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Is there really still a chance for staying below 1.5 °C global warming?

Filed under: — stefan @ 22 September 2017

There has been a bit of excitement and confusion this week about a new paper in Nature Geoscience, claiming that we can still limit global warming to below 1.5 °C above preindustrial temperatures, whilst emitting another ~800 Gigatons of carbon dioxide. That’s much more than previously thought, so how come? And while that sounds like very welcome good news, is it true? Here’s the key points.

Emissions budgets – a very useful concept

First of all – what the heck is an “emissions budget” for CO2? Behind this concept is the fact that the amount of global warming that is reached before temperatures stabilise depends (to good approximation) on the cumulative emissions of CO2, i.e. the grand total that humanity has emitted. That is because any additional amount of CO2 in the atmosphere will remain there for a very long time (to the extent that our emissions this century will like prevent the next Ice Age due to begin 50 000 years from now). That is quite different from many atmospheric pollutants that we are used to, for example smog. When you put filters on dirty power stations, the smog will disappear. When you do this ten years later, you just have to stand the smog for a further ten years before it goes away. Not so with CO2 and global warming. If you keep emitting CO2 for another ten years, CO2 levels in the atmosphere will increase further for another ten years, and then stay higher for centuries to come. Limiting global warming to a given level (like 1.5 °C) will require more and more rapid (and thus costly) emissions reductions with every year of delay, and simply become unattainable at some point.

It’s like having a limited amount of cake. If we eat it all in the morning, we won’t have any left in the afternoon. The debate about the size of the emissions budget is like a debate about how much cake we have left, and how long we can keep eating cake before it’s gone. Thus, the concept of an emissions budget is very useful to get the message across that the amount of CO2 that we can still emit in total (not per year) is limited if we want to stabilise global temperature at a given level, so any delay in reducing emissions can be detrimental – especially if we cross tipping points in the climate system, e.g trigger the complete loss of the Greenland Ice Sheet. Understanding this fact is critical, even if the exact size of the budget is not known.

But of course the question arises: how large is this budget? There is not one simple answer to this, because it depends on the choice of warming limit, on what happens with climate drivers other than CO2 (other greenhouse gases, aerosols), and (given there’s uncertainties) on the probability with which you want to stay below the chosen warming limit. Hence, depending on assumptions made, different groups of scientists will estimate different budget sizes.

Computing the budget

The standard approach to computing the remaining carbon budget is:

(1) Take a bunch of climate and carbon cycle models, start them from preindustrial conditions and find out after what amount of cumulative CO2 emissions they reach 1.5 °C (or 2 °C, or whatever limit you want).

(2) Estimate from historic fossil fuel use and deforestation data how much humanity has already emitted.

The difference between those two numbers is our remaining budget. But there are some problems with this. The first is that you’re taking the difference between two large and uncertain numbers, which is not a very robust approach. Millar et al. fixed this problem by starting the budget calculation in 2015, to directly determine the remaining budget up to 1.5 °C. This is good – in fact I suggested doing just that to my colleague Malte Meinshausen back in March. Two further problems will become apparent below, when we discuss the results of Millar et al.

So what did Millar and colleagues do?

A lot of people were asking this, since actually it was difficult to see right away why they got such a surprisingly large emissions budget for 1.5 °C. And indeed there is not one simple catch-all explanation. Several assumptions combined made the budget so big.

The temperature in 2015

To compute a budget from 2015 to “1.5 °C above preindustrial”, you first need to know at what temperature level above preindustrial 2015 was. And you have to remove short-term variability, because the Paris target applies to mean climate. Millar et al. concluded that 2015 was 0.93 °C above preindustrial. That’s a first point of criticism, because this estimate (as Millar confirmed to me by email) is entirely based on the Hadley Center temperature data, which notoriously have a huge data gap in the Arctic. (Here at RealClimate we were actually the first to discuss this problem, back in 2008.) As the Arctic has warmed far more than the global mean, this leads to an underestimate of global warming up to 2015, by 0.06 °C when compared to the Cowtan&Way data or by 0.17 °C when compared to the Berkeley Earth data, as Zeke Hausfather shows in detail over at Carbon Brief.

