Energy and Human Ambitions on a Finite Planet, 2021a

9 Climate Change 156
1880, sea level has risen about 230 mm At the current rate, we would
expect a comparable addition by 2100 for a total of 0.5 m.
But the current rate is not likely to be the right measure, since warming
air temperatures result in a faster rate of ice melting. Positive feedbacks
also accelerate ice melt. For instance, melted pools of water on top of
the ice are darker than ice, increasing the rate at which solar energy is
absorbed.
We can get a quick h<strong>and</strong>le on how much sea level rise might possibly be
in store, based on the fact that the vast majority of “permanent” ice on
the planet is in Antarctica <strong>and</strong> Greenl<strong>and</strong>. These two ice sheets constitute
2.7% <strong>and</strong> 0.3% of the globe’s surface area, respectively. From there, it is
easy to estimate sea level rise, because the ocean (71% of the globe) has
an area 26 times bigger than the Antarctic ice sheet <strong>and</strong> 210 times larger
than the Greenl<strong>and</strong> ice. What this means is that it takes 26 meters of ice
melting from Antarctica 64 to raise sea level by 1 meter, <strong>and</strong> 210 meters
off Greenl<strong>and</strong> to do the same. 65 See Figure 9.17 to underst<strong>and</strong> the logic
here.
64: ...ifuniformly distributed across the
continent
65: In the spirit of an approximate estimate,
we ignore the 10% difference in the density
of ice vs. water <strong>and</strong> assume that one cubic
meter of ice displaces one cubic meter of
water.
Figure 9.17: If sea area is x times that of an
ice-covered isl<strong>and</strong>, water level will rise by
1/x times the ice thickness if it all melts. The
diagram shows a way to think about this,
for x 3: by slicing the ice into x 3 layers
to redistribute the volume on top of the
water. In this case, a 30 m ice sheet would
raise sea level by 10 m if melted.
Now if we just knew the average thickness of each ice sheet, we could
figure out how much sea level would rise if all the ice melted. The
Greenl<strong>and</strong> ice sheet is estimated to be 2.85 million cubic kilometers,
translating to an average depth of 1.7 km. 66 210 m goes into this 8 times,
so we might expect something like 8 m of sea level rise if all the ice
in Greenl<strong>and</strong> melts. For Antarctica, the 26.5 million cubic kilometers
corresponds to an average ice thickness of 1.9 km, which is about 70 of
our 26 meter units, so we would expect about 70 meters of sea level rise
in the extreme case of Antarctica losing all of its ice.
The preceding paragraph <strong>and</strong>
Table 9.6 have enough information
(absolute <strong>and</strong> relative areas <strong>and</strong>
volumes) to figure out.
66: Divide volume by area.
Box 9.3: A more rigorous estimate. . .
It is relatively easy to find references estimating sea level rise potential
from complete melting of Greenl<strong>and</strong> <strong>and</strong> Antarctica. One such [60]
arrives at a 7.4 m rise from Greenl<strong>and</strong> melting, 58 m from Antarctica,
<strong>and</strong> 0.3 m from glaciers.
So why did we go through the crude estimation process? The goal
was to remove the mystery. 67 Once we have an estimate of the depth
<strong>and</strong> a ratio of surface areas, it is within our grasp to estimate the rise
ourselves.
[60]: Davies (2020), Calculating glacier ice
volumes <strong>and</strong> sea level equivalents
67: One goal of this book is to empower
students to independently check more authoritative
sources—much like 2+2 4 can
be personally verified <strong>and</strong> is not a matter of
faith.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.