What Does This Number Mean?

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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26 comments on “What Does This Number Mean?
  1. grindupBaker says:

    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.

  2. 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 :-)

    • Anna Fogdell-Hahn says:

      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

      • 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 :- )

        • 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.

          • Pete Roberts says:

            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.

      • Craig says:

        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.

        • Anna FH says:

          I knew somebody would start calculating on this! Thanks for the figures.
          Again, back to the fluctuation over seasons. Do we even see it over the night/day cycle that shows you the daily photosynthesis and nightly respiration. What does it tell you?
          To me it suggests that we could, with human capacity, aim for reducing the trend so that we never go over 400 ppm again.
          Trees? I was thinking plants to eat, or use for other purposes that will keep the C in the biosphere.
          For inspiration see the TED talk by Michael Pawlyn about using nature’s genius in architecture.

          • Craig says:

            Actually, I slipped a digit, it’s 10 times more trees than I thought. And if found a few other numbers that indicated that I was in the right order of magnitude.

            When you eat a plant, you metabolize it, turning it back into CO2. Crops are not CO2 sinks unless you bury the waste and byproducts. When you’re looking at biomass, only biomass that is not harvested (eaten or burned), and is buried, is actually removing permanently CO2 from the atmosphere.

            The seasonal fluctuations in the Keeling curve are caused by seasonal increases in biomass that draw down the carbon, then release it when the leaves fall and the biomass rots, etc. notice how small they are compared to the long-term trend?

            And the biomass of

          • Anna FH says:

            Yes, planting all these trees for a few people over a few years is impossible. But what if you divided it on all people alive on the planet and over a lifetime of 50 productive years? How much would each person need to contribute with each day?
            If you also count on that some of the carbon will be stuck in the dying material, some move to building blocks of animals and some used for building wooden stuff (estimate 25%)?
            We should of course do all we can to also stop the release, but at the same time we need to start working on the repair, which will take long term dedicated work. Compensate for loss of cultivation area due to building of cities, explore yet unplanted spaces and recycle biomaterial. We need to start this process in parallel.Are you with me on this?

  3. 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.

  4. Michael C says:

    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

  5. paresh patel says:

    i can remove 70ppm CO2 from the air through the my project in two years

  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. Joe Skovira says:

    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!

  8. Johnny says:

    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.

  9. James B Fuller says:

    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.

    • Chris G says:

      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.

      • James Fullers says:

        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.

        • Jimbo108 says:

          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.

    • Does that excess acid component not lead to oceanic acidification? This in turn leads to huge amounts of sulpfer gas – another greenhouse gas.

  10. 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.

  11. Jacques Sauvage says:

    May we know the effect on humans if any?

  12. Jim Powell says:

    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.

    • Jimbo108 says:

      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.

  13. Robert SanGeorge says:

    You are doing invaluable work for the planet!

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