For 250 million years, throughout the age of the dinosaurs, the Earth was warm and wet, almost tropical. Then around 40 million years ago, the planet began to cool. North America drifted away from Europe, India slammed into Asia, and new habitats were created. Antarctica iced up, temperate climates took over from some previously luxuriant tropics, and grassland and desert replaced temperate zones. Around 15 million years ago, the Earth cooled even more, leading to the glacial periods that have dominated life in the northern hemisphere ever since. What was it that reset the global ‘thermostat’? (see Graphic)
The Victorian father of geology, Charles Lyell, was optimistic about finding an answer to just such a question. Writing in 1875, in the 12th edition of his Principles of Geology, he reflected that, in the 45 years since he had first ‘tried to account for the vicissitudes of climate by reference to changes in the physical geography of the globe’, geologists’ knowledge of the subject had greatly increased. He speculated that if the land had been subdivided more, existing as islands, the Earth’s climatic history would have been more uniform. If there had been mountains higher than the Himalayas, especially in the higher latitudes, ‘there would be a greater excess of cold’, he wrote.
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With hindsight, Lyell’s hypothesis seems remarkably prescient. For as geologists and climatologists come ever closer to answering the ‘thermostat’ question, it seems that one of the world’s most prominent topographical features, the vast high plateau of Tibet, may have played a large part in reducing the Earth’s temperature. Some geochemists now suggest that the rising plateau, forced up from the seafloor as India crashed into Asia, was responsible for reducing the concentration of carbon dioxide in the atmosphere. The gas traps heat radiated from the Earth’s surface, so as the level of carbon dioxide dropped, they argue, the Earth radiated more of the energy absorbed from the Sun and began to cool. The idea is remains controversial, because it relies on a unique event to explain the ‘thermostat’ question.
During the past decade, scientists have devised mathematical models that replicate the ‘vicissitudes’ of the global climate. In 1983, Robert Berner of Yale University showed that there is a close relationship between the carbon dioxide in the atmosphere, the carbon stored in the oceans and on land, and the gas released by volcanic and other mantle activity.
Dynamic balance
Berner argued that there had to be a dynamic balance between the processes that remove carbon dioxide from the atmosphere and those that restore it. Without such a balance, the concentration of carbon dioxide in the atmosphere might increase and lead to a hot climate similar to that of Venus, where a high level of carbon dioxide traps almost all the incoming solar energy. Or the carbon dioxide could have disappeared altogether, making the Earth cool down dramatically.
In modelling the Earth’s carbon cycle over 600 million years, Berner found that he had also produced a mathematical version of the Earth’s climate history, which was generally endorsed by the geological record over this period. In the cycle, carbon dioxide in the atmosphere dissolves in rainwater to form a mild acid (carbonic acid), which reacts with limestones and granites to produce calcium, magnesium and bicarbonate ions and silica. These are eventually deposited as sediment on the seafloor and can become embodied in magma as a result of tectonic activity at the Earth’s crust. The carbon dioxide locked away in these transformed sediments is eventually released into the atmosphere during processes such as seafloor spreading and when rocks under stress undergo metamorphic changes. Carbon dioxide is also released by the decomposition of buried organic remains.
The model also takes into account changes in land area and mean elevation, changes in the atmospheric concentration of carbon dioxide, the effects of plants, and the rate at which sediments on seafloors release the gas.
After constructing a dozen equations from these variables, Berner explored how levels of atmospheric carbon dioxide changed over the past 600 million years (see Figure 1); he was even able to calculate short-term variations over just tens of millions of years. According to his model, the concentration of carbon dioxide more than 500 million years ago was 18 times higher than at present. Then over the next 200 million years, the level dropped to one that is similar to today’s, with the climate becoming glacial. Next followed a period of global warming, accompanied by or perhaps driven by an increase in atmospheric carbon dioxide. Then came yet another net downswing around 120 million years ago. The model’s predictions agree broadly with what the geological record of the past 600 million years suggests about the Earth’s climate.
