Climate change affects the amount of land usable by humans
in at least three different ways. Land is lost through sea level rise. Land is
lost because it becomes too hot for human use. Land is gained because it
becomes warm enough for human use. Exact calculations of the size of all three
effects, if possible at all, would require much more expertise and effort than
I am bringing to the problem so what I offer are Fermi
estimates, numbers based on very crude approximations. For all three estimates
I will be assuming warming of 3°C above current temperatures and sea level rise
of .6 m above present sea level, roughly what the latest IPCC report projects
for the end of the century under SSP3-7.0.
Land Lost to Sea-level Rise
The amount of land lost equals the length of coastline times
the amount by which it shifts in. For the total length of the world’s coastline
I found a figure of 356,000 km. The
amount by which coastline shifts in with a given amount of sea level rise depends
on the slope of the coastal land. I came across a figure of a hundred feet of
shift for every foot of sea level rise in a book discussing the situation on
the U.S. Atlantic coast; since I do not have figures for every coast in the
world, I will use that.
60m coastline shift x 356,000 km of coastline = 21,436 km2
That is my very approximate estimate of land lost to sea
level rise.
Land Lost to Rising Temperature
How much does temperature rise in hot parts of the world
with 3° more of global warming? Figure SPM.5b of the latest IPCC report
shows a map of projected average temperature change due to a 4° increase relative
to 1850-1900 in average global temperature, roughly 3° relative to current
temperature. Parts of the Earth that are both hot and densely populated appear
to warm by a little less than the global average. Table 11.SM.2 shows the effect of different
levels of global warming on maximum temperatures. It looks from that as though
3° of global warming would raise the maximum temperature of the relevant
regions
by about 3°. So if we knew at what temperature, average or maximum, the Earth’s
surface becomes too hot for human habitation, we could conclude that any area
currently within three degrees of that would, with our assumed level of global
warming, become too hot for humans.
The simplest approach to doing this is to compare a map of
global temperature (Figure1
) to a map of population density (Figure 2) and see
at what temperature population density goes to close to zero. Comparing the two
maps we observe that while the coldest areas of the globe are essentially
empty, the hottest are not; some, such as the Philippines, Senegal, and
Malaysia, are densely populated. If there is a temperature at which the Earth’s
surface becomes unliveable, these maps do not show it. Our estimate of the
amount of land lost by the direct effect of heating, calculated in this way, is
zero.
We may be able to do a little better by looking at data on
cities. The hottest city, by average temperature, is Assab, Eritrea, at 30.5°C,
with several others nearly that warm. Hence we can conclude that any city whose
average temperature after climate change is less than 30.5° will not be
unliveably hot while cities whose temperature is higher than that might be.
There are 28
cities with an average temperature of 28°C or more. Their a combined
population is about 33 million, which is roughly .7% of the urban population of
the world. If we use urban population ratio as a very rough proxy for total
population ratio and that as a very rough proxy for land ratio and calculate.7%
of the non-arctic land area of Earth, we get
149 million km2 (Land area) – 5.5 million
(Antarctica) - .8 million (Greenland) = 143 million km2
143 million km2 x.007 = 1 million km2
That gives us a very approximate upper bound for the amount
of land that becomes unlivable due to global temperature increasing by three
degrees. It is only an upper bound because we do not know that a city would be
unliveable at an average temperature of 31°, only that there are no cities that
hot.
Both of these calculations are based on average temperature.
Arguably what habitability depends on is be maximum temperature. If it gets
unendurably hot during a summer day, the fact that winter nights are cold is
little compensation.
Figure
3 is the equivalent of Figure 1 for maximum temperatures. The highest
temperature regions it shows include densely populated parts of India as well
as more sparsely populated parts of Africa and Arabia. Insofar as one can tell
from that map, there are no places large enough to show on the map where
maximum temperatures are too high for human habitation. It is possible that
some would be that hot after an additional three degrees of warning but the
combined evidence of Figures 2 and 3 suggests not, since some of the hottest
regions are densely populated.
I have been defining usable land as land humans can live on.
While there are parts of Earth that seem crowded, average land per person is
about five acres, so human populations are not limited by the amount of space
to put them in. They might, however, be limited by not enough land to feed
them, so it might make more sense to define usable land as land suitable for
growing crops.
Is there any significant amount of land that is too hot to
grow crops? So far as I can tell, there is not. Maps showing yield of various
crops can be found online;
some regions with high average and maximum temperatures show substantial
yields. The yields shown are averaged over countries, but a map
of agriculture in India shows crops being grown across areas within India
of both high average and high maximum temperature.
My conclusion from these calculations is that there is
probably no substantial amount of land area that will become either
uninhabitable or unable to grow crops solely because of temperature with global
warming of 3°C.
