Geology Reference
In-Depth Information
our planet. The model daisies would grow according to the equations in the
Nature
pa-
per and would compete for space on the planet's surface, thereby satisfying the evol-
utionary biologists' requirement that competition be part of Gaia. But the innovative
move that Lovelock made in his model was to wire in an explicit mathematical feedback
between life—the daisies—and their non-living environment—the surface temperature
of the planet.
Here we need a digression to explore the concept of
albedo
(from the Latin
alba
,
meaning white)
,
which, on a scale ranging from zero to one, scientists use as a measure
of how much solar energy is reflected by a surface. When exposed to the sun, a high
albedo (light) surface reflects most of the solar energy striking it without any transfer
of energy taking place, whilst a low albedo (dark) surface heats up because it absorbs
solar energy. The heat then radiates out to the surrounding air, warming the immediate
vicinity, and, ultimately, the entire planet.
In Daisyworld, dark daisies warm themselves and the planet by absorbing energy
from the sun which they radiate into their surroundings as heat, whilst white daisies
cool themselves and their world by reflecting solar energy back to space. Lovelock con-
structed these couplings between life and the non-living environment using well-known
equations from thermodynamics, the branch of physics that deals with energy flows.
Also included in the calculation of the planet's surface temperature was the albedo of the
bare ground, which was set, realistically, at a uniform grey. He then modelled the tight
coupling between the planet's non-living environment (the surface temperature) and the
growth of the daisy species by giving both the same realistic, bell shaped response to the
planet's surface temperature. Both grew best, as real plants do, at 22.5
0
C, and not at all
below 4
0
C or above 40
0
C. Finally, as for the real Earth, Lovelock modelled the sun to
gradually increase its output of energy as time went by.
Figure 13
gives the flavour of
how Dasiyworld works by focusing, for the sake of simplicity, on how only white dais-
ies regulate the planet's surface temperature.
Figure 13: Feedbacks between white daisies and the surface temperature of their planet. On the
left is the effect of white daisy cover on surface temperature. This is an inverse coupling: for a
