Analysis: Only five years left before 1.5C carbon budget is blown

In its most recent synthesis report, published in early 2014, the Intergovernmental Panel on Climate Change (IPCC) laid out estimates of how much CO2 we can emit and still keep global average temperature rise to no more than 1.5C, 2C or 3C above pre-industrial levels.

That same year, Carbon Brief used these estimates to calculate how many years of current emissions were left before blowing these budgets.

Updating this analysis for 2016, our figures suggest that just five years of CO2 emissions at current levels would be enough to use up the carbon budget for a good chance – a 66% probability – of keeping global temperature rise below 1.5C.


The IPCC estimates carbon budgets for 1.5C, 2C and 3C. For each temperature limit there are three budgets. The first gives a 66% probability of staying below the given temperature, the second a 50% chance, and the last a 33% chance.

Carbon budget: A carbon budget is the maximum amount of carbon that can be released into the atmosphere while keeping a reasonable chance of staying below a given temperature rise. The Intergovernmental Panel on Climate Change (IPCC) first adopted the concept of carbon budgets in its 2013 report. Budgets are typically expressed in gigatonnes of carbon (GtC) or carbon dioxide (GtCO2). To convert the former to the latter, multiply by 3.67.
Carbon budget: A carbon budget is the maximum amount of carbon that can be released into the atmosphere while keeping a reasonable chance of staying below a given temperature rise. The Intergovernmental Panel on… Read More

Strictly speaking, these aren’t probabilities, but are the proportion of all the model simulations that keep warming below that temperature limit.

The IPCC’s synthesis report presented the total carbon budget from the beginning of the industrial revolution and said what was remaining, as of the beginning of 2011.

Using data from the Global Carbon Project, Carbon Brief has brought these budgets up to date. In 2015, for example, worldwide CO2 emissions from fossil fuel burning, cement production and land use change were 39.7bn tonnes – slightly lower than the 40.3bn from 2014.

As of the beginning of 2011, the carbon budget for a 66% chance of staying below 1.5C was 400bn tonnes. Emissions between 2011 and 2015 mean this has almost halved to 205bn tonnes. The result is that, as of the beginning of 2016, five years and two months of current CO2 emissions would use up the 1.5C budget.

As it is now May, this means there are now fewer than five years remaining before the budget is blown. So, if the current rate of emissions continues, the 1.5C budget would be used up sometime in 2021.

The equivalent remaining budgets for a 66% chance of staying below 2C and 3C are 20 years and three months, and 55 years and six months (respectively) of current emissions. You can see all the budgets in our updated graphic at the top of the article, and the full spreadsheet with data sources here.

You can explore how the 1.5C, 2C and 3C carbon budgets have changed over time in the interactive chart below. Use the slider to move from 1959 to 2016.

And we’ve also created an animation of how the different carbon budgets have shrunk – starting at the Earth Summit in Rio in 1992 and finishing when 1.5C budget is expected to be used up in 2021.

Note that we calculate the remaining carbon budget in any one year by assuming that annual emissions from that point continue at the same level. This means that on some occasions the budget actually increases from one year to the next. This happens when a dip in annual global emissions means the reduction in cumulative emissions in future years is larger than the amount of CO2 removed from the budget for that year.

Multiple methods

It’s worth noting that there’s more than one way to construct a carbon budget. A paper published in Nature Climate Change earlier this year looked at the different approaches and their relative merits.

The simplest budget is one that only considers CO2.

CO2 has a near-linear relationship with temperature. This means every tonne of CO2 emitted to the atmosphere makes roughly the same contribution to global temperatures. It allows scientists to make a relatively simple estimate of the cumulative CO2 emissions that would produce a particular amount of warming – say, 1.5C or 2C.

However, we don’t only emit CO2 into the atmosphere. We also emit methane, nitrous oxide, ozone, hydrofluorocarbons, and a host of other greenhouse gases. These all have different warming impacts on the planet and different lifetimes in the atmosphere.

Scientists have two main approaches for calculating carbon budgets that take other greenhouse gases into account. The IPCC synthesis report includes budgets for both, which are summarised in this table:

Remaining carbon budgets from 1870 (top section) and 2011 (bottom section) in billions of tonnes of CO2. Credit: Table 2.2 of IPCC AR5 synthesis report.

The first is the snappily-titled “threshold exceedance budget”, or “TEB” for short. These are the type used for the “Complex models, RCP only scenarios” rows in the IPCC table.