Despite its overall success, however, the model fails to predict the timing of the downturn of the global thermostat 40 million and 15 million years ago. In Berner’s model, the greatest fall in carbon dioxide happens between 100 and 50 million years ago, yet the cooling in this period was small. Instead, the evidence derived from sediments suggests that the Earth cooled substantially between 50 million years ago and the present – something not predicted by Berner’s model. This discrepancy is so significant, argue Maureen Raymo of the Massachusetts Institute of Technology (MIT) and William Ruddiman of the University of Virginia, that a more subtle model is now needed. They believe that the explanation behind the current ice age must lie in something other than a simple balance between volcanic outgassing and weathering.
In the mid-1980s, when Raymo was a postgraduate at Columbia University in New York, she put forward a simple yet far-reaching proposal. When India and Asia collided, she argued, the rising up of the Tibetan plateau somehow altered climatic conditions so that the Earth radiated more energy than before, setting off global cooling. The idea was originally regarded with suspicion, but computer simulations show that this theory does produce a worldwide cooling effect that corresponds well with climate changes over the past 50 million years.
The Tibetan plateau lies between the Himalayas in the south and the Kunlun Mountains in the north. It covers about 2.2 million square kilometres, or 0.4 per cent of the Earth’s total surface area, with an average height of about 5 kilometres above sea level. Raymo likens the plateau to a giant boulder thrust into the atmosphere, so large that it profoundly disturbs the atmospheric circulation patterns in the whole of the northern hemisphere. In the late 1980s, John Kutzbach of the University of Wisconsin in Madison ran global climate models with and without the plateau and discovered that if there were no plateau, there would be no Indian monsoon.
This is the key to Raymo’s hypothesis. She suggests that the uplift of the plateau created the patterns of air circulation that bring water-laden air off the Indian Ocean in summer and deliver monsoon rains to the Indian subcontinent, including the Tibetan plateau’s southern ramparts. Dissolved in the torrential rain was carbon dioxide, forming weak carbonic acid. The higher the uplift, the greater is the volume of water-laden air drawn off the ocean by the stack of rising warm air, and the greater the rainfall. And the greater the rainfall, the more carbon dioxide is removed from the atmosphere. A combination of chemical weathering by the acid rain and fast physical erosion of the plateau’s rock delivers the carbon dioxide, in the form of bicarbonate ions, to the oceans.
In essence, Raymo saw the plateau as a giant carbon dioxide extractor, pumping the gas out of the atmosphere through rainfall and then dumping the by-product in the oceans. But, as she admits, there is one flaw in her theory: it does not explain why there is any carbon dioxide left in the atmosphere – her extractor pump would have emptied the atmosphere of the gas in less than 100 000 years.
Oversimplification
In the course of her work, Raymo found that Berner’s original model oversimplified the influence of the height of continents on the erosion caused by rain. The model assumed the erosion varied simply with land area – the greater the area, the greater the erosion. Raymo and Ruddiman drew on research done at MIT in the early 1980s, which surveyed the world’s largest rivers, such as the Amazon, to gauge their weathering effects. This work showed that the mountainous relief of a continent has a greater influence on weathering than its surface area. Most of the Earth is covered by well-weathered, mainly flat regions that are drained by large rivers carrying little dissolved material. In mountainous regions, by contrast, heavy rainfall and melting snows cause extensive physical erosion: not only is solid matter carried downstream, but new surfaces become exposed to chemical erosion by acid rain. The steeper the mountains, the greater the wash-down effect of the torrents, and the greater the chemical erosion.
According to Raymo and Ruddiman, the rising up of the Tibetan plateau is an extraordinary event in the history of the world’s climate. Very high, with steep sides, the plateau is also close to a warm ocean capable of providing huge quantities of rain. Eight major rivers drain it, including the Ganges, Brahmaputra, Yangtze, Indus and Mekong. Together, they carry 25 per cent of the dissolved material that reaches the world’s oceans – yet the area they drain, including the plateau, is less than 5 per cent of the Earth’s land area. So the effect of the plateau’s weathering should be greater than that suggested by its area alone. Also, because India is still pushing up against Asia, there is a positive-feedback effect: the higher the plateau rises, the greater the monsoon rains.
