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How has the Earth remained hospitable for life for billions of years? This question remains one of the most important in 21st-century science because the answer could help scientists understand the long-term consequences of human activities on the environment. In this Point/Counterpoint Sidebar, scientist James Lovelock presents his case for Gaia theory. The theory maintains that Earth is an interrelated system in which living things, together with Earth’s surface and atmosphere, evolve as a single entity. Further, Lovelock argues, this system functions to make the planet habitable for life. Earth scientist James W. Kirchner agrees that life on Earth is part of an interrelated system, but he argues that the factors regulating the environment are more complex than can be accounted for by Gaia theory.
By James Lovelock
For most of the 20th century scientists held that conditions on Earth are comfortable for life because, by good fortune, the chemical composition of our planet and its distance from the Sun are exactly right. If the Earth were closer to the Sun, conditions on Earth would be too hot, and if the Sun were farther away, the Earth would be too cold. Biologists since Charles Darwin’s day in the 19th century have taught that living organisms adapt to Earth’s conditions, and Earth scientists have long taught that geological forces alone determine conditions on the Earth.
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What if, contrary to these long-held beliefs, living organisms began to change the Earth soon after their origin 4 billion to 3.5 billion years ago? As a result of these changes to the Earth, organisms soon were living in a world of their own making. In this scenario, organisms adapt to a world whose material parts—the Earth’s surface, oceans, and atmosphere—are the products of the ancestors of today’s living organisms. This alternative view of the Earth is Gaia theory, in which all of life, together with the Earth's surface and atmosphere, evolve as a single entity. This single entity is able to sustain habitable conditions and compensate for adverse changes in the Sun's output of heat and in the Earth's surface composition.
In his book Physical Biology, published in 1925, American scientist Alfred Lotka first suggested the idea that there was more to evolution than simply the natural selection of organisms. He wrote: “It is not so much the organism or the species that evolves, but the entire system, species and environment. The two are inseparable.”
Gaia theory follows from Lotka’s conjecture. The theory is based on an evolutionary science that is as much about the rocks and oceans as about the living things that inhabit them. In this view, an Earth system evolves gradually for long periods under an ever-warming Sun. But as the Earth system evolves, sudden events punctuate the system’s gradual evolution. These events may be internal, such as the appearance of oxygen, a glaciation, or a species like humans; or the events may be external, such as the impact of planetesimals, planet precursors that orbited the Sun in the early solar system. Whether internally or externally driven, these events change the whole Earth system, affecting both the physical environment and living organisms.
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Lotka's view of evolution passed almost unnoticed in his time. It was not until the 1960s, when scientists at the National Aeronautics and Space Administration (NASA) began exploring our planetary neighborhood, that Lotka’s broader, transdisciplinary view of the Earth was revisited. I was a member of NASA's exploration team during that time. My NASA experience led me to propose, in a 1965 paper published in the British science journal Nature, that life and its environment are so closely coupled that the presence of life on a planet could be detected merely by analyzing the chemical composition of the planet’s atmosphere. I argued that living organisms would have to use the atmosphere as a source of raw materials and a place to deposit wastes. These actions would make the planet’s atmosphere recognizably different from the atmosphere of a lifeless planet, which is unreactive, like gas exhausted by combustion. My proposal is now an integral part of NASA’s astrobiology program and NASA scientists use this concept in the search for life on extrasolar planets.
When we look at the Earth's atmosphere in this way, we see that oxygen, methane, and nitrous oxide are some of the raw materials required by living organisms to function, and nitrogen and carbon dioxide are released as waste products into the atmosphere by living organisms. So dependent is Earth’s atmosphere on life that if some catastrophe removed all life from the Earth without changing anything else, the atmosphere and surface chemistry would rapidly (in geological terms) move to an atmospheric state similar to Mars or Venus—dry, chemically stable planets, with atmospheres dominated by carbon dioxide. By contrast, Earth is a cool, wet planet with an unstable atmosphere that somehow stays constant and always fit for life.
