WMOD00435

• Title: Limits on Global Warming
• Date: 5/9/1991
• Weather Modification - Doc Type: Report

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Table I. - Distribution of the earth's carbon among principal reservoirs.'

Reservoir

Mass stored
(gigatons)**

Mass stored
(Relative to
atmosphere)

All biota
Atmosphere as CO2
Dead surficial carbon
Recoverable fossil fuels
Ocean (as CO2, H2C03, bicarbonate
and carbonate ions)
Sedimentary organic matter
Calcium-magnesium carbonate
(mostly in sedimentary rocks)
Calcium carbonate (mostly in
sedimentary rocks)

560
720
3,000
4,000
42 , 000

0.8
1.0
4
6
58

15 ,000 ,000
25,000,000

35 ,000,000

21,000
35 , 000

49,000

* Based on Berner and Lasaga (1989).
** I gigaton (Gt) = 109 metric tons = 1015 g = 1 petagram (Pg). Dr. Arnold Court has
pointed out to me that 1 Gt is the mass of 1 kIn3 of water, a fact that helps one to visualize
the quantities listed in this table.

2.2 Short-term Carbon Cycles

The observed increase in atmospheric CO2 over the
past few decades has resulted from a small net difference
between opposing processes (Houghton and Woodwell
1989). Large transfers of carbon relative to the
atmospheric reservoir take place each year (fig. 1). Every
year some 100 Gt is removed from the atmosphere by
photosynthesis and sequestered as plant tissue, mostly
wood, while a like amount is replaced by respiration by
plants and animals and the decay of organic matter in the
soil. Every year just over 100 Gt of carbon is removed as
atmospheric CO2 dissolves in the ocean, and a slightly
smaller amount diffuses from the ocean back into the
atmosphere. The relatively small annual contributions of
carbon to the atmosphere from burning of fossil fuels (5
Gt) and from deforestation (2 Gt) are important because
there are no known offsetting effects. However, the
amount of carbon in the atmosphere is going up by "only"
3 to 3.5 Gt per year and the net absorption into the ocean
is roughly 2 Gt annually, which leaves 2 Gt (some
authorities say 4 Gt) per year unaccounted for.

l

The large annual exchanges between the ocean and
the atmosphere indicated in figure I arise mainly from the
fact that the equilibrium concentration of CO2 at the ocean
surface varies from place to place, principally because the
ability of water to hold a gas in solution decreases with
increasing temperature. Because atmospheric
concentrations are quite uniform, the general tendency is
for CO2 to be given off by surface seawater that is being
warmed and absorbed by water that is cooling. For
example, degassing is likely along the equator west of
South America, as cold upwelling water from the coast

:so 100

:so 2

,

1

CD RESPIRATION BY TREES AND PLANTS
@ PHOTosmTHESIS
<J) OCCAYlNG ORGANIC MAT7FR
~ OEroRE'STATlON
@ FOSSIL FUEL CONSUMPTION
@ OISSOLVED AT SEA SURFACE
<V OI./TGASSING FROIrI S~ SURFACE
@ HE:T GAIN BY oc,F;NoIS

102
6

100

7

-T-
2

Figure 1. - Short-term carbon transfers (gigatons per
year) among the atmosphere, ocean and the
earth's crust (adapted from Houghton and
Wood well, 1989).

107