1. Introduction - Gaia and Global
Change
It often seems obvious that life on
Earth lives at the mercy of powerful non-biological forces like volcanic
eruptions, storms, climate change, and even the movement of continents.
Over this semester you have learned how we believe matter and the
universe came about, how the solar system was formed, and how life on
earth emerged and diversified. Today much of the earth's surface is
covered by a layer of life, and everywhere on earth the influence of
living organisms has an effect. Recently there has emerged a
controversial theory, called the Gaia Hypothesis. It is based on the
idea that, over the long run of geological time, life may control the
powerful physical forces for its own good.
"The Gaia hypothesis states
that the lower atmosphere of the earth is an integral, regulated, and
necessary part of life itself. For hundreds of millions of years, life
has controlled the temperature, the chemical composition, the oxidizing
ability, and the acidity of the earth's atmosphere"
(Margulis, L and J. Lovelock. 1976. "Is Mars a Spaceship,
Too?" Natural History, June/July pp. 86-90)
2. The Hypothesis
and its Originators
The originators of the hypothesis were
James Lovelock and Lynn Margulis. Lovelock is a British independent
scientist and inventor with a background in human physiology. Margulis
was, in the 1970's, a microbiologist at Boston University. She is also
the originator of the theory that the eukaryotic cell arose by
endosymbiotic cell capture - this was a radical idea that has become
widely accepted, thereby giving Margulis a high degree of credibility.
The essential idea of the Gaia
Hypothesis is analogous to the thermostat in your home, or the
thermostat in your brain. You set the thermostat in your home to 65 °F
in order to keep a comfortable living environment. When the temperature
falls below this, the furnace is switched on. When the temperature in
the house reaches the target, the furnace is switched off. Something
more complicated, but with similar effect, goes on in our bodies.
Everyone of us is a comfy 98.6 °F now, and almost always. If our body
temperature deviates very far from a narrow range, we die. The human
body has a number of self-regulatory, or homeostatic, mechanisms.
The conditions for life as we know it
to exist also require a relatively narrow range of circumstances. How
does life modify the physical and chemical conditions of the
environment?
- Where does oxygen come from? Small
amounts emanate from volcanic activity, but usually it is combined
with other elements, e.g., as CO2 and H20. As
you have learned, the earth's original atmosphere contained almost
no oxygen, and the advent of photosynthesis some 2.5 billion years
ago is responsible for the presence of abundant oxygen in the
atmosphere (presently 21%). As explained in lecture, initially the
liberated oxygen combined with oxidizable minerals such as iron,
leaving a sedimentary record of red bands that tells us that a new
atmospheric chemistry was being brought about by life. Other
geological evidence suggests that oxygen levels on earth have been,
within a factor of roughly two, at near-present values for the past
billion years, during which complex multi-cellular life arose.
If oxygen were to reach a value of
30% of atmospheric gas composition, fires would occur whenever a
lightening bolt hit humid vegetation. The planet would be in serious
danger of burning up. What has kept oxygen from building up to
dangerous levels? Why has it gone from nearly zero to 21%, and then
stopped? One possible answer is the biological production of methane
by bacteria. A short-lived molecule, methane might combine with
oxygen to produce CO2, thus stabilizing oxygen
concentrations.
- We know that climate has changed a
great deal in the past, producing episodes of glaciation, and could
warm significantly in the future, due to atmospheric pollution.
However, climate change could be much more extreme. At least for the
past billion years it is unlikely that the earth's temperature was
more than 15 degrees warmer or 5 degrees colder than it is today.
Earlier temperatures are very uncertain. Calculations suggest that
the sun emitted perhaps 25% less heat energy some 4 billion years
ago, than it does today. Calculations also suggest that under this
faint early sun, the earth should have been a frozen ball. However,
life arose under these conditions, and there is geological evidence
of flowing water from this time. It has been suggested that a kind
of greenhouse warming was in effect at that time, involving such
gases as methane, ammonia, and carbon dioxide, and that this is
evidence of a kind of Gaian planetary temperature control mechanism.
And why hasn't the planet
overheated, since the sun has increased in luminosity over the past
4 billion years? Lovelock and Margulis argue that life solved this
one, also. A warming earth stimulated greater plankton production,
removing CO2 from the atmosphere. When the plankton died
they sank to the ocean floor, forming sediments, and thus removed CO2
from the system. Moreover, a warmer planet has more rain, which
means more erosion and more nutrient runoff to the oceans. This also
stimulates phytoplankton growth, removing CO2 from the
atmosphere as before. Thus, Gaia maintains a fairly constant climate
as the sun heats up.
Life has other influences over the
chemistry of the planet: methane and ammonia exist in their present
abundances because bacteria continually regenerate them by
decomposing organic matter.