The laws of chemistry predict that the long-term habitable conditions we enjoy on Earth are infinitely improbable, and we are forced to consider the more likely alternative: Something regulates the chemicals of the air. What is it? It has to be connected with life at the surface because we know that organisms determine the composition of the atmosphere. Furthermore, the climate depends on atmospheric composition and, in spite of the Sun increasing its heat by 30 percent, Earth has had a habitable climate ever since life began.
These facts led me to propose in 1969, at a meeting of the American Astronautical Society, that the biosphere regulates the atmosphere in its own interests. At about this time my friend and neighbor, Nobel laureate novelist William Golding, suggested that I call this hypothesis Gaia, the name of the ancient Greek Earth goddess. In 1972 I was joined in my Gaia research by American microbiologist Lynn Margulis. From her deep knowledge of the Earth's bacterial ecosystem, Lynn put flesh on what otherwise was the dry skeleton of Gaia drawn from physical chemistry. Together we published a paper in the Swedish journal Tellus proposing the biosphere to be an active, adaptive control system able to keep the Earth habitable.
Both Earth scientists and atmospheric scientists welcomed the Gaia hypothesis. But the idea was so contradictory to the views of evolutionary biologists that it was not long before one of them, Canadian biochemist W. Ford Doolittle, challenged the hypothesis. In a 1981 article in the journal CoEvolution Quarterly entitled “Is Nature Really Motherly?” Doolittle argued that global self-regulation would require organisms to have foresight and plan their future environment, which was teleology not science. In his 1982 book The Extended Phenotype, British biologist Richard Dawkins continued Doolittle’s argument, writing that global regulation by organisms could never have evolved because the organism itself was the unit of selection, not the Earth.
In time I found myself agreeing with these arguments. There was no way for organisms by themselves to evolve so that they could regulate the global environment. But when I remembered the strong evidence for self-regulation provided by the Earth's unstable atmosphere, I wondered, could there be another way for the Gaia system to evolve?
In 1981 I composed an evolutionary model called Daisyworld that was simply intended to show that self-regulation could take place on a planet where organisms and their environment evolve as a single system. The model was one that Alfred Lotka might have made had computers been available in his time. Daisyworld described a simple planet orbiting a star like the Sun. The planet grew warmer as it aged, and it was populated by light- and dark-colored daisies.
When I ran the model, the planet sustained a temperature close to ideal for daisy growth, in spite of large changes in the strength of the sunlight. According to the critics of Gaia theory, planetary self-regulation would require foresight by the organisms and/or competition between planets. On Daisyworld at least, self-regulation took place without needing either of these strange mechanisms.
Daisyworld was so strong an answer to the critics of Gaia that many scientists came to see Gaia as a threat to their branch of science and, wrongly, as contrary to the evolutionary theories proposed by Darwin. So persistent and intense has been the battle over the Daisyworld model that Philip Campbell, editor of Nature, commented in 2002, “Let no one underestimate the irritation that hypothetical simplicity can engender. Just consider Daisyworld … the irritation that the model has engendered stems both from a proper scientific skepticism about its relevance to the Earth, but also, at times, an improper defensiveness of traditional scientific boundaries.”
Gaia did not depend on Daisyworld alone. I worked with a number of colleagues to confirm Gaia. In addition to the evidence from the chemistry of our atmosphere, in the 1970s British chemist Peter Liss and I were able to confirm a Gaian prediction about the natural cycles of the elements iodine and sulfur. We showed that organisms in the ocean created the gas vapors methyl iodide and dimethyl sulfide. These gases traveled through the air to the land, where they were utilized in ecosystems.