Figure: Difference between modeled and observed warming in 2015, with respect to the 1861-1880 average. Observational data has had short-term variability removed per the Otto et al 2015 approach used in the Millar et al 2017. Both RCP4.5 CMIP5 multimodel mean surface air temperatures (via KNMI) and blended surface air/ocean temperatures (via Cowtan et al 2015) are shown – the latter provide the proper “apples-to-apples” comparison. Chart by Carbon Brief.

As a matter of fact, as Hausfather shows in a second graph, HadCRUT4 is the outlier data set here, and given the Arctic data gap we’re pretty sure it is not the best data set. So, while the large budget of Millar et al. is based on the idea that we have 0.6 °C to go until 1.5 °C, if you believe (with good reason) that the Berkeley data are more accurate we only have 0.4 °C to go. That immediately cuts the budget of Millar et al. from 242 GtC to 152 GtC (their Table 2). [A note on units: you need to always check whether budgets are given in billion tons of carbon (GtC) or billion tons of carbon dioxide. 1 GtC = 3.7 GtCO2, so those 242 GtC are the same as 887 GtCO2.] Gavin managed to make this point in a tweet:

Add to that the question of what years define the “preindustrial” baseline. Millar et al. use the period 1861-80. For example, Mike has argued that the period AD 1400-1800 would be a more appropriate preindustrial baseline (Schurer et al. 2017). That would add 0.2 °C to the anthropogenic warming that has already occurred, leaving us with just 0.2 °C and almost no budget to go until 1.5 °C. So in summary, the assumption by Millar et al. that we still have 0.6 °C to go up to 1.5 °C is at the extreme high end of how you might estimate that remaining temperature leeway, and that is one key reason why their budget is large. The second main reason follows.

To exceed or to avoid…

Here is another problem with the budget calculation: the model scenarios used for this actually exceed 1.5 °C warming. And the 1.5 °C budget is taken as the amount emitted by the time when the 1.5 °C line is crossed. Now if you stop emitting immediately at this point, of course global temperature will rise further. From sheer thermal inertia of the oceans, but also because if you close down all coal power stations etc., aerosol pollution in the atmosphere, which has a sizeable cooling effect, will go way down, while CO2 stays high. So with this kind of scenario you will not limit global warming to 1.5 °C. This is called a “threshold exceedance budget” or TEB – Glen Peters has a nice explainer on that (see his Fig. 3). All the headline budget numbers of Millar et al., shown in their Tables 1 and 2, are TEBs. What we need to know, though, is “threshold avoidance budgets”, or TAB, if we want to stay below 1.5 °C.

Millar et al also used a second method to compute budgets, shown in their Figure 3. However, as Millar told me in an email, these “simple model budgets are neither TEBs nor TABs (the 66 percentile line clearly exceeds 1.5 °C in Figure 3a), they are instead net budgets between the start of 2015 and the end of 2099.” What they are is budgets that cause temperature to exceed 1.5 °C in mid-century, but then global temperature goes back down to 1.5 °C in the year 2100!

In summary, both approaches used by Millar compute budgets that do not actually keep global warming to 1.5 °C.

How some media (usual suspects in fact) misreported

We’ve seen a bizarre (well, if you know the climate denialist scene, not so bizarre) misreporting about Millar et al., focusing on the claim that climate models have supposedly overestimated global warming. Carbon Brief and Climate Feedback both have good pieces up debunking this claim, so I won’t delve into it much. Let me just mention one key aspect that has been misunderstood. Millar et al. wrote the confusing sentence: “in the mean CMIP5 response cumulative emissions do not reach 545GtC until after 2020, by which time the CMIP5 ensemble-mean human-induced warming is over 0.3 °C warmer than the central estimate for human-induced warming to 2015”. As has been noted by others, this is comparing model temperatures after 2020 to an observation-based temperature in 2015, and of course the latter is lower – partly because it is based on HadCRUT4 data as discussed above, but equally so  because of comparing different points in time. This is because it refers to the point when 545 GtC is reached. But the standard CMIP5 climate models used here are not actually driven by emissions at all, but by atmospheric CO2 concentrations. For the historic period, these are taken from observed data. So the fact that 545 GtC are reached too late doesn’t even refer to the usual climate model scenarios. It refers to estimates of emissions by carbon cycle models, which are run in an attempt to derive the emissions that would have led to the observed time evolution of CO2 concentration.

