20
activity or deep sea vents, atmospheric O 2 would be unaffected because the domi-
nant form of outgassed carbon is CO 2 , with the O drawn from abundant oxygen in
Earth’s crust. Measurement of atmospheric O 2 with sufficient precision to quantify
the minute, putative decline was an instrumental challenge first overcome by Ralph
Keeling (Keeling et al. 1996 ), the son of Charles David Keeling. The slight decline
in atmospheric O 2 recorded in Fig. 1.7b provides strong quantitative evidence that
combustion of fossil fuel is the driving factor behind rising CO 2.
The final human fingerprint involves the isotopic composition of atmospheric CO 2.
The most common form of carbon has an atomic mass of 12 (^12 C), due to the presence
of six protons and six neutrons in the nucleus.^15 Carbon can exist in two other forms:
carbon 13 (^13 C) and carbon 14 (^14 C), due to the presence of either 7 (^13 C) or 8 (^14 C)
neutrons in the nucleus. The chemical properties of a compound are determined by the
number of electrons, which equals the number of protons if a compound is neutral
atmospheric O 2 , whereas the O in CO 2 produced by combustion of coal originates mainly from
atmospheric O 2.
(^15) Atomic mass is the sum of the number of protons and neutrons.
340
360
380
400
South Pole
Mauna Loa, Hawaii 19.82°N
a
600
500
400
300
200
100
Mauna Loa, Hawaii 19.82°N
b
1980 1985 1990 1995 2000 2005 2010 2015
8.6
8.4
8.2
8.0
7.8
7.6
7.4
Mauna Loa, Hawaii 19.82°N
c
CO
(ppm) 2
02
/N
ratio (per meg) 2
(^13) δ
C (per mil)
Fig. 1.7 Human fingerprint on rising CO 2. (a) Time series of CO 2 mixing ratio from Mauna Loa
Observatory (black) (Keeling et al. 1976 ) and the South Pole (Tans et al. 1990 ); (b) ratio of atmo-
spheric O 2 to N 2 measured at the Mauna Loa Observatory in units of per meg, where 1 per
meg = 0.00001 % (Keeling et al. 1996 ); (c) abundance of^13 C in atmospheric CO 2 at Mauna Loa
relative to a standard in units of per mil, where 1 per mil = 0.1 % (Keeling et al. 2005 ). See Methods
for further information
1 Earth’s Climate System