by Fred Pearce
Do plants and animals really work in harmony or are they at war for control of the Earth's climate?
It is a breathtakingly simple idea. Our planet's atmosphere, and ultimately its climate, is the result of a battle between the two great kingdoms of the living world--animals and plants. Animals breathe in oxygen and breathe out carbon dioxide, while plants do the opposite. Carbon dioxide is a greenhouse gas, so it is a short step to concluding that animals warm up the planet, and plants cool it down. And if one side or the other gets the upper hand, we could be headed for a hothouse world or the next big freeze. It's amazing that nobody has thought of it before.
For years, the prevailing notion has been very different. The natural culprits in any atmospheric imbalance are seen to be the belching of volcanoes and the weathering of rocks, while animals and plants are thought to live in harmony, each inhaling the other's breath. But Greg Retallack, a soil scientist at the University of Oregon in Eugene, has other ideas. He says that wherever and whenever you look, the "breath" of plants and animals doesn't balance out. This imbalance occurs, he says, on timescales from a few months to hundreds of millions of years. And he is about to publish evidence that this has driven the atmosphere's chemistry and temperature almost from the day the first bugs started rooting around in the primeval swamp.
Retallack's story is one of a great evolutionary conflict between plants and animals, in which the competitors range from termites and dinosaurs to grass and lignin--the stuff that makes plant cell walls stiff and stops trees from toppling over. In this scheme of things, humans are just another bunch of critters mucking about with the carbon cycle, with plants no doubt already plotting their revenge.
If Earth's unique atmosphere was simply a result of gaseous eruptions from the planet's own core, then it has far less CO2 than it should have, and far more oxygen, an extremely reactive chemical that should have bound to other elements long ago. Not only that, but if you go back through the geological record, the temperature at the surface seems tightly tied to the chemical make-up of the air.
What is it that makes the composition of our atmosphere so unusual, and what gives it the small, but crucial, variations that drive the world's climate? The mainstream view, put forward almost 20 years ago by Bob Berner of Yale University, is essentially geological. It holds that there is a "dynamic balance" for a gas such as CO2. Volcanoes give out CO2 and weathering processes use it up, eventually dumping the carbon in sediments on the ocean floor. The balance between these two processes, so the theory goes, keeps reactive oxygen in the atmosphere, and any slight imbalance can trigger changes in climate.
An alternative explanation is that life is rather more actively involved in our planet's air conditioning system. The Gaia theory, first presented by British scientist James Lovelock, holds that living organisms have in some way taken charge of the atmosphere, stabilising its chemical make-up to favour life on Earth. The planet's thermostat is in biological rather than geological hands and the atmosphere, says Lovelock, has become a "cybernetic extension of life".
While Lovelock's idea of a giant self-regulating organism remains highly controversial, no one doubts that life is a major force on the chemistry of the atmosphere. For example, the actions of living organisms in soils speeds up Berner's weathering processes many times over--and the warmer it is, the more active these organisms are. Without life, there would be no dynamic balance.
But Gaia's cybernetics is not a matter of smooth micro-tuning, says Retallack. The thermostat is crude and its action is often violent. Fossils and sediment isotope ratios show that temperatures and CO2and oxygen levels in the atmosphere have all fluctuated significantly in the distant past, with CO2 levels rising as oxygen levels fall and vice versa. Maybe an analysis of these fluctuations, of the planet's "breath", can reveal what is going on.
The first pungent whiff of this breath came in the late 1950s when Charles Keeling of Scripps Institution of Oceanography in La Jolla, California, began measuring CO2 levels in the clean air on top of Mauna Loa, a dormant extinct volcano in Hawaii. Keeling discovered that throughout the year the CO2 levels fluctuated by around 2 per cent. Researchers now agree that this is due to the seasonal cycling of CO2 into and out of living things. Plants and other organisms that grow through photosynthesis consume CO2 from the air, especially in spring. But during autumn and winter, photosynthesis largely stops and the photosynthesisers are eaten by soil bacteria, fungi and animals. They exhale CO2, pushing atmospheric levels back up again. Because most of the vegetation on the planet is in the northern hemisphere, the atmosphere loses CO2 in the northern summer and gains it again in the winter.
