pathways and using a simple coupled climate–carbon-cycle model to estimate the resulting climate change. As many parameters in the model are uncertain, Phil. Trans. R. Soc. A (2011) Downloaded from rsta.royalsocietypublishing.org on November 30, 2010
48 N. H. A. Bowerman et al. a likelihood-based method is used to identify the values that give the best agreement with observations of the recent past or model studies with more complex coupled climate–carbon-cycle models [11]. For the majority of this paper, we run one simulation for each selected emission pathway, using the parameters that were previously found to give the best agreement with observations and more complex models. The model is run between the years 1751 and 2500. By running large ensembles containing hundreds of different emission pathways, we can begin to analyse trends across emission pathways. This method allows us to ask questions, such as: ‘What is it about an emission pathway that controls the resulting peak rate of global mean temperature increase?’ (a) Emission pathways We use emission pathways that follow the algorithm outlined by Allen et al. [11]. This gives the rate of change of future emissions according to the equations below: ⎧ H (t) for t < t0, ⎪⎨ aebt for t0 ≤ t < t1, Ea = ⎪⎩ cedt2 +ft for t1 ≤ t < t2, geht for t ≥ t2, where Ea is the carbon emissions in year t, H (t) is historical emissions data, a, b, c, d, f, g and h are constants, and t0 is the year at which historical data are replaced by emission pathways. The parameters b and h, representing the initial rate of exponential growth and final rate of exponential decline, depend on the specification of the emission pathways and are allowed to vary between emission pathways; t1 and t2 are the times of transitions, and also vary between emission pathways. The remaining constants are determined by the requirement that emissions are continuous everywhere, and that rates of change of emissions are continuous, where t > t0. Note that Ea is measured in tonnes of carbon, as opposed to tonnes of CO2. To convert to tonnes of CO2, one would simply multiply our emissions and cumulative emission values by a factor of 44/12. To create the emission pathways, we select a number of values of parameters b, h, t1 and t2. Each combination of these parameters represents a different possible emission pathway. We select parameter options such that there are 12 750 possible emission pathways of the type outlined here. We choose the ranges of the parameters to give a range of emission pathways with cumulative emissions to 2200 between 0.7 and 3 TtC. We choose the parameters so that the majority of emission pathways have a maximum rate of emissions decline of less than 4 per cent per year, but we also consider a smaller number of pathways that decrease by up to 10 per cent per year. We also develop a new set of pathways, extending those described above. These pathways have ‘emissions floors’ to represent the emissions that are potentially technologically, economically or politically unfeasible to mitigate. Although these are expressed in terms of CO2 alone, they could also be assumed to include the impact of the other principal long-lived anthropogenic radiative forcing agent, N2O. Including the impact of further non-CO2 forcing agents is difficult. Because 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
Page 3 and 4: Phil. Trans. R. Soc. A (2011) 369,
Page 5 and 6: Editorial David Garner Phil. Trans.
Page 7 and 8: 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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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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Sea-level rise and its possible imp
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162 R. J. Nicholls et al. expected.
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174 R. J. Nicholls et al. global ad
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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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