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Myles Allen: Can we hold global warming to 1.5°C?

Posted on 15 December 2015 by Guest Author

This is a re-post from Carbon Brief

A guest post from Prof Myles Allen, a professor of geosystem science at the Environmental Change Institute, School of Geography and the Environment and Department of Physics, University of Oxford, and Director of the Oxford Martin Net Zero Carbon Investment Initiative.

The negotiators in Paris appear to have agreed to aim to limit warming to “well below” 2C, and even “to pursue efforts to limit the temperature increase to 1.5C”. But given the most likely value of human-induced warming is over 0.9C already and increasing at almost 0.2C per decade, is stabilising at 1.5C realistically possible?

On one level, the answer is very simple: if 2C is possible, then so is 1.5C, albeit less likely, because we do not know precisely how the climate system will respond to future emissions, and still less how future emissions will respond to mitigation policies. If reducing emissions turns out to be easier than many people fear, or the response of the climate system turns out to be at the lower end of the current range of uncertainty, then the policies that would have limited warming to 2C might well buy us 1.5C instead.

But what are the chances of meeting this new 1.5C goal? Some simple round numbers may help to put this question in perspective. Cumulative emissions of carbon dioxide (CO2) are the dominant driver of long-term temperatures. Past emissions, amounting to about 2tn tonnes of CO2, have already committed us to about 1C of warming. If we limit net future emissions to another trillion tonnes of CO2, which the IPCC Fifth Assessment Report considers to be technically feasible, that gets us close to 1.5C of warming due to CO2 alone.

At one level, the challenge is very simple. Stabilising temperatures requires net zero CO2 emissions. So to stabilise at 2C, emissions need to peak now and fall, on average, by 10% of their peak value for every tenth of a degree of warming from now on. To stabilise at 1.5C, they need to fall, on average, by 20% per tenth of a degree of future warming. Right now, the world is warming by a tenth of a degree every 5-10 years, but of course that would slow as emissions fall.

And CO2 is not the only pollutant causing warming, although it is the most persistent. Almost all the IPCC’s scenarios project that other sources of pollution (methane, soot and the like) will add at least another 0.5C to this, taking the total to 2C. But we are only just beginning to work out how to reduce these other emissions, and in any case, it is the warming caused by CO2 that is particularly dangerous because it is so hard to reverse.

This is illustrated by the figure, adapted from figure 2 of a recent Policy Brief, published by the Oxford Martin School. Drawing on the modelling tools used in the IPCC Fifth Assessment Report, it shows that if we follow the IPCC’s most aggressive mitigation path (“RCP3PD”) for CO2 – adjusted to begin reductions today – then on a mid-range estimate of the climate response, temperatures stabilise around 2C. If, in addition, we take immediate action to reduce methane and soot emissions, which UNEP and others have argued is not only possible but would bring significant health benefits as well, it is possible to stabilise temperatures at 1.5C.


Red line: Mid-range warming response to the IPCC’s RCP3PD most aggressive mitigation scenario, which involves reducing CO2 emissions to zero and below in the second half of the 20th century. Blue line: Mid-range warming response to RCP3PD supplemented by rapid reductions in “Short-lived Climate Pollutant” (SLCP, including methane and soot) emissions over the period 2015-2035. Source: Oxford Martin School.

Possible does not mean straightforward. The RCP3PD scenario involves a substantial element of industrial-scale CO2 disposal: rapid deployment of carbon capture and sequestration (CCS) on fossil fuel plants, followed by large-scale deployment of Biomass Energy with CCS to draw CO2 out of the atmosphere in the second half of this century. It still has not been demonstrated that CO2 disposal on this kind of scale is even possible, and early progress in CCS deployment has been slow.

Likewise, Rogelj et al. (2015) argue that reducing non-CO2 human-induced warming below that in RCP3PD may not be possible, but options for reducing methane and soot emissions have been explored much less thoroughly than CO2. But if, once CO2 emissions are firmly on a path to net zero, we also succeed in substantially reducing methane and soot emissions, and the climate system response turns out to be in the lower half of the current range of uncertainty, then stabilising temperatures at 1.5C, while far from guaranteed, is clearly not out of the question.

