Time is Short for the Carbon Budget

The “carbon budget” is an estimate of how much CO2 we can still emit, but still have a good chance to keep global warming from going over the 1.5°C limit into “dangerous” territory. The budget has recently been revised (upward, thank goodness) to about 420 GtCO2 (420 billion tons of carbon dioxide).

Staying within the 1.5°C limit doesn’t make us “safe” — there are still consequences of climate change, dangerous and costly, and we’re already paying the price despite not having hit 1.5°C yet. But going above 1.5°C takes us into what is best described as: nobody wants to go there.


If all of the “budget” amount of 420 GtCO2 went into the atmosphere, it would raise CO2 concentration by 50 ppm (parts per million). But when we emit CO2, only about half of it remains in the air. The other half is absorbed, mainly by the oceans and by plant life. So, our 420 GtCO2 budget translates to an increase of about 25 ppm in atmospheric concentration.

The air right now has about 410 ppm CO2. Adding another 25 will bring that number up to 435 ppm. In my opinion, that’s the number we should be looking at. Instead of a 420 GtCO2 emissions budget, we should be talking about a 435 ppm CO2 concentration limit.

That’s the kind of limit we can actually keep track of, with precision and accuracy.

We’ve been monitoring CO2 concentration continuously since 1958, with the groundbreaking work of Charles Keeling. And what have we found? Here’s the data from the Mauna Loa atmospheric observatory in Hawaii:

It rises and falls each year, decreasing when plants absorb CO2 from the air during growth in spring and summer, increasing when plants return it to the air during decay in fall and winter. But in addition to the seasonal cycle, there has also been an inexorable upward trend, perhaps best visible if we show just the yearly averages:

Yes, it’s going up. That’s because of us. Worse yet, the increase has been getting faster and faster.

Here’s the same data (yearly averages), but with the scale expanded so I can show where the carbon budget is (using my “concentration” budget, not the “emissions” budget):

Now for the kicker: let’s look at what the CO2 concentration will be if we keep on doing what we’re doing:

Notice that we’ll exhuast the “carbon budget” in the year 2029.

That doesn’t mean temperature will hit the 1.5°C limit that year. There’s lag in the system, so it’ll be another decade or so for temperature to catch up. But catch up it will.

That’s why kids are marching — because time is short. That’s why the Green New Deal is gathering momentum — because time is short. That’s why scientists are raising the alarm more than ever — because time is short.

We have ten years to change our path. If we keep going like we’re going, then not only will we hit the limit soon, we’ll have so much momentum going it’ll be nearly impossible to avoid even worse…


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18 responses to “Time is Short for the Carbon Budget

  1. Sometimes forgotten is that CO2 is not the only greenhouse gas we are emitting. That’s why I periodically go to to check out the Annual Greenhouse Gas Index (AGGI):
    https://www.esrl.noaa.gov/gmd/aggi/

    As of 2017, CO2 was about 405 ppm, but the greenhouse equivalent for all the greenhouse gases was 493 ppm.

  2. Zeke Hausfather

    Actually the lag in the climate system really isn’t accounted for in carbon budget calculations. They are largely based on running a no-policy climate model scenario until temperatures exceed 1.5C, then adding up cumulative emissions up to that point. This is one of the reasons why observation and GCM-based carbon budgets (which calculate cumulative emissions once 1.5C is reached) and integrated assessment models (which optimize for limiting warming to 1.5C in 2100) end up differing a bit in their implied carbon budget: https://www.carbonbrief.org/analysis-how-much-carbon-budget-is-left-to-limit-global-warming-to-1-5c

    • Excellent link; very informative.

    • I’m curious to know: what do you think of setting limits based on atmospheric CO2 (in ppm) rather than emissions (in GtCO2)? Also, what is your best guess (as rigorous, or as fast and loose, as you choose) of the 1.5C limit in terms of ppm CO2?

  3. bostonblorp

    Well, fingers crossed! If we’re not even meeting Paris it’s hard to see a 1.5C budget being any more likely. Until substantial personal sacrifice is required it’s hard to see any plans or budgets as being anything more than hand waving.

    • I’m not sure that “substantial personal sacrifice” is actually what’s required–except, of course, on the part of Big Fossil, whose financial stake is all too comprehensible.

      For the rest of us, we may need to make adjustments, all right. But few of them are going to entail true sacrifice. In fact, less consumerism may well equal *more*, not less, personal happiness.

      In any case, it’s systemic change that we need.

    • If you mean “drastic change” by “personal sacrifice”, you’re right (IMO). I think that those who believe we can keep under 1.5C, or even 2C without changing almost everything we do are being overly optimistic. Of course, eventually everything would have to change as economies can’t keep growing indefinitely and even the average lifestyle can’t be sustained indefinitely.

      That new higher estimate for CO2 is almost certainly the median point in a range, and I think we could even be past the budget, at the bottom of the range. So it is very possible that we don’t even have 10 years but, whatever the real figure is (known only in hindsight), surely it’s obvious that we need to see emissions drastically reducing right now. Are we seeing that? Are there policies in place, worldwide, that could deliver that? Kevin Anderson has estimated that, on the basis of equity, developed nations would have to reduce their emissions by over 10% per year from now, allowing developing nations to peak their emissions a bit later. That kind of decrease can’t be done without changing the way our societies work and without huge “personal sacrifice”, would be my contention.

