Where is the carbon going

Figure 31.3 is a gross simplification. For example, humans are causing additional flows not shown on this diagram: the burning of peat and forests in Borneo in 1997 alone released about 0.7 GtC. Accidentally-started fires in coal seams release about 0.25GtC per year.

Nevertheless, this cartoon helps us understand roughly what will happen in the short term and the medium term under various policies. First, if carbon pollution follows a "business as usual" trajectory, burning another 500 Gt of carbon over the next 50 years, we can expect the carbon to continue to trickle gradually into the surface waters of the ocean at a rate of 2GtC per year. By 2055, at least 100Gt of the 500 would have gone into the surface waters, and CO2 concentrations in the atmosphere would be roughly double their pre-industrial levels.

If fossil-fuel burning were reduced to zero in the 2050s, the 2 Gt flow from atmosphere to ocean would also reduce significantly. (I used to imagine that this flow into the ocean would persist for decades, but that would be true only if the surface waters were out of equilibrium with the atmosphere; but, as I mentioned earlier, the surface waters and the atmosphere reach equilibrium within just a few years.) Much of the 500 Gt we put into the atmosphere would only gradually drift into the oceans over the next few thousand years, as the surface waters roll down and are replaced by new water from the deep.

Thus our perturbation of the carbon concentration might eventually be righted, but only after thousands of years. And that's assuming that this large perturbation of the atmosphere doesn't drastically alter the ecosystem. It's conceivable, for example, that the acidification of the surface waters of the ocean might cause a sufficient extinction of ocean plant-life that a new vicious cycle kicks in: acidification means extinguished plant-life, means plant-life absorbs less CO2 from the ocean, means oceans become even more acidic. Such vicious cycles (which scientists call "positive feedbacks" or "runaway feedbacks") have happened on earth before: it's believed, for example, that ice ages ended relatively rapidly because of positive feedback cycles in which rising temperatures caused surface snow and ice to melt, which reduced the ground's reflection of sunlight, which meant the ground absorbed more heat, which led to increased temperatures. (Melted snow - water - is much darker than frozen snow.) Another positive feedback possibility to worry about involves methane hydrates, which are frozen in gigaton quantities in places like Arctic Siberia, and in 100-gigaton quantities on continental shelves. Global warming greater than 1 ° C would possibly melt methane hydrates, which release methane into the atmosphere, and methane increases global warming more strongly

years from today 100

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years from today a

0 200 400 600 800 1000

years from today

Figure 31.4. Decay of a small pulse of CO2 added to today's atmosphere, according to the Bern model of the carbon cycle. Source: Hansen et al. (2007).

than CO2 does.

This isn't the place to discuss the uncertainties of climate change in any more detail. I highly recommend the books Avoiding Dangerous Climate Change (Schellnhuber et al., 2006) and Global Climate Change (Dessler and Parson, 2006). Also the papers by Hansen et al. (2007) and Charney et al. (1979).

The purpose of this chapter is to discuss the idea of fixing climate change by sucking carbon dioxide from thin air; we discuss the energy cost of this sucking next.

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