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Comments 1 to 17:

  1. "value of human-induced warming is over 0.9C"

    Isn't it closer to 1.1 C?

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  2. What is your estimate for the amount that the world's temperature will raise if/when we stop spewing enormous amounts of polluting (but insolation-shielding) aerosols into the atmosphere? 

    I've heard estimates from .2 C to 2 C, but I haven't kept up to date on the latest studies on this.

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  3. wili @2, trend is relevant when we hit peaks just as much as when we hit troughs.

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  4. But - there is a lag. 400ppm CO2 is enough to bring us to around 1.5C increase in equlibrium.  Therefore we can not accept any more CO2 emissions.  Which seems unlikely to happen. :(

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  5. What's more, at 1.0C we are already seeing significant instability in the Greenland and West Antarctica ice sheets, pointing to up to 10m sea level rise in an uncertain period, even if we stay below 1.5C.

    Can we go back 20 years and try again?

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  6. ArnotSmith @4, there is also a lagged natural drawdown of CO2 concentration so that if we ceased all emissions, to a first order approximation, temperatures will remain constant.  The gradual increase of temperture to ECS will be balanced out by the gradual drawdown of CO2.  That means that if we could genuinely eliminate all emissions, including all anthropogenic NOX or CH4 emissions, even at 560 ppmv we would have a 50/50 chance of limiting temperature rise to 2 C (470 ppmv for 1.5 C).

    As it happens, with out sequestration, it is impossible to eliminate all emissions, particularly of agricultural CH4 and NO2 so that we would hope to reach zero net emissions of CO2 significantly prior to that.  That is not going to happen on the COP21 agreement.  Therefore our chance of keeping temperatures below 2 C, and certainly for below 1.5 C depend on economically viable, large scale sequestration of atmospheric CO2 in the second half of this century.  Absent that sequestration, we are looking at 2.5-3.5 C for midrange TCR estimates, and must relly on the fortunate fact that the 2 C cutoff is somewhat arbitrary.  Going above 2 C will be worse than staying below it, but incrementally so rather than a dicontinuity resulting on complete catastrophe.  That is, it will be bad, and will result in significant loss of life, but is unlikely to result in the end of civilization. 

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  7. Tom, are you trying to say that the increased temperatures this year are due to El Nino and that soon after we will return to the longer-term trend of near linear increases in GW?

    Do you expect that we will see any acceleration of that heating anytime in the coming years and decades?

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  8. wili @7, high temperatures in 2015 relative to post 2000 values is primarilly due to the strong El Nino, and yes, they will revert to trend.  The trend they will revert to is most probabibly about 15% less than the model predicted trend of 0.2 C per decade.  That trend will increase overtime, but gradually at first so it is not likely to be much above that till after 2030.

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  9. I think we need to differentiate between what is possible in an ideal world, where humans stop acting irrationally (i.e. stop acting like humans). Such a world doesn't exist and so 1.5C is impossible. 2C? Well, again, it pretty much requires significant declines starting now but the Paris agreement concerns emissions from 2020, so even steeper declines are needed from that point. If the world doesn't prove it's taking this seriously by 2020 (by already having peaked emissions) then 2C will also be impossible.

    From some of the comments and policy decisions I've heard from politicians in a couple of countries, since Paris, I don't think they are taking it seriously.

    IMO, of course.

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  10. TonyW, I think whether we hold warming below 1.5C or 2C depends entirely on economics (no change in human behaviour required)... just as the COP21 accord and the US/China deal preceding it never would have happened if the cost of renewable power hadn't fallen enough to make those plans economically viable.

    If new breakthroughs drop the cost of wind/solar power to 20% of current (well below fossil fuels) within five years then I think keeping warming under 1.5C becomes a real possibility. If renewable energy prices stay about where they are currently then even the existing COP21 targets are in danger and we may pass 3C.