The annual increase in atmospheric CO2 now slightly exceeds the decrease because we are burning fossil fuels and adding an extra dose. But Retallack wondered if such oscillations also occur over longer timescales. Could the Earth's climate have been shaped by this struggle for dominance between plants and animals, or more properly between the photosynthesisers and respirers--the consumers and producers of CO2?
Here is how the history of life on Earth looks from this perspective. The story starts over 500 million years ago. Although the Sun was weaker than it is today, the Earth was warm because there were greenhouse gases in profusion. CO2 levels were 20 times higher and life was having a spring party. This was the age of the Cambrian "explosion". All sorts of animals were occupying the planet. Retallack, an expert on fossilised soils, says soils were full of respirers emitting CO2 into the air. Fossil soils from the time are riddled with the remains of millipede burrows. But the party didn't last--not for the animals, at any rate. For between 450 and 350 million years ago, CO2 levels crashed. It was the big chill.
What happened? Retallack believes that in the evolutionary struggle for supremacy, plants had their revenge. Fossils show that this was when vascular plants that contained lignin first emerged. Lignin made cell walls rigid. It allowed plants to grow large, and the first trees emerged.
Big, green and out there
"Lignin gave plants an edge," says Retallack. With more and bigger plants growing in unprecedented profusion across the planet, rampant photosynthesis was sucking the CO2 out of the air. And as this vegetation sank roots deep into the ground, the weathering of rocks and the formation of soils also went into overdrive, consuming more CO2. The hungry respirers, unable to digest lignin, could do nothing about it. In a world where CO2 levels were probably lower than today's, ice caps formed at the South Pole. Swamp forests were burying huge amounts of atmospheric carbon by creating thick peat bogs and coal deposits. It is only today that the carbon in these coal seams is being released back into the air, thanks to the burning of fossil fuels.
The animals eventually fought back through further evolution, says Retallack. The heroes were termites and dinosaurs, which learned how to eat lignin and prospered. Fossil soils from that time, says Retallack, are full of termite nests and the tracks of large dinosaurs, especially giant vegetarians such as the sauropods. Animals regained the upper hand for perhaps 200 million years. Their heavy breathing and destruction of vegetation raised CO2 levels in the air to more than three times current levels. It was tropically warm and wet across most of the planet. The greenhouse had returned.
But plants weren't finished yet. After the dinosaurs disappeared 65 million years ago, wiped out by an asteroid impact or other calamity, plants seized their chance. The emergence of the first grasses was the breakthrough. Grass doesn't hold much CO2 itself, but it can create mollisols, soils that are very rich in organic matter and hence carbon. "Typically they are 10 per cent organic matter down to a depth of a metre, whereas forest soils are only that rich down to about 10 centimetres," says Retallack. So a grassland ecosystem can, despite appearances, contain more carbon than a forest ecosystem.
Over the past 40 million years or so, tall grasslands spread across the globe, taking over many formerly forested zones. These ecosystems, Retallack argues, took control of the planetary thermostat, securing lower CO2 levels for their own advantage. New grazing animals evolved to live on and coexist with the grasses. "The co-evolution of grasses and grazer created a carbon-hungry ecosystem of a kind never before seen," says Retallack. "I think mollisols are saving our skins right now. Without them the world would be a lot hotter."
As the Earth cooled under the influence of grasslands, it seemed to hit an era of abrupt swings into and out of ice ages, beginning about 5 million years ago. Could this too be explained by the battle between plants and animals? Retallack thinks it could, but many scientists disagree.
Ice ages come along roughly every 100,000 years. The conventional theory is that these are caused by a wobble in the Earth's tilt known as a Milankovitch cycle, which also lasts 100,000 years. Although it doesn't make much difference to the overall amount of solar radiation reaching the Earth, it does alter where it falls. And then something in the climate system kicks in to amplify the effect and plunge the planet into and out of glaciations. Trouble is, no one knows what that feedback is.
