What Does This Number Mean?

Repost of April 2013 entry

The Mauna Loa carbon dioxide (CO2) record, also known as the “Keeling Curve,” is the world’s longest unbroken record of atmospheric carbon dioxide concentrations.  


This record, from the NOAA-operated Mauna Loa Observatory, near the top of Mauna Loa on the big island of Hawaii, shows that carbon dioxide has been increasing steadily from values around 317 parts per million (ppm) when Charles D. Keeling began measurements in 1958, to nearly 400 ppm today.

Scientists make CO2 measurements in remote locations to obtain air that is representative of a large volume of Earth’s atmosphere and relatively free from local influences that could skew readings. The quality of data is verified before daily average values are determined.

Concentrations are currently approaching the symbolically important value of 400 parts per million. The continued rapid rise in CO2 ensures that levels will rise far beyond 400 ppm before they stabilize.  If the pace of the last decade continues, carbon dioxide will reach 450 ppm by the year 2040. Carbon dioxide is the most important man-made greenhouse gas, produced mainly by the burning of fossil fuels such as coal, oil and natural gas. The pace of rise depends strongly on how much fossil fuel is used globally.

Although the Mauna Loa record extends back only 55 years, a record extending 800,000 years has been obtained from samples of old air preserved as bubbles in the Antarctic ice sheet.  These records from the Antarctic ice sheet are referred to as ice-core records of atmospheric carbon dioxide and two ice-core studies have been used to create the figures on the front page showing atmospheric carbon dioxide before the Mauna Loa record. The reference for the ice-core record extending back 800,000 years is: Lüthi, D., et al. 2008. High-resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature, Vol. 453, pp. 379-382. doi:10.1038/nature06949 The reference for the ice-core record extending back 2,000 years is: MacFarling Meure, C., et al. 2006. The Law Dome CO2, CH4 and N2O Ice Core Records Extended to 2000 years BP. Geophysical Research Letters, Vol. 33, No. 14, L14810 10.1029/2006GL026152. These data sets can be accessed from the Carbon Dioxide Information Analysis Center (CDIAC), Oak Ridge National Laboratory, U.S. Department of Energy.

Prior to the Industrial Revolution, natural climate variations caused atmospheric CO2 to vary between about 200 ppm during ice ages and 300 ppm during the warmer periods between ice ages. At the dawn of the Industrial Revolution, around the year 1780, the CO2 concentration was about 280 ppm, so CO2 had already risen by around 40 ppm before Keeling began his measurements.  Anyone who has breathed air with less than 300 ppm CO2 is now over 100 years old!   An even longer but much less accurate record of atmospheric CO2 can be obtained using other geochemical methods.  These suggest that the last time atmospheric CO2 was over 400 ppm was at least as far back as the Pliocene, three to five million years ago, before humans roamed the earth and when the climate was considerably warmer than today.

UPDATE: Day-to-day numbers can fluctuate due to variations in the air masses moving over Hawaii. That’s why researchers look at monthly averages which tend to smooth out these synoptic events. And as one can see on the Keeling Curve, the overall trend is superimposed on the seasonal cycle. Since the seasonal cycle tends to peak in May, researchers expect that the highest values are yet to come in 2013. However due to natural variability in the air flowing across the Pacific, researchers can’t predict exactly when the planet will cross the 400 ppm threshold on a daily or monthly average basis.


This website provides daily updates, analysis, and information on the state of climate. Follow @Keeling_curve to get daily updates of the CO2 value. Through this site, the public can also help support the continuation of the iconic Keeling Curve and of complementary measurements of atmospheric oxygen made at Scripps. These measurements enable society to witness climate change and inform strategies to address it. Click below to go to the UC San Diego giving fund set up specifically to fund ongoing operations.

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54 thoughts on “What Does This Number Mean?”

  1. To me, two points are particularly important:
    1. Not once since 1958 has the Keeling Curve gone down.
    2. Even though the greenhouse effect is a fact of physics, the global warming deniers ask us to believe that the Keeling Curve can keep rising, but for some reason that they cannot explain, the temperature curve will stop rising and decline.

    1. Jim, in addition to your observation, I would also comment that there are no large anomalies due to volcanic eruptions. This is a direct contradiction to what many climate deniers like to claim.