To calculate a TEB, scientists simulate global temperatures in Earth system models according to a pathway of future emissions that considers all greenhouse gases. Scientists run the model until global temperature rise crosses a given threshold – say 1.5C. They then work out the cumulative CO2 in the atmosphere at that point – and this is the carbon budget.

The other gases are, therefore, taken into account when calculating how the Earth’s climate reached 1.5C of warming, but the resulting budget is still only expressed in CO2.

Of course, this assumes that emissions stop immediately once the threshold temperature is reached, which is essentially impossible in the real world. It also assumes there is no further warming once emissions have stopped, yet recent research shows this isn’t the case, says Dr Joeri Rogelj, a research scholar at the Energy Program of the International Institute for Applied Systems Analysis (IIASA), who is lead author on the Nature Climate Change study. He explains to Carbon Brief:

“This means these budgets are a bit of an overestimate of the carbon we have left to burn because temperatures would continue to warm for about a decade after we stopped emitting CO2.”

The TEB is the approach used to calculate the carbon budget we presented above. In the IPCC’s calculations, they assume emissions continue along the RCP8.5 pathway – where greenhouse gas emissions aren’t curbed – and simulate the impact on global temperatures in 20 different models.

The second approach for carbon budgets that take other gases into account is the “TAB”, or “Threshold Avoidance Budget”. These are the type used for the “Simple model, WGIII scenarios” rows in the IPCC table above.

In calculating TABs, scientists simulate many scenarios in a simple model and only pick scenarios that don’t exceed the temperature in question. From these scenarios, they then estimate a carbon budget for staying below that temperature.

Therefore, rather than using one scenario in lots of models, the TAB approach uses lots of scenarios in one model.

However, as most scientists have been working on how to keep temperatures below 2C or 3C, there aren’t very many scenarios for 1.5C. For example, there is no IPCC budget using this approach that has a 66% chance of keeping below 1.5C.

Many of the scenarios that do keep temperature rise below 1.5C assume application of negative emissions technologies to help offset emissions from human activities. These technologies, such as bioenergy with carbon capture and storage (BECCS), remove CO2 from the atmosphere and store it on land, underground or in the oceans. However, as Carbon Brief recently explored in our negative emissions series, questions still remain over the feasibility of large-scale application of these technologies.

One of the reasons why there are so few IPCC scenarios that keep temperatures consistently below 1.5C is that many allow for a situation in which emissions overshoot the 1.5C budget, and are brought back in line later through the use of negative emissions.

The lack of available scenarios has also been identified as a key challenge for authors of the planned IPCC special report on 1.5C. Future scenario work is expected to remedy this gap, says Rogelj, as more scenarios in line with 1.5C are being published.  

So, both the TEB and TAB approach to calculating carbon budgets have their strengths and weaknesses. But which one will scientists favour for future IPCC reports? Rogelj suggests both:

“I expect we will continue to use both in the future. However, to inform policymaking, it makes most sense to derive carbon budgets from scenarios that actually limit warming to below a particular temperature limit.”
Main image: 1.5C carbon budget graphic. Credit: Rosamund Pearce for Carbon Brief.
Categories: Global temperatureIn FocusInfographicsScienceTemperature
Tags: 1.5Ccarbon budgetcarbon dioxideGlobal Carbon ProjectIPCC

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    I have recently received a reply from DECC (http://ow.ly/oK0n300mhVv ) which has

    "the models used vary in what they include, and some feedbacks are absent as the understanding and modelling of these is not yet advanced enough to include. From those you raise, this applies to melting permafrost emissions, forest fires and wetlands decomposition." and

    "DECC (as for the IPCC) has not made any direct estimate [on how “missing feedbacks” would change the values of these remaining carbon budgets]. Understanding of the feedbacks currently excluded from climate models is constantly evolving, though the majority of these feedbacks, if included, would likely further reduce the available global carbon budget."

    Has anyone any estimates, even guesses, on how much the budgets soould be reduced?

    • It's refreshing that the DECC admits that their models don't include many of the feedbacks that have already kicked in. Methane emissions alone are going to cause global temperatures to skyrocket in the very near future, as will the upcoming blue ocean event in the Arctic.

      In short, by my estimation the budget should be reduced to nil.

  • Geoff, I have written about carbon cycle feedbacks and carbon budgets.
    See also a his Skeptical Science piece (look at the Appendix, in particular)

    My estimates are that carbon-cycle feedbacks could reduce "conventional" emissions budgets by about 25% and, coincidentally, it doesn't matter whether you are looking at high or low emissions pathways. I should caution, though, that this is very much a back-of-the-envelope estimate. I think there's a great need for scientists to do a proper, integrated study on this.

