The death of the Amazon



Mark Lynas



Excerpt from Six Degrees by Mark Lynas

I have in my filing cabinet a well-thumbed and rather tatty printout of a paper that was published in Nature back in November 2000. Stapled onto the back of it is a page of scribbles written by me at the time, expressing startled disbelief at its contents. This paper, perhaps more than any other I had read, convinced me that this book needed writing – not just because of what it said, but because of the reception it got. Despite containing one of the most alarming projections ever published in the scientific literature, barely a ripple was caused in media or political circles. There should have been panic on the streets, people shouting from rooftops, statements to parliament and 24-hour news coverage. There were none of these things. The paper, modestly entitled ‘Acceleration of global warming due to carbon cycle feedbacks in a coupled climate model’, and written by a team from Britain’s Hadley Centre, was largely ignored.

I was reminded of it again during the 2005 Amazonian drought, the worst in decades, a time when fresh water had to be flown into villages which normally sit beside massive rivers. Scanning the headlines, it felt as if the Hadley Centre paper’s projections were already coming true-half a century too soon. Fires were erupting in areas that had never burned before. Whole stretches of meandering river had dried up and turned into baked mud. Had Amazonia already reached the tipping point? Thankfully, this time the great forest came back from the brink. The drought lifted before the year’s end, and rainwater slowly trickled back into the main river’s tributaries, bringing relief to the parched trees. The fires were dampened, and the world’s most diverse and precious ecosystem was saved – for now.

The Hadley Centre’s Nature paper should have rung worldwide alarm bells. First, it showed that global warming could begin to generate its own momentum if a previously unforeseen positive feedback – a vicious circle by which warming would release more greenhouse gas, causing more warming and thereby more gas to be released in an unstoppable spiral – came into effect. This, the ‘carbon cycle feedback’ referred to in the paper’s title, would potentially leave human beings as powerless bystanders in a devastating runaway global warming scenario. Second, the Hadley paper revealed that the main furnace of this positive feedback would burn not in the industrial capitals of the world, but in the remote heart of South America, beginning with the near-total collapse of the Amazonian rainforest.

The paper’s authors, led by the climate modeller Peter Cox, had reached this terrifying conclusion by taking what now sounds like an obvious step to make their global model more realistic. Whilst previous models had treated rising temperatures as a simple linear process, Cox’s team realised that land and ocean systems would not remain static during rapid global warming. They would themselves be affected by the changing climate. In the case of the oceans, warmer seas absorb less CO₂, leaving more of it to accumulate in the atmosphere and further intensify global warming. On land, matters would be even worse. Huge amounts of carbon are currently stored in the globe’s soils, the half-rotted remains of long-dead vegetation. According to generally accepted estimates, the soil carbon reservoir totals some 1600 gigatonnes, more than double the entire carbon content of the atmosphere. As soil warms, bacteria speed up their work to break down this stored carbon, releasing it back into the atmosphere as carbon dioxide. Whereas most climate models treat the land surface as inert, Cox’s team for the first time included this ‘positive feedback’ of carbon releases from warming soils and vegetation-and reached an astonishing result.

According to the newly updated model, a three-degree rise in global temperatures-something that could happen as early as 2050-effectively reverses the carbon cycle. Instead of absorbing CO₂, vegetation and soils start releasing it in massive quantities, as soil bacteria work faster to break down organic matter in a hotter environment, and plant growth goes into reverse. So much carbon pours into the atmosphere that it pumps up atmospheric concentrations by 250 parts per million (ppm) by 2100, giving a further 1.5°C boost to global warming. In other words, the Hadley Centre’s team had discovered that carbon cycle positive feedbacks could tip the planet into a runaway global warming spiral by the middle of this century, much earlier than anyone had so far suggested. By 2100 global warming in the Hadley model rose from 4°C to 5.5°C, perilously close to the IPCC’s worst-case scenario. That is why my scribbled notes expressed such shock and dismay when I first read the paper back in 2000.

Politicians may not have stirred, but other scientists did sit up and listen. Following the Hadley Centre’s lead, American climate modellers at the National Center for Atmospheric Research in Colorado added a carbon cycle component to their model and also found a decrease in the amount of carbon stored in globally warmed soils. A French team repeated the Hadley experiment, again with a different model, and got the same result. Another team, this time based in both the US and Italy, found that atmospheric CO₂ jumped by 90 ppm and global warming increased by 0.6°C when they added carbon cycle feedbacks to their model. These results may differ in their magnitude, but the direction they point in is the same.

