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The Keeling Curve: Part II

Posted on 5 November 2021 by Evan

Monitoring our progress to Net-Zero

In the context of IPCC goals and company initiatives, “Net-Zero” means achieving the point where an entity emits no net GHGs. That entity could be a single company,1  an entire country,  or the entire Earth; the ultimate goal of the Paris Accord. Two related questions are

  • What does it mean to achieve Net-Zero?
  • How do we know when we’ve reached Net-Zero?

Deceptively simple questions with not quite so simple answers.

What does it mean to achieve Net-Zero?

Humans must remove from the atmosphere the same amount of CO2 that we emit. That is the meaning of Net-Zero. If we do this, the environment has a carbon-removal matching plan such that it will remove additional CO2 on our behalf, for free, so that if/when we reach Net-Zero Emissions, atmospheric CO2 concentrations will actually start to decrease!

How cool is that. The atmosphere is ready to help out with a carbon-removal matching plan, as long as we do our part. Although no countries have yet achieved Net-Zero Emissions, some companies, such as Google, claim to have already become carbon neutral, which means essentially the same thing as achieving Net-Zero Emissions. But here is an important distinction.

  • Carbon free means that you don’t emit the carbon, such as by using solar and wind for electricity generation.
  • Carbon neutral means reduction of carbon emissions as much as possible, by using carbon-free technology, and then offsetting the remaining carbon emissions using methods that remove as much CO2 from the atmosphere as we emit. One practice is to buy carbon offsets, typically covered by planting trees or land restoration. Other methods include substituting for fossil fuels, biofuels made from recycled carbon.

Carbon free is what we would like, but because running completely carbon free is probably not possible, carbon neutral is needed to reach Net-Zero Emissions.

A problem using trees to offset carbon emitted today is that it represents a promise to remove tomorrow what we emit today. That is, once the carbon is emitted, a significant portion will remain in the atmosphere for 1000’s of years, or until we physically remove it. That’s what trees and other carbon-removal technologies are supposed to do at some future date. However, even though we plant trees, we cannot guarantee their long-term survival nor that they will survive land-use policy changes by future administrations.

We can praise companies that make the effort to become Net-Zero, but even if well-intentioned marketing divisions of large corporations convince their users and shareholders that they are now a green, environmentally-responsible company, nature is not so easily convinced. And nature is the only judge of our efforts that matters. How can we monitor what nature is seeing as the sum total of our efforts to reach Net-Zero status?

How do we know when we've reached Net-Zero?

Although we can imagine how to assess the approach of a single company to Net-Zero status, how do we assess if an entire country has reached this goal? And then, the entire world?

When the IPCC and climate scientists refer to “Net-Zero”, they are implicitly referring to entities achieving Net-Zero Emissions, meaning that we remove from the atmosphere as many GHGs as we emit. Along the road to this ultimate goal, Net-Zero Accumulation represents a more readily verifiable way-point than Net-Zero EmissionsNet-Zero Accumulation simply means that the long-term trend of the Keeling Curve stabilizes at some concentration. Any concentration. Although our ultimate objective should be Net-Zero Emissions, considering that in 2021 the long-term trend of the Keeling Curve is accelerating upwards, achieving Net-Zero Accumulation would be a monumental milestone, on its own, on the road to Net-Zero Emissions.

What will it take to achieve Net-Zero Accumulations?

Figure 1 represents the Keeling Curve as polynomial fits projected into the future along a Business-As-Usual (BAU) trajectory (Fig. 1A) and along a trajectory leading to Net-Zero Accumulation by 2040 (Fig. 1B). 2040 is chosen arbitrarily as a check on our progress towards the ultimate goal of achieving Net-Zero Emissions by 2050. That is, if we don’t achieve Net-Zero Accumulation by 2040, then we must increase our decarbonization efforts such that we reach the ultimate goal of Net-Zero Emissions by 2050: by definition, we will achieve Net-Zero Accumulation before achieving Net-Zero Emissions.

The trend lines in Fig. 1 are shown as thick lines, to emphasize the difficulty in bending these curves. Fossil fuels are engrained in every aspect of our lives, and removing our dependence on them will not be easy. That is the reason for showing the trend lines in a manner that illustrates resistance.

