The Ecologist

Dr Simon Lewis

Dr Simon Lewis is a tropical forest expert from the University of Leeds

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Tom Levitt

27th May, 2011


Tom Levitt: Can you explain how the Amazon could start adding more emissions to the atmosphere than it soaks up?

Dr Simon Lewis: The remaining intact Amazon rainforest isn’t currently at equilibrium, as on average, in a normal year, these forests are getting bigger. That is, carbon dioxide is being removed from the atmosphere and the carbon incorporated into the trees. The rate is about 1.5 billion metric tonnes of carbon dioxide per year. Therefore, these forests are providing an important free service to humanity, reducing the rate and magnitude of climate change. The exact reasons for this increase in carbon storage are debated, but the leading candidate is the amount of carbon dioxide in the atmosphere itself, which is fertilising these forest trees. In a drought that is severe enough to kill many trees, the dead trees start to rot, returning the carbon to the atmosphere as carbon dioxide.

These emissions offset some of the usual carbon uptake. So, if the climate changes in the Amazon to a regime with more severe and frequent droughts, then the dead trees may be numerous enough to cancel-out all the usual carbon uptake, and perhaps even add carbon dioxide to the atmosphere. This could happen if the Amazon starts to suffer from three or more severe droughts per decade. Worse still, this could set into motion a ‘positive feedback’ mechanism, by which the droughts could lead to greater emissions of carbon dioxide, further exacerbating the problem, which could lead to the loss of rainforest in some areas of the Amazon. Our current emission pathways are essentially playing Russian roulette with a substantial portion of the world’s largest rainforest.
TL: What is the biggest cause of deforestation in the Amazon?

SL: It is difficult to define single causes of deforestation, and care must be taken to note not only the proximate drivers, for example, forest removal for soya bean production, but also the ultimate drivers of such decisions, for example, economic drivers like the price of agricultural commodities, and policy choices, such as government infrastructure projects that open areas to become within reach of national and international markets. The key proximate driver of deforestation in the Amazon is agricultural expansion – for cattle and soya – itself allied to road-building. However, over the past 6 years the deforestation rate in the Amazon has decreased substantially (by about 50 per cent in Brazil), driven by law enforcement, the intensification of agricultural production on land that has already been deforested and campaigns by NGO’s to stop companies expanding production into rainforest lands.

TL: Could the Amazon, on its own, cause runaway climate change?

SL: If the forests of the Amazon were all removed, then the repercussions would be felt worldwide in terms of the disruption of usual climatic regimes. In terms of climate change, this would perhaps add 100-200 billion tonnes of carbon to the atmosphere. We humans currently emit about 10 billion tonnes, so we would add 10-20 years worth of today’s level of emissions. This hypotheically shows that it will be the direct emissions of fossil fuels that will most strongly determine the rate, magnitude and impacts of climate change this century and beyond. Furthermore, given that the rainforest has persisted in some form in what is now the Amazon region for at least 55 million years, the rainforest will probably persist under projected climate change if greenhouse gas emissions are reduced. The key problem is the climate change-direct human impact interaction, which is novel in the history of the Amazon forest as humans only arrived there about 12,000 years ago.

TL: Can you explain what you mean by the climate change-human impact interaction?

SL: It can take several forms. Firstly, droughts allow more forest to be burnt and then cleared, as it is possible for more land to be burnt when it is dry. Secondly, people use fire as a land management tool, but in severe droughts fires can get  out of control and encroach forests. Thirdly, degraded forests, like logged-over or fire-impacted forest, can dry-out more easily, which make them more susceptible to fires. Fourthly, large-scale deforestation can reduce local rainfall, which could exacerbate future droughts if deforestation continues. This complex set of interactions could see much larger impacts on the forest compared to analysing any one trend, like climate change or deforestation rates, in isolation. 

TL: Out of the Amazon rainforest, Arctic and Antarctic ice-caps and Tundra permafrost - which is closest to its 'tipping point'?

