How much carbon dioxide can we release?

Climate sensitivity is the amount of warming that would arise from doubling the concentration of carbon dioxide in the atmosphere. The IPCC estimates that it is between 2.0˚C and 4.5˚C, with 3.0˚C as the most likely value. They also warn that, because of feedback effects, “values substantially higher than 4.5˚C cannot be excluded.” Basically, this number refers to how many degrees of warming would arise from raising the concentration of CO2 in the atmosphere from the pre-Industrial level of 290 parts per million (ppm) to 580 ppm: 50% above today’s level of 385 ppm. The higher the number, the worse the consequences from any particular level of emissions.

We can combine that figure with the maximum amount of warming we are willing to tolerate and come up with a figure for how much more carbon dioxide humanity can release, all told. Using the 2˚C ceiling adopted by the European Union and endorsed by 200 of the world’s top climate scientists, Andrew Weaver worked out what those limits would be for different climatic sensitivities:

  • 2.0˚C sensitivity – 1314 billion tonnes (gigatonnes) of carbon – 4822 gigatonnes of CO2
  • 3.6˚C sensitivity – 661 billion tonnes (gigatonnes) of carbon – 2426 gigatonnes of CO2
  • 4.5˚C sensitivity – 484 billion tonnes (gigatonnes) of carbon – 1776 gigatonnes of CO2
  • 8.0˚C sensitivity – 163 billion tonnes (gigatonnes) of carbon – 598 gigatonnes of CO2

The 2˚C and 4.5C figures are the top and bottom of the IPCC’s probable range. The 3.6˚C figure is the one considered most probable by those running the University of Victoria climate model. The 8˚C figure illustrates the impact of much higher sensitivities on how much can be emitted.

Current annual carbon dioxide emissions about ten gigatonnes of carbon (36.7 gigatonnes of CO2) per year. Note that the figures above are how much CO2 can be emitted in total before the whole world becomes carbon neutral – certainly not about how much can be emitted before we need to begin cutting. Those totals need to include all future emissions from developed and developing states alike, between this year and whichever year the world achieves carbon neutrality.

The 4.5˚C scenario implies a peak concentration of 445 ppm – slightly lower than a commonly cited ‘safe’ level. Some people – notably James Hansen – have argued that an even lower stabilization concentration is necessary to avoid runaway climate change.

Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford. Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here.

30 thoughts on “How much carbon dioxide can we release?”

  1. The new numbers, which some scientists called “scary,” were a surprise because experts thought an economic downturn would slow energy use. Instead, carbon dioxide output rose 3% from 2006 to 2007.

    That amount exceeds the most dire outlook for emissions from burning coal and oil and related activities as projected by a Nobel Prize-winning group of international scientists in 2007.

  2. Global carbon emissions jumped 3% in 2007

    The report shows a continuation of the grossly unsustainable growth rate in CO2 emissions since 2000, which is nearly four times the growth rate of the 1990

    Scientists were surprised and dismayed because the increase “exceeds the most dire outlook for emissions from burning coal and oil and related activities” projected by the IPCC and because the increase occurred despite rising fossil fuel prices

  3. Humanity has emitted about 488 gigatonnes of carbon since 1750. That means the total carbon emissions allowable for industrial human society are between 1802 gigatonnes of carbon and 651 gigatonnes, depending on climate sensitivity.

    If sensitivity is 2.0˚C, we have already burned 27% of our total carbon allowance.

    If sensitivity is 3.6˚C, we have burned 42%.

    If sensitivity is 8.0˚C, we have burned 75%.

    Taken in that context, the emissions trend described in the comments above is especially disturbing.

  4. That’s tricky to answer.

    You need to consider (a) how many generations there will be between now and stabilization and (b) what population growth will look like, both globally and state-by-state.

    A fair division would probably need to take into account a fair number of factors, including historical contribution to the problem and ability to pay. Arguably, any per-capita allotments should be indexed to a base year, so as to avoid encouraging additional unsustainable population growth.

