James Hutton wrote in 1785 that the Earth is alive. For example, the circulation of blood is similar to the water cycle.
"A book on a serious topic should be written so that any intelligent lay person could read it. The English language is surely flexible enough to permit the explanation of almost anything that we can understand."
To figure out if there is a planet-sized life-form, we must first know the telltale signs of life. Life is basically negentropy. When something differs greatly from the background, enough to be a recognizable structured form, that is likely either life, or a product of life.
We can imagine an 'equilibrium world' where all the Earth's elements are mixed until all the natural chemical reaxions have gone to completion.
Life means chemical disequilibrium. Disequilibrium is the state from which energy can be extracted. We know the proportions of elements on Earth, so in theory they should be spread out equally in compounds they naturally bond to form, like CO2.
A bird's nest is not life, but is a product of life. Similarly, the Earth's chemically unlikely atmosphere is not alive, but a habitat made by Gaia. It would be easier to detect this
Negentropy inside the skin, entropy outside.
"One of the most characteristic properties of all living organisms, from the smallest to the largest, is their capacity to develop, operate and maintain systems which set a goal and then strive to achieve it through the cybernetic process of trial and error. The discovery of such a system, operating on a global scale and having as its goal the establishment and maintenance of optimum physical and chemical conditions for life, would surely provide us with convincing evidence for Gaia's existence."
If the atmosphere is a circulatory system, there must be 'carrier compounds' that shuttle essential elements around. This is one of the testable conclusions of the Gaia theory. "It was rewarding to find evidence that both were conveyed from the oceans, where they are abundant, through the air to the land surface, where they are in short supply."
The troposphere (lowest layer) is warmer at the bottom, hotter at the top, but the stratosphere (next layer up) is the inverse. Because of this, hot air does not rise easily in the stratosphere and it remains quite stable. In the ionosphere (above the stratosphere), most molecules split, except for CO and nitrogen. Exosphere is above that, and is so thin it's similar to space. Hydrogen escapes into space, but is replaced by hydrogen coming in on solar winds.
There are intermediate gases involved in atmospheric chemistry, like CO and all the free radicals.
Life removes excess CO2 from the air by photosynthesis and other processes. The more CO2 there is, the more it takes in - responsively, cybernetically.
Atmospheric CO2 is balanced by reacting with water to make bicarbonic acid. There is 50 times as much carbon dioxide in this form than in the air. If the level in the air falls, it comes out of the sea. There is also abiological balancing performed by rocks. Life uses an enzyme (carbonic anhydrase) to speed up the absorption of CO2 and make it form chalk and limestone. The mechanical breakup of soil by life provides more surface area for carbon exchange with the air.
CH4 and CO2 should combine to make CO2 and offgas H2 to space. But life is producing a billion tons of CH4 a year to balance it. Without life, there would be no CH4 left in the air. Gaia makes this from anaerobic bacteria and farting animals. The Gaia hypothesis allows us to consider that gases of the atmosphere have purposes, so what is the purpose of methane? It sweeps oxygen and heavy metal methylates from anaerobic swamps. It oxidizes to water vapor in the stratosphere. When the hydrogen from that escapes, the oxygen is added to the atmosphere, so CH4 makes oxygen.
Oxygen concentration determines how easily combustion can happen. More oxygen would make for a more energetic chemistry. Nearly all oxygen released into the air is used again quickly by the biosphere.
Oxygen is taken up by things rusting, but new oxygen is released when the carbon from carbon-oxygen compounds in dead plants binds to rock. Buried carbon can either become methane and do the chemistry just described, or it can remain in the ground. Lovelock thinks the proportion between these two uses regulates O2 levels.
UV light splits O2 in the stratosphere, and some of it reforms as O3. Nitrous oxide decomposes in stratosphere to make nitric oxide which splits ozone. It regulates O3 levels. Methyl chloride produced by the biosphere may do the same.
Methane oxidizes in the lower atmosphere. Without this O2 sink, O2 concentration would increase 1% every 12000 years, and the planet would burn.
N3 in the air builds mass/pressure and keeps things inert. It would have been taken into the sea as nitrate ions without Gaia. Life takes nitrogen out of the sea into the air. This controls salinity (nitrate salts) and reduces the toxic effects of the nitrate ion itself on sea life.
NH3 regulates the acidity of the planet. It limes the world. "In the absence of ammonia the rain everywhere would fall at a pH close to 3". NO2 and NH3 behave similarly to CO2 and CH4; life is producing the NH3 to make up for what is lost.
The biosphere now makes a billion tons of NH3 a year. "This quantity is close to the amount needed to neutralize the strong sulphuric and nitric acids produced by the natural oxidation of sulphur and nitrogen compounds: a coincidence perhaps, but possibly another link in the chain of circumstantial evidence for Gaia's existence."