    As to needing to peak global emissions by 2020. It is possible last year was the peak. 2014 had a tiny increase over 2013 and estimates for 2015 range from just barely over to slightly under 2014. Either way, peaking by 2020 seems entirely possible.

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  11. Thanks, Tom.

    So do you rule out the possibility of a 'step change' or discontinuity happening at sometime?

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  12. A nice piece on abrupt change:

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  13. Tom Curtis @6. For some days now I've been puzzling over your comments about natural drawdown of CO2.

    I was under the impression (quite possibly incorrectly!) that natural drawdown is a slow process that takes place over thousands of years. If that is true, we would surely expect temperatures to carry on rising for some time after we stop emitting greenhouse gases due to thermal inertia and long term feedbacks. Also, if there is appreciable release of CO2 and methane from the permafrost and clathrate, this would be expected to take greenhouse gas concentrations and hence temperatures higher still.

    Are you saying there are mechanisms for natural drawdown which operate on a short enough timescale to counteract this warming "in the pipeline"? If so, what are they and please can you point me to some further reading on the subject?


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  14. paulchevin @13, a slightly simplifief schematic of the rate of draw down of CO2 over time, with the main processes distinguished, is provided by David Archer:

    As you can see, the initial drawdown through 'Ocean Invasion' is very rapid, and occurs over a few hundred years.  As it happens, the vast majority of the rise in temperature to the Equilibrium Climate Response occurs on approximately the same time scale.  Consequently, if there are no new net emissions, the effects approximately balance.

    Despite that rapid initial drawdown, a significant fraction of CO2 remains in the atmosphere for 100s of thousands of years due to the very slow rate of chemical weathering ('Reaction with ignious rocks').  The amount that remains over that period will be somewhere between 10 and 30% depending on which model is most accurate, and (more importantly) the quantity of total emissions.  If we restrict total emissions to a trillion tonnes of Carbon, the range is about 8-20% retained, while for

    (Figures from Archer et al 2009)

    1 trillion tonnes of Carbon is the effective limit to keep global temperatures below 2 C, while 5 trillion tonnes represents a more or less BaU scenario.  That is the more important distinction because we cannot control which model will turn out to be most accurate, but we can control our total emissions.

    Finally, and as an aside, the 'Reaction with CaCO3' also almost completly restores pH levels so that ocean acidification is a geologically temporary problem - but biologically a very long term problem indeed.

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  15. Thanks Tom. I appreciate you taking the time to post that.

    It appears that David Archer's model is based on a single large addition of CO2 to the atmosphere and it therefore isn't directly comparable to the gradual increase (currently around 2ppm per year) that we're seeing in reality. Much of the CO2 we've added to the atmosphere over the past century or so will surely have already undergone the most rapid phase of ocean absorption.

    Thanks again,


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  16. paulchevin @15, that is correct.  However, even there still remains a significant further reduction at the rapid rate over the first few centuries.

    Consider the first figure in my comment @14.  It appears to show a slug of 2,500 billion tonnes of Carbon, sufficient to raise the atmospheric CO2 by 1170 ppmv.  If that did not appear as a single slug, however, but over 150 years, only the retained fraction of 45% of that increase would remain in the atmosphere.  That still represents an increase of 530 ppmv, for a total atmospheric concentration of 810 ppmv.  That is still significantly more than the ~640 ppmv remaining at the end of the ocean intrusion, for a total increase of 360 ppmv over the preindustrial levels.  The ratio of the increase (360/530, or 68%) gives a rough idea of the further reduction from current levels over the next two centuries or so if we ceased all emissions.  That ratio is also approximately the ratio between the Transient Climate Response and the Equilibrium Climate Sensitivity, the transition to which occurs over approximately the same timescale.

    For what it is worth, although I first noticed this correlation on this simple level, which is very approximate, since then at least two papers have been published showing the same thing with a combination of carbon cycle and climate modelling.  The following graph is from Matthews and Caldiera (2008)

    And here is a clearer image:

    (See also Matthews and Solomon 2013)

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  17. 1.5 degrees is technically possible but likely way to costly.  