  5. Climate change and the greenhouse effect
    A briefing from the Hadley Centre
    December 2005

    The reduction in natural carbon sinks, and their
    eventual change into carbon sources, allows more
    man-made CO2 to remain in the atmosphere, and
    so concentrations build-up faster — a positive
    feedback between the climate and the carbon cycle.
    Under one business-as-usual emissions scenario,
    CO2 concentration in the Hadley Centre model was
    predicted to rise to 750 ppm by 2100, accompanied
    by a warming over land of about 5 °C. When the
    feedback between climate and the carbon cycle was
    included in the model, CO2 was predicted to rise to
    1,000 ppm, and global mean temperature over
    land to 8 °C, under the same emissions scenario.
    This first estimate of the strength of the feedback
    has since been repeated by other modelling
    centres, which unanimously agree that climate
    change will reduce the natural absorption of CO2
    by the biosphere, although they find a variety of
    different strengths, so more research is needed to
    reduce uncertainties. But the potential for
    enhancement of global warming from this feedback
    has been clearly demonstrated.

  6. How important are non-CO2 gasses for working this out?

    For instance, how important are methane and nitrous oxide?

  7. Here is a chart I made, showing Weaver’s estimates of how much humanity can emit without exceeding a 1/3 chance of having temperatures rise by more than two degrees. A larger temperature increase is reckoned by most to be a ‘dangerous’ level of anthropogenic climate change

    On the left is a scenario where the sensitivity of the climate to greenhouse gasses is very low (just two degrees of increase when CO2 concentrations double). Each bar leftwards shows a progressively more sensitive planet, with the two middle options being the current high and low estimates of the IPCC. The rightmost bar shows total emissions to date.

    It is important to note that the grey bars show the total amount of greenhouse gas emissions humanity can emit across all time, from the first usage of fossil fuels to the achievement of carbon neutrality.

  8. How much Co2 is released just by humans – discounting factories, transportation, the production of food, etc… Just the human bodies? Because, presumably, we can’t do much to reduce that part.

  9. As animals, we are part of a biosphere that basically balances out. The plants we eat (or which are eaten by the animals we eat) draw a quantity of carbon from the air equivalent to what we exhale.

    Indeed, in any particular year, this natural flux is of a far greater magnitude than human emissions. What makes human emissions problematic is that they do not have a counterbalancing sink.

    Burning biomass (wood, etc) is a similar situation.

  10. Two books that do a good job of discussing how the climate functions in the long term are:

    Alley, Richard. The Two Mile Time Machine. Ice Cores, Abrupt Climate Change, and Our Future.

    Morton, Oliver. Eating the Sun: How Plants Power the Planet.

    The former concentrates more on physical processes, and the latter on biological ones. They both give a sense of scale, explaining how the climate was dramatically different in the distant past and why it changed.

    For example, there is the oxidation of atmospheric sulfer and iron (and the emergence of atmospheric oxygen) during the Proterozoic eon. Also, the causes and consequences of the Permian-Triassic Extinction Event – the largest die-off of life in Earth’s history.

  11. Carbon cuts ‘only give 50/50 chance of saving planet’

    As states negotiate Kyoto’s successor, simulations show catastrophe just years away

    By Michael McCarthy, Environment Editor

    The world’s best efforts at combating climate change are likely to offer no more than a 50-50 chance of keeping temperature rises below the threshold of disaster, according to research from the UK Met Office.

    The key aim of holding the expected increase to 2C, beyond which damage to the natural world and to human society is likely to be catastrophic, is far from assured, the research suggests, even if all countries engage forthwith in a radical and enormous crash programme to slash greenhouse gas emissions – something which itself is by no means guaranteed.

  12. How Much Should We Leave in the Ground?
    Posted May 6, 2009

    Total conventional fossil fuel reserves therefore contain 818 billion tonnes of carbon.

    Even ignoring all unconventional sources and all other greenhouse gases and taking the most optimistic of the figures in the two Nature papers, we can afford to burn only 61% of known fossil fuel reserves between now and eternity.

    Or, using Meinshausen’s figure, we can burn only 33% between now and 2050. Sorry – 33% minus however much we have burnt between 2000 and today.

    So the question which arises is this: which fossil fuel reserves will we decide not to extract and burn? There is, as I have argued before(9), no point in seeking to reduce our consumption of fossil fuels unless we also seek to reduce their production. Yet, apart from the members of OPEC (who do it only to shore up the price), no government is attempting to limit the amount of fuel extracted. Far from it; they all pursue the same strategy as the United Kingdom: to “maximise economic recovery”(10).

    The test of all governments’ commitment to stopping climate breakdown is this: whether they are prepared to impose a limit on the use of the reserves already discovered, and a permanent moratorium on prospecting for new reserves. Otherwise it’s all hot air.