Nitrous oxide takes oxygen from soil to air.
The amount of hydrogen determines pH. So the level of hydrogen must be right for an environment to be friendly to life.
Over time, hydrogen escapes to space and a planet's chemistry becomes oxidizing.
Another balancing act of early life: CH4 turning to CO2 and sulphides turning to sulphates during the Great Oxygenation Event made things more acid. Gaia must have alkalized things somehow.
Earth has never been "wholly unfavorable to life" for one minute of the past 3.5 billion years. But the sun was 30% dimmer then. Chemistry must have changed, because otherwise the planet would've frozen then.
A greenhouse gas like CO2 or NH3 was needed to keep us warm then; CO2 is more likely. Albedo could have also compensated. Man and other organisms use multiple mechanisms - sweating, shivering - to regulate their temperature. Gaia could have turned the dials on albedo and greenhouse gases in the same way.
Two cooling processes happened together:  CO2 and NH3 were absorbed by the biosphere,  snow and ice made the planet whiter. Something must have balanced these cooling factors.
Hydrogen-rich environments cause reduction; hydrogen-poor environments cause oxidation. Things rust in our air because it is low in hydrogen.
We run by combusting oxygen, but it could have been done with hydrogen.
Life began in hydrogen-rich, reducing conditions. This kept O2 out of the air. The sun was dimmer then than it is now, but CO2 kept us warm.
The switch from H-fueled reductive life to O-fueled oxidative life "was probably the most critical period of all in the history of life on the Earth." It was caused by hydrogen escaping to space, possibly because the biosphere offgassed it. Then there was no atmosphere for a while, then a new one came out of the ground.
The sea is a sink of dissolved gases that can be taken or given from the air to keep things in balance.
The corpses of diatoms are a cybernetic silicon sink. More silica > more diatoms > more silicon-removal. "and this is well known to occur"
More sulphur is washed into the sea than can be accounted for by run-off. Where is the extra sulphur coming from? Lovelock thinks dimethyl sulphide is going from the sea, through the air and being washed back. Sea life methylates sulphur to turn it into a gas to get rid of it. (Life may also use this method to get rid of poisons like mercury and arsenic.) There are algae (like polysiphonium fastigiata) in shallow waters that specialize in this. Other seaweed may convey iodine to the land in the form of methyl iodide, acting like Gaia's thyroid gland.
Shallow seas also add oxygen to the air by burying carbon from CO2.
Methyl iodide reacts with the chlorine ions in the sea to make methyl chloride, which reduces the level of O3 in the air.
Lovelock has not found a marine source that would convey phosphorous to land. He speculates migratory birds and salmon do it. He likewise believes that something in the sea is volatilizing selenium to convey it to land.
"Many algal species can assume both salt and fresh-water forms."
When dissolved in water, the ionic bonds that form salts like sodium chloride become too weak to hold them together, and the constituent ions wander about freely.
Large biological molecules dock to other large biological molecules by electrostatic attraxion. If there are too many dissolved salt ions around, these ions smother the docking sites. Conversely, if salinity is too low, the reaxions happen too easily and too often. Wacky salt levels at cell membranes play similar mischief with ion pumps that take things in and out of cells.
There is evidence that the salinity of the seas has not changed much for perhaps billions of years.
One theory is that the salt in the sea was gradually washed in from the land over time and had no way out. The salinity of water in living bodies reflects the salinity of seawater at the time they evolved. Lovelock doesn't think much of the theory, because we know 540 megatons wash in yearly, and that is too much to fit this theory. (A problem made worse when you factor in the salt added by volcanic action on the seafloor.) This picture is missing a way of getting salt out of the sea; Lovelock thinks this must be under a Gaian control mechanism. Previous theories have been mostly abiological. One theory is that positive ions settle to the seabed, while negative ions crystallize and form rock salt in isolated lagoons. If lagoons are somehow created by life, that would be a Gaian control mechanism. Lovelock proposes that coral reefs could form lagoons. He also thinks that enough silica deposited on the seafloor could create volcanoes by increasing pressure and heat (insulation).
As a further complication, the sea keeps a balance of positive and negative salt ions, but the run-off from the land contains more positive ions. So the removal system must preferentially remove positive ions.
Lovelock believes he has a testable hypothesis to see if it Gaia is cybernetically controlling sea-salt. If there are organisms that are particularly sensitive to rising salinity, they could die when salinity rises, and as their bodies fall to the sea-bed, they trap salt. But this can only account for part of the salt-sink we are looking for.