Even if all these models are wrong, another significant danger lies beneath the tropical forest lands of Indonesia, Malaysia and Amazonia: peat. For thousands of years, dead vegetation has built up beneath the living forest, in places forming peat layers tens of metres thick. But this peat layer is only kept stable because it is waterlogged: in Indonesia, it was burning peat that contributed most of the 2 billion tonnes of extra carbon which hit the atmosphere during the devastating fire season of 1997-8. Much of it smouldered underground for months, still releasing carbon even once the overland fires had been put out by the returning rains. This is another potentially devastating carbon cycle feedback: if rainfall patterns shift in a globally-warmed future, leaving these flammable mounds of peat tinder-dry over tens of millions of hectares of south-east Asia and Amazonia, then vast amounts of extra carbon will enter the atmosphere, further aggravating global warming. Modelling results examining this potential feedback are inconclusive but worrying: one 2007 study found that 7 out of 11 models investigated predicted a decrease in dry-season rainfall over the Indonesian peatlands, whilst 6 out of 11 predicted a similar decrease over Amazonia.

This latter result illustrates the uncertainty surrounding projections of future rainfall changes: whilst the Hadley Centre’s model shows a dramatic drying trend over Amazonia, some models show less of a change, and others even project increasing rainfall. Which of them is right could hardly be more important, for this colossal ecosystem is home to half the world’s biodiversity, and accounts for a tenth of the net primary productivity (the photosynthetic output of plants) of the entire planetary biosphere, in an area of just 7 million square kilometres. Fed by Andean snowmelt and seasonal torrential rains (of over 2.5 metres per year in some regions), the Amazon River contains 20 per cent of all the water discharged into the world’s oceans-ten times the volume of the Mississippi. The energy released by this huge amount of precipitation plays a major role in the circulation of the whole world’s weather. As the science writer Peter Bunyard explains: ‘The functioning of the Amazon basin as a hydrological power engine is a critical component of contemporary climate.’

But Amazonia is already under siege, irrespective of global warming. Over half a million square kilometres-an area the size of France-has been deforested, and more is chopped down each year to make way for cattle ranching and soya plantations. The human population encroaching on the forest has increased tenfold in the last half-century, and each new road the Brazilian government forges into pristine areas is quickly surrounded by new ‘herringbone’ patterns of deforestation. Slash-and-burn agriculture is also a serious threat, as half a million land-hungry peasants converge on Brazil’s last great wilderness in search of a better living for themselves and their families. Illegal logging is rampant: when Greenpeace asserted that 80 per cent of logging was conducted illegally, the Brazilian government – instead of issuing furious denials – threw up its hands and agreed.

Even if all this destruction stopped tomorrow, the Hadley Centre’s model suggests that the Amazon rainforest would still be doomed unless global warming levels off at two degrees. If the world crosses this crucial tipping point, the model simulates a tidal wave of destruction, beginning in north-eastern Amazonia and advancing steadily south and westwards across the continent. Modelled rainfall declines almost to zero in some areas by 2100. Temperatures soar to Saharan highs, reaching on average 38°C. Once the collapse is complete, the interior of the Amazon basin is essentially desert, devoid of any significant vegetation at all. Only a small amount of grassland and savannah persists on the outermost edges.

Every fire season gives a preview of how this collapse would unfold in reality. Amazonian trees are used to constant humidity and have no resistance to fire. They are very different from trees in other forest ecosystems which are not only adapted to the occasional burn, but need regular fires to keep the forest healthy. In contrast, Amazonian trees, with no evolutionary experience of burning, continue to die long after the flames have passed. Regeneration is slow, allowing sunlight to penetrate the canopy and further dry out the forest floor. Bird and animal species show catastrophic population declines. And rainfall is further suppressed by the clouds of smoke that hang for months over the stricken forest.

When the final conflagration takes place, it will be on a different scale from anything witnessed so far. The closest comparison might be the 1998 fires across Indonesia, which blanketed several countries in choking smog for many months. In Brazil, Venezuela, Colombia, eastern Peru and Bolivia, life will become increasingly difficult as the air becomes an unbreathable mixture of searing hot gases and smoke. The sun will be blotted out by the leaden pall hanging overhead, whilst a grey drizzle of light ash falls from the sky.

From space, satellites might witness gigantic walls of flame marching through the last areas of untouched forest. Thousands of indigenous people – the Yanomami, the Ashaninka, and other tribes who have known this forest as their only home since prehistory – are driven out. Deprived of their livelihoods and culture, unable to make sense of the sudden disappearance of all they have ever known, they will pine for their lost world. For these people, the Earth itself will have vanished. For once the firestorm has passed, white, grey and black piles of ash, surrounded by seared and smoking timber, will be all that remains of the mighty Amazon.

A new, unrecognisable landscape is born. In the deepest parts of the basin, where once the only sound was the howling of monkeys and the rustling of leaves, a moaning wind has arisen. Dust gathers in the lee of burned-out tree stumps. Nearer to the ground, a gentle hissing sound is heard. Sand dunes are rising. The desert has come.



