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Are CO2 levels increasing?

What the science says...

Currently, humans are emitting around 29 billion tonnes of carbon dioxide into the atmosphere per year. Around 43% remains in the atmosphere - this is called the 'airborne fraction'. The rest is absorbed by vegetation and the oceans. While there are questions over how much the airborne fraction is increasing, it is clear that the total amount of CO2 in the atmosphere is increasing dramatically. Current CO2 levels are the highest in 15 million years.

Climate Myth...

CO2 is not increasing

"...there is the contention by Wolfgang Knorr of the Department of Earth Sciences at the University of Bristol in England that carbon dioxide levels in the atmosphere are about where they were 160 years ago." (as quoted by Ken Ward Jr.)

The 'airborne fraction' refers to the amount of human CO2 emissions remaining in the atmosphere. Approximately 43% of our CO2 emissions stay in the atmosphere with the rest being absorbed by carbon sinks. But is the airborne fraction increasing? A paper published in November 2009 found no statistically significant trend (Knorr 2009). Anthony Watts labeled this result the "Bombshell from Bristol" - A potentially devastating result for anthropogenic global warming. Was it such a shock? The 2007 IPCC verdict on the airborne fraction was "There is yet no statistically significant trend in the CO2 growth rate since 1958 .... This 'airborne fraction' has shown little variation over this period." (IPCC AR4) I'm not sure the move from "not much happening" "to "still not much happening" warrants the label "bombshell".

The airborne fraction is calculated from the rate of human CO2 emissions and changes in atmospheric CO2 concentration. The global increase in atmospheric CO2 has been directly measured since 1959 and can be calculated from ice cores for earlier periods. Primarily, CO2 emissions come from fossil fuel combustion with a lesser contribution from land use changes. Fossil fuel combustion is calculated from international energy statistics. CO2 emissions from land-use changes are more difficult to estimate and come with greater uncertainty. Land use emissions are estimated using deforestation and other land-use data, fire observations from space and carbon cycle modeling.

There have been several recent studies determining the airborne fraction. Trends in the sources and sinks of carbon dioxide (Le Quere 2009) examines the airborne fraction from 1959 to 2008. This period was chosen as we have directly measured atmospheric CO2 levels over this time. Fossil fuel emissions rose steadily in recent decades, contributing 8.7 ± 0.5 gigatonnes of carbon in 2008. This is 41% greater than fossil fuel emissions in 1990. CO2 emissions from land use was estimated at 1.2 ± 0.4 gigatonnes of carbon in 2008. Note the proportionally higher uncertainty compared to fossil fuel emissions.

Over this period, an average of 43% of each year's CO2 emissions remained in the atmosphere although there is much year-to-year variability. The noise in the airborne fraction was reduced by removing the variability associated with El Nino Southern Oscillation (ENSO) and volcanic activity. They found the airborne fraction increased by 3 ± 2% per decade. This is a slightly increasing trend although only barely statistically significant .

Knorr 2009 extends this analysis back to 1850 by combining direct CO2 measurements from Mauna Loa and the South Pole with CO2 data derived from Antarctic ice cores. This enabled Knorr to compare CO2 emissions to atmospheric CO2 levels for the past 150 years.

Figure 1: Observed increase atmospheric CO2 derived from direct measurements, taking the average of Mauna Loa (Hawaii) and the South Pole (thin solid line) and two ice cores: Law Dome (dashed thin line) and Siple (thin dotted line). This is compared to total anthropogenic emissions (thick solid line) and 46% of total emissions (thick dashed line). (Knorr 2009)

Knorr finds that since 1850, the airborne fraction has eemained relatively constant. When CO2emissions were low, the amount of CO2absorbed by natural carbon sinks was correspondingly low. As human CO2 emissions sharply increased in the 20th Century, the amount absorbed by nature increased correspondingly. The airborne fraction remained level at around 43%. The trend since 1850 is found to be 0.7 ± 1.4% per decade.

There are several differences in methodology between Knorr 2009 and Le Quere 2009. Knorr's result does not include the filtering for ENSO and volcanic activity employed by Le Quéré. However, when Knorr does include this filtering in his analysis, he finds a trend of 1.2 ± 0.9% per decade. This is smaller than Le Quere's result but is statistically significant.

