An introduction to climate change - Climate Change: Biological and Human Aspects - page 10

(early 21st century) global climate computer models to reproduce the likely effect of
water vapour over a period of warming was given credence in 2005 by a US team
of atmospheric scientists led by Brian Soden. They compared satellite observations
between 1982 and 2004 at the 6.3 m wavelength - which is part of water's absorption

spectrum and especially useful for measuring its presence in the upper troposphere -
with climate models. The satellite measurements and the models showed a good
correlation.
Clouds (the suspension of fine water droplets in regions of saturated air) complic-
ate the picture further still. Being reflective, they tend to cool the surface during the
day and at night act as an effective greenhouse blanket. However, there are various
types of cloud. The picture is complex and our understanding incomplete, hence
climate models provide only an approximation of what is going on, but they are
revealing approximations nonetheless. However, it has long been thought that bio-
logical processes affect cloud formation by releasing volatile organic compounds
such as monoterpenes (C 10 H 16 compounds such as pinenes, which give pine forests
their smell) and sequesterpenes that in turn determine particle formation around
which droplets can form, hence clouds. Clouds above a large expanse of boreal
forest can affect the local temperature by as much as 5-6 ◦ C. Nonetheless it is still
early days in developing a clear picture. Indeed, one puzzling feature has been a
mismatch between the amount of volatile organic compounds produced by plants
and that found in the air above some boreal forests. In 2009 a German-Finnish
team led by Astrid Kiendler-Scharr demonstrated, using a plant chamber contain-
ing birch ( Betula pendula ), beech ( Fagus sylvatica ), spruce ( Picea abies ) and pine
( Pinus sylvestris ), that isoprene was also released and that some species are high
isoprene emitters. One of these species was oak ( Quercus robur ), which the team
included in their study. They found that isoprene, with its hydroxyl radical (OH • ),
inhibits particle formation and so counters the release by other plants of volatile
organic compounds which have cloud-forming action. In short, the effect that a
forest will have on cloud formation (and hence temperature and water cycling) will
depend on the forest's species mix. This work is all the more relevant from a climate
change perspective (let alone an ecological management climate perspective) in that
the amount of isoprene that vegetation produces is affected by light and temperat-
ure. Such studies are now illuminating the construction of global climate computer
models, which in the 21st century increasingly have a biological component. One
problem is that that there are literally many tens of thousands of organic volatiles pro-
duced by plants and teasing out which ones are important from a climate perspective
will take time. (We will return to climate change and the water cycle later in this
chapter.)
Given that overall the Earth's atmosphere (with its mix of various greenhouse
gases) broadly confers a 43 ◦ C greenhouse warming effect (because, as we have seen,
the airless Moon is cooler), the question remains as to how much warming has
been conferred since the Industrial Revolution as a result of the human addition of
greenhouse gases. We shall come to this in Chapter 5. Nonetheless, it is worth noting
for now that mathematicians Cynthia Kuo and colleagues from the Bell Laboratory
in New Jersey, USA, statistically compared instrumentally determined changes in
atmospheric CO 2 concentrations between 1958 and 1989 and global temperature

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