WELCOME to the Anthropocene. It’s a new geological era, so take a good look around. A single species is in charge of the planet, altering its surface features almost at will. And what more natural than to name this new era after that top-of-the-range anthropoid, us?
Coined a couple of years ago to describe the last two centuries of our planet’s evolution, the word Anthropocene is catching on among a new breed of scientists who study Earth systems – how our planet functions and maintains itself in a fit state for life. And they are coming to believe that some of these systems are close to breakdown. If that is true, then the gradual global warming predicted for the next century will be the least of our worries.
The big new discovery is that our planet does not do gradual change. The way it works is far cruder and nastier. “Most people think we live in a benign Copernican world in which the sun always rises and the seasons don’t change. It’s nice, but not true,” Bill Clark from Harvard University told a workshop at Dahlem, Berlin, Germany earlier this year. He and other Earth systems scientists have been watching how humanity is changing key planetary processes such as the carbon and nitrogen cycles, the growth and demise of glaciers and forests, the chemistry of the atmosphere and the circulation of the oceans. From their Olympian vantage point, they fear that we may be on the verge of triggering “abrupt, nasty and irreversible” change. They fear the Anthropocene may well end in tears.
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We have already had one lucky break, 20 years ago, when a hole suddenly opened up in the ozone layer over Antarctica, stripping away the continent’s protective shield against ultraviolet radiation. We were lucky that it happened over Antarctica; lucky that we spotted it before it spread too far; and lucky that half a century before, industrialists had picked chlorine compounds rather than the very similar bromine compounds to put in aerosol sprays and refrigerators. That fortuitous decision saved us from a much more virulent ozone-destroyer that could have stripped away the ozone layer before we knew what was happening. Many of the scientists who worked to unravel the cause of the ozone hole are among the most vehement in issuing the new warnings. They know how close we came to disaster.
True, life on planet Earth is probably not in jeopardy. But at the very least we risk making living conditions very uncomfortable for ourselves. Remember, the whole of human civilisation has developed during 10,000 years of tranquil climate – a long balmy spring after the last ice age. We are simply not used to big and sudden changes. But they will be coming. Will Steffen, the Stockholm-based director of the International Geosphere Biosphere Programme, a worldwide network of Earth system scientists, says: “The planet may have an Achilles’s heel that can cause abrupt changes. And if it does, we badly need to know about it.”
The first clues that things can take a sudden turn for the worse come from climatic records of the past million years, which show that, before our balmy spring, the planet repeatedly swung in and out of ice ages – and did so with startling speed. The ice ages seem to be triggered by wobbles in the planet’s orbit that marginally change the distribution of solar heating of the Earth. One of these wobbles, which recurs every 100,000 years or so, coincides with the ice ages. This is curious, because it is actually the weakest of three so-called Milankovitch wobbles. But the link is unmistakable, say glaciologists. They believe that the weakest wobble has the biggest effect because the Earth systems amplify its impact, turning a minor cooling into a major freeze.
The main cause of this amplification is the growth of ice caps in the northern hemisphere, says Tony Payne of the University of Bristol, UK. Once a small local cooling has caused a small growth in ice, the advancing ice will accentuate the cooling by reflecting more solar heating back into space than the darker, bare ground it replaces. This process also seems to slow the release of carbon dioxide into the atmosphere from vegetation. And it may change ocean circulation, again reducing the amount of CO2 in the atmosphere. CO2 is a greenhouse gas – perhaps the planet’s most important thermostat. Less CO2 in the air means lower temperatures. Put all this together and what begins as a minor change in the planet’s passage through the universe triggers the growth of ice caps across the northern hemisphere. At their greatest extent, those ice caps measure more than 40 million cubic kilometres, and hold so much water that sea levels are 120 metres lower than today.
These glaciations last for tens of thousands of years. But eventually, some similar combination of wobble and feedback, this time operating in reverse, will destabilise the ice caps and bring the ice ages to an end. The details are not clear but, says Andy Watson of the University of East Anglia, UK, “an extraordinary picture is emerging of a complex but ordered dance” involving ice caps, atmospheric chemistry, oceans and climate. This, he says, is not just scientifically intriguing, “it also offers a unique opportunity to probe the dynamics of the Earth’s climate system. And it may help us decide if we are pushing the system towards a catastrophic outcome”.