Does it all matter?

We still live in a world on a path to 3 or 4 °C global warming, waiting to finally turn the tide of rising emissions. At this point, debating whether we have 0.2 °C more or less to go until we reach 1.5 °C is an academic discussion at best, a distraction at worst. The big issue is that we need to see falling emissions globally very very soon if we even want to stay well below 2 °C. That was agreed as the weaker goal in Paris in a consensus by 195 nations. It is high time that everyone backs this up with actions, not just words.


Technical p.s. A couple of less important technical points. The estimate of 0.93 °C above 1861-80 used by Millar et al. is an estimate of the human-caused warming. I don’t know whether the Paris agreement specifies to limit human-caused warming, or just warming, to 1.5 °C – but in practice it does not matter, since the human-caused warming component is almost exactly 100 % of the observed warming. Using the same procedure as Millar yields 0.94 °C for total observed climate warming by 2015, according to Hausfather.

However, updating the statistical model used to derive the 0.93 °C anthropogenic warming to include data up to 2016 gives an anthropogenic warming of 0.96 °C in 2015.


Statement by Millar and coauthors pushing back against media misreporting. Quote: “We find that, to likely meet the Paris goal, emission reductions would need to begin immediately and reach zero in less than 40 years’ time.”

118 Responses to “Is there really still a chance for staying below 1.5 °C global warming?”

  1. 101
    Scott Strough says:

    Protecting grasslands is cool but my plans are to convert row crops back to grasslands. It’s a far more ambitious goal, 1000’s of times bigger than the ACR.

    I am glad that simply protecting an established grassland from the plow is now backed up well enough by the science to be sold as an offset. I have stated many times the current consensus that grassland soils saturate after a decade ow two is emphatically wrong if managed properly. Glad to see that there finally is enough evidence to prove that well enough to sell offsets. If the grass is growing above, then carbon is being pumped into the soil below. It is a function of the way perennial grasses and their symbiotic AMF grow. It’s different than a forest which is based on biomass, and biomass does saturate. I would love to get a hold of the case studies they base that on though.

    As for your earlier question what is to be done? I don’t have all your answers. Sorry. I study and plan and research, but I am just one man. I can’t even get my own research project funded, so obviously I have little idea how to fund anything of global consequence. Mans got to know his limitations.

    However, should I ever actually complete my proof of concept, I do have a small farmer business plan that could be highly profitable and scale-able even to very large farms. And a plan to sell carbon offsets…..which would fund buying farms across the country and placing a new young and eager intern on that farm to start his own family farm. He should be able to quickly buy it from me and then I can purchase new farms for new young future farmers. + the offsets providing funding for new farms to buy and place interns. .. rinse and repeat.. If the interns then become active in their own communities and also become demonstration farms too…training their own new interns…. I believe we can make a significant play nationwide, and even worldwide in the remaining 60 or so years we have to accomplish this.

    I need to try because not succeeding is not an option. Even if AGW wasn’t a thing, I would still have to try. Otherwise we all find out what it is like to be a Syrian or Ethiopian these days. Except multiplied worldwide. But even though I have been putting in 100% of my effort non stop since I figured out what was happening, I have to date failed at every grant request and only raised a grand total of 10 dollars on my go fund me page. So clearly you need to talk to someone with a different and better skill set than me. I can show you in the field what to look for and monitor, but certain things I am apparently not anywhere near as good at as I once thought. Politics, high finance social media etc..I just work way too hard for way too limited results.

