Geology Reference
In-Depth Information
constant solar luminosity, as white daisy cover increases, surface temperature decreases. On the
right is the bell-shaped effect of surface temperature on white daisy cover: to the left of the op-
timum growth temperature (the negative feedback regime) the coupling is direct; to the right of it
(the positive feedback regime) the coupling is inverse. The brightness of the sun (not shown) in
part determines which feedback regime applies.
What Lovelock had done was to create a model with six tried and tested equations from
physics and biology to represent a simple but nevertheless realistic
feedback
between
life and its environment. In doing so, he had created a new way of modelling the Earth
as an integrated system of life and climate. Daisyworld was a bold and novel move—no
one had ever attempted this, despite the injunction in 1925 by Alfred Lotka, the founder
of theoretical population ecology, that species should not be modelled without reference
to their non-living environment. As he prepared to run the model on his computer for
the first time, Lovelock had only a faint inkling of what might happen. Daisyworld was
a shot in the dark—had it failed, Gaia might well have languished in the obscurity ac-
corded to quaint but now irrelevant scientific concepts, such as the theory of phlogiston.
As it happened, Daisyworld produced a startling result
(Figure 14)
.
As the young,
cool sun shone down on Daisyworld, dark and light daisies began to grow as soon as the
bare ground became warm enough to favour germination. Under the cool sun, dark dais-
ies warmed themselves closer to the optimum growth temperature, and so they spread,
out-competing the light daisies that cooled themselves into oblivion. Dark daisies grew
explosively into the barren expanses of the planet through positive feedback, darkening
its entire surface and warming the world exponentially fast as they spread. As time went
by, Daisyworld entered a long, stable period of negative feedback. Light daisies thrived
increasingly well, because under a brightening sun they gained a slight advantage over
the dark daisies by being able to cool themselves closer to the optimum growth temper-
ature. When the sun reached its middle age, there were equal numbers of dark and light
daisies. As the sun's brightness increased even further, dark daisies slowly vanished as
they warmed themselves far above the temperature for optimum growth, leaving room
for an expanding coverage of the better-adapted light daisies.
What amazed and delighted Lovelock most of all was the thin red line on his com-
puter screen that traced the overall temperature of the planet as the sun increased its
output of energy. The line revealed two startling emergent properties: first, the overall
temperature of the model planet had remained remarkably constant over a vast period of
time despite the shifting populations of daisies and an ever-brightening sun, and second,
the temperature had settled on a value just below the optimum for daisy growth.