What has most startled researchers is the speed of events. By geological standards it is a frenetic dance. At certain stages, temperatures may have plunged by between 6 and 8 °C within two decades. It appears, says Watson, that the Earth’s climate during the era of ice ages had two “stable states”. There was no smooth transition between them. The planet simply jumped periodically from one state to the other.
Detailed analysis of gas bubbles trapped in ice cores shows that these two stable states are anchored by two levels of CO2 in the atmosphere: around 190 parts per million during the glaciations, and 280 ppm during the interglacials. No other concentrations persisted for long. Researchers believe this shows that a large element of the “dance” into and out of ice ages involves a reallocation of carbon between the oceans, land and atmosphere. Indeed, it may have driven the process. And that raises critical questions for the Anthropocene. In the past two centuries, humanity has pushed atmospheric CO2 levels up by a third to 373 ppm, a figure that continues to rise by approximately 16 ppm a decade. So how will Earth’s systems respond?
Dramatic feedback
Conventional thinking about global warming predicts that rising emissions of CO2 will produce a steady rise in atmospheric concentrations and an equally steady rise in temperatures. But the evidence of the ice ages suggests that our future climate may not be like that. Instead, rising CO2 emissions could excite dramatic feedback in the system. That might be good news: it might allow the system to absorb all that excess CO2 and damp down warming. Much more likely though, says Steffen, is that it will accelerate the changes. “If the ice age era seemed to gravitate between two steady states, maybe in future we will gravitate to a third steady state.”
An obvious concern is that global warming might unlock natural stores of carbon, such as forests and soils, creating sudden surges of greenhouse gases into the air. Currently, forest ecosystems are absorbing CO2 because global warming is accelerating photosynthesis. But studies at the UK’s Met Office Hadley Centre for Climate Prediction in Exeter predict that rates of carbon decomposition in wood and soils will soon match uptake. By 2050, forests and soils will be net contributors of CO2 to the atmosphere. Peter Cox a modeller at the Met Office predicts that by then the whole Amazon rainforest could be dying and releasing its store of carbon into the air. That, he calculates, would be enough to increase the expected rate of global warming by 50 per cent.
Another vulnerable store of greenhouse gases is the methane frozen in the Siberian permafrost. A natural product of past biological processes, this methane could be released as the permafrost thaws. Methane is a more powerful and faster-acting greenhouse gas than CO2, and there is enough in the Siberian store to raise global temperatures by several degrees. It may even have happened before. Around 55 million years ago, way before the ice ages, trillions of tonnes of methane erupted from somewhere on the Earth’s surface, triggering global warming of 10 °C or more. And this happened in a time of gradual global warming disturbingly similar to the conditions we have today.
But climatic lurches do not need big releases of greenhouse gases. The oceans are an equally important planetary thermostat. The basic circulation system – known as the thermohaline circulation – is capable of switching itself on and off with startling abandon, with potentially big impacts on climate. The circulation is driven by formation of ice in the far north Atlantic. As the ice forms, it leaves behind super-saline seawater. This water, being very dense, plunges to the ocean floor and starts a circulation that reaches all the main oceans. The original water eventually resurfaces about 1000 years later as the warm flow of water in the North Atlantic known as the Gulf Stream.
What might turn off the circulation? Likely candidates include a cessation of ice formation in the north Atlantic or a big increase in fresh water reaching the sea, perhaps from melting ice on land. Both are likely outcomes of global warming. Indeed, recent studies have found that water in the north Atlantic may already be freshening. Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany calculates that this could shut down the system in around 100 years’ time. Such a cataclysm could initially cool Europe down – which might by then come as a relief – but it would also leave the tropics and the southern hemisphere sweltering. And, since the circulation is responsible for removing large amounts of CO2 and heat from the air, it would probably accelerate global warming.
