Present climate and biological change - Climate Change: Biological and Human Aspects - page 237

even when they get some past of present detail wrong, we can use this to get an idea
of the potential types of error in their future forecasts.
5.3.2 UncertaintiesandtheIPCC'sconclusions
The above discussion demonstrates that we need to be aware of uncertainties and view
the IPCC's conclusions in each of their successive assessment reports with these in
mind. Related to the mid-Cretaceous warm-Arctic problem above, one of the major
concerns is why the greatest warming actually measured during the 20th century
took place at high latitudes. This is a concern doubly worrying because, as noted, the
global climate models do not yet properly reflect such warming (although they are
improving). This is likely to be reconciled once we have a better idea of how heat is
distributed about the planet: the previous subsection's suggestion of low-latitude air
movement is but one idea.
Another concern is that of a phenomenon nicknamed global dimming. This at
first may seem paradoxical in a world that is supposed to be warming up. Yet those
concerned with agricultural biology have known for some years that the amount of
sunlight reaching parts of the Earth's surface has been in marked decline. Normally
weather instruments for temperature and humidity are kept in the shade, but water
evaporation is not just a matter of temperature and humidity: it is also a matter
of sunshine. The relevance of this has been demonstrated by a series of long-term
(multi-decade) measurements of what is called 'pan evaporation' taking place in
several countries. Pan evaporation refers to the daily evaporation of water from a
pan exposed to the elements including sunlight and is of importance to biologists
studying agricultural systems, as pan evaporation is relevant to irrigation methods. It
has therefore been monitored for a number of decades in various countries. What has
been found is that pan evaporation has been decreasing markedly in many countries
for more than 50 years. Sunlight, separate to temperature, excites water molecules
and so enhances evaporation. In 2002 two Australian researchers, Michael Roderick
and Graham Farquar, discovered that reduced pan-evaporation rates appeared to be
linked to a decrease in sunshine: hence the expression global dimming.
What appeared to be happening is that fine-aerosol pollution from urban areas and
industry is providing seeds for the formation of clouds that consist of finer-than-usual
droplets (the indirect aerosol effect as opposed to the direct aerosol effect of the
aerosols without the atmospheric water). These clouds are more reflective than usual.
Also, the finer droplets are less likely to result in rain.
The consequences of dimming were considerable. First, there is the human implica-
tion: it is now considered by some that global dimming caused less ocean evaporation
in parts of the northern hemisphere, such that in the 1980s and 1990s it resulted in
the failing of the monsoon and mass famine in east Africa. But it also means that if
there is a greater cooling force than we thought over much of the temperate northern
hemisphere, then there must be an even greater warming force than we thought for
there to have been the net result of warming in the 20th century. In 2004 a small
team of US researchers led by Joyce Penner reported that they had a measure of this
indirect aerosol effect by comparing conditions in two separate areas of that country,
one of which had clean air and the other of which was polluted with aerosols. They