  • This doesn't make any sort of sense to me. My understanding is that there's a 40-year delay between cause and effect when it comes to CO2. In other words, today's levels reflect the CO2 emitted in the 1970s. Since CO2 emissions have increased since then, I don't understand how we could possibly have any sort of CO2 budget left (never mind the foolishness of factoring nonexistent/unproven negative emissions technologies).

    The article does mention other gases have to be taken account, but I don't think enough attention is being paid to methane, which is 28 times more potent as a heat-trapping gas CO2 (though dissipates within decades, unlike CO2, which lingers about a thousand years).


    One last point: We're already almost at the 1.5C threshold, with temperatures this year getting ever closer. NASA and the other often release temperature levels relative to the mid-20th century average, but the ceiling is set based changes since the late-1880s, so you usually have to add at least 0.22C to the figure released to be able to compare apples to apples.


    Granted, El Nino has had a great impact but has also set in motion a number of events that will unleash methane from the permafrost, such as the number of huge wildfires currently taking place around the globe.

    • My impression is that this analysis does not include the longer term feedbacks that continue to increase greenhouse gases. You are right in assuming that if the human caused increase in greenhouse gases miraculously stopped, warming would still continue. This analysis only includes the immediate effects.

      • I agree, and I think it's a big problem. Time and time again, the actual data blows the models out of the water and the models aren't updated fast enough.

        The reality is that we don't have a carbon budget left. We're on the verge of a blue ocean event in the Arctic, which is another factor not taken into consideration in this analysis. Arctic sea ice reflects 80% of sunlight, whereas a dark ocean surface absorbs 90%. The ice will come back but it will be increasingly thinner and less resilient, and it'll only be another decade or two before the arctic is free of ice all year round.

    • LiveLight. you said on your website "I eat less meat and do more exercise, but I still have a car, a nice warm house in winter, foreign holidays, and all the stuff I could ever need"

      I find it hard to believe you get down to a 2 tonnes a year CO2e. A normal car creates three more tonnes a year and making it creates much more. Obviously you don't travel by air or eat beef.

      Your carbon calculator looks interesting. I have related (but if I'm truthful unsuccessful) project: the Green Rtion Book (http://www.greenrationbook.org.uk. Istill would be interested to know where you got the numbers. Your site says "Figures are taken from DEFRA and other reputable sources". I'm rather suspicious of DEFRA since they seemed to have buried the work they commissioned from Adrian Williams on beef, lamb and other foodstuffs.

      But I get some confidence from your statement "So for example, the emissions figure for the
      diesel or petrol you put in your car includes the emissions from drilling the oil and refining it."

      • Hi Geoff, many thanks for your interest. In calculating my footprint via the LiveLight calculator, I've tried to use the 'whole process' costs wherever possible. Hence the use of CO2e rather than Co2. I also try to use values that include emissions from manufacturing. So my calculations for my car include an amount for the manufacture of the car as well as extracting, refining and burning the fuel to make it go.

        I spend about £20 on gas, and £30 on electricity at home each month. I drive about 8,000 miles a year in a pretty efficient family car, that really only gets used for long journeys. So home energy accounts for roughly 2 tonnes CO2e, the car (as you suggest) is 3 tonnes (low mileage, and production CO2e pro-rata'd over its useful lifetime of, hopefully, 10 years or more. I eat an organic and mostly local diet which accounts for 6 tonnes a year. I make mend and do, buy a load of second hand stuff, so I estimate that is about 1.5 tonnes a year for £6,000 of spend (mostly clothes). With a bit in for finance, the total emissions for my household come out at around 13 tonnes of co2e. But this is for a family of five in the household, so my personal emissions share of this is 2.6 tonnes. Just over, dammit! But I am confident of getting to 2 tonnes in the very near future. All of my electricity is from Good Energy, which is 100% renewable, which could be badged as 'zero carbon', but I've chosen to use the UK grid average carbon intensity, as I feel this is a fairer reflection.

        I'm genuinely looking to improve these calculations, and if you were interested, I'd gladly share the detail. You clearly understand what is involved. But you are probably sorry you asked now !!! All the best.

  • In other words, get ready for a lot more warming. I think our resources are better spent on preparation for a warmer climate because preventing it is completely futile.

    I read a study recently evaluating the effectiveness of policy from an economic standpoint. Using climate models, the study found that all global efforts to prevent warming (Paris, CPP, Energiewende etc all combined) have an mitigating effect of just 0.05 degrees by 2100. In other words, does pretty much nothing despite the huge economic cost.