From New Scientist: “My verdict on Gaia hypothesis: beautiful but flawed”

NewScientist

New Scientist

26 January 2015
Toby Tyrrell

We now have the evidence to pass judgement on James Lovelock’s wildly popular notion that life engineers hospitable worlds, says oceanographer Toby Tyrrell

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Climate cycles seem incompatible with the Gaia notion of a hospitable planet (Image: Michael Appelt/Anzenberger/eyevine)

LIFE has steered Earth’s environment over billions of years, helping to keep it stable and comfortable for living things. That’s the crux of James Lovelock’s Gaia hypothesis, which addresses enduring questions such as “how does our planet work?” and “how is it that Earth has remained continuously habitable for more than 2 billion years?”

Gaia is a fascinating hypothesis, but is it right? Working out the answer is particularly significant as we battle to be stewards of a planet with a human population of 7 billion and rising. If we don’t understand how our planet’s environment works, how can we know the best way to preserve it?

I first became interested in Lovelock’s idea when I read his book Gaia: A new look at life on Earth and became intrigued by the idea that our planet could regulate itself. That interest influenced my career, eventually leading me to oceanography. But at the same time, I wasn’t sure that things could really be as simple as Lovelock had suggested.

Lovelock made powerful arguments in favour of the Earth as a self-regulating system, but I could see no conclusive proof that the hypothesis was correct. So I began to investigate its feasibility in more detail, carrying out research into pertinent questions such as whether the ocean’s nitrate levels could be regulated by a “nitrostat” – a natural stabilising mechanism analogous to a thermostat.

I was also fortunate to attend several conferences on Gaia, in Valencia, Spain, and in Oxford, along with luminaries such as the evolutionary biologists Bill Hamilton and John Maynard Smith; Lynn Margulis, who established endosymbiosis, the idea that one organism can live symbiotically within another; and Heinrich “Dick” Holland, an expert in the great oxygenation event, when oxygen first accumulated in the air. These meetings were a cross-disciplinary melting pot and led to an exciting exchange of ideas of a breadth we seldom find today.

As I did more work, I realised that although there was a wealth of literature on Gaia, no one had made a thorough investigation of the whole hypothesis. I decided to attempt one. My approach was to dissect the Gaia hypothesis into component assertions, scrutinising each in turn.

Lovelock’s books and articles propose three main arguments for Gaia: (1) that Earth is an extremely favourable habitat for life; (2) that life has greatly altered the planetary environment, including the chemical composition of the atmosphere and the sea; and (3) that Earth’s environment has remained fairly stable over geological time.

Climate record

I tested these assertions against the latest scientific evidence. In the decades since the Gaia hypothesis was proposed, our knowledge has grown enormously. We now have long climate records from ice cores; we have devised experiments to test the idea of kin selection – that it can make evolutionary sense for an organism to sacrifice its own reproductive success to boost that of a close relative; and we can reconstruct ice-age landscapes and vegetation. I tried to determine not only whether each assertion seemed correct in light of modern data but also whether, if correct, it constituted strong evidence in favour of the Gaia hypothesis over alternatives.

Analysis of the first claim – that Earth’s environment is favourable for life – led me to look at ice ages. The environmental scientist Stephen Schneider considered these to be a strong argument against Gaia. I, too, found plenty of evidence that they are rather unfortunate times for life. During past ice ages the amount of terrestrial vegetation was reduced to about half that of warmer interglacial periods, and about three-quarters of the area now covered by shallow seas – the most productive parts of the ocean – became dry land when the sea level fell.

The main driver of ice ages is not life but Milankovitch cycles, the periodic variations in the way the Earth orbits the sun, which are purely a matter of physics. However, life is implicated in the low temperatures which allow ice ages to occur because life is involved in the carbon cycle, which in turn controls atmospheric carbon dioxide and thus Earth’s greenhouse warming.

Also unhelpful for life are the forms in which nitrogen is found on Earth. Vast quantities of it are present in the air and sea as inert molecules made of two nitrogen atoms, which only nitrogen-fixing microbes can make use of; much rarer are forms more easily used by life, such as nitrates. This leads to widespread nitrogen starvation despite the element’s superabundance. Earth’s nitrogen cycle is run almost entirely by microbes – but the outcome is the exact opposite of what ought to happen on a Gaian planet, on which life would be expected to engineer more favourable environmental conditions.

When I looked into Lovelock’s second claim, I found it to be supported. There is plenty of evidence of biological alteration of the global environment. For instance, life affects the planetary albedo – the degree to which Earth reflects solar energy back out to space – through the generation by ocean microbes of dimethyl sulphide, a chemical that influences cloud formation.

However, this effect, long held up as confirming Gaia, has turned out to be relatively weak (New Scientist, 29 June 2013, p 32). And there’s another catch. Although Lovelock’s second claim is clearly correct, it is not a clincher for Gaia because it could equally well support a competing idea. The “coevolution of life and planet” hypothesis posits that life and the environment influence each other but with no requirement that the outcome improve or maintain Earth’s habitability. There is no compelling reason to favour Gaia over this alternative.

What about Lovelock’s third claim, that the Earth’s environment has remained fairly stable over geological time? This is contradicted by evidence for climate cycles punctuated by ice ages. We also have evidence of long-term variations in the concentrations of the major ions in seawater, and of snowball/slushball Earth events, when our planet may have completely frozen over. There is also the great oxygenation event itself, which caused a mass poisoning of anaerobic organisms.

I also considered other topics, such as whether there is any obvious mechanism for Gaia. The hypothesis would be instantly more plausible if it could be seen to arise naturally out of evolution. Although I found no convincing reasons to believe this, I did come across some fascinating cases of “Gaias in miniature”. For instance, the interiors of termite mounds and wasps’ nests are strongly thermoregulated, experiencing much smaller day-night temperature fluctuations than the air outside. These stable internal temperatures come about partly because of how these social insects orientate their nests, but also through corrective group behaviours when brood temperatures drop too low or rise too high.

These are great examples of Gaia in action, but they do not lead us to expect that something similar must happen at the global scale. It turns out that communal regulation of a shared environment has so far been observed only in closely related individuals, whereas the global biota is the opposite – genetically extremely diverse.

My research led to a clear outcome: that the Gaia hypothesis is not an accurate picture of how our world works. Unfortunately, our planet is less robustly stabilised than Gaia implies, and therefore more fragile. In some ways it is a shame that this beautiful idea doesn’t hold true, but it is far better that we tackle environmental issues based on an accurate view of how our Earth system operates rather than a flawed one.

See the full article here.

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