How much CO2 emission is too much?

This week, representatives from around the world will gather in Nairobi, Kenya for the latest Conference of Parties (COP) meeting of the Framework Convention of Climate Change (FCCC) which brought us the Kyoto Protocol. The Kyoto Protocol expires in 2012, and the task facing the current delegates is to negotiate a further 5-year extension. This is a gradual, negotiated, no doubt frustrating process. By way of getting our bearings, a reader asks the question, what should the ultimate goal be? How much CO2 emissions cutting would it take to truly avoid “dangerous human interference in the climate system”?

On the short term of the next few decades, the line between success and excess can be diagnosed from carbon fluxes on Earth today. Humankind is releasing CO2 at a rate of about 7 Gton C per year from fossil fuel combustion, with a further 2 Gton C per year from deforestation. Because the atmospheric CO2 concentration is higher than normal, the natural world is absorbing CO2 at a rate of about 2 or 2.5 Gton C per year into the land biosphere and into the oceans, for a total of about 5 Gton C per year. The CO2 concentration of the atmosphere is rising because of the 4 Gton C imbalance. If we were to cut emissions by about half, from a total of 9 down to about 4 Gton C per year, the CO2 concentration of the atmosphere would stop rising for awhile. That would be a stunning success, but the emission cuts contemplated by Kyoto were only a small step in this direction.

Eventually, the chemistry of the ocean would equilibrate with this new atmospheric pCO2 concentration of about 380 ppm (the current concentration), and its absorption of new CO2 would tail off. Presumably the land biosphere would also inhale its fill and stop absorbing more. How long can we expect to be able to continue our lessened emissions of 4 Gton C per year? The answer can be diagnosed from carbon cycle models. A range of carbon cycle models have been run for longer than the single-century timescale that is the focus of the IPCC and the FCCC negotiation process. The models include an ocean and often a terrestrial biosphere to absorb CO2, and sometimes chemical weathering (dissolution of rocks) on land and deposition of sediments in the ocean. The models tend to predict a maximum atmospheric CO2 inventory of about 50-70% of the total fossil fuel emission slug. Let’s call this quantity the peak airborne fraction, and assume it to be 60%.

The next piece of the equation is to define “dangerous climate change”. This is a bit of a guessing game, but 2°C has been proposed as a reasonable danger limit. This would be decidedly warmer than the Earth has been in millions of years, and warm enough to eventually raise sea level by tens of meters. A warming of 2° C could be accomplished by raising CO2 to 450 ppm and waiting a century or so, assuming a climate sensitivity of 3 °C for doubling CO2, a typical value from models and diagnosed from paleo-data. Of the 450 ppm, 170 ppm would be from fossil fuels (given an original natural pCO2 of 280 ppm). 170 ppm equals 340 Gton C, which divided by the peak airborne fraction of 60% yields a total emission slug of about 570 Gton C.

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