In 1982 I worked with British scientists Andrew J. Watson and Michael Whitfield to reexamine a proposal by the American geochemists J. C. G. Walker, James F. Kasting, and P. B. Hayes. The Americans proposed that the weathering of silicate rock controls atmospheric carbon dioxide and, hence, climate. We agreed with their proposal, but we introduced Gaia into the equation by suggesting that the presence of organisms in the soil and on the rocks greatly increases weathering. Moreover, since the growth of organisms is temperature dependent, rising temperatures would help plants grow faster. And since plants use carbon dioxide in photosynthesis, an increase in plant growth would result in a decrease of atmospheric levels of carbon dioxide and a reduced atmospheric temperature.
In 1986 the American climatologist Robert Charlson and I had one of those thrilling moments of discovery that makes life as a scientist so worthwhile. I was looking for another mechanism of Gaian climate control. At the same time, Charlson was seeking the source of the sulfuric acid droplets he had found far out over the Pacific Ocean. These tiny acid droplets are the nuclei on which water condenses to form clouds. Charlson knew that these droplets did not come from pollution or volcanic sources, and he wondered where they could come from.
When I told Charlson that there was dimethyl sulfide in the oceanic air, together we realized that the sulfuric acid could come from the atmospheric oxidation of dimethyl sulfide. And then the awesome possibility came to us that the clouds over the oceans might owe their existence and opacity to the sulfur gas emitted from algae living on the ocean surface. Here was a vast Gaian climate-regulating mechanism. With Charlson's colleague Stephen G. Warren, and the German chemist Meinrat Andreae, we published our ideas in a 1987 Nature paper. The World Meteorological Organization recognized the importance of this research by awarding us the Norbert Gerbier Prize in 1988.
By now there was sufficient physical evidence and models of a self-regulating Earth to justify calling Gaia a theory, which can be stated as follows: “The evolution of organisms and their material environment proceeds as a single tight-coupled process from which emerges an environment, self-regulated for habitability.”
The concept of emergence concerns the transformation that occurs when an assembled system, such as a television, a computer, or a virus, is switched on and becomes “alive,” becoming more than the sum of its parts. Emergence is familiar to us all, but scientists, used to thinking in the classical way through cause and effect, still find this concept hard to understand.
Ordinarily, scientific theories are judged by the accuracy of their predictions and by their usefulness to working scientists. Gaia theory has made four other successful predictions in addition to those I have described, and research on Gaia theory and its applications now employs thousands of scientists worldwide.
So, why is Gaia theory only rarely cited in scientific journals? The answer lies in the strength and persistence of the criticisms and possibly the fact that Gaia was for a while used as a New Age icon. Consequently, the name became unfashionable among scientists, and they soon discovered that papers and grant applications that included the word Gaia were rejected.
The wonderful thing about science is that in time the truth prevails. However much the Gaia name was disliked, the evidence and models deriving from Gaia research could not be ignored. As a result, scientists began to research and model a self-regulating Earth under the pseudonym Earth System Science. In 2001, at a conference on global change hosted by the International Geosphere Biosphere Program and other agencies, the Amsterdam Declaration was issued. It declared: “The Earth System behaves as a single, self-regulating system comprised of physical, chemical, biological and human components.”
This declaration is well on the way to an endorsement of Gaia theory, although it fails to say what the Earth System regulates, or at what level. But it was as far as the conference attendees would go, and it is a considerable step forward from the research performed in the 1970s, in which the study of the Earth was separated into individual sciences. Since 1995 biologists have also begun to accept the existence of global-scale self-regulation, but still they can see no way for it to happen by Darwinian natural selection.
It took the eminent Darwinist, the late William Hamilton, to see this as a challenge, not an objection. In 1998 Hamilton cowrote, with my friend and successor, British environmental scientist Tim Lenton, a paper entitled “Spora and Gaia,” published in the journal Ethology, Ecology & Evolution. Hamilton and Lenton proposed a selective advantage for algae that produced dimethyl sulfide. This advantage would be to stimulate wind through cloud building, and so spread algae spores more efficiently. Hamilton changed from being a strong opponent of Gaia theory to seeing it as a theory facing the same scientific bias that the Copernican theory did in the 16th century.