  13. The Carbon Counter

    We are seeking to raise public awareness of climate change. As investors, we know the importance of measurement so that we can track progress. Our approach is to start by understanding the quantity, or concentrations, of long-lived greenhouse gases which are building up in the world’s atmosphere and are leading to global warming.

    We have turned to key scientific sources to calculate the current atmospheric concentrations of greenhouse gases and to monitor how quickly these concentrations are increasing. Our starting point has been the Intergovernmental Panel on Climate Change’s (IPCC) Fourth Assessment Report, released in 2007, which details the important long-lived greenhouse gases that contribute to global warming and are driven by human activity resulting in emissions. Using the work from the IPCC, we collaborated with scientists at the Joint Program on the Science and Policy of Global Change at the Massachusetts Institute of Technology (MIT) to update the findings and track these gases in the most timely and up-to-date manner possible. This is a process that relies on regular measurements taken at dozens of sites across the world.

  14. “A study by Canadian and British scientists adds further support to the notion that it is the total ‘slug of carbon’ (the cumulative emissions total) that matters most to global temperature change over timescales of decades to centuries. A method is proposed for estimating the global climate response to accumulated carbon emissions.”

    Reference: Matthews, H.D., N. Gillett, P.A. Stott and K. Zickfeld. 2009. The proportionality of global warming to cumulative carbon emissions. Nature Vol 459, June 11, 2009, pp 829-833

  15. One paper, by a team led by Myles Allen, shows that preventing more than two degrees means producing a maximum of half a trillion tonnes of carbon (1830 billion tonnes of carbon dioxide) between now and 2500 – and probably much less. The other paper, written by a team led by Malte Meinshausen, proposes that producing 1000 billion tonnes of CO2 between 2000 and 2050 would deliver a 25% chance of exceeding two degrees of warming.

    official reserves of coal, gas and oil amount to 818 billion tonnes of carbon.

    The molecular weight of carbon dioxide is 3.667 times that of carbon. This means that current reserves of fossil fuel, even when we ignore unconventional sources such as tar sands and oil shale, would produce 3000 billion tonnes of carbon dioxide if they were burnt. In other words, if we don’t want to exceed two degrees of global warming, we can burn, according to Allen’s paper, a maximum of 60% of current fossil fuel reserves by 2500

  16. “A recent paper in Nature showed that if we are to avoid a warming of more than 2 degrees we can afford to use no more than a quarter of proven, economically recoverable fossil fuel reserves by 2050. At our present pace we will have burned through our carbon budget by 2030.

    Staying within that budget requires some straightforward policy prescriptions. We must end subsidies for fossil fuels and make polluters pay the full cost of their impact on society; establish binding targets for the adoption of renewable energy, with support from feed in tariffs and priority grid access; and introduce efficiency standards that apply to our appliances, buildings and vehicles.”

  17. “Speaking to an invitation-only conference at New Mexico’s Santa Fe Institute, Schellnhuber divulged the findings of a study so new he had not yet briefed Chancellor Angela Merkel about it. If its conclusions are correct—and Schellnhuber ranks among the world’s half-dozen most eminent climate scientists—it has monumental implications for the pivotal meeting in December in Copenhagen, where world leaders will try to agree on reversing global warming.

    Schellnhuber and his WBGU colleagues go a giant step beyond the findings of the Intergovernmental Panel on Climate Change, the U.N. body whose scientific reports are constrained because the world’s governments must approve their contents. The IPCC says that by 2020 rich industrial countries must cut emissions 25 to 40 percent (compared with 1990) if the world is to have a fair chance of avoiding catastrophic climate change. By contrast, the WBGU study says the United States must cut emissions 100 percent by 2020—in other words, quit carbon entirely within 10 years. Germany and other industrial nations must do the same by 2025 to 2030. China only has until 2035, and the world as a whole must be carbon free by 2050. The study adds that big polluters can delay their day of reckoning by “buying” emissions rights from developing countries, a step the study estimates would extend some countries’ deadlines by a decade or so.”