It's a natural phenomenon. Our planet is littered with plutonium and uranium from a supernova that happened nearby when it was forming.
Life is used to radioactivity. During early evolution it was more intense than now because the radioactive rocks had not yet decayed. This radiation probably speeded up evolution by splitting bonds. There was no ozone layer to block solar radiation then either.
Even early, pre-oxygen life must have used sunlight for energy i.e. must have photosynthesized.
Lovelock is confident in the ability of life (though not necessarily human life) to survive without an ozone layer, or through a nuclear war. A team of scientists checked out Bikini Atoll and marine ecology there was not very much disturbed. The US National Academy of Sciences reported in 1975 that 10000 megatons of warheads going off would affect man-made ecosystems for a little while, and things would be basically back to normal after 30 years. "It seems that to delete life from our planet without changing it physically would be well-nigh impossible."
Rachel Carson wrote Silent Spring on the damage pesticides do. "pesticide residues were present in all creatures of the Earth, from penguins in Antarctica to the milk of nursing mothers in the USA"
Pollution is inevitable. "To grass, beetles, and even farmers, the cow's dung is not pollution but a valued gift. In a sensible world, industrial waste would not be banned but put to good use."
"The very concept of pollution is anthropocentric and it may even be irrelevant in the Gaian context." "Our uncertainties about the future of the planet and the consequences of pollution stem largely from our ignorance of planetary control systems." "any attempt to understand the consequences of air pollution would be incomplete and probably ineffectual if the possibility of a response or an adaptation by the biosphere was overlooked. The effects of a poison on man are greatly modified by his ability to metabolize or excrete it."
"We have increased the carbon cycle by 20 per cent, the nitrogen cycle by 50 per cent, and the sulphur cycle by over 100 per cent. As our numbers and our use of fossil fuels grow, these perturbations will grow likewise. What are the most likely consequences? The only thing we know to have happened so far is an increase in atmospheric carbon dioxide of about 10 per cent and also an increase in the burden of haze attributable to particles of sulphate compounds and soil dust."
Krakatoa released chlorine into the atmosphere, enough to destroy 30% of the ozone layer. Yet somehow Gaia dealt with it. So why worry about ozone-destroying chemicals from industry? The biosphere produces other ozone-destroying chemicals like nitrous oxide and methyl chloride.
Slash-and-burn agriculture is releasing methyl chloride pollution, 5 million tons a year of it, which may be greater than the amount that comes off the sea.
Pollution alone can be absorbed, but if we simultaneously pollute and disrupt ecosystems, especially tropical forests and shallow seas, we run into trouble.
"shifting balance of power" between Gaia and Man. "The larger the proportion of the Earth's biomass occupied by mankind and the animals and crops required to nourish us, the more involved we become in the transfer of solar and other energy throughout the entire system. As the transfer of power to our species proceeds, our responsibility for maintaining planetary homeostasis grows with it"
"all attempts to rationalize a subjugated Earth with man in charge are as doomed to failure as the similar concept of benevolent colonialism." "The Gaia hypothesis implies that the stable state of our planet includes man as a part of, or partner in, a very democratic entity"
Three principles for dealing with Gaia:  She keeps homeostasis  Her vital organs are in the tropics and shallow seas  She can regulate some things quickly and some things slowly; we're in trouble if we make rapid changes to something she can only correct slowly.
An interesting question is whether our collective intelligence and the actions of our species are influenced by Gaia, e.g. by being programmed by biophilic instincts. We alone could potentially protect the planet from a meteor strike, or expand life "beyond the planet.
People who live with nature, like Eskimos, have a body of oral tribal knowledge which is drawn from the natural world around them. Civilization has a much bigger, stored, tribal knowledge that does not much come from the biosphere. Most urban knowledge is about man-man interactions, not man-species interactions.
"the universe appears to be littered with life's chemicals", "it seems almost as if our galaxy were a giant warehouse containing the spare parts needed for life". For modern evidence of this, see here, and here
Lovelock downplays the Ice Ages, saying the glaciers only came to 45 degrees. Is that still believed?
"My father never told me why he believed that everything in this world was there for a purpose, but his thoughts and feelings about the countryside must have been based on a mixture of instinct, observation, and tribal wisdom. These persist in diluted form in many of us today and are still strong enough to power environmental movements which have come to be accepted as forces to be reckoned with by other powerful pressure groups in our society. As a result, the churches of the monotheistic religions, and the recent heresies of humanism and Marxism, are faced with the unwelcome truth that some part of their old enemy, Wordsworth's Pagan, 'suckled in a creed outworn', is still alive within us." (These movements also have recently incorporated environmentalism into their values.)
Computer models show that a more biodiverse ecosystem is more stable.