Keeling Curve trend lines projected into future as thick lines that illustrate resistance to change

Figure 1. A) Business-As-Usual (BAU) projection of CO2 and B) required adjustment to BAU trajectory to achieve Net-Zero Accumulation by 2040. The fat curves emphasize the difficulty of counteracting the BAU trend.

Here’s an exercise you can do to visualize the task we’re up against. Find a large tree branch that is bent upwards, in the shape of the Keeling Curve. One end of the branch represents 1958, the start of the Keeling Curve, and somewhere in the middle represents today. The branch should match the shape of the curve between those two years and continue bending upwards, similar to the shape in Fig. 1A.

When selecting your branch, remember that we are a planet of 8 billion people whose lives must be realigned to this Net-Zero challenge. Think of each person representing a single strand in the curved branch you select. The more people, the thicker the branch. Do not pick a wimpy branch.2

Now bend the branch so that it is level with the ground, similar to the shape of Fig. 1B. This represents Net-Zero Accumulation, because it corresponds to a stabilization of the Keeling Curve. Continue bending the branch until it is curved downwards. That is something like our approach to Net-Zero Emissions. Did you get the branch to start bending down by 2050?

And by the way, breaking the branch is not allowed, because that would represent massive disruption. Achieving Net-Zero, whether Net-Zero Emissions or Net-Zero Accumulation must be done in a non-destructive manner.

Get the picture?

What if we “only” achieve Net-Zero Accumulation by 2050? This would not be as good as achieving Net-Zero Emissions, but would be way better than the BAU trend we are currently riding. If we were able to achieve Net-Accumulation by 2040 or 2050, there is a good chance that CO2 concentration would allow us to stay close to the 2°C warming. Getting to Net-Zero Accumulation by 2050 would, in itself, be an enormous and significant achievement, the accomplishment of which we can readily verify.

In any case, charting our progress towards Net-Zero Emissions is a task for international agencies that monitor the emissions of every country. However, anybody with Internet access can monitor our approach to a Net-Zero Accumulation way-point: you only need to track the progression of the Keeling Curve.

Or is it that easy?

The effect of natural variability

We fit trend lines to data and make nice, smooth projections into the future. But atmospheric CO2 concentration data is not this well behaved. Year-to-year variability means that trend lines must be fit to multiple years of data to tease out the statistically-significant data trends. How does this affect our ability to assess our approach to Net-Zero Accumulation?

Although a rigorous treatment of this subject is well beyond the scope of this paper, we can get a good idea of how natural variability affects our view of the data as follows. Using the trend lines shown in Fig. 1, I added artificial, natural variability to the future projections, by using the data variability from the data from 2000 to 2020, shown as circles in Fig. 2. I applied this natural variability to the future projections shown in Fig. 2 (represented by the squares and diamonds). Note how this causes the square and diamond “data points” in Fig. 2 to vary up and down about the trend line in a similar manner that the real data from 2000 to 2020 varies up and down about the trend line. This exercise teaches us two things.

Keeling Curve showing the effect of natural variability on trends

 Figure 2. A) Future projections along a BAU trajectory and B) assuming that CO2 concentrations stabilize by 2040.

Figure 2 shows that even when comparing the worst-case scenario, BAU, against the best-case scenario (stabilization of CO2 by 2040), and assuming we begin aggressive CO2 stabilization in 2021, because of annual variability in measured CO2 , it is not possible to distinguish a difference in these two very extreme cases until about 2030. At least not for casual observers. It may be that statisticians can tease out a difference before 2030, but the point is that it takes time to definitively see the effect of carbon-reduction efforts on the Keeling Curve.

The second feature to note is that natural variability can make it appear that CO2 stabilizes by 2031, a full 10 years before the stabilization date used to create the lower trend line. This is illustrated by the three solid diamond “data points” in the lower curve, with a horizontal trend line drawn through them.

Whereas monitoring our approach to Net-Zero Accumulation is a useful check on our progress towards the bigger goal of Net-Zero Emissions, trends over at least 5 years are needed to verify stabilization of atmospheric CO2 concentrations.