As Gaia theory evolved it changed scientific wisdom. Just consider how separate the biological and Earth sciences were in the 1970s. The American geologists Frank Press and Raymond Siever wrote in their authoritative 1974 textbook Earth: “Life depends on the environments in which it evolved and to which it has adapted.”
And the evolutionary biologist John Maynard Smith wrote in his 1975 book The Theory of Evolution, “The study of evolution is concerned with how, during the long history of life on this planet, different animals and plants have become adapted to different conditions, and to different ways of life in those conditions.”
For most of the 20th century, these and other eminent scientists saw evolution as wholly biological and thought that organisms adapted to a given geological environment. Consequently, they ignored the massive changes that organisms made to their environment. True enough, during the 1940s and 1950s the biogeochemists Alfred Redfield, Vladimir Vernadsky, and G. Evelyn Hutchinson each were becoming aware that something was wrong with the conventional wisdom. These scientists individually proposed that organisms were more than mere passengers on the planet. But no one, apart from a few early followers of Gaia theory, researched a global system able to sustain a habitable Earth.
Large theories in science usually take 40 years before acceptance, and perhaps Gaia should wait its turn. Unhappily, we are now making such rapid changes to the Earth's surface and atmosphere that the distinguished British environmentalist Sir Crispin Tickell has warned that waiting for Gaia theory to become accepted by all scientists may be an unaffordable luxury. To understand and perhaps prevent the adverse consequences of human activities on Earth, geologists and biologists must cease working in separate buildings whilst merely imagining that they are part of something larger.
In their 1999 book The Earth System, American scientists Lee R. Kump, James F. Kasting, and Robert G. Crane show how it can be done by providing a new way of organizing information from a variety of science disciplines. Likewise, British climatologists Peter Cox and Richard Betts employed a variety of sciences when they used Gaia theory to predict the consequences of global change. We face unknown dangers in the coming century, and we are more likely to deal with them successfully if Earth scientists and life scientists unite and use the fruitful Gaia theory while it is still ripe.
To scientists interested in the true nature of the Earth it hardly matters what the science is called, but to nonscientists the name Gaia is already in their minds and made comprehensible by its metaphor, the living Earth. The public understanding of science benefits from powerful names and metaphors; indeed this is how neo-Darwinism succeeded through its equally powerful metaphor, the selfish gene. In a speech given in 1994 at Independence Hall in Philadelphia, Pennsylvania, the former president of the Czech Republic, Václav Havel, stated that Gaia, unlike previous theories of science, had ethical consequences. He said, “According to the Gaia Hypothesis, we are parts of a greater whole. Our destiny is not dependent merely on what we do for ourselves but also on what we do for Gaia as a whole. If we endanger her, she will dispense with us in the interests of a higher value—that is, life itself.”
About the author: James Lovelock is an independent scientist, a Fellow of the Royal Society, and an Honorary Fellow of Green College, Oxford University, in the United Kingdom. He is the author of several books including Gaia: The Practical Science of Planetary Medicine (1991), The Ages of Gaia (1988), Gaia: A New Look at Life on Earth (1979), and an autobiography, Homage to Gaia: The Life of an Independent Scientist (2000).
By James W. Kirchner
Go outside on a warm spring afternoon. Feel the soft breeze and the warm sunshine; hear the birds sing. The world seems just about perfect.
And indeed, the world seems almost perfectly suited to our needs as organisms. For example, think about the air in that warm spring breeze. Our atmosphere is about 21 percent oxygen. A few percent less, and we would not survive; a few percent more, and fires might rage out of control. The oxygen concentration of Earth's atmosphere has remained close to its present level for millions of years. And the oxygen in Earth's atmosphere is a biological byproduct produced by plants during photosynthesis. It is clear that there is a system that regulates the oxygen level in Earth's atmosphere, and biological processes are part of that system.
Or think about the temperature on a warm spring afternoon. The Earth's surface temperature is controlled by several factors: the brightness of the Sun; the whiteness or darkness of the Earth's surface (which affects how much sunlight is absorbed); and the strength of the greenhouse effect, a natural process through which the atmosphere traps heat and keeps the Earth warmer than it would be otherwise.