  2. Thank you for your work. As a artist/songwriter, I’m trying to do education about Climate Change with our music videos: see them at http://www.greatleap.org. We are trying to show how small changes in our lifestyle can make an impact. We need more info about what ordinary people can do to help. Thanks.

  3. We get data, but I would like to see more on what the 13 & 14 Carbon molecules end up as. Like carbonic acid or the oceans cycle effect of these carbons in particular.
    I know plants can not use this form.

    1. James, that is not true. There are chemical processes, like one of the flavors of photosynthesis (and not the other), where there is a statistical bias toward lighter or heavier atoms, but they still work in the reactions. It is not a 0 or 1 thing, real numbers are included.

      1. Chris, I appreciate your reply. Maybe I stated the 13C and 14C isotopes in less than identifiable detail. When I was referring to the plant process that plants do not absorb as quickly or not at all due to the weight of the man made 13,14C isotopes making them slower. As is Know in many scientific studies 12C is Absorbed readily before 13,14C can approach synthesis. The result is more CO2 13,14C. Now What Happens to this type over time? A question I proposed we find out.

        1. A slight correction to your comment is in order: The key isotopes in photosynthesis are C12 and C13 only. C14 is not, because it is so rare and more importantly, constantly being created by cosmic rays bombarding the atmosphere. The C12/C13 ratios are the ones that show that the current increase in CO2 in the atmosphere is from fossil sources, not from volcanic or other sources.

  4. So what’s the plan? I mean, realistic, concrete plans for individuals to do, not just take shorter showers or stop driving. I don’t think that’s enough anymore. We need to reverse the trend.

  5. This is an excellent site! Do you have data for historical concentrations of atmospheric C14? I think it would be interesting to visualize the C14 concentration as another indicator of anthropogenic carbon dioxide. Please keep up the good work! Thank you!

  6. Awareness is key. I have been following disturbing realities of C13&C14 for some time understanding the danger is not so much the atmosphere as it is the oceans holding the heat in deeper waters. This is scary stuff. Our oceans currents get disrupted and who knows what will be the outcome. Greenland is now losing billions of gallons of water every day during summers.
    Mauna Loa is a high pristine area imagine what Chicago is putting out.

  7. I’m amazed that your work has somewhat of a hard time being adequately funded! It is obviously of crucial importance for our whole planet. But then, as we all know: SNAFU. (And I really mean the “F” word).
    In your sequence of graphs, it would be useful to have one between 1700 to present and 800000 years, say the past 5000 years or so. There must be correlations between the state of earlier civilizations and the global climate; such a graph would be enlightening. That’s only logical!
    Live Long and Prosper

  8. These results show clearly the tight connection CO2 fluctuation has with biomass and seasonal variation of photosynthesis. Therefore there is a lot we can do. By a massive joint effort to increase plant biomass we should be able to decrease the CO2 levels again. Please join me in the “plant a plant for the planet” project.

  9. When I was giving lectures about indoor climate in the 90:s I spoke about CO2 since there is a wellbeing value at 800 ppm and a do something threshold at 1000 ppm. The curve was climbing and at that time it was somewhere around 355-360 ppm. I did question, without knowing any answer, what we were to do if the level was continuing to climb, making it impossible for us humans to evacuate our outbreathed CO2 through ventilation with fresh air, since this fresh air already contained much CO2. Now, 20 years after, the Keeling curve tells me the level is 400 ppm right now. We, that is you and I, must do something! Please do your part, I will do mine.
    With the best, Mikael :-)

    1. Would you not agree that it should bepossible to trap the CO2 back into plants again? I mean, that is where it has been released from by humans./Anna

      1. Hej
        To plant a tree, or many, is something valuable of many reasons but to halt the climate change I think there must be a lot more done concerning reduction of releasing more CO2. Mikael :- )

        1. I was thinking with quite higher ambitions here, with a substantial part of earth population planting many plants each (in addition to other CO2 reducing activities). I think the yearly fluctuation clearly show this dependence on plant biomass. It is easier to make people contribute with cultivation than reducing car miles.

