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Arguing for population reduction Case study: Climate change Hilary Greaves Philosophy, Oxford “Population ethics: Theory and practice” workshop Oxford, 29 November 2014 Outline • Generalities: Arguments for population reduction meet population axiology • The case of climate change – The rhetoric – The science – Reasons(?) to favour population reduction • Summary and conclusions Arguing for population reduction • “Arguing for population reduction”: Arguing that the optimal instantaneous population in the short to medium term, given the actual empirical state of the world, is lower than we are likely to get under ‘business as usual’ scenarios. – Population Matters – World Overpopulation Awareness – Zero Population Growth Population axiology: A recap • ‘Population axiology’: A ranking of states of affairs in terms of better/worse overall, where those states of affairs can differ over the number of people who ever live. • Examples: A B – “Average utilitarianism”: A is better than B if the average well-being level in A is higher than that in B – “Total utilitarianism”: A is better than B if the total well-being level in A is higher than that in B • It is obviously more difficult to convince a total utilitarian to reduce population size than it is to convince an average utilitarian. – But: If you can convince the total utilitarian, you can convince anybody… A taxonomy of argument-types • Type 1: Arguments from a fixed resource flow (food, water, physical space, renewable energy…) – Usually average-utilitarian in character • Type 2: From long-run considerations – Overpopulation now would lead to lower quantity/quality of life in the long run • Climate-change case: climate change could lead to a loss of “5% of global GDP each year, now and forever” (Stern Review, 2007) • “Environmental degradation, including climate change and resource depletion, is steadily reducing the number of people the Earth can indefinitely sustain.” (“Population policy and the environment” position statement, Population Matters, January 2014) – Somewhat dependent on population axiology, but could convince (even) the total utilitarian... • Type 3: From an exogenous cap – Instantaneous population is going to reduce anyway; it’s just a matter of choosing the nice or the nasty way for this to happen – Virtually independent of population axiology Total utilitarians care about the long run Scenario 1: Higher population now, lower quantity and quality of life in future Scenario 2: Lower population now, higher quantity and quality of life in future Quality of life Quality of life Time Time Population size Population size Climate change and overpopulation: The rhetoric • “At one level, this is all about basic mathematics. We roughly know the total volume of greenhouse gases we can put into the atmosphere over the next few decades if we are to stay the right side of the two-degree-centigrade increase (by the end of the century) which scientists tell us we absolutely mustn’t go above. That total volume has to be divided up between the total number of people doing the emitting… I believe it’s part of our duty to the next generation… to advance the compassionate, progressive case for a full-on global campaign to put the world on a downward population trajectory just as fast as we can.” (Jonathan Porritt, “Population and climate change”, New internationalist, NI 430, Jan-Feb 2010) Two interpretations of Porritt’s argument • “In order to avert dangerous climate change, we need to stay within total emission rate R… • “Type-1” interpretation: “… and this generates a cap on average per-capita emissions of R/n. Individual instantaneous well-being is increasing in per-capita emissions. Therefore, the lower the instantaneous population, the better.” • “Type 2” (long-run) interpretation: “… But our chances of succeeding in reducing average per-capita emissions by an amount a are a decreasing function of a. Therefore, our chance of averting dangerous climate change is a decreasing function of instantaneous population size. Therefore, the lower the instantaneous population the better.” Getting the science right: Cumulative emissions • Key fact: The peak amount of warming (over pre-industrial temperatures) depends on cumulative carbon emissions alone, not the timing of those emissions. Who cares about the rate, then? • Given the relationship between peak warming and cumulative emissions, it’s not immediately clear(?) why one would care about merely slowing down emissions, if the slow-down occurred in such a way as to hold cumulative emissions fixed. •‘Green’ scenario: reduce instantaneous population; hold fixed, for each n, the carbon emissions of the nth person. •Two questions: • Would such a ‘mere slowdown’ be any advantage? • Would the result of population reduction be this ‘mere slowdown’? Some bad reasons to favour slower climate change: (Alleged) intrinsic importance of time • “Time integral of the instantaneous average” utilitarianism: more time for “humans” to enjoy the pre-change, more benign, climates (w=average instantaneous well-being) • Discounting future well-being: suffering in the further future matters less than near-future suffering (P=instantaneous population size; = discount rate) A better reason: The importance of adaptation • Let T(t) = number of degrees warming at time t • Thanks to the importance of adaptation, some climate change damages are (if other things are equal) more strongly linked to dT/dt than they are to T itself. – Examples: damages due to extreme weather events, sea level rise, climatic zone shift… • Hence, even a mere slowdown in climate change might reduce climate change damages. • However: reducing instantaneous population size is likely also to slow down many processes of adaptation (fewer people working in R&D, etc.). – Exceptions include: adaptation by other species… Might a reduction in instantaneous population reduce even cumulative emissions? • By allowing “more time” for the R&D required to “get society off carbon”?? – But again: It’s unclear (at best) whether population reduction would yield “more time for R&D” in the relevant sense. • By allowing higher average per capita consumption of renewable energy… – …thereby (for a fixed level of technological knowledge and capital and per capita energy consumption) decreasing per capita emissions – …and thence (holding fixed the first, the second, … the nbillionth… person’s energy consumption) decreasing cumulative emissions. Summary and conclusions • Many arguments for population reduction require controversial population-axiological assumptions. • “Long-run” arguments do not, and are therefore among the stronger arguments in the population-reduction-campaigner’s arsenal. • There may be a good long-run argument, for population reduction, from considerations of climate change. • The details (however) are fairly intricate – this is not just “basic mathematics”. • The most promising versions of the argument are those that emphasise (i) a fixed supply of renewable energy per unit time (so that lower instantaneous populations are likely to translate into lower percapita carbon emissions), and/or (ii) adaptation effects that do not rely on further technological progress. Appendix Example of a Type 1 populationreduction argument • “A diet that includes 40 grams of animal protein per day is probably optimal. With this consumption, animal protein would provide about half the total protein intake; the average daily food supply per capita would be 3000 kilocalories, of which approx. 650 would be of animal origin. If this criterion is adopted, the world must be considered as overpopulated, as the global average animal protein consumption is 29 grams per capita per day.” (Bernard Gilland (economist), in a paper linked to the World Overpopulation Awareness website) Example of a “Type 2” populationreduction argument • “It is a fact that current growth (10,000 more people per hour) will stop one day, simply because a finite planet cannot sustain an infinite number of people. But it can only stop in one of two ways: either sooner, the humane way, by fewer births - family planning backed by policy to make it available and encourage people to use it; or later, the 'natural' way, by more deaths famine, disease and predation/war.” (“The ethical implications of population growth”, position statement issued by Population Matters, May 2011) Clear case of a “dT/dt” adaptation effect: Species extinction • It’s certainly the case that there would be fewer extinctions of nonhuman species if climate change were slowed down • But the speciesextinction effect is unlikely to be a very significant fraction of total climate change damages.