Reproduction and the carbon legacies of individuals
Introduction
Much attention has been paid to the ways that people’s home energy use, travel, food choices and other routine activities affect their personal emissions of carbon dioxide and, ultimately, their potential contributions to global warming O’Neill and Chen, 2002, Bastianoni et al., 2004, Bin and Dowlatabadi, 2005, Vandenbergh and Steinemann, 2007, Wei et al., 2007. For example, a variety of “carbon calculators” allows individuals to estimate their lifetime emissions of greenhouse gases (U.S. Environmental Protection Agency, 2007).
While population growth is obviously a key component of projections of carbon emissions at a global level (Dyson, 2005, Harte, 2007, IPCC, 2007), there has been relatively little emphasis on the environmental consequences of the reproductive choices of an individual person. Obviously, the choice to reproduce contributes to future environmental impacts. There are the immediate effects caused by each offspring over his or her lifetime (Hall et al., 1994), but, should the offspring reproduce, additional impacts could potentially accrue over many future generations.
Some authors have discussed the “externalities” of childbearing (i.e., the consequences of an individual’s reproduction that are borne by society, rather than the individual himself), focusing on the costs of abatement of the impacts caused by the individual’s descendants Cline, 1992, O’Neill et al., 2001. For example, O’Neill and Wexler (2000) consider emission scenarios that lead to particular stabilization concentrations of CO2, and they model the abatement costs incurred by the addition of a single individual (and his or her descendants) to the population.
We explore the effects of an individual’s reproductive behavior by tracing a single female’s genetic contribution to future generations and weighting her descendants’ impacts by their relatedness to her. We apply this approach to emissions of fossil carbon dioxide with the goal of quantifying the carbon legacy of an individual and examining how it is affected by the individual’s reproductive choices.
Section snippets
The carbon legacy of an individual
Our basic premise is that a person is responsible for the carbon emissions of his descendants, weighted by their relatedness to him. For a descendant that is n generations removed from the focal individual, the weight is . So, for example, a mother and father are each responsible for one half of the emissions of their offspring, and 1/4 of the emissions of their grandchildren. Fig. 1 illustrates this simple idea.
We will refer to the weights that indicate the relatedness of a descendant to
Approaches to estimating the carbon legacy
We calculated trajectories of genetic units vs. time for genetic lineages of in each of the 11 most populous countries in the world, listed in Table 1. We used two approaches, one based on a simple analytical model and one involving numerical simulation.
Genetic units and person years
Fig. 3 shows trajectories of average genetic units vs. time for lineages under scenarios of constant fertility (fixed at the 2005 value of 2.05 children per woman) and medium-variant fertility in the United States. As in the example of the U.S., any country having net reproductive rate exceeding one daughter per woman (roughly corresponding to fertility exceeding two) will have lineages with exponentially increasing person years, unless fertility decreases in the future. Even under the
Acknowledgments
We are grateful to three anonymous reviewers who identified a key shortcoming of an earlier version of our paper and offered many constructive suggestions for improvements. M.S. was supported by NASA through contract 1206715 administered by the Jet Propulsion Laboratory.
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