21(i.e., uncharged). Hence^12 C,^13 C, and^14 C are all considered to be different forms of
carbon because they all contain six electrons. Most importantly, biological properties
of a compound are mass dependent: our bodies prefer^12 C over the other two heavier
forms of carbon that are digested, because lighter molecules diffuse more readily
through our capillaries. The term isotopic composition, as used here, refers to the rela-
tive abundance of^13 C in a sample of atmospheric CO 2 compared to the sum of^12 C,^13 C,
and^14 C in the same sample, and is expressed using the notation δ^13 C.
Figure 1.7c shows a time series of δ^13 C recorded at MLO (Keeling et al. 2005 ).
The downward decline of δ^13 C means atmospheric CO 2 is getting isotopically lighter
over time. In other words, at the start of the time series in 1980, the relative propor-
tion of^13 C to^12 C in atmospheric CO 2 at Hawaii was larger than today. This serves
as our final fingerprint because the carbon content of fossil fuels, which formed
from the decomposition of plants on geologic time-scales, are isotopically light
relative to contemporary atmospheric CO 2 (Whiticar 1996 ). If rising levels of atmo-
spheric CO 2 during the time period shown in Fig. 1.7c had been due primarily to
volcanoes, atmospheric CO 2 would have been expected to have gotten isotopically
heavier (Rizzo et al. 2014 ), which is the opposite of what has been observed.
It is stated in Sect. 1.1 that over geologic time scales, atmospheric CO 2 is controlled
by volcanic activity and deep sea vents. Yet CO 2 shows no volcanic influence over the
time of the modern instrument record. To further illustrate the lack of recent volcanic
influence, the orange triangles in Fig. 1.6b have been placed at the time of eruption of
Mount Agung, El Chichón, and Mount Pinatubo, the three largest eruptions over the
past six decades. The growth of atmospheric CO 2 during the years of these eruptions
(1963, 1982, and 1991) is unremarkable compared to other years: in fact, the growth of
51015202530354045
CO 2 FF+LU (Gt yr^1 )10123(^4) [CO
2
MLO CO
2
SPO]=0.142×CO
2
FF+LU 1.65
R^2 = 0.95
[CO
ML 2
O CO
SPO 2
] (ppm)
Fig. 1.8 Human fingerprint on hemispheric gradient of CO 2. Difference in annual average CO 2 at
Mauna Loa Observatory (MLO) (Keeling et al. 1976 ) minus CO 2 at the South Pole Observatory
(SPO) (Tans et al. 1990 ) (vertical axis) versus annual total emission of CO 2 to the atmosphere from
the combustion of fossil fuels (Boden et al. 2013 ) and land use change (Houghton et al. 2012 ). Data
span the time period 1959–2015. Numerical results of a linear least squares fit as well as the cor-
relation coefficient are also given. See Methods for further information
1.2 The Anthropocene