The Horrifying Spectre of Irreversible Runaway Climate Change

Image via WikipediaBy Robin Whitlock

The UK's Independent newspaper today (4th February 2011 (1))is carrying a truly terrifying story on its front page. Apparently the Amazon rain forest suffered an appalling drought last year which caused it to become an emitter of carbon rather than a carbon sink. For those unused to the terminology this means that whereas in previous years the rain forest absorbed carbon dioxide from the atmosphere, it is now beginning to release it.

According to the story, scientists believe that it will release some eight billion tonnes of carbon into the atmosphere, which is more than the current annual total emissions of the United States. This switch from carbon sink to carbon emitter occurred because many of the trees dried out and died.

So why is this such an appalling development in the ongoing saga of climate change? The answer can be found in an article written by Craig D. Allen of the US Forestry Department (2). He explains that what is occurring with increased regularity is something called 'climate-induced forest dieback'. This is important because trees grow relatively slowly but can die very quickly, which means that their rate of replacement is slower than the rate of their destruction.

Trees absorb carbon dioxide from the atmosphere and produce oxygen, therefore the more trees that die more quickly the greater the amount of carbon is released into the atmosphere. This is known among climatologists as a positive forcing or a positive feedback in the sense that it is something that is highly likely to accelerate the rate of climate change rather than reducing it.

Where trees die out en mass, for example in response to drought, an entire regional environment can be quickly and severely affected. There is increasing evidence that this is already starting to happen in other parts of the world, for example in Canada where Kurtz et al (2008) have shown that forest dieback, insect and disease mortality and fire impacts have turned the Canadian forests into a producer of carbon rather than a carbon sink. Lewis (2005) suggests that widespread drought throughout this century could transform the world's forests into a massive carbon source. This would grossly accelerate the rate of warming as more and more carbon is released into the atmosphere.

How does dieback actually occur with higher temperatures? This is shown in a study by Henry D. Adams et al (2009). In trials with pinus edulis pine trees, Adams and his team showed that higher temperatures of around 4 degrees C shortened the time to drought-induced mortality by nearly a third. There are two processes involved here, 1) carbon starvation as a response to prolonged water stress, and 2) sudden hydraulic failure under water stress (cavitation).

Carbon starvation occurs where trees close their pores (called 'stomata') in order to maintain safe levels of xylem pressure (xylem is a form of tissue within the structure of trees and plants that is responsible for the transportation of nutrients around the system). This stops photosynthesis and forces the plant to rely on stored carbohydrates to support the maintenance of tissue. It seems that higher temperatures increases the metabolism, forcing the tree to consumer its stored resources at a faster rate than normal. Eventually the tree runs out of food and death results. Hydraulic failure occurs where air bubbles block the transportation of stem water.

Some experts have noticed that increasingly trees are dying faster than expected in droughts around the world and this has led to the belief that higher temperatures are responsible(4).

This positive feedback, alongside others such as the loss of albedo caused by melting sea-ice and the release of methane from melting permafrost, adds to the likelihood that the earth will be pushed closer and closer to irreversible 'runaway' climate change unless we as a species take urgent measures to curtail and reduce greenhouse emissions. Runaway climate change is the term used to describe the point in time at which climate change is impossible to stop and at which it starts to increase in severity under its own momentum as more and greater positive feedbacks add to greenhouse gas emissions, rather like a giant snowball running downhill, hence the term 'snowball effect'.

Climatologists and environmentalists that the danger of this occurring becomes more likely as we approach a level of 2 degrees C beyond the temperatures experienced in pre-industrial times. An even greater danger of runaway becomes likely as temperatures move towards 4 degrees C. An article in The Guardian in November grimly concluded that at the present time there is very little chance that the 2 degree limit can be maintained and that therefore a move towards 4 degrees is likely.

James Hansen, the well known climatologist and scientific adviser to NASA, has talked about 'the Venus syndrome', a situation in which runaway climate change gathers such momentum that warming of the planet carries on all the way towards the death of the life preserving capabilities such that the Earth becomes a dead planet - like Venus (5). This is a very controversial idea and Hansen has, as a result been accused of excessive 'doom mongering'. Nevertheless, the movement of the Amazon basin towards being a carbon source, is a very worrying development indeed, of which everyone of us should take note.

(1) Steve Connor, Special Report: Catastrophic drought in the Amazon, The Independent Newspaper, 4th February 2011

(2) Craig D. Allen, US Geological Survey, 'Climate-induced forest dieback: an escalating global phenomenon?

(3) Adams, H. D. et al (2009) 'Temperature sensitivity of drought-induced tree mortality portends increased regional die-off under global change-type drought', Proceedings of the National Academy of Sciences, (doi:10.1073)

(4) Goodall, S. (2009) 'Trees die quicker in drought when temperatures are hotter', Carbon Commentary website.

(5) Hansen, J. (2009) 'Storms of my grandchildren: The truth about the coming climate catastrophe and our last chance to save humanity', London:Bloomsbury

Robin Whitlock is a British freelance writer with a special interest in environmental issues, particularly climate change. He has been an environmentalist since the early 1990's and writes regularly on the subject of renewable technology for a technical recruitment consultancy website.

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