Last month, the Intergovernmental Panel on Climate Change (IPCC) released its fifth ‘assessment report’, which provide a detailed look at the science of climate change.
The latest report is the first to include an assessment of a “carbon budget” – a finite amount of carbon that can be burnt before it becomes unlikely we can avoid more than two degrees of global warming.
So how big is the carbon budget, how was it calculated, and how does it help us understand the challenge of limiting climate change?
A budget for two degrees
In 1992, the United Nations Framework Convention on Climate Change (UNFCCC) decided the objective of global climate policy should be to stabilise humans’ influence with the climate below the level at which it can be considered “dangerous”.
In its new report, the IPCC includes a calculation of how much carbon we can emit and still have a reasonable chance of staying below two degrees.
It calls this amount a carbon budget. The budget is an upper limit on total human emissions, from the beginning of the industrial revolution until the day we stop burning carbon.
To stick to the budget, any fossil fuels that would take us over-budget will either have to be left in the ground, or the emissions captured before or after entering the atmosphere.
1000 billion tonnes
So how big is the budget? For it to remain likely that we stay below two degrees, the total amount of carbon released through carbon dioxide emissions must be less than 1000 billion tonnes, the IPCC says. ‘Likely’ here means a 66 per cent chance.
It’s possible to calculate a budget like this because carbon dioxide, which is the biggest contributor to global warming, has a predictable relationship with temperature. The warming we get is almost directly proportional to the total amount of carbon dioxide that accumulates in the atmosphere.
The budget is calculated using a measure of how sensitive the planet is to carbon dioxide called the transient climate response to carbon emissions ( TCRE). This is defined as the change in average global surface temperature for a given amount of carbon dioxide accumulated in the atmosphere.
The calculation takes into account processes that amplify or reduce the warming we see in the atmosphere, which are known collectively as feedbacks. These are complicated, introducing uncertainties to the calculation. Because of this, the carbon budget is expressed as the probability of staying below a certain temperature, rather than a guarantee that’s what will happen.
How much of the budget has been used up by carbon dioxide?
The IPCC report says that between the start of the industrial revolution and 2011, carbon dioxide emissions from human activity added about 531 billion tonnes worth of carbon to the atmosphere.
Uncertainty around that figure means it could be anywhere between 446 and 616 billion tonnes but broadly-speaking, this would mean that about half the carbon budget is already spent.
Including other gases shrinks the budget
While carbon dioxide is the main greenhouse gas resulting from human activity, it’s not the only thing causing the atmosphere to warm. Methane, CFCs, ozone, nitrous oxide and soot all have an overall warming effect.
To stick within the two degree target, this means the budget for carbon dioxide emissions ends up being less than the original 1000 billion tonnes.
In the draft Summary for Policymakers given to governments to review back in June, the IPCC scientists stopped short of including a concrete figure for how much these other gases tighten the budget for carbon dioxide emissions.
That was because their warming effect is less straightforward than for carbon dioxide. But after calls from governments for something more tangible to work with, the final version of the Summary for Policymakers included scientists’ best estimate of what the reduced budget would be.
800 billion tonnes
The Summary for Policymakers states that taking into account additional warming factors, the amount of carbon that can be released through carbon dioxide emissions – in total – comes down to about 800 billion tonnes.
That figure comes from scientists’ estimate of the total warming expected from the non-carbon dioxide factors, which comes to about 0.4 degrees Celsius by the end of this century.
Given the target of staying below two degrees of warming overall, that leaves 1.6 degrees of warming to come from carbon dioxide. Using the relationship between carbon dioxide and temperature, staying within that budget means carbon dioxide emissions over the industrial era cannot contribute more than 800 billion tonnes of carbon.
The numbers here relate to the IPCC’s lowest emissions scenario, RCP2.6, which was specifically designed to show how warming can be limited to two degrees. This means the IPCC have made assumptions about the non-carbon dioxide factors, which could be higher or lower in scenarios with weaker or more stringent mitigation of non-carbon dioxide factors. The upcoming report of the IPCC’s Working Group 3 will look at this in more detail.
How far, how fast?
With a budget for carbon dioxide emissions of 800 billion tonnes worth of carbon, and assuming that we had already put 531 billion tonnes into the atmosphere by 2011, it’s more accurate to say we’ve spent two thirds of the budget, not half.
And with about 270 billion tonnes of carbon left in the budget, and current carbon dioxide emissions contributing around 10 billion tonnes per year, the budget is set to be exhausted in about 25 years.
If there are other sources of warming that aren’t accounted for in climate models, the budget could be smaller still. For example, models don’t currently include permafrost methane emissions – as there’s too much uncertainty about them. Including these in the budget could shrink it further.
Adjusting the budget
If we want a better chance of staying below two degrees, a smaller carbon budget improves the odds.
On the other hand, Governments also asked the IPCC to calculate what running a bigger risk of surpassing two degrees would do to the budget. The Summary for Policymakers says expanding the budget to 840 billion tonnes of carbon would give a 50:50 chance. Upping it to 880 billion tonnes reduces the chance to a third.
Rather startlingly, that means that with current emission rates of 10 billion tonnes per year, the difference between a carbon budget of 800 and 880 billion tonnes is just eight years worth of current emissions. That bigger budget halves the chance of staying below two degrees, the IPCC says, from 66 per cent to 33 per cent.
Last month’s calls for greater clarity over the IPCC’s new budget calculations could be taken as a sign that governments are looking for clearer signs on how far emissions cuts need to go and how quickly. But it remains to be seen the level of risk they’re willing to accept, or how a worldwide cap on emissions might be divvied up between nations.
Something to consider, as the 2015 deadline for reaching an international climate agreement approaches.
A note on units: Because carbon dioxide is the main contributor to warming, the budget is sometimes expressed in terms of tonnes of carbon dioxide rather than carbon. Carbon dioxide has a higher mass than carbon, so the budget for carbon dioxide emissions can be expressed either as 800 billion tonnes of carbon, or 2936 billion tonnes of carbon dioxide - but it's essentially the same thing.