Uncertainty, scepticism and the climate issue - Climate Change: The Facts - page 106

10 Uncertainty, scepticism and the climate issue
Garth W. Paltridge
The World Meteorological Organization of the United Nations took its first steps towards
establishing the World Climate Program in the early 1970s. 1 Among other things, it held an
internationalworkshopinStockholmtodefinethemainscientificproblemswhichneededto
be solved before reliable climate forecasting could be possible. The workshop defined quite
a number of problems, but focused on just two.
The first concerned an inability to simulate the amount and character of clouds in the
atmosphere. Clouds are important because they govern the balance between solar heating
and infrared cooling of the planet, and thereby are a control of Earth's temperature. The
second concerned an inability to forecast the behaviour of oceans. Oceans are important
because they are the main reservoirs of heat in the climate system. They have internal,
more-or-less random, fluctuations on all sorts of time-scales ranging from years through to
centuries. These fluctuations cause changes in ocean surface temperature that in turn affect
Earth's overall climate.
The situation hasn't changed all that much in the decades since. Many of the problems
of simulating the behaviour of clouds and oceans are still there (along with lots of other
problems of lesser moment) and for many of the same reasons as were appreciated at the
time. Perhaps the most significant is that climate models must do their calculations at each
point of an imaginary grid of points spread evenly around the world at various heights in
the atmosphere and depths in the ocean. The calculations are done every hour or so of
model time as the model steps forward into its theoretical future. Problems arise because
practical constraints on the size of computers ensure that the horizontal distance between
modelgrid-pointsmaybeasmuchasadegreeortwooflatitudeorlongitude—thatistosay,
a distance of many tens of kilometres.
That sort of distance is much larger than the size of a typical piece of cloud. As a
consequence, simulation of clouds requires a fair amount of inspired guesswork as to what
might be a suitable average of whatever is going on between the grid-points of the model.
Even if experimental observations suggest that the models get the averages roughly right
for a short-term forecast, there is no guarantee they will get them right for atmospheric
conditions several decades into the future. Among other problems, small errors in the
numerical modelling of complex processes have a nasty habit of accumulating with time.
Again because of this grid-point business, oceanic fluctuations and eddies smaller than
the distance between the grid-points of a model are unknown to that model. This would
not be a problem except that eddies in turbulent fluids can grow larger and larger. A small
random eddy in the real ocean can grow and appear out of nowhere as far as a forecasting