Trillion tonne baby

James Hrynyshyn (The radical "trillionth ton" meme) suggests there may be just 20 years of emissions at business-as-usual rates, not 40, because initial estimates do not take account of emissions from agriculture and forestry. At present I don't know if this is right. [1], [2]

Hrynyshyn also likes the following from the lead authors of the "trillionth ton" papers in the Nature climate package (previously noted here):

A tonne of carbon is a tonne of carbon, whether released today or in 50 years time. Emitting CO2 more slowly buys time, perhaps vital time, but it will only achieve our ultimate goal in the context of a strategy for phasing out net CO2 emissions altogether.

At some point in the past few years, without any fanfare, we burned the half-trillionth tonne. Somewhere out there, in a coal seam, hydrocarbon reservoir or some as-yet-undiscovered exotic form of fossil carbon, lies the trillionth tonne. Its fate, perhaps more than any other consequence of climate-change policy, is inextricably linked to the risk of dangerous climate change. Where will it be in the twenty-second century?

But here's another question. The trillionth tonne 'meme' (would 'frame' be a better word?) takes its energy from the idea that there is something especially significant - fateful - in the odds of a global average temperature increase of more than 2 C crossing a threshold to above one in four. Why?

Aren't the odds already unacceptably high? A one in a twenty chance of, say, a plane crashing would probably keep you off the plane -- unless the downside of not getting on the plane was almost certainly worse (crazy men with guns swearing to kill you right now, for example). What eventualities can we see as likely to be worse than those resulting from a more than 2 C rise this century, and how might they be affected by sharply reducing emissions very soon, or reduced by not doing so?

Footnotes

[1] George Monbiot makes an estimate of how much fossil fuel we can burn. Clearly, it is less than known reserves. As the Stern Review (2005/06) pointed out:

Increasing scarcity of fossil fuels alone will not stop emissions growth in time. The stocks of hydrocarbons that are profitable to extract (under current policies) are more than enough to take the world to levels of CO2 concentrations well beyond 750ppm, with very dangerous consequences for climate-change impacts.

[2] (added 7 May) See Planet of doubt