It will take just six years of current emissions to exhaust a carbon budget that would give a good chance of keeping global warming below 1.5 degrees Celsius, based on figures from the Intergovernmental Panel on Climate Change (IPCC).
The IPCC’s new budget, revealed earlier this month, calculates the remaining amount of carbon dioxide humans can emit and still hope to cap global warming at less than 1.5 degrees Celsius above preindustrial levels.
Limiting warming to 1.5 degrees has become a political rallying call for some nations and Non-Governmental Organisations (NGOs).
The IPCC’s calculations suggest hopes of preventing temperatures from ever crossing the 1.5 degree threshold are slim to none. But the IPCC highlights that options to temporarily exceed the target and return to lower temperatures later in the century could still be on the table.
A new report, a tighter budget
The concept of a “carbon budget” first appeared in the IPCC’s report on the physical basis of climate change, released last September.
The IPCC calculated how much carbon we can emit and still have a good chance of limiting global warming to below two degrees above pre-industrial levels. Two degrees is the UNFCCC internationally-accepted point beyond which the risks become unacceptably high.
But while the international community uses two degrees as the rule-of-thumb threshold for “dangerous” warming, many low-lying and island nations have pushed for a lower limit to be adopted.
In response, the IPCC undertook calculations in the Synthesis Report to provide a carbon budget to stay below 1.5 degrees of warming.
Crunching the numbers
This table from the IPCC’s Synthesis Report contains the numbers. It’s divided along the top into different temperature targets. As well as columns for keeping warming below 1.5 degrees and two degrees, there’s also one for three degrees.
The IPCC calculates three budgets for each temperature target, each gets its own column. The first gives a 66 per cent probability of staying below the stated temperature, the second a 50 per cent chance, and the last a 33 per cent chance. These probabilities are based on the number of model simulations that keep warming below the given temperature limit.
(Source: Table 2.2, page 61 of the full length report)
Taken together, the scenarios in the table range from relatively optimistic – a good chance of staying below 1.5 degrees, to a less hopeful scenario which offers only a poor chance of staying under three degrees.
Six years and counting
Carbon dioxide is the main contributor to warming. But the IPCC’s budget also allows for some extra warming from other factors like methane, nitrous oxide and fluorinated gases.
At the ambitious/optimistic end of the spectrum, securing a two thirds chance of keeping warming below 1.5 degrees requires total emissions over the industrial period to be capped at no more than 2250 billion tonnes of carbon dioxide, the IPCC says. Subtracting historical emissions from this leaves a remaining budget in 2011 of 400 billion tonnes of carbon dioxide.
We brought this budget up to date using data from the Global Carbon Project. Four extra years of emissions shrinks the budget by a further 157 billion tonnes, leaving 243 billion tonnes of carbon dioxide left for us to emit from 2015 onwards.
We are currently emitting about 40.3 billion tonnes of carbon dioxide a year, from fossil fuel burning, cement production and land use change. That suggests we have just six years of business-as-usual emissions before the budget giving us a two-thirds chance of staying below 1.5 degrees is exhausted.
As the level of climate ambition drops, the numbers get less dramatic – but still demonstrate the scale of the challenge. As the graphic above shows, if we want a 50 per cent chance, the budget stretches to ten years at current emissions rates. And for a 66 per cent chance of limiting warming to below two degrees, we have the equivalent of 21 years of current emissions left in the budget.
We’ve summarised our calculations in the table below, but you can see the full spreadsheet with data sources here.
Since emissions couldn’t become zero overnight, fossil fuel use would need to be steadily phased out over a number of years to stay within any one of these budgets.
But the IPCC considers another possibility.
A limited number of scenarios allow concentrations of carbon dioxide in the atmosphere to temporarily overshoot the level that would keep warming below 1.5 degrees.
In these scenarios, global temperature peaks during the 21st century and is brought back down to 1.5 degrees by 2100, making use of technology known as Carbon Capture and Storage (CCS) to reduce the amount of carbon dioxide reaching the atmosphere.
To stay on track to meet the 1.5 degrees target, emissions would need to come down by between 70 and 95 per cent compared to 2010 levels by 2050, turning negative (meaning more carbon is removed than emitted) by the end of the century.
But CCS doesn’t yet exist on commercial scales and the potential of the technology to meet ambitious temperature targets remains untested and highly uncertain, the IPCC points out.
Crunching the numbers, the scale of the challenge to keep warming below 1.5 degrees looms large. It’s not impossible – if we consider 1.5 degrees as a long term target rather than a hard limit that must not be crossed.
That said, it would require a major overhaul of energy generation and a rapid scaling up of CCS. With so much uncertainty over the latter, early and deep emissions cuts would hedge against the fairly hefty risk that the technology doesn’t meet expectations.
In the real world, global emissions are rising – and expected to carry on doing so. This makes the leap to a 1.5 degree pathway even harder to envisage. But the fact that the IPCC has extended its budget calculations this far means we can at least see what it means in practice.
Six years worth of current emissions would blow the carbon budget for 1.5 degrees
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