the wood faster than they regenerate. The annual net terrestrial storage of carbon only
applies to the global average.
(^) Another fate of some fossil-fuel CO 2 may be chemical conversion to soluble form
by weathering of silicate minerals. As CO 2 levels rise, acidity of rain and soil water
increases by formation of carbonic acid, H 2 CO 3 . This can attack a wide array of
silicates, for example wollastonite, CaSiO 3 :
(^) The products are carried to the sea, incorporated in shells, mostly of foraminifera
and diatoms, then deposited in sediments. Chemical weathering is believed to have
been very important in limiting and sometimes reducing CO 2 levels in the atmosphere
over long geological time scales. For example, a reduction is believed to have
occurred over the past 12 million years (Myr), because the uplift of the Himalayas,
which started 12 Mya ago, exposed great expanses of new rock surface to weathering
at a high rate. The importance of enhanced chemical weathering due to current
anthropogenic CO 2 increase has been little addressed. Detecting the increase in calcite
deposition resulting from gradually progressing CO 2 levels would be difficult, since it
could happen anywhere in the global ocean (as coral-reef build-up, foraminiferan
shell deposition or inorganic oolites in tropical waters) and only need remove about 1
GtC yr−1 to be important. Moreover, the pH lowering in the sea resulting from
increased carbonic acid (from fossil-fuel CO 2 in the atmosphere) acts to reduce calcite
deposition rates in reefs and coccolithophores (Riebesell et al. 2000). Sorting out the
magnitudes and interactions of constituent processes in global warming is a
considerable challenge, but one being widely addressed.
(^) It is certain that much of the “lost” fossil-fuel carbon ends up in the ocean, but it is
less certain exactly what processes mediate its dissolution. According to marine
chemists, the general answer is fairly simple: the transfers that matter are atmosphere–
ocean exchanges involving CO 2 solubility. The sequestration occurring as we add
fossil fuel CO 2 to the atmosphere is essentially a non-equilibrium process. A map of
CO 2 invasion and evasion rates (Plate 16.1), based on station data for CO 2 contents
(air and water) and exchange rates, shows that much of the influx is in areas of deep-
water formation: (i) around the Antarctic, where sinking is driven by chilling coupled
to salt release during ice formation, and (ii) the Norwegian Sea and/or the Irminger
Sea, where sinking is driven by arctic refrigeration of salty Gulf Stream water. In
these areas, the new levels of CO 2 (at the fossil-fuel enhanced pCO 2 of the
atmosphere) equilibrate with the cold, polar water that carries an increased
concentration to depth as it sinks. Evasion sites, most dramatically the eastern tropical
Pacific, Arabian Sea, and deep southwestern basin of the Bering Sea, return the gas to