Geology Reference
In-Depth Information
lier simulations (the most extreme results) showing temperature increases ranging from
2
0
to 11°C for a doubling of carbon dioxide. The implications of this new work are that
the high-end predictions of severe warming, although not very likely, must be taken very
seriously.
The key effect that the researchers varied in this new work was the behaviour of
clouds, which have been called the 'Achilles heel' of climate models because they are
notoriously difficult to represent mathematically. When carbon dioxide warms the air,
more water evaporates from the oceans, so in a warmer world we would expect to find
more water vapour in the atmosphere. Water vapour is a powerful greenhouse gas in its
own right, so increasing it could lead to even more evaporation and hence to even higher
temperatures—a positive feedback on warming. But things aren't that straightforward,
because water takes on many guises as it journeys around our living planet. It can mani-
fest as liquid water, or as a solid, as in ice and snow, or as ten major kinds of cloud, each
of which can have an overall warming or cooling effect both regionally and globally, de-
pending on the altitude at which the clouds form, how long they last, and how white they
are. For example, those wispy cirrus or mare's tails that form high up on clear days may
well be overall warmers, whilst the dense low marine stratus clouds, which give Britain
its well deserved reputation for virtually unending gloom, are coolers. What clouds will
do in a warmer world is still a mystery, but as the warm-end outliers of new simulations
show, there is a chance that clouds will disperse sooner under higher temperatures, or
that the few clouds that do manage to appear will quickly drop their rain and vanish.
Both scenarios would increase the warming feedback as more sunlight reaches Gaia's
surface and as the greenhouse effect of the additional water vapour delays the escape of
infrared radiation to space. There are some alarming indications from the real world that
these sorts of things are already happening in the tropics, where skies have become less
cloudy since the late 1980s. No one is certain that this effect is due to climate warming,
but many climate scientists suspect that there is a connection, and many are concerned
that the IPCC climate models don't cope reliably with clouds. There seems to be an
emerging consensus that the models with the most unrealistic cloud feedbacks are the
ones that produce low-end results.
Concerns about the likely underestimates of the IPPC predictions in the TAR promp-
ted the British government to convene a major international conference at Exeter in
February 2005 entitled 'Avoiding Dangerous Climate Change'. The findings of this con-
ference are alarming. In the words of the Steering Committee report, the conference
agreed that: “Compared with the TAR there is greater clarity and reduced uncertainty
about the impacts of climate change across a wide range of systems, sectors and soci-
eties. In many cases the risks are more serious than previously thought.” Conference
participants deemed that “increasing damage” was likely, with temperature increases of
