Eight years’ worth of current emissions halves the chances of staying below two degrees warming
Limiting global warming to two degrees above pre-industrial levels will require “substantial and sustained” cuts in greenhouse gas emissions, the Intergovernmental Panel on Climate Change (IPCC) said in its latest report.
It went a step further by proposing a “carbon budget” – a first for the IPCC reports. This is a total amount of carbon we can put into the atmosphere while still having a good chance of staying below the critical two degree threshold.
How scientists work out the budget is quite complicated – we have written a more detailed briefing here. But here are the most important bits.
Two thirds of the budget is already spent
According to the IPCC’s calculations, 800 billion tonnes is the maximum amount of carbon we can release through carbon dioxide emissions to still have a 66 per cent chance of limiting warming to two degrees – a probability the IPCC terms “likely”.
This budget allows for some additional warming to come from emissions other than carbon dioxide, including methane, CFCs, ozone, nitrous oxide and black carbon.
Carbon dioxide emissions over the industrial era have put about 531 billion tonnes of carbon into the atmosphere – which means we’ve already ‘spent’ about two thirds of the budget.
If emissions were to continue at current levels, the remaining budget – about 270 billion tonnes – would be exhausted in about 25 years.
To stay within budget, any fossil fuels that would put us over budget would have to be left in the ground – or the emissions captured before or after entering the atmosphere.
Halving our chances
If we burn more carbon than the 800 billion tonne budget, the chance of staying below two degrees gets smaller.
According to the IPCC’s figures, upping the carbon budget to 880 billion tonnes halves our chances of staying below two degrees of warming – from 66 per cent to 33 per cent.
If it’s difficult to imagine what the difference between a big number and another big number really means, put it this way – carbon dioxide emissions are currently about 10 billion tonnes per year.
So halving the chances of staying below two degrees would give us an extra eight years worth of current carbon dioxide emissions – not much.
Rather than the carbon budget being exhausted in about 25 years from now, which is roughly what would happen if emissions stay high, accepting this lower probability of remaining below two degrees would stretch the timeline to about 33 years.
A narrow window
Some scientists have criticised the expression of the need to bring down emissions as a budget. Climatologist Ken Caldeira told Joe Romm of the climate analysis blog ThinkProgress, the tendency with a budget is to spend first, think later. He says:
“If you look at how our politicians operate, of you tell them you have a budget of XYZ, they will spend XYZ. Politicians will reason: ‘If we’re not over budget, what’s to stop us spending? Let the guys down the road deal with it when the budget has been exceeded.’ The carbon dioxide budget framing is a recipe for delaying concrete action now.”
Caldeira says the climate system responds only slowly to a change in greenhouse gas emissions, so it’s important to make changes now if we’re to see a significant difference in the next few decades.
Indeed, the IPCC’s scenario for how to limit warming to two degrees – known as RCP2.6 – assumes strong mitigation in the next few decades and negative emissions by 2070. That means taking more carbon out of the atmosphere than we’re putting in.
Cutting emissions now would give a better chance of keeping more than two degrees of warming an “unlikely” prospect – but the IPCC’s calculations suggest it’s a pretty narrow window however you slice it.