The greenhouse effect depends on the atmospheric concentrations of so-called “greenhouse” gases, including carbon dioxide and methane. Just like oxygen, carbon dioxide and methane are biological byproducts. Carbon dioxide is produced during respiration, and methane is generated by bacteria as they decompose organic matter. So biological processes are important in regulating the greenhouse effect, and therefore in regulating the Earth's temperature.
And this regulatory process has to work just right. If there is too much greenhouse gas in the atmosphere, the Earth could become too warm to support life as we know it. Too little greenhouse gas in the atmosphere, and the Earth might become so snowy that its white surface would reflect a lot more of the Sun's light back into space. This would make the Earth colder still, which would lead to even more snow and ice accumulating on the Earth’s surface, which would make the Earth even colder. This vicious cycle could lead to what scientists call a Snowball Earth—a globe almost covered with ice. There is growing evidence that our planet became a Snowball Earth at least twice in the distant past, but this has not happened for at least 500 million years. Instead, the system that regulates the composition of the atmosphere, and thus Earth's surface temperature, has kept the environment habitable throughout the history of life on Earth.
Understanding how Earth's global environment regulates itself is one of the great scientific challenges of the 21st century. Why is the oxygen concentration in the atmosphere a life-supporting 21 percent, and not 11 percent or 31 percent? What keeps it at that level? What keeps the global average temperature nearly constant from year to year? Over the past 400,000 years, despite four major ice ages, the Earth's temperature has never been more than about 10° Celsius (18° Farenheit) cooler than it is now, nor more than 3° Celsius (5.4° Farenheit) hotter. How does Earth's climate-regulating system work? And how will this system respond to our growing human population, and to the environmental effects of our growing industrial economy? We urgently need to understand the processes that regulate the global environmental system.
The Gaia hypothesis is one attempt to understand the Earth as a system. Years ago, Jim Lovelock proposed the name Gaia, the Greek goddess of the Earth, to describe the coupled system that regulates the Earth's environment. Most scientists don't like the idea of personifying the Earth system as a deity, but most would agree that we need to understand planet Earth as a system, rather than a set of disconnected components.
Most scientists also agree that organisms are important in controlling environmental conditions on Earth. We cannot understand why the atmosphere has the composition that it does, or why Earth's climate is what it is, without understanding the effects of biological processes such as respiration, photosynthesis, and decay of organic matter. Without life, Earth's atmosphere would be very different, and so would its climate. Lovelock and his followers have been vigorous advocates for the idea that biological effects help shape Earth's environment, and the scientific evidence clearly supports this view.
Not only do living organisms help shape the environment, but environmental conditions also shape how life evolves over the long term. Thus life and the environment evolve together, as a coupled system, through geologic time. Life is not just a passive passenger on Spaceship Earth. Organisms do not simply adapt to fixed environmental conditions; instead, they alter their physical and chemical environment, which in turn shapes their further evolution. This coupled system of life-plus-environment could potentially evolve in ways that we would never expect from analyzing its components separately from one another. The Gaia hypothesis has been an important reminder that we always need to consider the largest scales and pay close attention to the linkages between organisms and their environment.
So what's wrong with Gaia? If the Gaia hypothesis meant only that life can have a big effect on its environment, and therefore the biosphere and the global environment evolve together through time, most scientists I know would stand up and cheer.
But Lovelock and his followers have always said that Gaia means more than just that life and the environment form a coupled system. At one time they argued that the environment was controlled “by and for the biosphere.” They abandoned that idea years ago, in the face of heated criticism from the scientific community. But Gaia proponents still maintain that biological processes stabilize the global environment, and that biological effects modify the environment in ways that make it more suitable for life.
The scientific evidence for this concept is sketchy. Indeed, some biological processes do stabilize the global environment, but others make the global environment less stable. Likewise, some biological processes do enhance the environment for life, but others degrade it. It is reckless to generalize.
