Tim Flannery's The Weather Makers warned of a number of "tipping points" in the global climate system which could dramatically accelerate climate change. One of these was the Amazon rain forest. While we think of the Amazon as one of the wettest places on Earth, isotope studies have shown that it effectively creates its own rain. Moisture blown over from the Atlantic Ocean is constantly absorbed and effectively transported west by transpiration (and then ultimately flows east again in the rivers). Any disruption in this cycle - either in the amount of moisture input, or in the level of transpiration - could therefore have dramatic effects, and models have predicted that the Amazon could disappear and turn into a desert if we see the sorts of temperature rises and climate shifts we are expecting if CO2 emissions continue unchecked. And this in turn would release vast amounts of carbon from rotting trees and warmer soils.
The models Flannery talked about predicted that this could start happening around 2040, and be complete by 2100. But it might be happening sooner than we think - and the culprit is drought. A story in today's Independent (and syndicated to the Herald) reports that the Amazon is highly sensitive to drought, and could suddenly transform to a desert if severe drought persists for two successive years. Here's the interesting bit:
When Dr Dan Nepstead started the experiment in 2002 - by covering a chunk of rainforest the size of a football pitch with plastic panels to see how it would cope without rain - he surrounded it with sophisticated sensors, expecting to record only minor changes.
The trees managed the first year of drought without difficulty.
In the second year, they sunk their roots deeper to find moisture, but survived.
But in year three, they started dying.
Beginning with the tallest the trees started to come crashing down, exposing the forest floor to the drying sun.
By the end of the year the trees had released more than two-thirds of the carbon dioxide they have stored during their lives, helping to act as a break on global warming.
Instead they began accelerating the climate change.
How much acceleration? According to the article, the Amazon is estimated to contain 90 billion tons of CO2 - and that if this was released, it would increase the rate of global warming by 50 per cent.
And now for the worse news: the Amazon looks set to be entering its second consecutive year of severe drought.
What's worrying is that, of the identified tipping points - Greenland melting, the Gulf Stream shutting down, the Amazon drying up, West Antarctica breaking lose, and Siberia emitting its vast amount of stored methane - we're getting strong warning signs from all of them. And any of them will speed up the process even further, making it far more likely that the others will happen.
But what's really worrying is that, while we know that these tipping points exist and that positive feedback is a real possibility, we don't know exactly where the thresholds are - and given the inertia in the climate system, we may already have passed them.