  2. 102
    Mal Adapted says:

    Scott Strough:

    I need to try because not succeeding is not an option. Even if AGW wasn’t a thing, I would still have to try.

    I’m most assuredly sympathetic with your urgency, Scott. OTOH, I’ve found a sense of proportion to be among the few unequivocal benefits of advanced age. It’s been useful for discerning personal priorities.

    Leaving out all conceivably superfluous words, my usual lengthy monologue follows. First are three scant paragraphs of personal backstory: if you skip them, please don’t attack me for not saying something I said therein ;^).

    Although I abandoned theism by age 12, I never lost a congenitally geekish fascination with the property of self-organization inhering in the Universe. My adult meta-physical foundation is ontological monism. Science, with all its cultural elaborations, is IMHO telling the greatest story that can ever be told: that of the unfolding over 13.8 GY of all phenomena including my self-referencing mind, from information packed into the primordial singularity. I’m hooked on the suspense. Curiosity will keep me going when nothing else does!

    At age 8, I learned of the scientific theory of Evolution. I was already a boy naturalist, and Charles Darwin was my instant, permanent hero, although I learned much that was unknown to him of Evolution’s complexities subsequently in academic training to the doctoral level. I also learned at that time of what’s now being called the 6th Great Extinction. I recall with clarity my outraged reaction. Existential grief is one of my justifications for later leaving a PhD program in Ecology and Evolution after two years; though to be sure, stumbling into an easier and relatively grief-free way to make a decent living was decisive at the time.

    In the present, I’m foremost a ‘treehugger’* by emotional affinity and a conservationist by political ideology; other socialized costs of ‘civilization’ matter to me at lower priority. My subjective propensities feel integral to over-arching engagement with the Universe, nevertheless it would be hypocrisy to claim they’re intersubjectively verifiable. Economically, I’m a ‘free-market socialist’: AFAICT, all real or probable economies are mixed.

    Please keep reading, you’re approaching the point:

    As an intersubjectively verifiable phenomenon, AGW is indeed ‘a thing’. In hindsight it’s to be expected from a species that evolved culture, i.e. non-genetic propagation of individually adaptive behavior over generations, as Homo sapiens has. I regard AGW as the most consequential change in the global environment for life since the End-Pleistocene deglaciation, and as AGW is occurring an order of magnitude more rapidly, I anticipate its ramifying impacts will be that much more subjectively dramatic. As I’m reasonably confident of living until 2050, I’m focused on slowing the warming as much as possible in the interval.

    Scott, inarguably, regenerating the anthropogenically depleted humic carbon pool would mitigate global warming as effectively as leaving the same amount of fossil carbon in the ground at the same rate would. In my holistic judgment however, and taking my understanding of human cultural adaptation fully into account, collectively terminating the large-scale anthropogenic transfer of fossil carbon to the atmosphere has a greater probability of success in the near term. As for predicting humanity’s actual collective response, I’ll not do that! I’m only one human, so you’ll need to ask the other 7.5 billion people now living and their demographically probable offspring. None of mine, thankfully, since I have none. The buck stops here.

    * lol, Chrome’s spellchecker suggested ‘thuggery’. See, this conservationist does have a sense of humor ;^D.

  3. 103
    Mal Adapted says:

    Michael Roddy:

    Hardwoods are not a common consumer or industrial product, and are not seen on many shelves in building supply stores, not to mention toilet paper and paper towels, courtesy of Canadian old growth softwood forests.

    Much could be done here on the demand side here in the US. We have little control over what happens in Brazil or Indonesia. Especially since America’s credibility is at an alltime low.

    Quite right, Mike. To be fair, my home state of Wisconsin grows whole forests of aspen and balsam poplar on short rotation for paper and engineered wood products. It’s my understanding, however, that the furniture industry is a major driver of demand for tropical hardwoods.

