I think their scenario is unrealistic. What they're saying is that a decline in the world's oil supply will prevent a lot of CO₂ from getting into the atmosphere if coal is deliberately restricted.

So they're saying that we'll go sick and just burn all the oil we have, limited only by its physical availability, but have great restraint with our coal.

It just doesn't make sense. If we go sick with oil, why wouldn't we go sick with coal? If we have restraint with coal, why wouldn't we have restraint with oil?

Surely as oil becomes scarce we'll see more use of coal?

For reference, the IPCC 2004 report told us that 21st century emissions of 1,800Gt CO_2e would give us 450ppm, and 4,100Gt would give us 1,000ppm.

However, already from 2001-7 we've added 315Gt, taking these levels to 1,500Gt and 3,800Gt CO_2e.

But only 55% of emissions come from burning fossil fuels. It's fair to assume that other contributions will at best remain in proportion. In practice they'll probably rise; the places short of fossil fuel will probably cut down more forests, and deforestation already contributes around 17.5% of greenhouse gases.

So for example, with our current 49Gt CO_2e emissions annually, and with 17.5% or 8.6Gt coming from deforestation, well 8.6Gt for 92 years is 789Gt CO_2e. That is, just cutting down forests pretty much takes us to 450ppm, forget about the fossil fuels.

Hansen and Kharecha of course do say that there should be forestation programmes; but again that suggests planning and restraint. Why are we going to have planning and restrain for forests and coal, but not for oil?

But looking just at the fossil fuels, being optimistic and assuming they rise or decline in that same proportion, so that fossil fuels remain 55%, we can set the 450ppm level as requiring the burning of fossil contributing around 800Gt, and the 1,000ppm level requiring around 2,000Gt CO_2e.

So burning fossil fuels and adding 800Gt of CO₂ will give us trouble but not catastrophic, but if we hit 2,000Gt CO₂ from them then all bets are off.

Considering our reserves and the emissions burning them gives, we have,

Oil reserves of 1317Gbbl@ 450kg CO2e/bbl give 593Gt CO2e
Coal reserves of 998Gt@ 2,350kg CO2e/t give 2,854Gt CO2e
Natural gas reserves of 6,074T cu ft@ 52.2kg CO2e/1,000 cu ft give 316Gt CO2e
For total emissions of 3,763Gt CO2e

Which is to say, if we burn up all our reserves we'll completely screw ourselves. Well, we all knew that already.

Current coal consumption is 6Gt, oil 31Gbbl, and gas 100T cu ft.

To keep contributions by 2100 under 800Gt requires a decline every year in use by 4% in each of the three. We would then have used only 15% of coal reserves, 55% of oil reserves, and 39% of gas reserves. It would mean that over any decade, use would decline by a third. This seems remarkably restrained over a period of 92 years.

To keep contributions under 2,000Gt requires only a decline of 1% annually. This is essentially 10% in any decade. This seems more plausible.

Hansen and Kharecha's scenario with one lot of fossil fuels being guzzled up like mad and another lot being restrained makes no sense. More plausible is that use of all of them increases, or all declines.

Only a strong decline - of 4% each year for 92% - gives us eventual CO2 of around 450ppm; business as usual or a slight decline of 1% annually takes us past 1,000ppm. Burning the lot in this century means catastrope.

Now, I've previously pointed out that any particular individual in the West can make their impact only about one tonne of CO2, down from the 12 tonnes which is average for the US and Australia, or 5-10 tonnes for most of Europe. For everyone to be able to do this might take a decade or so of improvements in renewable energy availability, mass transit and so on.

Things individuals can control like transport and diet make up about half of all emissions. So total Western emissions could decline by around 40-45% over those ten years without any significant discomfort or decline in lifestyle for the average Westerner. The other half of society contributing emissions, in industry, commerce and agriculture, would then have to get their shit together and act.