Knorr also finds the 150 year trend while Le Quéré looks at the last 50 years. This may be significant. If the airborne fraction is increasing, it is possibly a recent phenomenon due to natural carbon sinks losing their absorption ability after becoming saturated. Several studies have found recent drops in the uptake of CO2 by oceans (Le Quere 2007, Schuster 2007, Park 2008). However, with such a noisy signal, this is one question that will require more data before being more fully resolved.

Lastly, some perspective. There are still areas of uncertainty associated with the carbon cycle. Because of this uncertainty, scientists are currently debating whether the airborne fraction is steady at 43% or slightly Increasing from 43%. Unfortunately, some skeptics use this uncertainty to hold the position that the airborne fraction is closer to 0%.

Intermediate rebuttal written by John Cook

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

Additional videos from the MOOC

Andy Skuce: The CO2 rise is man man-made

Interviews with various experts

Expert interview with Corinne Le Quéré

Last updated on 8 July 2015 by pattimer. View Archives

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Comments 26 to 42 out of 42:

  1. Id say since each year goes up 2 percent, so we avg near 391 ppm this year, 393 ppm in 2012, 395 ppm 2013, 397 ppm 2014, 399 ppm 2015. I think 2016 we make it. 2000 369.40 0.12 2001 371.07 0.12 2002 373.17 0.12 2003 375.78 0.12 2004 377.52 0.12 2005 379.76 0.12 2006 381.85 0.12 2007 383.71 0.12 2008 385.57 0.12 2009 387.35 0.12 2010 389.78 0.12 In May 2010 we came near 393, 2009 made it to 390, 2008 made it to 388 ppm. This is the months. Id say 2011 395 ppm 2012 397 ppm 2013 399 ppm 2014 401 ppm
  2. Not 2 percent, but 2 ppm.
  3. Isn't the percentage of CO2 retained in the atmosphere increasing. I took the NOAA MLO CO2 annual increments from 1959 to 2010, calculated the percentage change for each year from 1960 to 2010, graphed that, and found that percentage of CO2 retained in the atmosphere (given here as 43%) is generally increasing during that period. Does this mean that the ocean and land CO2 sinks are becoming less effective?
    Response: [DB] Perhaps I've missed it, but I have not seen anything conclusive yet (there is some natural variability in the uptakes). It is indeed being studied closely, for that event is what some models have indicated in a Business-As-Usual situation.
  4. Sir: Homo s. has had his brain cage increase from 500cc to 1700cc over the short period of 3million years. He will be able to endure just about anything nature throws at him at the year 2100. Why link temperature and atmos CO2 and end there? We know plants and animals can cope better at "high" temperatures, and we know higher CO2 is supportive of more plants. (World food sources have increased, relative to all else,in the last 60 years) Homo S. died down to (near extinction?) about a mear 2000 during the last ice age. So why not get this discussion around to what's the best of both? Instead of they're both bad...which seems to be the thread. acorn1
    Response: [Daniel Bailey] You're relatively new here, so let me take a moment to welcome you & to give you a quick tour and idea of how things work best here. We encourage you to read the Newcomers-Start-Here post, followed by the Big-Picture post, so you can get a synopsis of what's going on. I also recommend you watch this video on why CO2 is the Control Knob of temperatures. When you have questions, use the Search function in the upper left corner of every page to find out if there's an answer ready-made for you in the form of a blog post investigating that (chances are there is). Questions are to be posed on those threaded posts. If you still have questions, find the most appropriate post & ask it there. Someone will get back to you. Lastly, keep the Comments Policy in mind when formulating your questions (remember to use the Preview function to ensure readability). Thanks!
  5. Acorn1 - perhaps look at: CO2 is plant food and Its not bad. I don't know of any science that supposes humans as a species is going to die out. However, climate change that is too fast will be seriously bad for many individuals of the species. Its about the rate of change not about what's an optimal temperature. Have a look at the scenarios of what 2100 could look like and think about whether this seems to good thing, but its best to do so on the appropriate thread rather than throwing a bunch of skeptic talking points into a single post.
  6. Regarding my own comment 28. I'll look at my "analysis" again. But as I was lying in bed last night, and again this morning, I concluded that I was being overly-simplistic. I implicitly assumed/ was provided a constant rate of Carbon emissions increase (1%) from 1960 to 2010, and that probably is not the case. The 1% figure is no doubt an average rate of increase.
  7. Re: koyaanisqatsi (31) This graphic shows the rise in emissions as well as the attribution: Data through 2003, so it's a bit dated. The Yooper
  8. Koy: The EIA has data for annual worldwide carbon emissions, with country breakdown if you are so inclined. 1980: 18.4 Gtons; 2009: 30.4 Gtons. Net= +12.0 Gtons/yr. Over 30 years, that is actually an average growth rate of 1.7% per year.
  9. John, you might update the article with the recent news about CO2 increasing despite the recession. I recall NETDR claiming that recessions cause dips in CO2. While that's possible (and may still be possible with this double dip or super-dip), so far it hasn't been evident.
  10. The global recession did cause a dip in CO2 emissions for a couple of years, but emissions started rising again in 2010 and the emissions dip was much too small to have any impact atmospheric concentrations... which have continued increasing at about 2ppm per year through the recession.
  11. Stop the presses! The Mauna Loa November CO2 data is in: [Click to enlarge] 6 more months of increases before the next peak in May 2012. My WAG (Wild-A**-Guess) for then: 396+
  12. Sorry if this is the wrong comment string. I am having trouble finding quantitative answers to questions related to how temperature increase lags behind CO2 concentrations. Specifically: 1) if CO2 emissions stopped today, what global average temperature would we eventually reach, and 2) how soon would we reach it?Thanks
  13. Ritchieb1234,