Of course, Earth's environment has been stable enough for life to survive for billions of years, but is that because life stabilizes the environment? Or is it in spite of the fact that life destabilizes the environment?
Let's look at how the biosphere is likely to react to climate change. This is an area of cutting-edge research, so scientists are not absolutely certain about these questions yet, but here are five examples of what we think we know:
1. Increasing carbon dioxide concentrations in the atmosphere should make plants grow faster. Because photosynthesis consumes carbon dioxide, this should help bring carbon dioxide concentrations back down. This should help stabilize the climate.
2. Warmer temperatures may lead to drying in some areas. If deserts take over areas that were formerly forests or grasslands, this will make the planet's surface color lighter. A lighter planet will reflect more sunlight. This should help stabilize the climate.
3. As the climate gets warmer, high-latitude forests should expand toward the poles, covering areas that were formerly arctic tundra. Forests are darker than tundra, so the planet will absorb more sunlight, making the planet warmer still. This would tend to destabilize the climate.
4. Warmer temperatures may lead to more frequent wildfires. The net result of more frequent wildfires is that younger, smaller trees replace older, bigger trees. More carbon dioxide is generated through burning the old, big trees than is consumed by growing smaller trees to replace them. The net effect will be to release more carbon dioxide into the atmosphere, amplifying the warming trend. This would tend to destabilize the climate.
5. As soils become warmer, decay rates of organic matter in soils will accelerate. Decay of organic matter in soils generates carbon dioxide, methane, and nitrous oxide. These are all greenhouse gases, so they will make the planet warmer. This would tend to destabilize the climate.
As these five examples show, some biological responses to global warming will tend to counteract global warming and stabilize the climate. Other biological processes will tend to amplify global warming and destabilize the climate.
Which processes—the stabilizing ones or the destabilizing ones—have a bigger effect on the global environment? The best available scientific evidence indicates that on balance, all of these biological processes will have the net effect of amplifying global warming and destabilizing the climate, just the opposite of what the Gaia hypothesis would predict.
If this is true, then it is very important that we recognize it. If we naively believe that biological processes will stabilize the climate, we will underestimate the long-term consequences of global warming. Here's a specific example. For years, many people have believed that the biosphere would regulate atmospheric carbon dioxide levels: Higher carbon dioxide concentrations should make plants grow faster (and thus consume more carbon dioxide), and this should bring the carbon dioxide concentrations back down. Plants do in fact respond this way, but not nearly as much as one might think. Concentrations of carbon dioxide in the atmosphere have increased by about 35 percent since the Industrial Revolution. But according to the best available scientific data, plants have responded by consuming carbon dioxide only about 2 percent faster! Biological processes respond to atmospheric chemistry, but contrary to the Gaia hypothesis, they do not tightly regulate it, as far as we can tell from the scientific evidence.
Gaia's proponents have mostly resorted to theoretical computer models, rather than real-world data, to support their case. In Lovelock's Daisyworld model, for example, black daisies grow faster on a cold planet, thus making the surface darker and warming it up. Conversely, white daisies grow faster when Daisyworld is hot, making the planet's surface lighter and cooling it. The shifts in the daisy populations keep the planet's surface close to the biologically optimal temperature. The Daisyworld model clearly behaves like the Gaia hypothesis says it should, but is it a reliable guide to understanding how the real world works? Probably not. On balance, global warming is expected to make Earth's vegetation cover darker (as forests spread toward the poles), making the planet warmer still, just the opposite of what the Gaia hypothesis would predict.
Do organisms function as a global thermostat, adjusting the chemistry of the atmosphere to make the Earth warmer when it's too cold, and cooler when it's too hot? We now have good scientific data to answer this question. By painstakingly analyzing layers of ice in the Antarctic ice sheet, international teams of scientists have compiled a 400,000-year record of Earth's climate and atmospheric chemistry. Three important compounds recorded in the Antarctic ice are carbon dioxide and methane (which are greenhouse gases, helping to keep the planet warm) and sulfur aerosols (which aid in cloud formation, and thus help to keep the planet cool). All three of these compounds are biological byproducts.
