In the run-up to the Copenhagen climate summit in early December, I am dismayed by some of the lax targets being proposed for reducing carbon emissions. Some of the targets sound aggressive because they are expressed as a reduction in emissions. The problem is that emissions is a flow, but we need to reduce atmospheric concentration, which is a stock. Below is a live simulation (the first to be posted here) which illustrates the difference:

The box on the left is emissions (in gigatons of carbon equivalents per year), which countries are aiming to reduce. Currently at 7 GtCe/year and increasing at about 2.5% per year, this number may be reduced by about 2% per year based on some of the targets being bandied about. Which is a nice start.

The problem is the box on the right, which is the concentration of greenhouse gases in the atmosphere. This is currently at 430 parts per million (ppm), and increasing by about 4 ppm per year. We need to keep this below about 500 ppm to avoid a temperature increase above the critical 2 degrees.

Concentration is a stock because it is the cumulative sum of all inflows (emissions) and outflows (removal of carbon from the atmosphere, natural or otherwise). Unless carbon capture and storage becomes big, removal will be driven by the lifetime of carbon in the atmosphere, which is about 125 years; so about 0.8% of carbon leaves the atmosphere each year, absorbed by the earth.

Even if we decrease emissions, if they are above the rate of carbon removal, carbon concentration will still increase. Try clicking on the emissions growth rate and typing in a negative number, say -20 for a 20% reduction in emissions each year. The model will be updated instantly to show a dramatic reduction in emissions, but carbon concentration will still increase (although more slowly than before).

It's like debt: reducing the deficit (annual shortfall between income and expenses) is a good thing, but debt will still go up each year unless you have a negative deficit. With the debt, it is even worse because your budget surplus needs to cover interest as well.

The model suggests that we need to increase global emissions by no more than about 1.8% per year to stabalize concentrations below 500 ppm this century. Current targets are nowhere near this.

Note: the numbers above are from memory, and I will confirm them and perhaps revise this page in a few days.

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