    Assuming that we will be able to stabilize the non-CO2 greenhouse gases (CH4, NO2, etc.) at their current levels (because reducing them will be really difficult in a more affluent and more populous world) and assuming that we can eventually phase out coal, it looks to me like we are already close to seeing a 2°C world this century based on current greenhouse gases in the atmosphere (not that 2°C is a meaningful target):

    1. The aerosols from burning coal significantly dampen the temperature increase caused by CO2 emissions. According to Dr. Michael Mann, since the burning of coal must be ended to meet any meaningful temperature increase target, a more realistic target of atmospheric CO2 is 405 PPM, which will be reached in a few years (

    2. We are already at 400 PPM for CO2, 450 PPM CO2e if the Kyoto greenhouse gases are included, and 480 PPM CO2e if all IPCC greenhouse gases are considered (

    3. The temperature increase at the end of 2015 was about 1.0°C and an additional .1°C increase is expected for 2016. In addition, the aerosols from the burning of fossil fuels are masking another .5°C of temperature increase. And the Earth’s current energy imbalance will likely lead to another .5°C increase over the coming decades based on the greenhouse gases already in the atmosphere. (If we can reach “net zero” anthropogenic CO2 emissions, the oceans will absorb significant amounts of CO2, but this will likely offset by natural emissions from global warming feedbacks (permafrost thawing, etc.))

    4. With all of the weird weather that we have been getting from 1.0° C temperature increase and with another .5°C to 1°C “baked in”, it would seem that current atmospheric concentrations of greenhouse gases are already too high

    If we are already close to seeing a 2°C world this century based on current greenhouse gases in the atmosphere, then in order to keep the global temperature rise this century to well below 2 °C above pre-industrial levels (per the COP 21 agreement) we need to remove the CO2 equivalent of all future greenhouse gas emissions from the atmosphere.


    Greenhouse gas sources and sinks from 2015-2100 (GTCO2e):

    1522 - Net of all of the greenhouse gases emitted after 2015, assuming they peak in 2025 and are reduced linearly at 3 percent per year (net zero in 2058) (because we are very close to the atmospheric concentrations of CO2 and CH4 that will result in a 2.0° C temperature increase) (=580+942) (There will likely need to be significant CCS to meet the “net zero” goal, but the associated costs are not included here)

    320 - Greenhouse gases emissions from 2058-2100 that need to be captured and sequestered (8*40) (IEA – – assumes the annual amount of CCS needed is stabilized in 2050, whereas it is likely to increase)

    500 - GHG equivalent emissions from climate feedbacks from 2020-2100 (440 GTCO2e from permafrost and 60 GTCO2e from other sources)

    -450 - CO2 absorbed by the oceans from 2050-2100 as net CO2 emissions approach zero. Oceans currently absorb 30-50% of CO2 from fossil fuel emissions ( Assuming that there are “net zero” fossil fuel emissions in 2058 and using a 40% absorption rate of 2010 emissions in 2050 of about 12 GTCO2, which would be reduced in half by 2100, the total CO2 absorbed by the oceans from 2050 would be 50 * (12 + 6) /2, or 450 GTCO2 (this is just a “swag” but is probably in the right ballpark)

    1892 - Total CO2 to be sequestered for a 2 degree world


    To get an idea of the costs involved, simply multiply an optimistic lower bound of the expected dioxide removal costs for CO2 in 2050 ($100/ton – about ¼ of what the Natural Resource Council predicts for direct air capture (DAC) ( times the 2010 greenhouse gas emissions (about 50 GTCO2e). The result shows future generations would need to spend about $5 Trillion ($100 * 50 / 1000) to remove the CO2e for just one year of current emissions. Since emissions will not come down any time soon, we can expect that future greenhouse gas emissions that need to be captured and sequestered will be likely more that 2,000 GTCO2e, resulting in CDR costs likely in excess of $200 Trillion in this century for a 2° C world and likely more than $300 Trillion for a 1.5° C world.

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