Analysis: Just four years left of the 1.5C carbon budget

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Four years of current emissions would be enough to blow what’s left of the carbon budget for a good chance of keeping global temperature rise to 1.5C.

That’s the conclusion of analysis by Carbon Brief, which brings the Intergovernmental Panel on Climate Change’s (IPCC) carbon budgets up to date to include global CO2 emissions in 2016.

Our infographic above shows how quickly the budgets for 1.5C, 2C and 3C will be used up if emissions continue at the current rate. For 1.5C, this could be a soon as four years’ time.

CO2 emissions

The IPCC has previously 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. These are known as carbon budgets.

For each temperature limit there are three budgets, each corresponding to a different probability of staying below that limit: 66%, 50% and 33%. (Strictly speaking, these aren’t probabilities, but are the proportion of all the model simulations that keep warming below that temperature limit.)

Now that provisional data is available for 2016, we can see what shape the budgets are in after another year of emissions.

Update for 2016

According to the Global Carbon Project, CO2 emissions from fossil fuel burning and cement production in 2016 totalled 36.4bn tonnes, a rise of less than 1% compared to last year. This is the third year in a row of very small emissions growth, tentatively suggesting they may be showing signs of peaking.

Including emissions from land use change – only an estimate at present – puts total CO2 emissions at 39.9bn tonnes for 2016. This is slightly below the 41.1bn tonnes from 2015, which saw large land use emissions as a result of peat fires in Asia.

The animation below shows how the carbon budgets look at the end of 2016. With the current rate of emissions, there are now just four years and one month left in the budget for 1.5C.

You can see the calculations and full results here.

Data visualisation and animation by Rosamund Pearce for Carbon Brief, featuring the voice of Jocelyn Timperley. Data from the IPCC.

You can read more about the different methods of constructing carbon budgets in an earlier Carbon Brief article.

Pathways for the 21st century

Another way to look at our CO2 emissions is to compare them with scenarios for the rest of this century.

Scientists have developed a set of pathways for how technology, energy and land use, and the concentration of greenhouse gases in the atmosphere could change over the centuries ahead. The four “Representative Concentration Pathways” (RCPs) each provide a plausible description of the future, based on socio-economic scenarios of how the global society grows, develops, and uses land and energy.

Scientists then use these pathways as inputs for climate models to generate projections of climate change impacts and assess the effects of cutting emissions. By using the same scenarios, scientists from different research centres around the world can directly compare their results.

The four RCPs are named after the amount of “radiative forcing” they cause by the end of the century – this is the change in energy that, on balance, warms the Earth as a result of a stronger greenhouse effect.

The chart below shows how actual emissions from 1959 up to the present day compare with each of the four RCP pathways for the rest of the century. At present, we are tracking closest to RCP8.5, the highest of the four.

Historical data from the Global Carbon Project and scenarios from the Potsdam Institute for Climate Impact Research, plotted by Carbon Brief.

Below is a brief summary of each RCP, which describe how the concentration of CO2 in the atmosphere changes under each pathway. (Note, the chart above shows the annual CO2 emissions, not the total amount of CO2 in the atmosphere.)

RCP2.6 (also sometimes referred to as “RCP3-PD”): A “peak and decline” scenario where stringent mitigation and Carbon Dioxide Removal technologies mean atmospheric CO2 concentration peaks and then falls during this century. By 2100, atmospheric CO2 reaches around 420 parts per million (ppm) – about 20 ppm above current levels. In this scenario, global temperatures are likely to rise by 1.3-1.9C above pre-industrial levels by 2100.

RCP4.5: A “stabilisation scenario” where policies are put in place so atmospheric CO2 concentration levels off around the middle of the century, though temperatures do not stabilise before 2100. These policies include a shift to low-carbon energy technologies and  the deployment of carbon capture and storage. In RCP4.5, atmospheric CO2 sits at 540 ppm by 2100 – roughly 140 ppm higher than now – and the global temperatures are likely to rise by 2-3C above pre-industrial levels.

