An introduction to climate change - Climate Change: Biological and Human Aspects - page 20

This has led to speculation that it may be possible to use oceanic iron fertilisation to
sequester atmospheric carbon. However, modifying the base of some of the planet's
major ecosystems, such as in this way, may well carry with it unacceptable ecological
risks. Furthermore, it is one thing for winds to carry a global load of minerals to
fertilise the oceans and quite another for humans to do so. Indeed, it appears that
the energy required to distribute the iron over the ocean surface would roughly
equate in fossil fuel terms with the carbon assimilated. Even so, it does appear
that in the past natural changes in the carbon cycle, almost certainly involving the
marine component, have had a major effect on the global climate (Coale et al.,
1996).
1.4 Naturalchangesinthecarboncycle
We know that atmospheric carbon dioxide plays a major role in contributing to the
natural greenhouse effect and we also know that this natural greenhouse effect has
varied in strength in the past. Perhaps the most pronounced evidence comes from
Antarctic ice cores. Snow falls in Antarctica to form ice and in the process tiny
bubbles of air from the atmosphere become trapped and sealed within it. As more
snow falls, more ice with bubbles builds up. By drilling a core into the ice it is possible
to retrieve atmospheric samples of times past. Indeed, cores at one spot, Vostok
in eastern Antarctica, have provided an atmospheric record going back well over
100 000 years. This is a long enough time to cover the last glacial-interglacial cycle
(and more; a glacial is the cooler part of an ice age, compared with the warmer
interglacial, such as the one we are presently in; see Chapter 3). The ice at Vostok is
well over 2 km thick and the cores retrieved between the mid-1980s and the present day
have shown clearly that concentrations of carbon dioxide and methane were far lower
during the cool glacials than they were during the warmer interglacials. Using the
ice water's deuterium ( 2 H) concentration these palaeoconcentrations can be directly
compared to the estimated difference in temperature between the oceans from which
water at that time evaporated and when it fell as snow. This is because it takes more
energy (heat) to evaporate water containing the heavier deuterium isotope of hydrogen
( 2 H) than water containing the common isotope of hydrogen ( 1 H). Therefore, a plot
of the ice core's deuterium concentration gives an indication of regional temperature.
Such temperature changes, it can be seen, closely correlate with carbon dioxide
and methane concentrations (see Figure 1.6). Such ice-core evidence suggests that
atmospheric carbon - carbon dioxide or methane - really is linked to climate. Because
we know from laboratory analysis that these gases are greenhouse gases (absorbing
long-wave infrared radiation), we can deduce that this is the mechanism linking them
to climate. Equally importantly, because we know that the atmospheric concentrations
of both these gases are affected, if not determined, by the carbon cycle, we have a
direct link between the carbon cycle and climate. As one of the carbon cycle's
key drivers is photosynthesis, we can see that life is clearly linked to the global
climate.