The composition of the atmosphere clearly plays a role. Air bubbles trapped inside ice cores show that the rise and fall of temperatures corresponds to fluctuating levels of greenhouse gases such as CO2. Levels of CO 2 jump by around 30 per cent as the world emerges from each glaciation. This seems to suggest that these gases amplify the Milankovitch wobbles.
But Retallack goes further. He wonders if the fluctuations in CO2 could be a result of natural battles between the Earth's living kingdoms. "I am not impressed with the Milankovitch theory. I think ecosystems are in control," he says. For one thing, the wobble in the Earth's tilt does not obviously match actual temperature changes, says Retallack. Equally troubling, the wobble produces a smooth change in solar radiation, whereas the glaciations are not smooth. The world cools gradually over tens of thousands of years, and then warms again in less than a tenth of the time. "Something snaps," Retallack says. But what? And would it snap anyway, without the planetary wobbles?
Dishing the dirt
Retallack admits there are no straightforward answers to this. But he believes soils hold vital clues. His research has revealed strong fluctuations in the make-up of soils in the middle of continents as the ice ages come and go. "They switch from humid grasslands to dry sagebrush and back." Soils in the wet periods are full of earthworm pellets. In dry times they contain cicada burrows. Retallack believes this shows that the carbon economy of these soils is synchronised with global CO2 levels.
How does this follow? The conventional view is that these changes merely represent the response of ecosystems to climate change. But Retallack believes it may be the other way round: the ecosystems drive the glaciations, as carbon enters soils when grasslands dominate and leaves again in sagebrush eras. The jury is still out on this idea. But Retallack is not alone in thinking biology could be important in ice ages. Lars Franzen of Gothenburg University in Sweden has argued that the formation of peat bogs, which draw large amounts of CO2 out of the atmosphere, could pull the world into glaciation--and push it out again as the bogs decay under the ice. In any event, with evidence growing that biology can drive climate change on other timescales, it cannot be dismissed.
Retallack's ideas were unveiled at the Geological Society of America's annual meeting in Reno last November. It was a difficult audience: geologists are, after all, used to seeing the processes they study as the drivers of the planet's environment. Even Gaians are playing hard to get. One, Lee Klinger of the National Center for Atmospheric Research at Boulder, Colorado, says the story cannot be as simple as Retallack suggests: "He makes no mention of the ocean biota, which certainly impact CO2. Nor does he properly consider changes in peatlands, which contain one or two orders of magnitude more carbon per unit area than either forests or grasslands." Lovelock sees another gap: "He has ignored Gaia's farts. Methane was crucial to the health of the early atmosphere and still plays an important role." But both Klinger and Lovelock think Retallack should pursue his idea further. "I wouldn't want to put him off," says Klinger.
Retallack says he has come across a huge reluctance to publish some of his claims. "In particular, the idea of grasslands causing cooling has excited great opposition, even though I have a huge amount of evidence to support it," he says. The data will appear shortly in The Journal of Geology, after being rejected by a series of major journals. "I have data from 2000 soil samples, from Oregon and the Great Plains in the US to Kenya and Pakistan. They all show the rise of mollisols as grasslands evolved, till they covered about a fifth of the planet. That's a lot. You'd expect them to have an effect," he says.
Retallack is at pains to say he does not discount the power of geology. He is no Gaian purist. He even admits that the geology-based theories are right now "probably closer to proof" than his own. "Meteorite impacts, volcanic eruptions, hot-spring degassing and Milankovitch control are all well accepted by most scientists," he says. "But I think there is a middle way." He believes that biology has played a crucial role in the switchback of climate change over much of our planet's history. And, being a pedologist, he is convinced that the evidence lies in the soil.
From The New Scientist, 16 June 2001, page 30. Retallack, an Australian paleopedologist at the University of Oregon, says of his thesis, "the idea of grasslands causing cooling has excited great opposition, even though I have a huge amount of evidence to support it."