  18. Live countdown to the trillionth tonne

    22 Oct 09

    A new website launched today by Oxford University tracks how fast we are approaching total global emissions of a trillion tonnes of carbon – a level which, if reached, recent research suggests will result in dangerous global warming in excess of 2°C., hosted by the Oxford e-Research Centre, currently predicts that the trillionth tonne will be reached in March 2045, but this date is advancing as manmade carbon emissions gradually accelerate

  19. For starters, any public dialogue that talks about “percentage reductions in emissions” by a certain date is misleading. Because of the long residence time of CO2 in the atmosphere, it makes far more sense to talk about the amount of CO2 remaining to be released before we hit a peak CO2 concentration. Let’s call this the “remaining cumulative carbon emissions” method. After those emissions, we essentially need to emit zero carbon. This way of looking at the climate was first popularized by Krause, Bach, & Koomey, in an excellent book called “Energy Policy in the Greenhouse” (1992). It was revisited as a tool of understanding the climate challenge in two great Nature magazine articles this year. (Nature magazine is probably the most prestigious, and rigorous, of all the academic journals.) In one of those, Meinshausen et al., used this method of analysis to look at how you would limit the planet to 2 degrees C of warming.”

  20. The open question is why the industry persists in denial in the face of an endless body of fact showing climate change is the greatest danger we’ve ever faced.

    Why doesn’t it fold the way the tobacco industry eventually did? Why doesn’t it invest its riches in things like solar panels and so profit handsomely from the next generation of energy? As it happens, the answer is more interesting than you might think.

    Part of it’s simple enough: The giant energy companies are making so much money right now that they can’t stop gorging themselves. ExxonMobil, year after year, pulls in more money than any company in history. Chevron’s not far behind. Everyone in the business is swimming in money.

    Still, they could theoretically invest all that cash in new clean technology or research and development for the same. As it happens, though, they’ve got a deeper problem, one that’s become clear only in the last few years. Put briefly: Their value is largely based on fossil-fuel reserves that won’t be burned if we ever take global warming seriously.

    When I talked about a carbon bubble at the beginning of this essay, this is what I meant. Here are some of the relevant numbers, courtesy of the Capital Institute: We’re already seeing widespread climate disruption, but if we want to avoid utter, civilization-shaking disaster, many scientists have pointed to a 2 degrees C (3.6 degrees F) rise in global temperatures as the most we could possibly deal with.

    If we spew 565 gigatons more carbon into the atmosphere, we’ll quite possibly go right past that reddest of red lines. But the oil companies, private and state-owned, have current reserves on the books equivalent to 2,795 gigatons — five times more than we can ever safely burn. It has to stay in the ground.

    Put another way, in ecological terms it would be extremely prudent to write off $20 trillion worth of those reserves. In economic terms, of course, it would be a disaster, first and foremost for shareholders and executives of companies like ExxonMobil (and people in places like Venezuela).

    If you run an oil company, this sort of write-off is the disastrous future staring you in the face as soon as climate change is taken as seriously as it should be, and that’s far scarier than drought and flood. It’s why you’ll do anything — including fund an endless campaigns of lies — to avoid coming to terms with its reality. So instead, we simply charge ahead. To take just one example, last month the boss of the U.S. Chamber of Commerce, Thomas Donohue, called for burning all the country’s newly discovered coal, gas, and oil — believed to be 1,800 gigatons worth of carbon from our nation alone.

  21. GLOBAL warming isn’t a prediction. It is happening. That is why I was so troubled to read a recent interview with President Obama in Rolling Stone in which he said that Canada would exploit the oil in its vast tar sands reserves “regardless of what we do.”

    If Canada proceeds, and we do nothing, it will be game over for the climate.

    Canada’s tar sands, deposits of sand saturated with bitumen, contain twice the amount of carbon dioxide emitted by global oil use in our entire history. If we were to fully exploit this new oil source, and continue to burn our conventional oil, gas and coal supplies, concentrations of carbon dioxide in the atmosphere eventually would reach levels higher than in the Pliocene era, more than 2.5 million years ago, when sea level was at least 50 feet higher than it is now. That level of heat-trapping gases would assure that the disintegration of the ice sheets would accelerate out of control. Sea levels would rise and destroy coastal cities. Global temperatures would become intolerable. Twenty to 50 percent of the planet’s species would be driven to extinction. Civilization would be at risk.

  22. “In the IPCC [Intergovernmental Panel on Climate Change] SR-1.5 reports that came out last year, it says on page 108 in chapter 2 that to have a 67 per cent chance of staying below 1.5 degrees of global temperature rise — the best odds given by the IPCC — the world had 420 gigatonnes of CO2 left to emit back on Jan. 1, 2018.

    And today, that figure is already down to less than 350 gigatonnes.”

    Greta Thunberg, in her own words, at the Montreal climate march

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