This does not mean that there is no area that will become
either uninhabitable or unable to grow crops as a result of global warming,
only that there is no area where it will happen solely because of temperature.
Looking at Figure 2, one observes a wide region of northern Africa with almost
nobody living there — the Sahara. That area is less hot than some populated
regions, so temperature is not the entire reason it is empty, but it can be,
almost surely is, part of the reason, so increased temperature might expand it.
On the other hand, the latest IPCC report suggests the
possibility that climate change might have the opposite effect:
Some climate model simulations suggest that under future
high-emissions scenarios, CO2 radiative forcing causes rapid greening in the Sahel and
Sahara regions via precipitation change (Claussen et al., 2003; Drijfhout et al., 2015). For example, in
the BNU-ESM RCP8.5 simulation, the change is abrupt with the percentage of bare soil dropping from 45%
to 15%, and percentage of tree cover rising from 50% to 75%, within 10 years (2050-2060) (Drijfhout et
al., 2015). However, other modelling results suggest that this may be a short-lived response to CO2 fertilization
(Bathiany et al., 2014).
In summary, given outstanding uncertainties in how well the
current generation of climate models capture land-surface
feedbacks in the Sahel and Sahara, there is low
confidence that an abrupt change to a greener state will occur in these regions before 2100 or 2300.
Figuring out all consequences of climate change for the
amount of land available for human use is a much more complicated problem than
I am trying to solve.
Land Gained Due to Rising Temperature
Human land use at present is limited by cold, not heat, as
shown on Figure 2 above — the equator is populated, the polar regions are not.
It follows that global warming, by shifting temperature contours towards the
poles, should increase the amount of land warm enough for human habitation. Making
Antarctica habitable would require a lot more than three degrees of global warming
and the southernmost land masses north of it are already inhabited, so any land
gains from warming will be in the northern hemisphere.
Figure 11.SM.1 of the sixth IPCC report shows minimum temperature
of areas such as North America and Northern Asa going up by between 2 and 3.4
degrees per degree of global warming. Since warming is greater in colder
climates, I take 3 degrees per degree as a reasonable guess for the increase in
temperature in the northern part of those zones. It follows that three degrees
of global warming will increase the temperature in the colder parts of those
zones by about nine degrees. To estimate how much land will shift from not
quite habitable to at least barely habitable we need two numbers — what length
of the contour dividing barely habitable from not quite habitable is over land
and how far a nine degree increase in temperature will shift it.
It seems likely that habitability depends more on minimal
temperature than on average temperature. Figure 4 shows temperatures in
January, which should be close to the minimum, with contours every five degrees
— much more precise information than Figure 1 provides for average temperatures.
Combining the temperature information on Figure 4 with the population density information
on Figure 2, the border of habitability appears to be at about -15°C. Nine
degrees of warming will raise the January temperature of land currently at -24°
to -15°, so shift the land between those two contours from not quite habitable
to barely habitable. I estimate the distance between the -15° and -25° contours
to average about 800 km, making the distance between -15° and -24° about 720 km,
and the length over land of those contours to total about 15,000 km. Hence the
area between them is about 10,800,000 km2.
This land is being warmed from not quite habitable to at
least barely habitable, from a population density of less than two per square
km to a population density of more than two but in some areas less than ten. At
the same time, the land a little farther south is being warmed from barely
habitable to more than barely habitable, and the land south of that … . Combining those effects, 10.5 million
square km is a rough estimate of the increase in fully usable land.
The analysis so far has used population density as the
measure of habitability. As I suggested earlier, it may make more sense to use
the ability to grow crops. Crop production maps for Canada and Russia show
crops growing in about the same areas that appear habitable by population
density, so I have not tried to redo the calculation on that basis.
Conclusion
On the basis of these calculations, I find, for the effect
of climate change by the end of the century under SSP3-7.0:
Loss of usable land by flooding due to sea level rise:
21,436 km2
Loss of usable land due to the direct effect of warming: Probably
close to zero, with one calculation giving an upper bound of one million km2.
Increase of usable land due to the direct effect of warming:
10.8 million km2.
All of these numbers are very approximate but they imply a
large net increase, due to climate change, in the amount of land usable by
humans — more than twice the area of the United States. They also imply that nearly
five hundred times as much land is gained through warming as is lost through
sea level rise, which makes it odd that only the latter is commonly included in
discussions of the effects of climate change.
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This is a draft of a chapter for a book I am working on. I am looking for two sorts of comments:
1. Easy ways of doing my calculations better. There are obviously ways I could make my results more accurate by more complicated calculations but since I don't really care if the real number is twice mine or half it, that isn't worth doing. On the other hand, if there are ways just as easy but smarter, giving a more reliable result, I am interested.
2. Major mistakes. My conclusions are pretty dramatic and I want to know if they are, for some reason, wildly wrong.