There may be a string of similar hair-trigger climate systems across the planet that are ripe for change. In the Pacific Ocean, a warmer world could create a near-permanent El Niño, with dried-out rainforests and rain-swept deserts across the tropics. And it is not only the climate that is vulnerable in the Anthropocene. Some scientists warn that the ozone layer could yet spring big surprises. Then there is hydroxyl, a highly reactive and short-lived oxygen gas that reacts with common pollutants, removing them from the air. With ever more pollution to clean up, says Steffen, it could become over-worked, causing near-permanent smogs. There is no reliable way of estimating global concentrations of hydroxyl, so we could easily get caught unawares.
Similarly, abrupt change could happen to sea levels, and soon. The Greenland ice cap, which has survived the 10,000 years since the last ice age virtually unaltered, could now be close to a threshold that triggers total meltdown. Jonathan Gregory of the Hadley Centre calculates that warming of less than 3 °C – likely in that part of the Arctic within a couple of decades – could start a runaway melting that will eventually raise sea levels worldwide by seven metres.
What distinguishes all these Doomsday scenarios is that they are examples of benign planetary systems that, if pushed too far, can jump to a new and unpleasant method of operating. And anywhere near the jumping point, a small push can have a huge impact. For a while, the Amazon rainforest may enjoy global warming; but beyond a certain point, a fraction of a degree more warming could kill it. Likewise the thermohaline circulation works just fine, moderating global warming, till one last melting iceberg shuts it down.
Many of these systems also show another disturbing feature. Once you pass the critical point, the change in operation is irreversible. Like someone contemplating suicide on a cliff top, a step back before the edge will save the day – but not afterwards. Once global warming has released methane accumulated in Siberian permafrost over thousands of years, for example, no amount of subsequent cooling can put it back. And, says Rahmstorf, once the thermohaline circulation has shut down, even big reductions in CO2 levels in the air might not restart it.
It is not all bad news. While the Amazon rainforests may be close to drying out, the Sahara could be on the verge of a sudden flowering. The key to both the Amazon and Saharan situation is that vegetation and rainfall reinforce one another. Rainfall allows vegetation to grow. And vegetation in turn, by absorbing rain and then humidifying the air, stimulates more rain later. According to Martin Claussen of the Potsdam Institute, the dynamic interaction between vegetation and climate in the Sahara is on a permanent knife-edge, able to flip between dry desert and wet woodland in just a few years. That, he says, explains why some 5000 years ago it switched within a few decades from being a land of woodland and vast marshlands full of crocodiles to being fit only for camels. And, he says, global warming could push it back again.
How should we go about assessing the potential threats to our habitable world? One obvious step is to monitor the hotspots for change. To that end, representatives from 30 countries met in Washington DC in August to discuss setting up an integrated Earth Observation System. Concentrating on regions where the serious action could happen, their aim is to “take the pulse of the planet”, using a global network of satellites, aircraft and terrestrial data collection sites to measure things like air temperature, evaporation from the oceans, melting and formation of ice, and carbon releases from forests.
Quick and dirty models
Another crucial need is to develop better computer models that reproduce the essential features of the Earth, and then search for the danger points where small changes can have big and irreversible consequences. Cox, a master of the Met Office’s world-renowned General Circulation Model of the Earth’s climate, has an unexpected take on this. These GCMs are virtually useless for finding the danger points in Earth systems, he says. They are so complicated that they take three months to do a full run. They give a wonderfully detailed breakdown of just one possible future, but that’s not much help here. Instead, Cox wants to test as many different futures as possible to find out if one of them will produce a specially unpleasant outcome.
“What we need are simple models of basic systems,” he says. “They require little or no computer time, and you can perform endless runs just to see what happens.” Such models are already good enough to replicate past shutdowns of the thermohaline circulation and the “green Sahara” phenomenon. They are providing strong evidence that we could be close to destroying the Amazon rainforest, triggering a meltdown in Greenland and pushing the Pacific climate into a near-permanent El Niño. They could provide a route map through the minefield of global change.
Our future may depend on it. During the past 10,000 years, various human civilisations have plundered and destroyed their local environments, making sizable areas of the planet almost uninhabitable. We got over it. As one civilisation fell, another usually rose. But the rules of the game have now changed. In the Anthropocene, human influences on the environment are increasingly global. “Before, if we screwed up we could move on,” says Steffen. “But now we don’t have an exit option. We don’t have another planet.”