RCP6.0: A “climate-policy intervention scenario” where emissions peak around 2060 and then decline. CO2 concentration continues to rise through the rest of the century, but at a slower rate. There are only “very modest” efforts towards mitigation between 2010 and 2060, but improvements in energy intensity and a global market for emissions permits help limit atmospheric CO2 to 670 ppm by 2100. In RCP6.0, global temperatures by 2100 are likely to be 2.6-3.7C above pre-industrial.

RCP8.5: A scenario of “comparatively high greenhouse gas emissions“ brought about by rapid population growth, high energy demand, fossil fuel dominance and an absence of climate change policies. This “business as usual” scenario is the highest of the four RCPs and sees atmospheric CO2 rises to around 935 ppm by 2100. The likely range of global temperatures by 2100 for RCP8.5 is 4.0-6.1C above pre-industrial levels.

You can read more about each RCP in the original journal articles published in Climatic Change (see links in the RCP names above). All the papers, except for RCP6.0, are free to read, download and print.

Scientists are also developing four more RCPs to fill in the gaps between and around the four already in use. Following its inclusion in the Paris Agreement, one of these new RCPs will explicitly investigate staying below 1.5C. However, these model runs will not be available in time for the IPCC’s Special Report on 1.5C, which is due to be published in September 2018.

Categories: EnergyGHGs and aerosolsGlobal emissionsGlobal temperatureIn FocusInfographicsScience
Tags: 1.5Ccarbon budgetIPCC

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  • Just 66% certain of staying below a certain amount of global average temperature rise? Three years ago David Spratt at Climate Code Red found that we had already exceeded our remaining carbon budget to stay below 2 C / 3.6 F of global average temperature rise above the common 1880 baseline if we wanted to be 90% certain.

    Since 2014 there have been numerous new scientific findings on various sources of methane that were previously unaccounted-for by IPCC AR5, such as excess methane from landfills, excess methane from natural gas-fired powerplants, quite a bit of excess methane leaking from millions of abandoned oil & gas wells, as well as methane coming from reservoirs, and even from slow flow spots in rivers. Of course AR5 did not include any finding of methane coming from various rapidly-thawing sources in the Arctic either.

    The total impact to the IPCC's 2014 remaining carbon budget to stay below 2 C above 1880 of the new methane source findings above is in the 15-20% range plus or minus a couple of percent.

    In-addition to all the recent findings on various uncounted sources of methane two quite recent major scientific findings, the first on increased respiration and activity of soil microbes as temperatures rise in December in Nature Climate Change, and the other a finding on substantially deeper permafrost thaw than had been expected, together by themselves have cost more than 25% of the IPCC's 2014 remaining carbon budget to stay below 2 C above 1880.

    This is the newer finding from just a month ago:

    "As the World Gets Hotter, Soils May Emit More CO2 Than Previously Thought:
    The amount of greenhouse gases released from the earth due to climate change has been a subject of great debate — and new research suggests it could boost global emissions 30 percent", The Seeker, March 9, 2017:


    Now here is the kicker. Other recent science has found that the amount of global average temperature rise is a bit misleading, as of course the average includes our oceans as well as land in the southern hemisphere, which has not been nearly as affected by average temperature rise as land inland off of oceans in the northern hemisphere as well as both land and sea in the Arctic.

    So whatever global average temperature rises-to figure a temperature impact of 50% greater to double inland off of oceans across the northern hemisphere and as much as three times global average temperature rise across the Arctic, where rapidly-thawing permafrost and thawing shallow seabeds are emitting tremendous amounts of natural carbon and methane, with methane trapping 115 times as much heat as CO2 in our lower atmosphere over the first 5 years following emission, and as much as 258 times over the first two months after methane emission.

    The real budgetary emergency and the myth of "burnable carbon", by David Spratt, Climate Code Red, May, 2014.


    Here is David Spratt's take on the question of a 1.5 C remaining carbon budget.

