An introduction to climate change - Climate Change: Biological and Human Aspects - page 30

some climate change, they are not the dominant factors of current global warming.
Even so, they cannot be easily dismissed because, as we shall see, these factors could
make a serious contribution to climate change in the future. In particular, circulation
changes can help flip climate regimens between semi-stable states.
1.7 Thewatercycle,climatechangeandbiology
Something noted above, and to which we will occasionally return later in this topic, is
that in a warmer world we could reasonably expect more evaporation from the ocean.
Again, as noted, this expectation is at least in part corroborated by computer models
of atmospheric water vapour as well as satellite observations: so, global warming
affects the water cycle. This leads us to another expectation that we would reasonably
anticipate from more evaporation, that there would be more precipitation (rain and
snow) and thus increased river flow. But how reasonable are these expectations?
The Earth's climatic system is complex. Furthermore, biology is not just affected
by a changing climate; biology, as we shall see, plays a key part in affecting the
nature of climate change and in affecting some of the consequences of that change.
Not all these complications are biological, but the complexities need to be taken into
account.
First of all, although a warmer world will lead to more evaporation (other factors,
such as the complete solar spectrum, remaining constant), more evaporation does not
in itself necessarily mean more precipitation. To take an extreme example, Venus is
a far warmer planet than the Earth and its atmosphere imparts a far more power-
ful greenhouse effect. Indeed, Venus is so warm that water exists solely in the
form of water vapour. There is no rain to soak the ground on Venus and there
are no oceans. Second, although a warmer world results in more ocean evapora-
tion, if, hypothetically speaking, part of this extra water vapour does return as a
partial increase in precipitation this does not mean that river flow would necessarily
increase. It could be, hypothetically, that the increased evaporation means that the
excess precipitation evaporates before it reaches the rivers (we shall return to this in
Chapter 6). Alternatively, it may be that rainfall would increase mainly over the
oceans, so leaving river flow unaffected. Third, other factors and indeed biological
processes may well play their part. So it is important to identify these factors (or as
many as possible) and to try to quantify them.
Climate models and satellite observations both suggest that the total amount
of water in the atmosphere will increase at a rate of 7% per degree Celsius
(% ◦ C − 1 ) of surface warming. Yet climate models have predicted that global precipit-
ation will increase at a much slower rate of 1-3% ◦ C − 1 . However, a 2007 analysis by
Frank Wentz and colleagues of satellite observations carried out between 1987 and
2006 does not support this prediction of a milder response of precipitation to global
warming. Rather, the observations suggest that precipitation and total atmospheric
water have increased with warming at about the same rate (around 6% ◦ C − 1 )overthe
past two decades, even if not all of the extra evaporation falls back as rain (some may
stay in the atmosphere). Having said this, Wentz's team were aware that their results
were preliminary and that more work is needed in this area.