Raymo cites other evidence to support her idea. Pollen analysis shows that the vegetation of Tibet, and therefore the climate, has changed dramatically since the plateau was formed, in line with cooler temperatures. More importantly, studies based on the isotopes of strontium, osmium and oxygen have indicated how levels of carbon dioxide in the atmosphere have changed over the past 600 million years. These studies confirm that temperature-enhanced weathering effects coincided with the collision of India and Asia.
Strontium behaves chemically like calcium. It undergoes the same chemical weathering cycle as calcium and magnesium carbonates and silicates, and is also deposited on the seafloor. Strontium has two isotopes, strontium-86 and strontium-87, which occur in subtly different abundances in different rocks. The ratio of strontium-87 to strontium-86 can be measured to an accuracy of 1 part in 100 000. When the strontium eventually arrives at the seafloor, deposits from different rock sources are thoroughly mixed up. But if one source predominates, then the ratio of the abundance of strontium-87 to strontium-86 in the sediment moves towards the ratio of strontium isotopes in the source rock. So, argued Raymo, the strontium isotope record should reveal where the sediment had come from. And this in turn could show whether the weathering history of the plateau tied in with Raymo’s theory that there was rapid cooling 15 and 40 million years ago.
Evidence from isotopes
Strontium enters marine sediment from land from two main classes of rocks – silicates and limestones. Limestones have a low ratio of strontium-87 to strontium-86, ranging from 0.706 to 0.709, while silicates have a high one, with some granites having ratios greater than 1. So enhanced weathering of granites would increase the ratio in ocean sediment.
Strontium also enters the oceans through the leaching of newly created rock emerging from places such as the mid-Atlantic ridge. This strontium has a low ratio of strontium-87 to strontium-86. But according to Frank Richter of the University of Chicago, the process accounts for only around 25 per cent of the total deposits of strontium in the ocean floor; so the rest must be delivered by rivers because the ratio of strontium isotopes in the river sediment is high, high enough to account for the net ratio of strontium isotopes in the ocean. Richter found that the strontium ratio in the oceans began to increase rapidly around 40 million years ago, having been steady for about the previous 60 million years. It then rose even higher about 20 million years ago (see Figure 2).FIG-mg18804403.GIF Richter’s analysis is supported by the dating of rapid erosion in some parts of Tibet using argon isotopes.
Although the strontium ratio in the sediment could have been altered by processes outside the Tibetan plateau, such as erosion of the Canadian landmass or even glacial erosion in the Antarctic, the picture painted by this isotope pattern is supported by that of another isotope system, that of osmium. In continental rocks, the ratio of osmium-187 to osmium-186 is roughly ten times greater than the ratio of these isotopes in mantle material. Karl Turekian from Yale University suggests that if osmium-containing rocks were being leached from land into the sea, sediment records should contain evidence of changes in rates of deposition of its isotopes into the oceans. Indeed, the osmium records show similar net changes to the strontium record over the past 60 million years.
Raymo’s theory is also supported by a third isotope system, that of oxygen-18 and oxygen-16. Oxygen data for the past 55 million years shows a marked change over time. As global cooling occurs, the ratio of oxygen-18 to oxygen-16 in marine calcite sediments rises. Nick Shackleton from the University of Cambridge has shown that over the past 55 million years the ratio has tended to increase, with most rapid changes occurring about 36 million and 15 million years ago.
Other evidence in favour of Raymo’s theory comes from climate models worked out by Kutzbach, along with Warren Prell of Brown University, Rhode Island, who were investigating which factors most influenced the Indian monsoon. Last year, in computer experiments carried out at the National Center for Atmospheric Research in Boulder, Colorado, they looked at how the presence of mountains affects the monsoon. The results were sensational: with no mountains, the temperature over southern Asia would have been 12 °C higher. But with half mountains and full mountains, the land temperature falls and the precipitation increases as the elevation increases. This increases soil moisture and water runoff, which in turn encourage chemical weathering.
