Feedback Frenzy

February 20, 2009

Rainclouds over Los Angeles, Feb 2009, Reuters

Rainclouds over Los Angeles, Feb 2009, Reuters

2008 might become known as the year of the feedback. Study after study emerged, suggesting that future changes to the Earth’s climate would not simply happen gradually. We would not enjoy the luxury of a slow transition to what would still be an uncomfortably warmer future. Unfortunately, the deeper scientists peered into the workings of the climate, the more they realised that sudden transitions were much more likely to occur.

Feedbacks occur when one phenomenon (e.g. warming) leads onto another (for example, forests dying) which then leads back into more warming. There are many that we are aware of, and are coming to fear – including the aforementioned demise of tropical forests, the melting of northern permafrost to release methane, the loss of Arctic albedo leading to increased heat absorption (as the sea is darker than the ice that it replaces.)

Taken together, they form a formidable package, and taking them together – as aspects of an interconnected earth system – is what we have to try to do if we are to grasp the enormity of anthropogenic climate change.

One of those feedbacks should be in the news as I write this. That’s because scientists Andrew Dessler and Steven Sherwood (from Texas A&M and the Climate Change Research Centre at the University of New South Wales respectively) have just published the findings of their research on the subject in the journal Science.

Dessler and Sherwood contend that rising temperatures will lead to greater humidity in the atmosphere, as more water evaporates from oceans and lakes. As the water vapor produced is also a greenhouse gas, this will have the effect of raising the temperature still further. As Dessler states, “It’s a vicious cycle – warmer temperatures mean higher humidity, which in turn leads to even more warming.”

According to their research, the water vapor feedback is accurately factored into climate models which predict a global temperature increase of between 2 and 4 degrees centigrade.

In Dessler’s opinion this necessitates urgent action, saying that “everything shows that the climate models are probably getting the water vapor feedback right, which means that unless we reduce emissions, it is going to get much, much warmer on our planet by the end of the century.”

These findings, like those reported by Swiss researchers in 2005, challenges those who argue that increased evaporation due to rising temperatures would lead to increased cloud formation (at certain latitudes), and would therefore result in a cooling effect, as the light coloured cloud would reflect sunlight, lessening the amount of heat entering the Earth’s atmosphere.

Some researchers have argued that “negative feedbacks” within the Earth’s systems (such as the clou-cooling system mentioned above) will cancel out climate change caused by greenhouse gas emissions. Research like this suggests that such prophecies are dangerously complacent.

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