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 CO 2 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 CO 2 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 CO 2 concentration is higher than normal, the natural world is absorbing CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 , and sometimes chemical weathering (dissolution of rocks) on land and deposition of sediments in the ocean. The models tend to predict a maximum atmospheric CO 2 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 CO 2 to 450 ppm and waiting a century or so, assuming a climate sensitivity of 3 °C for doubling CO 2 , 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.

How much is 570 Gton C? We have already released about 300 Gton C, and the business-as-usual scenario projects 1600 Gton C total release by the year 2100. Avoiding dangerous climate change requires very deep cuts in CO 2 emissions in the long term, something like 85% of business-as-usual averaged over the coming century. Put it this way and it sounds impossible. Another way to look at it, which doesn’t seem quite as intractable, is to say that the 200 Gton C that can still be “safely” emitted is roughly equivalent to the remaining traditional reserves of oil and natural gas. We could burn those until they’re gone, but declare an immediate moratorium on coal, and that would be OK, according to our defined danger limit of 2°C. A third perspective is that if we could limit emissions to 5 Gton C per year starting now, we could continue doing that for 250/5 = 50 years.

One final note: most of the climate change community, steered by Kyoto and IPCC, limit the scope of their consideration to the year 2100. By setting up the problem in this way, the calculation of a safe CO 2 emission goes up by about 40%, because it takes about a century for the climate to fully respond to rising CO 2 . If CO 2 emission continues up to the year 2100, then the warming in the year 2100 would only be about 60% of the “committed warming” from the CO 2 concentration in 2100. This calculation seems rather callous, almost sneaky, given the inevitability of warming once the CO 2 is released. I suspect that many in the community are not aware of this sneaky implication of restricting our attention to a relatively short time horizon.

Note: responding to suggestions in the comments, some of the numbers in the text above have been revised. November 7, 2:31 pm. David