24 October 2009

350? Too Audacious a Target?

Today is the International Day of Climate Action, an event organised by 350.org. It aims, as I understand it, to raise awareness for climate change and the fact that we have exceeded the safe limit of amount of carbon in the air: 350 parts per million (ppm). The organisers advocate people globally to participate in an action that displays the number 350 prominently, and the Singapore arm of the movement intend to take an aerial photograph of supporters forming a massive 350 in Hong Lim Park.

However, I see a potential confusion here. Firstly, from what I know, this ppm thingy is a measure of how much carbon is in the air. It is a number that takes into account numerous factors, including stuff like carbon removal by forests. If you perform an ideal experiment and measure the composition of the atmosphere, this number is what you'll get for carbon dioxide.

Then there's another thing: carbon emissions, which is one part of this composition picture. Carbon emissions is the amount of carbon dioxide released into the atmosphere by human activities. While this number is, of course, linked to the atmospheric composition, it is not directly connected. A drop in emissions today will not ensure a drop in carbon dioxide composition in the atmosphere tomorrow.

Think of it this way: imagine a bucket with a source and a sink. While water flows in from the source, it is drained by the sink, and these two forms a balance. In an oversimplistic view, this is the pre-industrial age atmospheric carbon picture (oversimplistic because there are ice ages and all sorts of other factors that cause carbon composition to fluctuate) - with the source being the (natural) emissions, the sink being carbon removal capabilities like forests, and the amount of water being the amount of carbon dioxide in the atmosphere. In comes the industrial age, and the source is increased tremendously and the sink decreases. The water level thus rose and is still rising. So even if we turn off our "extra" source and put in more sinks, it's gonna be some time before the water level reverses and returns to the original level. In fact, there is a projection which predict that this reversal will not come before a thousand years have elapsed.

In short, carbon composition will lag behind changes in emissions. Already, the composition of carbon dioxide in the atmosphere is 390 ppm, way higher than the target, and in all realistic hopes, 350 ppm is too fanciful a number to dream for. Looking at the top chart below, obtained from an article by Michael Raupach et al. in the Proceedings of the National Academy of Sciences,

we can see that we have to cut our emissions drastically to even achieve 450 ppm (dashed green line) in the next century. Even 650 ppm looks way far off (dashed blue line). The various A and B lines are projections based on different scenarios (of which, if you want to know, can be found in the article). Even if we switch to clean technology completely - as described by A1T (solid green line) - we cannot even reach 450 ppm in a hundred years' time (though we will be captured by the error bars of 650 ppm). But the most realistic projection is A1B (orange line), which is based on a balance of fossil fuel and clean technology.

Now, much as I advocate environmentalism, I feel uncomfortable about brandishing 350 as the number. In all honesty, no climate scientists are able to tell you in certain terms what the magical number ought to be. In fact, 350 ppm is the lowest safety limit I've heard of; other predictions range up to 650 ppm, but I think that number is really skirting the cliff's edge. Nonetheless, that is already a difficult target to reach, as evidenced by the chart above. So there is no reason for inaction. Especially if you care.

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