Perhaps life regulates the physical
and chemical environment of the planet so as to maintain suitable
planetary conditions for the good of life itself. If so, then the
planet can be thought of as a single, integrated, living entity with
self-regulating abilities. This is the radical view that Lovelock
and Margulis have espoused. It can be thought of as the "strong
Gaian model."
- Oxygen: was first produced more than
3 billion years ago. Why did it take 2 billion years to build up? It
took 2 billion years for all of the available reactive chemicals of
the earth's surface, such as iron, to be oxidized to produce the
sedimentary layers known as red beds. When it finally did build up,
it forced then-existing life, which was poisoned by oxygen, to
retreat to anaerobic (lacking oxygen) environments such as the
bottom of swamps. Granted, this set the stage for the development of
the ozone layer, which shields us from ultraviolet light, and
permitted the evolution of aerobic life, but it was very
self-sacrificing on the part of the major life forms existing some 2
billion years ago. This is hardly homeostatic (homeostasis:
the ability or tendency of an organism or cell to maintain internal
equilibrium by adjusting its physiological processes).
- Climate: geochemists have offered an
alternative feedback mechanism for temperature control, which is
accomplished inorganically [University of Michigan geochemists
Walker, Hayes and Kasting]. As the sun warms, rains increase. Water
in the atmosphere combines with CO2 to form a weak
carbonic acid solution, and this could have weathered silicate
minerals in the rocks on land. Formation of new sediments would
store carbon, thus reducing its abundance in the atmosphere. Gaian
advocates fight back by arguing that soil bacteria play a role in
the weathering reaction. This is unresolved.
- The idea that climate and life
influence one another is profoundly important. In some form or
another, it has been recognized for a long time. Life and climate
"grew up together" and influenced one another over most of
earth history. But this is not to say that life somehow manages and
self-optimizes its own environment. It is this idea -- the
"strong form of Gaia" -- that is most controversial.
Jim Kirchner (see "suggested
readings") argues that there are really many Gaian hypotheses.
- Influential Gaia,
the weakest of the hypotheses, asserts that biota have a substantial
influence over certain aspects of the abiotic world, such as
temperature and the composition of the atmosphere.
"The Gaia hypothesis.. states
that the temperature and composition of the Earth's atmosphere are
actively regulated by the sum of life on the planet" (Sagan and
Margulis, 1983).
- Co-evolutionary Gaia
asserts that the biota influence their abiotic environment, and that
the environment in turn influences the evolution of the biota by
Darwinian process.
"The biota have effected
profound changes on the environment of the surface of the earth. At
the same time, that environment has imposed constraints on the
biota, so that life and the environment may be considered as two
parts of a coupled system" (Watson and Lovelock, 1983).
- Homeostatic Gaia
asserts that the biota influence the abiotic world, and do so in a
way that is stabilizing, by negative feedback linkages.
"The notion of the biosphere
as an active adaptive control system able to maintain the earth in
homeostasis we are calling the 'Gaia' hypothesis" (Lovelock and
Margulis, 1974).
- Teleological Gaia
asserts that the atmosphere is kept in homeostasis, not just by the
biosphere, but by and for the biosphere.
"the Earth's atmosphere is
more than merely anomalous; it appears to be a contrivance
specifically constituted for a set of purposes" (Lovelock and
Margulis 1974).
- Optimizing Gaia
asserts that the biota manipulate their physical environment for the
purpose of creating biologically favorable, or even optimal,
conditions for themselves.
"...it is unlikely that chance
alone accounts for the fact that temperature, pH and the presence of
compounds of nutrient elements have been, for immense periods, just
those optimal for surface life. Rather, ... energy is expended by
the biota to actively maintain these optima" (Lovelock and
Margulis, 1974).
Kirchner argues that the weak forms of
the hypothesis are not new and that the strong forms are not correct or
not testable. He suggests that many people point to evidence for one of
the weak forms of Gaia, and then go on to claim a stronger form.
6. Modeling Gaia
You can model feedbacks using the
classic Gaia example of Daisyworld with
Stella or using this interactive
Java applet. The latter is especially useful to get a
first-order understanding of changing parameters. The Stella model
permit more sophisticated analysis.
- The Gaia Hypothesis states that life
on earth controls the physical and chemical conditions of the
environment (the biotic controls the abiotic)
- The hypothesis was formulated by
James Lovelock and Lynn Margulis
- The hypothesis points to stable
conditions, such as oxygen levels and climate, as evidence that
living organisms maintain a life-sustaining environment
- The hypothesis has been defined and
argued in numerous ways, and has as many critics as adherents. It is
in need of more explicit formulation before it can be examined and
tested as a
true scientific theory.
- Two models emerge:
The model that life influences planetary processes (i.e., it has a
substantial effect on abiotic processes) has become known as the
weak Gaia hypothesis. This model is widely supported.
The original Gaia hypothesis, that life controls planetary processes
(i.e., life created Earth's system), has become known as the strong
Gaia hypothesis. It is not widely accepted.
Review
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