          1. I think we would struggle to plant new trees at the rate deforestation is taking place around the world where mature trees are being destroyed. Young trees would also take years to reach a size when they could replace the C02 take up of those being destroyed. Perhaps a more immediate remedy is to reduce the appalling wastage of energy that takes place in unnecessary travel, needless lighting & production of junk we don’t really need.

      2. Doing some (very) rough calculations based on yearly greenhouse gas emissions (~30 billion tonnes C), density of dry wood (.35), carbon content of wood (.45gC/g dry weight), and size of a big Giant Redwood (1e3m^3), one would have to create, every year, ~20e6 of some of the biggest, oldest trees we know in order to tie up the carbon we’re digging out of the ground and burning. Put another way, we’d have to cover an area the size of Rhode Island with a Redwood National Park … every year.

        This is carbon that has been buried for millions of years, not carbon that is cycling through the biosphere. And no, trees do not magically make carbon disappear. Carbon that isn’t incorporated into their mass returns to the atmosphere. Trees have a metabolism too, and metabolically “burn” much of the carbon they taken from the atmosphere.

  10. Someone makes a well-meaning tree-planting comment. They have no understanding of the quantities. Humans are adding 4.5% per decade of the carbon in the total 2,000 +/- 400 GtC land biomass to the atmosphere and it’s increasing. So, need to plant ~5% additional of all trees on Earth each decade. Nowhere to put them. Plus, need to bury their litter in subsoil when dead or it’ll go back in the air. Impossible task.

  11. Recently (week of 8/1/2014), the fluctuations of CO2 measured at Mauna Loa have appeared extreme. 4-6 ppm within ONE DAY! Is this normal? If not, what is causing this perhaps? Is the ocean or the land around the measuring station capturing and emitting more CO2 than normal? Normally we see a 7 ppm oscillation from summer to winter, but 6 ppm in ONE DAY! That seems like an aberation that should be analyzed.

    1. Nice comment. It’s fairly normal, especially in the summer time, to see strong afternoon “dips” in the CO2 concentration. The dips are generally caused by upslope winds that that are depleted in CO2 by photosynthesis at lower elevations. In the last few days have had very large swings of this sort showing the island influences can be strong. We do our best to exclude these island influences from the record by retaining only data when the concentration is stable for many hours. How unusual are the large recent swings? The magnitudes of the swings can vary significantly from year to year, with daily dips greater than 10ppm seen in some years. So the recent oscillations observed in a single day of about 6ppm are well within the range of normal variability.

      – Ralph Keeling and Stephen Walker of Scripps CO2 Group

  12. Hi, I’m graduate student and dealing with my thesis work nowadays. I was brainstorming on my own, then I decided to ask.
    Do you know the main reason(s) why CO2 concentration level in May is higher than other months, especially higher than winter months inwhich we consume more fuel or natural gas for heating (higher CO2 emission as a result of this consumption) ?
    Although biology is not my expertise area, I thought that plants also make more photosythesis reaction when they have fresh green leaf under whole day sun as in summer time whereas the green leaves of plants drop in winter.
    Thanks for your valuable comments.

  13. Thanks for reposting this. Explanations for seasonal and daily reading variations are interesting.

    I’ve been following your daily readings. As you folks reported, several averages at the end of April had to be corrected due to (I assume) bad calibration gas. I appreciate that it must be a continuing challenge to publicly publish daily averages. But could you share more about this incident? Also what is the expected error for daily averages?

  14. The primary calibration gas at Mauna Loa is typically changed about every 18 months or so. The instrument compares the unknown air with the known calibration gas so it is desirable to keep the calibration gas’ concentration close to the ambient air’s value. It would not be good, for example, to calibrate the instrument today with the 315ppm air that was characteristic of the late 1950s since air today is over 400ppm. Therefore the CO2 concentration of the replacement calibration gases is usually about 3ppm or so higher than the one it replaced, about matching the growth rate of CO2 in the air.

    The calibration tank change that occurred on 28th April 2015 replaced an approximately 393ppm tank with an ~396 ppm tank. This calibration concentration change was not included in the software that works up the data until early May, so the air data during that period appeared to decrease by about 3ppm.

    Errors in daily readings can come from a variety of factors including errors in the declared values of the calibration gases, which is of order of a few 1/10ths of a ppm and instrument variability, which is smaller than this. Other errors, such as omitting to account for a calibration gas change, can also occur.

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