    • As pointed out by Professor Kevin Anderson, if the richest 10% lived like the average European (not particularly onerous) then we’d have a 30% reduction in emissions just from that… not enough for sure but a huge step in the right direction… but apparently that’s just asking too much to give up private jets, lots of air miles etc

      That said, I have already reduced, I estimate mine at about 2t per annum.

  4. Global mean surface temperature (GMST) is only one of the important climate descriptors and its definition and measurement have been controversial.

    Others descriptors are ocean heat content, weather variability, remaining ice volume. Another descriptor should be a ‘danger index’ to take ‘dragon kings’ into account.

    Except that agreements need to be intentionally vague to get support, it would be better to set targets that are more precise and related to what can be monitored and enforced. (Enforcement’s a further discussion.) That’s not GMST nor atmospheric GHG concentrations. These are important descriptors but not attributable. Emissions, by and large, are attributable.

    If the politics could be solved, it would be better to have yearly schedules of allowable emissions for each country (and other actors) which could be monitored and enforced. Based on the assessment of evidence these should be regularly updated.

    OK, that’s simplistic but I do worried that GMST gets too much prominence. The Small Island Developing States are particularly threatened by ocean heat content and melting land-based ice. These are not directly dependent on GMST.

    See “Cheating with temperature” http://www.brusselsblog.co.uk/cheating-with-temperature/

  5. I’ve just been looking at remaining carbon budgets (with all their faults) and seeing what sharing them between all humans would look like.

    With a very crude calculation I estimated that humans of the UK are on course to be over 5 times over budget.

    http://www.brusselsblog.co.uk/the-uk-will-be-over-5-times-over-budget/

  6. Off-topic, but there isn’t one currently relevant for this query.

    Responding to an “it’s the sun” argument, I re-encountered the fact that there’s an annual insolation cycle varying by about 6.9% (peaking in January with Terrestrial perigee.) So is there a detectible seasonal cycle in GMST in response?

    I didn’t immediately find anything addressing this, and am curious.

    [Response: Of course there is a response, that’s as basic as physics gets. But there’s another factor too. During January that extra insolation falls on the southern hemisphere, which is mostly water — and that means HUGE thermal capacity, so the extra insolation isn’t enough to raise temperature that much. But in July, the weaker insolation hits northern-hemisphere land with its lower thermal inertia, resulting in even greater temperature increase. So the detectable cycle in GMST is the opposite of what it would be, if insolation were the only factor.]

    • Michael D Sweet.

      Tamino,
      I specially enjoyed your response to this question. I thought it was an interesting question and your answer demonstrated that scientists have already considered most questions and know the answer. The fact that land/water was more important than insolation was unexpected for me.

      • This is a part of what makes up Milankovitch cycles. And also part of why we ought to be cooling ever so slowly rather than warming rather quickly.

    • Thanks, Tamino–and the “other factor” illustrates the reason for my adjective “detectible”.

      Is this annual cycle usefully summarized anywhere? I didn’t see it in either the GISTEMP or HadCRUT papers (though in ‘scanning mode’ it would certainly be easy enough to miss, though GISTEMP does give an approximate annual absolute number.) And of course, reporting anomalies (so useful in many regards) does obscure this aspect of the record.

      [Response: It really comes down to absolute temperature, rather than anomalies. That makes it a lot harder. But I think the Berkeley data take a good approach to it, and I suspect their estimates would be most reliable.]

    • brettschmidt

      So would it be correct then to say that the increase in heat energy is greater in January, but the increase in temperature is larger in July?

  7. Thank you. I’ve begun trying to understand climate science and the carbon budget problem. This post, including discussion and links, has been enlightening.
    The first response, with its link and warning not to forget the other greenhouse gases, was very helpful. Maybe a CO2 equivalent or AGGI “budget” would be more appropriate than a “CO2 concentration budget”.
    I tried analyzing (see here: https://hswildman.home.blog/2019/03/11/a-problem-with-trends/) where current trends lead, but found they lead to two different answers. For the preceding 60 years, global temperature can be fit within a standard deviation of 0.16°C to 2.5 times the base 2 logarithm of the concentration of CO2 in ppm divided by 301.2. If this relation and the current CO2 trend continue then we could expect to reach the 1.5°C limit by 2039 with 456 ppm CO2 in the atmosphere. However, that was ignoring all of the other greenhouse gases. Global temperature for the past 60 years can also be fit within a standard deviation of 0.16°C to 1.6 times the base 2 logarithm of the ppm CO2 equivalent for all greenhouse gases divided by 319.4. If this different relation and more or less straight-line trend in the CO2 equivalent continue, then we reach the 1.5°C limit in 2062 with a 612 ppm CO2 equivalent mixing ratio of all greenhouse gases. The answers from trends, both too close for comfort, are 23 years apart. I guess following trends has pitfalls. It is better to have a model based in physical principals.