With Speed and Violence Fred Pearce - Global Commons Institute

absence—are most pronounced in deserts. Where there are no clouds, the
days are boiling, but the nights can get extremely cold, even in the tropics.
The temperature effects of clouds turn out also to depend on the nature of
the clouds. Their height, depth, color, and density can be vital, because
different clouds have different optical properties. The wispy cirrus clouds
that form in the upper atmosphere heat the air beneath, because they are
good at absorbing the sun's rays and re-radiating the heat downward,
whereas the low, flat stratus clouds of a dreary summer's day are good at
keeping the air below cool.
Researchers still know surprisingly little about how many and what sort of
clouds are above our heads. For instance, it has only recently emerged that
there may be many more cirrus clouds than anyone had thought. Many are
almost invisible to the naked eye, but nonetheless seem to be highly effective
at trapping heat. Some studies suggest that, taken globally, the cooling and
warming effects of clouds currently largely cancel each other out, with
perhaps a slight overall cooling effect. But nobody is sure. And even small
changes in cloudiness could affect planetary albedo substantially. If a
warmer world tipped clouds into causing greater warming, the effects could
be considerable.
So what is the prognosis? Again, a first guess is that extra evaporation will
make more clouds, because a lot of the water vapor will eventually turn into
cloud droplets. But even that may not be so simple. Evaporation doesn't just
lift water vapor into the air to create more clouds; it also burns off clouds,
leaving behind blue skies. And greater evaporation can also make clouds
form faster, so that they fill with moisture faster, make raindrops faster, and
dissipate faster. So, in a greenhouse world, fluffy cumulus clouds that we are