    I’ve seen handsome, finely crafted furniture and millwork made from ash, maple and oak harvested in Oregon’s Coast Range and Willamette Valley. Yet I’m aware of high quality commercial hardwood timber going unsold for lack of mill interest. I’m acquainted with people promoting the value of Pacific Northwest hardwoods, in order to foster stewardship of native woodlands and take pressure off tropical forests. If you’re interested, take a look at Zena Forest Products and An Environmental History of Zena.

  4. 104
    Mal Adapted says:

    Drat. That’s An Environmental History of Zena.

  5. 105
    Scott Strough says:

    What’s the old saying when building a tunnel? You start at that end and I’ll start at this end and we will meet in the middle somewhere.

    I have no emotional investment in which is bigger or faster or even better. I do not care if you do 60% and leave me with 40%, or vice versa. What I know is that to balance the carbon cycle both must happen in large enough %’s that we have at least a balance. Any % above that starts drawdown.

    Many many farmers as I mentioned before are converting their farms and there are programs like here in Oklahoma that cant help. Almost every state in the country has something similar. So it is possible you at your end may not need to dig that tunnel alone. We really do have the possibility to help a lot from this end.

    Remember that above program in right dead center of Inhofe’s back yard. So much like Texas has become one of the largest wind powered states, we could maybe be the model for agriculture, and both our states the icons of excesses and damages caused by both energy and agriculture. We are #’s 1-2 and 2-1 on those issues going all the way back to the oil booms and the dust bowl respectively.

    As for me personally, I will talk to a rather large sympathetic farm probably next week. Large enough we can do a proof of concept on the business plan at scale without even needing an investment. Just on profits alone. If they like the idea (and they have already expressed the idea sounds good in principle), then they will be hiring me as manager. So no worries mate. More than one way to skin a cat. Even if those negotiations fall though, there are literally millions of farms soon to change hands. I’ll find one that just needs a little coherent direction and it’s off to the races.

  6. 106
    zebra says:

    Mal Adapted #103,

    “lack of mill interest”

    Did you look at the prices for “finely crafted furniture” from the company you referenced?

    The issue is some version of commodification. The “mass market” has no place for a little nightstand that costs $2,000 dollars. And, with due respect to that enterprise, I can make you one of those for $600, with the same quality, and probably a more attractive finish.

    Unfortunately, there is not much demand between the relatively high end and the low end; you can buy durable wood furniture from overseas that most people think looks just fine, for much much less than my $600. If you couldn’t, then that factory wouldn’t be charging $2K, because there would be enough of a market to bring the price down to $600 or less, using that sustainable ash, oak, and maple or walnut.

    And this without considering the change in lifestyles and concepts of value. There are lots of older folks trying to give away good quality furnishings to their children and grandchildren with no takers. So why do you think a “middle class” person would buy something like that nightstand when they just have to spend the money on “minutes” and other such necessities?

  7. 107
    Alastair McDonald says:

    “We’re in this situation partly because of insufficient democracy in the USA. What if the US was an ordinary majoritarian democracy instead of having an electoral college system that skews voting impacts? Al Gore instead of Bush II; Hilary Clinton instead of Trump. Not to mention the ways in which the wealthy exercise outsize impact on American elections.”

    I agree! So we can’t rely on democracy or even the good sense of people and politicians to get us out of this mess. We have to shout louder!

  8. 108
    Mal Adapted says:

    Scott Strough:

    As for me personally, I will talk to a rather large sympathetic farm probably next week. Large enough we can do a proof of concept on the business plan at scale without even needing an investment. Just on profits alone. If they like the idea (and they have already expressed the idea sounds good in principle), then they will be hiring me as manager. So no worries mate.

    I can see why not. I don’t either, as long as the social cost of AGW is reduced by an accountable proportion of the revenue after your living is paid. I readily acknowledge you’re making a good case, Scott.

  9. 109
    Mal Adapted says:


    If you couldn’t, then that factory wouldn’t be charging $2K, because there would be enough of a market to bring the price down to $600 or less, using that sustainable ash, oak, and maple or walnut.