    Unravelling the myth of a "carbon budget" for 1.5C, David Spratt, Climate Code Red, Sept, 2016.


    How much temperature rise will we see? If we will recall, in April of 2014 Michael Mann forecast reaching 2 C above the common 1880 baseline by 2036, and 2,5 C by 2048 (Scientific American). At about the same time the US National Climate Assessment, (with its unusual baseline temperature to which a compensatory figure must be added to reach the 1880 baseline), forecast reaching 2.5 C by between 2055 and 2057. These two forecasts aren't terribly far apart.

    To them perhaps we should add the most-recent findings of the International Energy Agency, of 4.3 to 7.6 C by 2100, or of several other recent findings that have forecast a median roughly of 4.5 C also by 2100. Why limit ourselves just to what the IPCC has found considering that they don't include any science newer than 3 years old unless it is an emergency, and their political wing must sanitize their own scientist's findings lest any damage be done to the business interests of members of their political wing?

    If we include a recent nearly 200-page study of climate change impacts on Colorado's mid-century water supply done by senior PHD's at the University of Colorado's Western Water Assessment, as well as Colorado State University and NCAR, they are forecasting a median temperature rise just for Colorado, well inland off the ocean, of 4 1 C by mid-century (2.9 C to 5.3 C) along with a Statewide water supply deficiency of up to 600,000 acre-feet annually, By January, 2015 the study authors had already observed a 2.8 F rise in Colorado's average temperature above the 1880 baseline, though they use a 1965 baseline which itself must be compensated-for to reach 1880.

    If we use Michael Mann's forecast it is already too late to prevent 2 C worth of average warming above 1880 and 3-4 C inland off of oceans in the northern hemisphere, as there is more than enough carbon and CO2e already emitted to carry us to that much temperature rise, considering that natural carbon uptake will take 40-50 years to remove enough carbon and CO2e from our atmosphere to nearly stabilize rising temperatures, and the human race is still pumping more than 30 billion tons annually of CO2 into our atmosphere as well as immense amounts of methane, nitrous oxide, and a dozen other lesser greenhouse gases.

    So if we see 4 C worth of warming at both Lake Mead and Elephant Butte Reservoir by how much does that increase the annual evaporation rate? To a level where all water customers served by those two reservoirs (and many others just like them) will be forced to make immense service cutbacks while the cities that they serve keep growing at an out-of-control irresponsible rate. Do remember that Mexico City has more than 4 times the population of Colorado and that they are rapidly exhausting their sole remaining groundwater supply while the temperature keeps rising as does the surface water evaporation rate too.


    • Thanks for the great summary. I struggle to communicate the gravity of our situation to others. I'm spending less time reading the studies, and spending more time waving signs and speaking up. I don't know how to wake people up.

    • Most (all?) models don't include feedbacks, so we really should consider ourselves far more hooped than the IPCC, CarbonBrief or modellers would have us think.

    • Hi KingB, the modelling for the RCPs has a common starting point of the year 2000. The four RCPs then have the same emissions for the first five years before branching off in different directions from 2006. I've added the 2000-2005 RCP data to the chart above to make this a bit clearer (you can zoom into the map for a closer look). Thanks.

  • Has anyone done an analysis of how much of the world's existing fossil fuel reserves are unburnable to stay below 1.5C?

    • Almost all of it, right? We have a fairly good chance (66% of climate model runs) of staying below 1.5 degrees C of warming if we burn as we are now for 3-1/2 more years. That means according to a third of the model runs, we are doing worse. We could already be passing 1.5 degrees of warming. To me, this means we are now fighting to hold the line at 2 degrees, and have very little time to transition off of fossil fuels.

  • Thanks for this analysis. Can I just check if the key on the "Global CO2 emissions and scenarios for the 21st century" chart is correct? The highest emission scenario is labelled RCP2.6 and the lowest RCP8.5, which doesn't appear to correspond to the summaries of each scenario below.