5.10 How do you work out a carbon budget?

Because CO2 has a long residence time in the atmosphere, it is the cumulative emissions that matter.

There are at least two ways to work out global carbon emission budgets to avoid 2°C warming – and they arrive at similarly challenging budgets.

One method is to link cumulative emissions to ‘stabilisation targets’ for atmospheric CO2 concentrations, taking account of uncertainties in the carbon cycle and the climate response. This yields probabilistic relationships between concentrations of atmospheric CO2 and likely increases in global average mean surface temperature. Limiting cumulative emissions over 2000–2050 to 1440 GtCO2 gives a 50/50 chance of exceeding 2°C; limiting them to 1000 GtCO2 gives a 75% chance of avoiding 2°C. Emissions between 2000 and 2006 were 234 GtCO2, and current annual emissions are 36 GtCO2. If we assume that annual emissions continue at a constant rate of 36 GtCO2 (rather than increasing at 1–3% a year as they have been since 2000), then we would use up the 1440 GtCO2 budget by 2039 and the 1000 GtCO2 budget by 2027.

The carbon emitted from known fossil fuel reserves (i.e. known and commercially viable) would be 2800 GtCO2. This vastly exceeds the carbon budget described above, without worrying about new finds or ‘peak oil’. Avoiding 2°C means that most of this – coal in particular – must stay in the ground.

Another approach uses a combined climate and carbon cycle model to produce simulations spanning a range of climate futures consistent with historical change. The results show that emitting a total of 3670 GtCO2 (=1 trillion tonnes of carbon) between 1750 and 2500 is likely to lead to exceeding 2°C. Since the start of the Industrial Revolution in 1750 humans have released 1835 GtCO2 (or 500 GtC) into the atmosphere – half of the budget.

That's a bit higher than the 1440 or 1000 GtCO2 (390 or 270 GtC respectively) budgets above, but note the timescale is rather longer (the budget period runs out to 2500 rather than to 2050) – and it's still much less than burning known fossil fuels.

Turning that back into targets equates to at least 80% reductions in global emissions by 2050 (rather higher than the ‘challenging’ 50% currently forming the basis of the post-Kyoto talks to be held in Copenhagen in December 2009).

And even 2°C is far from ‘safe’ for many people.

These web-based resources provide additional information

  • George Monbiot discusses these issues.

  • RealClimate discusses ‘How long will global warming last’.

  • RealClimate, in ‘Simple question, simple answer… not’, explores an answer to the question of the amount of global warming that greenhouse gases will bring.

5.9 What's the relationship between emission reduction targets, global carbon budgets and global temperature increase?

5.11 How do the UK and Scottish Climate Change Acts compare with a 1000 GtCO2 budget for 2000–2050?