If the biosphere worked like a global thermostat, like the Gaia hypothesis says it does, the biosphere should drive up carbon dioxide and methane concentrations during ice ages (to try to make the Earth warmer), and it should drive up sulfur aerosol concentrations when conditions are warm (to try to cool the Earth down). But what the planet has actually done, over the last 400,000 years, is the exact opposite. Carbon dioxide and methane concentrations are highest during warm periods, and sulfur aerosol concentrations are highest during ice ages. In other words, the biosphere appears to work like a global thermostat that has been hooked up backwards: It tries to make the planet warmer when it is warm, and cooler when it is cool. Indeed, peaks in carbon dioxide concentrations correspond to peaks in global temperature as far back as geological records are available—over 250 million years. Once again, this is just the opposite of what the Gaia hypothesis would predict.
Do biological forces reshape the natural environment to make it more livable for its inhabitants? Certainly some biological processes appear to work this way. For example, nitrogen-bearing compounds, like nitrate and ammonia, are essential nutrients for plant growth, and these compounds are scarce in some ecosystems. Many plants depend on nitrogen compounds that they can extract from their soil. Other plants, called nitrogen fixers, can make their own nitrogen compounds from atmospheric nitrogen, with the help of specialized bacteria in their roots. Some of the nitrogen that these plants fix for themselves leaks out from roots or is released as leaves decay. In this way, the nitrogen fixers supply an essential nutrient to the entire ecosystem.
The supply of nitrogen can be regulated through competition between nitrogen-fixing plants and those that can't fix nitrogen. Where soil nitrogen compounds are scarce, nitrogen fixers should be able to grow more rapidly than plants that can't fix nitrogen. They will therefore become more abundant. But since fixing nitrogen takes a lot of energy, nitrogen fixers should have a disadvantage (compared to non-nitrogen-fixers) in places where nitrogen is already abundant in the soils. Thus nitrogen fixers should become common, and thus supply nitrogen, in ecosystems where nitrogen is scarce, but not where nitrogen is already widely available.
Nitrogen fixation is an example of a biological process that works just the way the Gaia hypothesis predicts—it is self-regulating and makes the environment more livable. But other important biological processes do not act this way. For example, natural populations tend to keep growing until they have depleted their environments of essential resources (such as nutrients, water, food, and light). These resources eventually disappear so that populations can't grow any more. This phenomenon, which has been recognized by scientists for at least 200 years, has been termed the biological plunder of resources.
Biological plunder is widespread in nature. Where food is abundant, populations can grow until they completely “foul their own nests” with their waste products. In this case the resource that is plundered is the environment's capacity to handle wastes. On land, organisms typically reproduce until they exhaust the available supply of water or nutrients, or until they are so crowded that they starve each other of sunlight. On the open ocean, plankton take up nutrients, die, and sink. This process of biological plunder removes nutrients from the water, resulting in nutrient starvation for other organisms and making most of the open oceans into virtual biological deserts.
Evolution tends to favor characteristics that enable organisms to more effectively consume resources or eliminate wastes, even if they degrade their environment in the process. For example, trees are highly evolved to catch sunlight, even if they shade their neighbors. Plants in arid zones are highly evolved to intercept moisture before it reaches their competitors. Some tree species (such as eucalyptus and black walnut) even conduct a form of chemical warfare against other plants, by dropping leaves or fruits that make the surrounding soils toxic for other species.
These strategies may offend our human sense of fair play, but they demonstrate an important fact about evolution: Evolution usually favors characteristics that benefit individuals, whether or not the characteristics benefit the ecosystem as a whole. Evolution, in a nutshell, is simply this: Each generation is inevitably dominated by individuals whose parents had lots of offspring, and they tend to inherit their parents' characteristics. Thus over time, characteristics that help individuals to have many offspring will become more common, whether or not they make the environment better for other species, or even other members of the same species. As a result, the natural world is full of examples of organisms that enrich themselves at the expense of their environments, just the opposite of what the Gaia hypothesis would predict.
