An introduction to climate change - Climate Change: Biological and Human Aspects - page 17

anhydrase works best on the 16 O isotope of oxygen and discriminates against 18 O.
Isotopic studies are therefore an increasingly important tool in understanding how
the carbon cycle works.
Ascertaining details of the carbon cycle is the subject of considerable ongoing
research. In terms of addressing the problem of increasing atmospheric carbon diox-
ide, the solutions will almost inevitably involve modifying carbon flows between
reservoirs such that the atmosphere's carbon burden will be reduced. We will return
to this towards the end of this section. Before doing so it is important to note that
this ongoing carbon cycle research does not just provide extra detail but still turns up
major surprises and even false surprises.
One such recent (2006) surprise was the possibility that plants in aerobic con-
ditions (with oxygen available) produce methane. Indeed, it was so surprising that
Nature ran a small article in its news section entitled 'How could we have missed
this?' The discovery was almost fortuitous in that it had been thought that all the
principal sources of atmospheric methane had been identified even if their indi-
vidual quantification needed to be refined. To ensure that it was methane from plants
(and not microbes) the researchers attempted to kill off bacteria on the plants with
radiation. They also removed methane from the air in the incubators in which the
plants were to be grown. Although the amounts of methane detected from individual
plants were small, globally it amounts to a significant source. The researchers who
made the discovery could only make a very rough estimate (as work has yet to be
done on an appropriately representative range of species and conditions) but they
thought that the annual atmospheric contribution could be between 60 and 240 mil-
lion t (Keppler et al., 2006), or between one-twelfth and one-third of the annual
amount entering the atmosphere. The result came as a surprise and was import-
ant in understanding the carbon cycle (hence for developing global climate models,
which increasingly include biological components). However, 4 years later Andrew
Rice of Portland State University in Oregon, USA, conducted a study suggesting
that trees act like chimneys, transporting methane produced by soil microbes up
from the roots through to leaves and that this effect could account for as much
as around 10% of methane emissions globally. This itself was a valuable develop-
ment as it could help explain why methane emissions are higher than expected from
wet tropical forest regions. Another advance in carbon cycle understanding came in
2009 by a team led by Eran Hood. They found that dissolved organic matter (rich
in carbon) in glacial run-off from 11 Gulf of Alaska watersheds was higher (and
more labile) than was thought. This was because most of the previous work done
on this aspect of the carbon cycle had been on watersheds where terrestrial plants
and soil sources dominate dissolved organic matter. This new insight suggests that
glacial run-off is an important source of labile reduced carbon to the planet's oceans.
Furthermore, the most glaciated watersheds the team studied were the source of the
oldest ( 4000 years old) and most labile (66% bioavailable) dissolved organic carbon.

All this is very important when considering a warming world in which glaciers are
retreating.
As for the year-on-year growth in atmospheric carbon dioxide since the beginning
of the Industrial Revolution (as stated above) this has been carefully charted. It
has been done so in two main ways. First, in recent times (since the middle of the

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