Download Arguing for population reduction

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.