An
assessment of the likely increase of CO2 in the atmosphere due
to climate change and if the Amazon Rainforest ceases to be a
CO2 sink.
The
C02 content of the atmosphere is usually expressed in parts per
million (ppm) by weight and the use of fossil fuels is expressed
as so many tons of carbon burned per year. At present the burning
of fossil fuels releases 7 billion tons of carbon into the atmosphere
each year in the form of carbon dioxide gas, C02.
C02
weighs 44 / 12 times the weight of carbon. This is derived from
the atomic weights
of carbon, 12, and oxygen, 16. The molecular weight (MW) of C02
is 12 + (2 x 16) = 44 and the MW of carbon is 12. So C02 is 44/12
= 3.67 times heavier than carbon per molecule.
Therefore
burning one billion tons of carbon produces 3.67 billion tons
of C02.
(A)
and so burning 7 billion tons of carbon will produce 26.7 billion
tons of C02
The
weight of the Earth's atmosphere can be calculated as follows.
Atmospheric pressure at sea level is on average 14.5 pounds per
square inch = 10 tons per square metre. This pressure is due to
the weight of atmosphere above an area at sea level of one square
metre.
The
radius of the Earth "r" is 5,925 km and so the surface
area of the Earth (land and ocean) is 4 x "pie" x "r"
squared = 4 x 3.142 x 5925 x 5925 = 441 million square kilometres
= 441,000 billion square metres.
Therefore
the weight of the Earth's atmosphere is 441,000 billion x 10 =
4.41 million billion tons.
Now
26.7 billion (the weight in tons of CO2 emitted into the atmosphere
each year, see (A) above), divided by 4.41 million billion gives
the fraction 6 /one million which means that the CO2 emitted to
the atmosphere each year from burning fossil fuels is equal to
6 parts per million (ppm) of the atmosphere by weight. (6 millionths)
It
can be seen therefore that burning 7 billion tons of carbon from
fossil fuels is now dumping 6 ppm per year of C02 into the atmosphere.
(B)
Prorata, burning one billion tons of carbon from fossil fuels
dumps 6 / 7 = 0.85 ppm of CO2 into the atmosphere.
As
explained in the above Article 1, the Amazon rainforest is probably
absorbing 2 billion tons of carbon per year. Removing this amount
of carbon reduces the C02 content of the atmosphere by 2 x 0.85
= 1.7 ppm. per year.
So
the Amazon rain forest is absorbing 1.7 ppm of the 6 ppm of the
total C02 being emitted by fossil fuel burning.
The
current understanding is that at the present level of concentration
of C02 in the atmosphere C02 is being absorbed by natural processes,
of which the Amazon rainforest is a major component, at the rate
of 3 ppm, i.e. one half of 6 ppm rate at which CO2 is being dumped
into the atmosphere from fossil fuel burning. If C02 emissions
are rising this will mean that year on year the C02 content of
the atmosphere will rise by at least one half the previous year's
rate of emission.
Therefore
at present C02 is increasing by 3 ppm each year (i.e. 6  3 =
3 ppm). If the level of fossil fuel burning rises by say only
25% (a much bigger rise is predicted) and if natural processes
do not increase their rate of absorption then the rate of increase
will become 3 + 25% of 6 = 4.5 ppm per year and if by 2050 we
loose the absorption by the Amazon rainforest the rate of increase
becomes 4.5 + 1.7 = 6.2 ppm per year, twice the current level.
At this rate C02 levels would increase by 50 x 6 = 300 ppm during
the 50 years from 2050 to the end of the century.
The
increase in C02 at today's rate over the 50 years from now to
2050 gives a further increase of 50 x 3 = 150 ppm.
So
the C02 content of the atmosphere by 2050 and 2100 could be as
follows:
Today's
level in say year 2,000
Increase
at today's rate up to 2050 = 50 years x 3 ppm
Increase
from 2050 to 2100 assuming 25% growth
in fossil fuel use and the Amazon rainforest
ceasing to be a C02 sink = 50 years x 6 ppm
Therefore total C02 in the atmosphere at 2100

=
350 ppm
=
150 ppm
=
300 ppm
=
800 ppm

This
total does not include C02 from Amazon rainforest fires, however
no doubt other forests will expand elsewhere in the world as their
conditions become more favourable so release of carbon by forest
fires in the Amazon rainforest will be offset by new trees elsewhere
but there will be a time lag. Also nontropical forests only absorb
CO2 during the spring and summer growing season whereas tropical
forests grow all the year round and tropical forests grow at a
faster rate and so absorb more CO2 than temperate forests.
If
10 years' growth of the Amazon rainforest were released in one
year's fires this would add an additional 10 x 1.7 = 17 ppm C02
into the atmosphere in that year.
If
the Amazon rainforest becomes savannah then 90% of the carbon
currently locked up in biomass would be released. Can we estimate
how much carbon this represents?
Assume
trees at 20 metre spacing, therefore 5 x 5 = 25 trees per hectare.
(100m x 100m)
Assume 10 tons of carbon per tree, therefore 25 x 10 = 250 tons
of carbon per hectare.
1
square km = 100 hectares. Therefore weight of carbon = 25,000
tons / sq. km.
The
total area of the Amazon rainforest = 4,000,000 sq. kms. approx.
Therefore
weight of carbon in trees = 25,000 x 4,000,000 = 100 billion tons
If
90% of this carbon returns to the atmosphere as CO2 this would
increase atmospheric CO2 by 0.9 x 100 x 0.85 (see (B) above) =
76 ppm.
The
increases in atmospheric CO2 levels described above are significant
increases when compared to historic levels (280 ppm in 1850 and
170ppm.in the recent geological past) and also the rate of change
is accelerating. We are entering unknown territory. However we
can project what might happen on the basis of what we do know
and the possibilities are awesome. These possibilities will be
described in future articles to be published soon.
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