What would a 4°C warmer world mean for the Amazon rainforest?

At a recent conference in Oxford, Richard Betts, the head of climate impacts at the UK’s Met Office Hadley Centre, launched a new report that warns that we could see a global increase in temperatures of 4°C as soon as 2055. Climate change could accelerate so rapidly because of feedback loops which are triggered by increasing greenhouse gas emissions and which in turn will cause new emissions.

“Four degrees of warming, averaged over the globe, translates into even greater warming in many regions, along with major changes in rainfall,” Betts said. An increase of 4°C global average temperature would mean a rise of up to 15°C at the North Pole. Sea levels would rise by up to 1.4 metres. Monsoon rains could fail. At the conference, two scientists looked specifically at the implications of 4°C warming for the Amazon rainforests: Yadvinder Malhi, a Professor of Ecosystem Science at Oxford University; and Wolfgang Cramer at the Potsdam Institute for Climate Impact Research in Germany.

In his presentation, Yadvinder Malhi pointed out that tropical land regions have already warmed by almost one degree since 1980. He asked the question “How will tropical organisms and ecosystems respond to this warming?” The short answer is not particularly well. “Rapid climate change is reweaving the web of life, not just in the tropics, but everywhere,” he said.

Malhi pointed out that both climate change and deforestation are serious threats to tropical forests:

“Sometimes you are asked what is the greatest threat to tropical forests, climate change or deforestation? Well both of those are happening. And really there is no choice. The greatest threat is the fact of the synergy of these two phenomena.”

While temperature increase by itself may not wipe out forests, reduced rainfall will. “If you dry a region enough, then you can no longer support a forest,” Mahli said. His research, “Exploring the likelihood and mechanism of a climate-change-induced dieback of the Amazon rainforest”, published earlier this year in the Proceedings of the National Academy of Sciences of the USA, found that tropical forests are likely to become more seasonal.

The spread of fire ignition associated with advancing deforestation, logging, and fragmentation may act as nucleation points that trigger the transition of these seasonal forests into fire-dominated, low biomass forests.

Mahli spoke of the links between damage to the forest from climate change and human-caused deforestation: “It may be this interaction between seasonal drought and ongoing fragmentation that pushes the Amazon to a potential die back,” he said. Mahli explained the importance of maintaining tropical forests as a “strategy for adapting to climate change”, and he ended his presentation on an almost optimistic note:

“[F]orest protection is also a component strategy for mitigation of climate change, the recognition of the role of tropical forest as stores of carbon and as sinks for carbon and this also presents an opportunity at the UN negotiations this year. It seems quite likely that either then or soon after that some deal will be done for reduced emissions from deforestation and forest degradation, where payments will be made and the amounts of potential finance available for tropical forest conservation and altering the economics of the degradation of many areas of the tropics maybe changed. But it opens up huge challenges as well. The finance is only part of the issue there. But tackling climate change ironically also provides an opportunity to build in a climate resilience plan for tropical forests into our planning for the 21st century.”

In his presentation, Wolfgang Cramer showed no signs of optimism. Here’s how New Scientist reported what he said at the conference:

The Amazon – gone
In a 4 °C world, climate change, deforestation and fires spreading from degraded land into pristine forest will conspire to destroy over 83 per cent of the Amazon rainforest by 2100, according to climatologist Wolfgang Cramer at the Potsdam Institute for Climate Impact Research in Germany. His climate models show global warming alone converting 30 per cent of the Amazon into degraded shrub land and mixed woodland by 2100. Even this grim estimate is based on the hopeful assumption that extra CO2 in the atmosphere will “fertilise” the forest, buffering it from drought. But we can’t be sure this will happen, says Cramer. “If we’ve overestimated the magnitude of CO2 fertilisation, we risk losing the entire Amazon.”

Cramer described the work of Peter Cox et al, published in Nature in 2000: “For about 10 years now we have been aware that climate warming is possibly having some impact on Amazonian rainforests.” Cox’s research showed for the first time that “you could have a drying effect that would lead to a massive loss of vegetation carbon. Effectively, it’s a very sort of bleak way to show that this would be a loss, almost total loss, of Amazonian rainforest.”

Cramer asked two questions during his presentations. First he asked, “Is it conceivable that climate-driven dieback of Amazonian forests could outrun deforestation?”

Cramer did not directly say yes or no to this question, but his response was not reassuring:

“If you want to be sure that you maintain those carbon stocks that are currently there [in the Amazon rainforest] then you shouldn’t embark on a climatic pathway that implies the risk of major dieback.”

His second question was, “And if so, at what degree of warming?” Cramer did not give a number in answer to this question, but once again, his response was disturbing:

“Beyond four degrees warming, we think that Amazon ‘dieback’ or severe degradation (we prefer that term) is a very significant risk.”

The implications for REDD and trading forest carbon are clear. Stopping deforestation is crucial. But so is reducing greenhouse gas emissions. Cramer pointed out that there is “a very significant risk” that very large areas of the Amazon will become “severely degraded” beyond 4°C warming. This would result in the release of vast amounts of CO2 into the atmosphere. If we want the carbon stored in the Amazon to remain where it is, we need to stop runaway climate change. To do that we need to stop burning fossil fuels. If we fail to reduce greenhouse gas emissions quickly and dramatically, we could see global warming of 4°C by 2055. Trading the carbon stored in the Amazon allows fossil fuel emissions to continue elsewhere and risks massively more emissions when the Amazon goes up in smoke.

Cramer’s research was carried out, “because of a direct request of the World Bank . . . to better understand the sensitivity of Amazonian rainforest to climate change,” he said. Let’s hope that the World Bank and other promoters of trading forest carbon are listening.