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Multicentury changes to the global climate and carbon cycle: results from a coupled climate and carbon cycle model

Govindasamy Bala, Ken Caldeira, Arthur Mirin, Michael Wickett, and Christine Delire · Journal of Climate 18, 4531-4544 · 2005

Key finding. Releasing the CO2 from all currently estimated fossil fuel resources warms global climate by about 8 K by the year 2300 and raises atmospheric CO2 to 1,423 ppmv, with emissions peaking above 30 petagrams of carbon per year early in the twenty-second century and nearly 50% of cumulative emissions still remaining in the atmosphere at 2300.

A line plot of carbon in gigatonnes, 0 to over 5,000, against year from 1900 to 2300. A black curve for cumulative emissions rises steeply through the twenty-second century and levels near 5,400. Below it, a red curve for atmospheric increase and a green curve for land uptake both approach about 2,000 to 2,400, while a blue curve for ocean uptake climbs slowly to about 900.
The gap between the black curve and the ones below it is what the sinks fail to take. By 2300 the atmospheric increase alone is close to half of everything emitted, with the land doing more of the absorbing than the ocean throughout. Figure 7 from Bala et al. (2005), Journal of Climate 18, 4531-4544. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

Emission scenarios normally run to 2100 and stop well short of the resource. That leaves open a simpler question: what if all the fossil carbon currently estimated to be recoverable were actually burned?

Answering it requires a model in which climate and the carbon cycle affect each other, since over centuries the land and ocean sinks respond to the warming they are helping to cause. This paper ran that case out to 2300.

What did we find?

The warming is larger than a fixed sensitivity would predict, because the model's sensitivity to radiative forcing increases as the simulation progresses.

Nearly half of everything emitted is still airborne in 2300, so the sinks do not catch up within the period even though they keep working.

Both soils and living biomass remain net carbon sinks throughout — the land biosphere does not flip to a source in this model.

That makes the result conservative in two respects at once. The model has relatively low climate sensitivity and strong land carbon uptake, and it still produces about 8 K.

Why does it matter?

It gives the fossil resource an upper bound expressed as a temperature. Reserves are normally quoted in tonnes or barrels; this converts the whole recoverable estimate into the climate it would produce.

The rising sensitivity is the uncomfortable detail. It means the damage per tonne is not constant along the path — the later tonnes do more than the early ones, which is the opposite of the intuition behind a fixed carbon price.

Because the model is favourable on both sensitivity and land uptake, the 8 K is better read as a floor for that scenario than as a central estimate.

Citation

Govindasamy Bala, Ken Caldeira, Arthur Mirin, Michael Wickett, and Christine Delire (2005). Multicentury changes to the global climate and carbon cycle: results from a coupled climate and carbon cycle model. Journal of Climate 18, 4531-4544.

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