Although we ultimately rely on professional Climate Scientists and the IPCC to inform us of how well we’re doing and to recommend needed course corrections along the way, as an independent observer you can monitor our progress by following the progression of the Keeling Curve as we navigate our way into the future.


1. Some companies have already declared they are Net-Zero, and some are even declaring that they will compensate for all legacy carbon emissions.

2. We realize that the carbon footprint varies greatly from country to country, and therefore the contribution to the thickness of the branch depends on the carbon intensity of each individual’s lifestyle, but in some simplistic sense, if the world was populated by 1/10th the people spread across similar income divides, the problem of bending the Keeling Curve would presumably be that much easier.

Additional Reading

Zeke Hausfather, Will global warming stop as soon as Net-Zero Emissions are reached?

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Comments 1 to 8:

  1. The last sentence of this essay is the answer: A reduction in human population from 8 billion to 800 million. An additional answer would be to eliminate industrial animal agriculture, the elimination of which would allow the planet's sustainable number of humans to rise to about 2.2 billion (BAU). Then, various steps could be required to trim GGEs further.

    Of course, none of this is going to happen...and like the above essay, such approaches are mere intellectual gymnastics in the understory of an ivory tower.

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  2. swampfoxh@1

    "and like the above essay, such approaches are mere intellectual gymnastics in the understory of an ivory tower."

    I won't disagree with you. I am merely clarifying what must happen to reach Net-0 Emissions so that people can monitor for themselves how well we are, or are not, translating talk into action.

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  3. I'm in total sympathy with your offering here. I do lament the fact that persons "like us" can devote so much of our lives to get at the essence of something very important, especially a matter of survival, yet knowing that humans are their own worst enemy. I don't see other species trying to commit suicide which makes me wonder if humans are native to planet earth.

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  4. Swampfoxh@1

    "...if the world was populated by 1/10th the people spread across similar income divides, the problem of bending the Keeling Curve would presumably be that much easier."

    But 1/10 the population continuing BAU is not "a solution to the climate change problem". The key words in that last Footnote are "...would presumably be ... easier". Reducing the most harmful aspects of the population is what is required. BAU will result in any population continuing to grow more harmful behaviours in pursuit of "More personal benefit". That BAU population would continue to face the harmful challenge of resistance to giving up developed comforts and perceptions of relative status. Without systemic change that problem would continue to grow no matter how small the population is.

    Read the 2020 Human Development Report. Things can get better. But it will not be as easy as reducing the total global population.

    The problem is people who get more personal benefit by being more harmful than Others being able to powerfully resist having to reduce how harmful they are because they would lose perceived status if they did that and resist making amends for harm that has been done for their benefit.

    Humanity should be able to enjoy millions of years of improving prosperity on this planet, without extending the problem to Mars and without unsustainable exploitation of the Moon or asteroids. That can only happen if human activity is governed and limited to be helpful development of new things and helpful un-development of harmful things. Harmful pursuers of more personal benefit are not helpful. And it is also harmful to hope that "new technological economic driven developments" will be helpful rather than be like the history of harmful profitable and popular developments that powerfully resist being undeveloped.

    The fundamentals of the system undeniably need to be significantly changed. Being richer cannot be allowed to excuse being more harmful. Being higher status needs to be restricted to people who are less harmful and more helpful than their peers of those who have lower status.

    Only when that systemic change of perceptions of status happens will there be a solution to the climate challenge and the other harm that is increasingly compromising the ability of future generations to thrive and survive. And it will also result in development and un-development that will allow a larger total human population to live sustainably live by fitting in as sustainable parts of a robust diversity of life on this planet.

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  5. OPOF@4

    Whereas I don't disagree with you're saying, one of the most difficult parts of the decarbonization problem is what to do about baseline emissions, which are mostly derived from agriculture. An estimate by Larkin et. al. (read here) seems to put baseline emissions at about 1 ton/person/yr CO2e. Current emissions are about 32 GT/yr (read here), meaning that baseline emissions are about 25% of current emissions. Beyond decarbonization of the energy sector, the baseline emissions seem to be one of the most difficult challenges, most likely requiring substantional negative emissions technology (NET) to offset emissions we cannot otherwise eliminate.

