Leading article: Beware the collapse of the planet's lungs

The year 2005 was an exceptionally dry one for the Amazon rainforest. Thousands of square kilometres of rainforest were destroyed. The level of the mighty Amazon river and its tributaries fell to the lowest levels since records began. Fish perished in the abnormally warm waters. Boats were grounded. Locals were forced to abandon their homes. It was the kind of drought that researchers would expect no more than once a century.

But then came the drought of 2010. As a new research paper published in the journal Science today reveals, last year's drought was even more severe than 2005. So Brazil has experienced two "once in a century" climatic events in a decade. Unsurprisingly, scientists are beginning to suspect that something is amiss.

A link between these crippling droughts and climate change cannot be proved. But increasingly common drought is consistent with what scientific models predict for a globally warmer world. Increasing Atlantic sea surface temperatures are expected to lead to lower rainfall in Brazil's great forest.

There is another deeply worrying trend. The amount of carbon dioxide pumped into the atmosphere by our societies has fallen in recent years because of the global economic downturn. But the latest readings suggest that CO2 levels in atmosphere still increased over that time. The fear is that this is being driven by a "feedback loop", whereby the impact of climate change itself accelerates climate change. In this case, as the climate heats up, rainforest trees fall and burn, releasing the carbon locked up in them. And this, in turn, accelerates warming further.

Again, the existence of a feedback loop is difficult to prove. But it fits predictions. Normally, rainforests function like great carbon sinks, absorbing a large proportion of the CO2 that human activity produces. But in 2005, thanks to deforestation, the Amazon became a net emitter of carbon dioxide. In that year, the rainforest is estimated to have emitted some 5 billion tonnes of CO2, almost as much as the entire output of the United States.

The pace of deforestation in the Brazilian rainforest appears to have slowed somewhat in recent years. But pressure on rainforests continues in equatorial regions elsewhere, from Congo to Indonesia. We need to preserve the world's existing arboreal lungs if we are to have any chance of avoiding runaway climate change. But human activity is still depleting this crucial natural asset, even as its role in climatic regulation shows ominous signs of breaking down.

The only viable strategy for preserving the world's rainforests that has been put forward is Reducing Emissions from Deforestation and Forest Degradation (REDD). This scheme would transfer money from high-income countries to developing countries, in order to dissuade them from cutting down their trees for profit.

But to work, this will require an overarching global climate-change treaty, with mandatory emission limits for each country. Without such a framework fiscal transfers between high-income and developing countries will never be substantial enough to affect behaviour in poor nations and the deforestation, by ranchers and loggers, will continue. Despite the great hopes, the United Nations climate change summit in Copenhagen in 2009 failed to deliver a binding global treaty. Last year's follow-up summit in Cancun was hailed by some environmentalists as a small step forward.

But the hour is too late for small steps. The world needs massive action, beginning immediately, to reverse the existing trends on emissions and deforestation. We also need to pray that it is not already too late.