But now, think back to that warm spring day mentioned at the outset, and how perfect it seemed. If Gaia is not generally correct—that is, if organisms don't necessarily make the global environment more habitable—why is it that the Earth seems so perfectly suited to life?
The best answer to this question comes from Douglas Adams, the late author of The Hitchhiker's Guide to the Galaxy(1979). Years ago I overheard a radio interview with Adams in which he was asked to comment on Gaia. His reply, as best I can remember it now, was roughly this:
“Imagine a puddle, waking up in the morning, and examining its surroundings (a brief pause here, to let the audience imagine this rather odd notion). The puddle would say, ‘Well, this depression in the ground here, it's really quite comfortable, isn't it? It's just as wide as I am, it's just as deep as I am, it's the same shape as I am... In fact, it conforms exactly to me, in every detail. This depression in the ground, it must have been made just for me!’”
In other words: The world seems so perfectly suited to us because we have been so well matched to it. Organisms have to adapt to their environmental conditions, or else they don't survive. Thus the particular forms of life that we see on Earth will always be ones that are reasonably well matched to their environments. Those that are not well matched to their environments will not thrive and will not be noticed.
To us, the Earth seems to be remarkably well suited to human needs. But evolution has made it virtually inevitable that we would think this. Simply put, the predecessors of ours for whom the Earth was too hostile all went extinct, so their traits weren't passed on to us. Instead, we've inherited the characteristics of individuals who were well suited to the environment in which they found themselves, and therefore were able to survive and reproduce. Perhaps this helps to explain how the bounty of nature has become a theme in human thought. We are the winners of the evolutionary lottery, so it is not surprising that we would think of ourselves and the life forms around us, with whom we share the winner's circle, as the beneficiaries of an environment that has been tailored to our needs.
The controversy over Gaia isn't just another dry academic debate, because diverse groups have used Gaia to justify their political agendas. Two groups that have embraced Gaia are environmentalists and, paradoxically, industrialists. Environmentalists have argued that harming any part of the environmental “organism“ could have far-reaching consequences, while industrialists have argued that since Gaia can keep the environment healthy, pollution control is unnecessary. Indeed, in the 1970s Jim Lovelock himself used Gaia to argue against controlling pollutants that destroy the ozone layer, saying, “We tend to forget that pollution is a way of life for many natural species ... Our capacity to pollute on a planetary scale seems rather trivial by comparison, and the system does seem robust and capable of withstanding major perturbations.” Lovelock later admitted that he may have been wrong, but his comments demonstrate the hazards of using unproven science as a basis for public policy.
Gaia's proponents have done a great service by championing the need to understand the Earth as a coupled system. Scientists now need to figure out how that system works, and it is crucial that they get it right. Human activities are altering the global environment in ways that scientists are just beginning to understand. Environmental science needs to advance as fast as possible, to keep pace with mankind's growing influence on the global environment.
In the human enterprise of science, the hardest task is to see things as they are, rather than as we wish they were. Gaia's vision of Earth as a harmonious whole, engineered by and for the organisms that live on it, is emotionally very appealing. But compared to Gaia's notions of global harmony, the real Earth system—as science brings it into clearer focus—is proving to be more complicated, more intriguing, and perhaps more challenging to our notions of the way things should be. Understanding the Earth system, in all of its fascinating complexity, is the most important scientific adventure of our time. We should get on with it, as free as possible from any preconceptions of the way the world ought to work.
About the Author: James W. Kirchner is professor of Earth and Planetary Sciences at University of California, Berkeley. He also directs Berkeley’s Central Sierra Field Research Stations. His primary areas of research include watershed hydrology and geochemistry, geomorphology, evolutionary ecology, and analysis of environmental data.
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