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  6. Evan@5

    I agree that "... the baseline emissions seem to be one of the most difficult challenges, ...". But I am reluctant to believe that substantial negative emissions technology (NET) will be developed and can harmlessly offset the harmful impacts of other technological developments that were not understood to be harmful when they were implemented and because popular and profitable (which made the harmful new ways powerfully able to resist efforts to limit or end the harm done).

    My baseline for understanding the problem may be the difference regarding our thoughts.

    My baseline understanding is that poorly governed and inadequately limited pursuits of personal benefit have over-developed many harmful unsustainable ways of living. Un-developing the harmful over-developed activity is required. And that would mean un-doing the incorrect beliefs that the developed perceptions of prosperity and superiority are "deserved" and "deserve" to be maintained. That may mean reducing GDP as the harmful over-development is removed from the system. And it means that it is harmfully incorrect to believe that the correction of harmful over-development should be delayed in order to maintain increasing GDP and to maintain harmfully over-developed perceptions of prosperity and enjoyable living.

    The reports you refer to do indeed indicate the magnitude of the problem and what to expect if the current system is not changed. The expectation that the harmfully incorrect over-developed food desires of wealthier people will be aspired to by everyone else is indeed the expected result if the systemic beliefs that cause the harmful pursuit of impressions of status are not changed. The belief that eating beef, and other meats, is a sign of status is just one of the many incorrectly developed beliefs that seriously compromise the nutrition and health of wealthier people.

    I would say that studies based on the perspectives of helpful people "from within the harmfully over-developed system of beliefs" are like ivory tower speculation. The reality is that the resistance to the required corrections is more powerful than the helpful people "thinking within the system" are acknowledging. What should be presented is the understanding that unless there are serious leadership actions that rapidly bring about significant systemic changes, including ending beliefs like "eating beef or other meat is a sign of superiority", there is no likelihood that impacts will be limited to 4C. A related ivory tower belief is that "non-profitable carbon removal" will be implemented at a meaningful scale (and a related ivory tower belief based on the incorrect belief that "new technology is helpful advancement" would be the failure to recognize the potential harm of industrial scale carbon removal technology or other "technological development" believed to be solutions that allow harmful unsustainable activity to continue longer).

    Without significant systemic changes the warming impacts are likely to exceed 6C, though the breakdown of global civilization, and the resulting global conflict and strife, may temporarily, or permanently, stall the harm done by the "endless harmful pursuit of More personal benefits and new likely to be harmful technological developments".

    The 2020 Human Development Report presents a current summary of understanding that contradicts developed worldviews and beliefs held by many people among the wealthier and more powerful portion of the global population. And the 2020 HDR is not investigating things in a New way. It is a continuation of a long history of efforts to better understand how to protect the future of humanity that included the 1972 Stockholm Conference.

    The worldview preferred by the wealthy and powerful has been constantly challenged by thoughtful people with interests that are not motivated by pursuit of personal benefit (for thousands of years). But more harmful wealthy and powerful people have repeatedly been able to quash or delay the advancements of civilization when that advancement would be contrary to the interests of the wealthy and powerful.

    The system aspects that need to change are the aspects that the harmful among the wealthy and powerful fight to establish and that they can take advantage of to defend their interests and increase their ability to be more wealthy and more powerful.

    One of the most insidious realities of the developed systems is the many ways that "Interests in Personal Freedoms" can be used against "Advancement of civilization's interests". The freedom to be more harmful and believe whatever excuses that behaviour is a significant part of the resistance to increased acceptance of climate science and the required limiting of the climate change harm being done to the future of humanity.

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  7. Re-reading does not always help.

    The opening to my comment@6 shold be:

    I agree that "... the baseline emissions seem to be one of the most difficult challenges, ...". But I am reluctant to believe that substantial negative emissions technology (NET) will be developed and can harmlessly offset the harmful impacts of other technological developments that were not understood to be harmful when they were implemented (and because they became popular and profitable which made the harmful new ways powerfully able to resist efforts to limit or end the harm done).

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  8. Related to my comments@6 and @4:

    The recent Opinion Piece "When it comes to climate change, the heavy hand of colonizers is as important as our carbon footprint", shared on the CBC News website, is a version of my understanding.

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