SL: The Arctic sea-ice will certainly disappear, as the trend is already clear and physical mechanism – air temperature warming – will continue. So in terms of an irreversible shift in the average state, then the sea-ice is the place to watch. For both the Amazon and the Tundra the situation is much more complex, as they both include lots of living organisms, which adapt to changing conditions. For example, trees might die due to droughts, but more drought-tolerant species will likely take their place, so ‘living tipping points’ are much more difficult to predict than those more purely physical parts of the Earth system.  

TL: What about rainfall? If the Amazon starts to die back will rainfall in neighbouring countries and further afield also be affected?

There is no good evidence so far that the Amazon is drying out. With a warming climate we expect more rainfall, as warmer air can hold more moisture, and there would also be more evaporation. The recent Amazon droughts were due to a change in the distribution of rain across the year, not the total amount. The problem was that the driest part of the year got drier. However, the Amazon forest does re-cycle rainfall, so very large-scale deforestation could cause reduced rainfall over the Amazon.

TL: Previous research that used NASA satellite data said the droughts had little negative impact on the Amazon and that the forest was resilient to the droughts. How did you research come to different conclusions?

SL: There has been some debate amongst scientists about what satellites that measure forest ‘greenness’ see regarding droughts in the Amazon, and what that greenness measure actually means. The assumption is that more greenness means more forest growth, but it could mean a loss of canopy leaves (exposing those below) or a flush of new leaves. Some scientists suggested that the Amazon got greener during the 2005 Amazon drought, but others argued that this was largely an artefact due to the corruption of the signal due to the amount of burning in the Amazon during the drought: with appropriate filtering of the data another group of scientists showed that there was no green-up. For the 2010 drought new results show that the forest did not green up in, indeed the canopy went brown over large areas in this more severe drought. The most likely explanation is that the forests start the drought with clear skies and therefore lots of sun and plenty of water in the soils and do grow well – but in an extended drought tree growth slows and eventually stops as water runs out. Ground measurements shows that growth is not largely affected over an entire year, but mortality increases strongly with drought intensity. Therefore caution is required regarding this type of satellite data, as it may not be detecting the most important ecological process.
TL: Do you believe more evidence about an Amazon die-back, could be enough to push momentum towards a global deal and emission cuts?

SL: No. The lack of a global deal on carbon emission and the failure to cut global emissions is due to long-term nature of the problem, the fact that it makes no sense for any one country to make concessions unless most others do, the centrality of fossil fuels to almost all aspects of modern life, and the immensely powerful vested interests in government and industry against taking meaningful action. In essence, countries and fossil fuel companies will not agree to what they view as trillions of dollars sitting under the ground, and leaving it there in perpetuity. They want the money now regardless of the longer-term impacts. 

TL: Are their any geoengineering solutions that could be applied to negate any die-back of the Amazon rainforest?

There are currently no geoengineering proposals that are sufficiently researched to assess whether they are a good idea or not, and on which ecosystems they may have positive or negative impacts. However, one lesson from recent research is that carbon dioxide gives some protection to plants from the impacts of drought (as plants increase the efficiency with which they utlise water when growing under higher carbon dioxide conditions), but other greenhouse gases do not provide this protective effect, which suggests that we should make stronger efforts to quickly and substantially reduce non-CO2 greenhouse gas emissions, such as methane, nitrous oxide, and black carbon.
You yourself have been at the centre of media mis-reporting on climate change - are the media now getting it right? Or is the scientific evidence being lost in controversy and political arguments?

SL: No they aren’t getting coverage right. Unfortunately the media are mostly ignoring the science, or selecting stories to fit their editorial prejudices on climate change. For example, scientific papers that may appear to underplay the severity of climate change will be reported in the Telegraph and Times, in the UK, and Wall Street Journal in the US, but these newspapers largely do not cover those stories that report the more alarming new results. Similarly, those newspapers with editorial lines that climate change should be taken seriously have better coverage, but do choose differing stories to cover. So the stories themselves may be reasonable, but the selection is highly skewed towards confirming already entrenched positions.

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