An introduction to climate change - Climate Change: Biological and Human Aspects - page 31

Time for a quick reality check. First, as already noted, the global climate is an
average made up of regional and annual variations. With a changing global climate
not only will some climatic components change in some places, and at some times
more than others, but it is possible that in some places (and/or times) there may
be a change in the direction of the overall trend. (This has been seized upon by
some, especially in the 1990s, to argue that global warming is not taking place. Such
arguments are based on selective and atypical data, with the fallacious claim that
the data are typical.) Second, there are other factors operating that are not related to
greenhouse gases. Temperature is not the sole mechanism behind evaporation: direct
sunlight (electromagnetic radiation of appropriate frequency) also plays a part. A
photon of sunlight can excite a water molecule, causing it to leave its liquid-state
companions and become vapour. As we shall see later in this chapter changes in the
amount of sunlight reaching the Earth's surface have been happening and have resulted
in so-called global dimming due to pollution particles. Another factor affecting water
becoming water vapour is biology.
The routes from liquid water back to water vapour are not restricted to straight-
forward evaporation but also include plant transpiration, as part of photosynthesis
in terrestrial plants. For this reason evapotranspiration (the total water loss from an
area through evaporation and vegetation transpiration) is important. Now let us return
from theory to reality.
A warmer world due to increased greenhouse gases will, among other things,
affect plant physiology. Plants exchange gas and water with the atmosphere through
openings on the surface of leaves called stomata. Stomata open and close to reg-
ulate photosynthesis in the short term. (In the longer term stomatal densities have
been shown to vary on more geological timescales with atmospheric carbon dioxide
concentrations; see Chapter 2.) In a warmer, more carbon dioxide-rich world with
higher rainfall that serves to enhance photosynthesis, all other things being equal,
we might expect plant homeostatic processes (physiological mechanisms acting to
keep functions stable) to dampen photosynthesis. If this were happening then we
would reasonably expect plant transpiration to decrease. This in turn would lower a
plant-covered water catchment's evapotranspiration and so more water would remain
in the ground to percolate through to streams. River flow would increase. So much
for theoretical expectations. The question then becomes how do the various factors
of changed precipitation, warmth, plant physiology and river flow interact?
It is possible to model the individual processes. We do have climate records of
temperature and precipitation covering many decades and measurements of solar
radiation reaching the Earth's surface, as well as those of river flow. We also know
about plant physiology and so can apply broad parameters to plant physiology over a
region, and include factors such as deforestation and land use. In short, we know both
actual river flow for principal catchments on each of the continents and, broadly, how
the various factors that contribute to river flow have changed over the last century.
A model of 20th-century continental water run-off has been constructed that reflects
actual river-flow measurements. It is then possible to examine the model conducting
a 'sensitivity analysis' (or 'optimal fingerprinting' or 'detection and attribution'; the
nomenclature varies with research group) to vary just one factor at a time and to see,
using statistics, how this causes the predicted run-off to differ from reality. It is known