    Well, you inspired me to do some additional homework, as I confess my knowledge of the PNW native hardwood market is around 15 years old. I didn’t find that $2K price for a nightstand at the website, but I did find the prices their sawmill is paying for logs. I wanted to compare those with current prices for tropical hardwood sawlogs at a West Coast port. In 15 minutes of iterative googling, I found lots of US importers selling tropical wood, but no log prices. Of course, one is easily led off on a tangent in this fashion. My curiosity was piqued by a hit dated Nov. 2016, on a Nova Scotia, CA Woodlot Management Home Study Program:

    There is a trend in Europe, and to a lesser extent, North America, to examine the `forestry’ behind each product being sold. Who is doing the examining? It began with the environmental movement, but today individual firms – furniture companies, large importers of softwood lumber, and consumer organizations – are taking on this task. Some believe that consumers have effectively shut down the tropical hardwood industry. We do know that in the hardwood trade, the switch from tropical (dark coloured) to temperate (light coloured) hardwoods has been monumental. The huge softwood lumber industry has not escaped either. Buyers question the practices of all Canadian suppliers, especially those from British Columbia.

    It appears my understanding that the furniture industry drives demand for tropical hardwoods may need updating also.

  10. 110
    Thomas says:

    BPL, then consider yourself BITTEN … and hard!


    I think this may have been written for you BPL and all your fellow patriotic American citizens … yes? (smiling)

  11. 111
  12. 112
    Thomas says:

    re #21 Ed … “….. people need to think carefully about what to do with their shares.”

    In your dreams maybe Ed. Nice “theory” but it’s not going to happen. Never happened before in human history, and it’s not going to happen now either. :-)

  13. 113
    Thomas says:

    Polly says: “… as people will assume a hedonist lifestyle”

    Um, when wasn’t that the case already Polly?

    The USA didn’t produce +25% of all AGW effects by living like Puritans the last 400 years.

  14. 114
    Thomas says:

    Dan H. “Oceans …. CO2 …. continuously reaching new equilibriums.”

    Um, yeah. You mean like the Dead Sea which is continuously reaching new equilibriums too?

    LOL. Oh boy. “Houston, we have a problem!”

  15. 115
    Thomas says:

    The ever adaptable “OTOH, I’ve found a sense of proportion to be among the few unequivocal benefits of advanced age.

    Mal’s prognosis is: “As I’m reasonably confident of living until 2050 …”

    Chickens and Counting?

    Not sure of the appropriateness of the word “advanced” in relation to “benefits” and “sense of proportion” either. But that’s just me. I’m a much more skeptical bugger at my advanced age. :-)

  16. 116

    Note Thomas’s method: If I make a remark he doesn’t like about an issue, he immediately attacks me. I respond in kind, he attacks me some more. That way he never has to acknowledge that he was wrong about anything.

  17. 117
    Kumudini says:

    As climate change leads to dramatic changes in the day to day weather patterns, with hurricanes and floods frequently headlining the news around the world, it makes you wonder why people are still turning a blind eye to this ginormous issue. Is it so difficult for people to take an active role in the preservation of our mother earth?

  18. 118
    Hank Roberts says:

    > Mark Goldes … breakthrough energy (above)
    His page features a Carl Sagan quote about genius.
    Well, probably not.

    Duncan’s paradox is a thermodynamic thought experiment proposed by T. Duncan in 2000 [1] that has connections to the second law of thermodynamics and catalysis. It was raised in response to an earlier theoretical proposal suggesting that certain gas-surface reactions (heterogeneous catalysis) can generate steady-state pressure gradients under low-pressure, sealed blackbody conditions.[2]

    Duncan’s paradox was devised to demonstrate the incompatibility of these purported pressure gradients with the second law of thermodynamics. Specifically, such pressure gradients would allow construction of perpetual motion machines of the second type; therefore, because such devices are presumed thermodynamically impossible, the enabling pressure gradients must also be impossible.

    They laughed at Columbus, they laughed at Fulton, they laughed at the Wright Brothers. But they also laughed at Bozo the Clown.” – Carl Sagan