In the carbon dioxide molecule, each oxygen atom is
doubly bonded to a carbon atom. The carbon atom
allocates two valence electrons to an sp l -hybrid , whose
lobes stick out in opposite directions, and the σ-bonds
they form give the molecule a linear O-C-O shape.
The two carbon 2p orbitals not involved in the hybrid
form π -bonds with the oxygen atoms on either side.
CO 2 is an acidic oxide (Figure 9.4). On dissolving in
water it forms a slightly acidic solution (Chapter 4).
Rain water in equilibrium with atmospheric CO 2 has a
pH of 5.7 (see Exercise 4.3), which partly accounts for
its effectiveness as a weathering agent. In the northern
hemisphere, however, this acidity has been reinforced
by oxides of sulfur and nitrogen introduced into the
atmosphere by the burning of fossil fuels.
CO 2 is believed to have been a major constituent of
the Earth's primordial atmosphere, as it is for Venus
today (CO 2 accounts for 96% of the Venusian atmos-
phere). Throughout most of the Earth's history, how-
ever, photosynthesis has 'drawn down' carbon from
the atmosphere into the biosphere. Photosynthesis is a
reaction by which plants and algae use chlorophyll
(Box 9.5) to generate the reduced carbon they need from
atmospheric CO 2 , or from CO 2 dissolved in the oceans:
Atmospheric CO 2 at Mauna Loa Observatory
Scripps Institution of Oceanography
NOAA Earth System Research Laboratory
Figure 9.2 Annual (thin line) and seasonally adjusted
(heavy line) variation in atmospheric CO 2 content measured
at the Mauna Loa observatory in Hawai'i (chosen for its
remoteness from centres of industrialization) from March
1958 to January 2014, as depicted on the Earth Systems
Research Laboratory website of the US National
Oceanographic and Atmospheric Administration (NOAA). 5
(Source: US National Oceanographic and Atmospheric
'greenhouse' gas (Box 9.7). For the past two centuries
the atmospheric concentration of CO 2 has been rising
due to fossil-fuel burning and deforestation, and it
exceeded 400 ppmv for the first time in 2013 (Figure 9.2),
higher than at any time in the last 4.5 Ma. This rise in
atmospheric CO 2 is the main cause of global warming.
The hemispheric balance between combustion and
photosynthesis shifts with the seasons, to the left in
winter and to the right in summer (Figure 9.2). Current
estimates suggest that the anthropogenic greenhouse
effect (Box 9.7) is warming the Earth by 0.2 °C per dec-
ade, which will have serious climatic and social cons-
equences if humanity fails to stem the rise very soon.
Carbon also forms a monoxide (CO) which occurs in
volcanic gases and in the atmosphere, but at much
lower concentrations than CO 2 .
xO 2 (9.3)
Such oxygenic photosynthesis - releasing the oxygen we
rely on today - has transformed the Earth's atmosphere
through the course of geological time, as described in
Chapters 10 and 11. It is the most complex, and most
recently evolved, form of photosynthesis, which relies
on water to release the electrons needed to reduce the
carbon in CO 2 . Earlier anoxygenic forms of photosynth-
esis relied on H 2 S or Fe as electron donors instead.
Aquatic biota (zooplankton and higher organisms)
have also fixed carbon in carbonate shells which have
accumulated as limestone. By progressively sequestering
carbon from the atmosphere into the crust, these processes
have together reduced the CO 2 content of the Earth's
atmosphere today to a few hundred parts per million.
The pre-industrial atmospheric concentration of CO 2
was about 280 ppmv. This represents a biologically
mediated balance between oxygenic photosynthesis
and respiration (which converts O 2 to CO 2 ). CO 2 is a
Carbon consists of two stable isotopes ( 12 C and 13 C) and
a short-lived radioactive isotope 14 C, which are dis-
cussed in Chapter 10.