Updating the remaining carbon budgets for 1.5˚C and 2.0˚C

More than a year has passed since Carbon Brief’s Carbon Brief’s Six years worth of current emissions would blow the carbon budget for 1.5 degrees  which, in November 2014, recalculated the IPCC’s remaining carbon budget for a global temperature rise of 1.5˚C.

So reducing the carbon budgets by emissions since in November 2014 (7 tonnes CO2) gives…

The remaining carbon budget for a 66% chance of avoiding 1.5˚C is now…

26 tonnes CO2 per person.

The remaining carbon budget for a 66% chance of avoiding 2.0˚C is now…

108 tonnes CO2 per person.

However, these budgets are too high because this only accounts for the effects of CO2 and do not take account of other greenhouse gasses. The World Resources Institute says:

“one can argue for an even smaller budget and additional emissions constraints because non-CO2 gases are not included in 1 trillion tonne C figure. For example, short-lived greenhouse gases, such as methane, are not included in – nor necessarily appropriate for – the 1 trillion tonne C budget approach because they play a secondary role in influencing long-term warming.

However, when non-CO2 forcings are taken into account, the budget is reduced and that budget may depend on the scenario studied. For example, according to one scenario studied in the IPCC AR5 (RCP 2.6), when non-CO2 greenhouse gases are considered, the budget drops much lower to 790 PgC.”

That means that the effect of other greenhouse gasses reduces the overall budget to 79% of the original. The remaining carbon budgets are measured in terms of CO2 so, as a rough estimate they should be reduced by 21%. This now gives…

The remaining carbon budget for a 66% chance of avoiding 1.5˚C becomes …

21 tonnes CO2 per person: 4 years to 1.5˚C

The remaining carbon budget for a 66% chance of avoiding 2.0˚C becomes…

85 tonnes CO2 per person: 16 years to 2.0˚C

But there are other reasons these may be too optimistic.

This is based on a postscript to “Is green growth a fantasy?”