How does this affect the rest of the northern hemisphere? In summer, hot dry air rises over the plateau and draws in moisture-laden air from the Indian Ocean, causing the monsoon rains. The column of hot, dry air then cools and sinks over nearby regions: for example, Mediterranean and Central Asian summers are the caused by this dry air. Meanwhile, Southeast Asia and India become wetter and stay warm. The desert landscape that lies to the north and east of the Tibetan plateau is also a result of the extraordinary uplift of the plateau.
In Europe, the models predict colder winters and colder summers and, as Kutzbach notes, the record of European vegetation over the past 20 million years does indicate a fall in temperature. We may have the Tibetan plateau to thank for the Gobi and Sahara, for the evolution of the grasses that were domesticated into wheat 9000 years ago, and for the ice ages that accompanied the evolution of mankind
Which came first?
Some researchers, however, are still not convinced by Raymo’s ideas. Peter Molnar of MIT and Philip England of the University of Oxford pose the most radical question: did climate change bring about the uplift of the mountain ranges rather than the other way around? They suggest that as material is removed from an uplifted area by weathering, the feature becomes lighter and more buoyant, floating higher on the mantle. The peaks of the uplifted area may thus appear to ‘grow’ as a result of their sides becoming eroded. So some phenomena, such as sharp incisions made by rivers into the rock that were used to infer that the uplift was recent, may themselves have been caused by climate change, rather than by the uplift itself. A gradual uplift driven by a plate collision may have been enhanced by weathering effects caused by climate change. However, Molnar and England admit they have no explanation for why the climate changed 40 or 50 million years ago, bringing about the erosion that cut deep valleys in the Tibetan region.
Raymo’s response is to suggest a further positive feedback mechanism involving weathering. As the Tibetan plateau rose and cooling took place over the whole globe, she says, glacier activity on other mountainous regions increased and added to the worldwide level of erosion, causing these regions to also ride higher on the mantle. The greater the erosion, the greater the global cooling effect.
However, the most serious criticism of Raymo’s theory came from Berner and Anthony Lasaga of Yale and Ken Caldeira and Michael Arthur of Pennsylvania State University. If the Tibetan plateau is such an effective remover of carbon dioxide from the atmosphere, they asked earlier this year, why is there any carbon dioxide left? Why has there been no runaway ‘icehouse effect’?
In Berner’s model, carbon dioxide is restored to the atmosphere by volcanism, in which the gas emerges from new rock created by seafloor spreading, and by metamorphism, in which rocks under the kind of stress present when India collided with Asia release significant amounts of the gas. As far as is known, the rate of seafloor spreading has not changed appreciably in the past 40 million years. So either increased metamorphism, or some other form of carbon dioxide restoration is coming into play to balance the carbon dioxide budget – albeit at a lower carbon dioxide level.
But there may be no need to find a new source to balance the carbon dioxide budget. Caldeira suggests that as global temperatures fell due to lower atmospheric levels of carbon dioxide, the rate of chemical weathering on mountainous regions well beyond the Tibetan plateau fell too. This created a new, stable, worldwide cycle of chemical weathering and carbon dioxide replacement.
Initially there was some tension between carbon cycle modellers such as Berner and geochemists such as Raymo and Ruddiman who sought evidence from sediment core data. But some modellers now seem to be won over. Berner considers Raymo’s central idea to be a useful one: in a new version of his model for atmospheric carbon dioxide (as yet unpublished) he has included the uplift ideas together with data drawn from studies of strontium isotopes.
We know that 70 million years ago Tibet lay under the sea, and we know its present state. But almost nothing is known of what happened in between. The last word on the issue of Tibet’s effect on our modern climate only awaits further study of its geology. Stratigraphers apply now.
David Paterson is a freelance journalist.