    Searching at RealClimate using the keywords "temperature change emissions stop" gives several interesting hits.  The first three or four are advertisements.

    This OP from 2010 seems to describe what you seek.  The original letter is here.  Follow the citations at Google Scholar to get more recent ideas.

    They have covered a lot of material in the old posts at Realclimate.  They are probably at a technical level that you would like.  Currently they post less because they have already covered so many issues.

  14. Michael SweetNice to hear from you again. Thank you for the references.
  15. "1) if CO2 emissions stopped today, what global average temperature would we eventually reach, and 2) how soon would we reach it?"

    Good questions.  Looking to the most recent summary at hand, Canada's Changing Climate Report for 2019:

    Section 3.4:

    "Global temperature change is effectively irreversible on multi-century timescales. This is because the total amount of carbon dioxide emitted over time is the main determinant of global temperature change and because carbon dioxide has a long (century-scale) lifetime in the atmosphere."


    "In summary, many aspects of climate change are irreversible on multi-century timescales. CO2 persisting for a century or more in the atmosphere is the main determinant of global mean temperature change, and global temperature will remain elevated even after emissions cease. GMST could be reduced only if human intervention could remove CO2 from the atmosphere over a sustained period."

    Section 3.5:

    "The difference between observed temperature and the multi-model average from roughly 2000 onward has been extensively analyzed (e.g., Fyfe et al., 2016) and is due to a combination of small errors in the observational record, decadal timescale internal variability, and incomplete early 21st century volcanic forcing in the models."

    Figure 3.9 shows the persistence of elevated temperatures after emissions cease:

    CCCR 2019

  16. As this is your first post, Skeptical Science respectfully reminds you to please follow our comments policy. Thank You!

    Thank you for the membership, i have followed the mauna loa curve for years and wondered what the yearly dip in co2 increase is caused by.

    This year the curve is straight up for the first time ever and i found a correlation; China.

    Since 2020 i have taken dayly screenshots of the "windy" website which monitors the co2 levels globally and focussed on china's dayly output, in short it showed increased activity just before the chinese new year celebration ( 12 feb to 26 feb ) and continues during the celebrations because of the pandemic demand for products.

    Mauna loa is directly downwind of china by some days and is in effect monitoring China's co2 output.

    You can check this out for yourselves on the "" and the noaa mauna loa co2 graph websites.

    Here is a link to my 3 youtube video's titled; "co2 levels explained"on youtube, it has all the info combined to show the relationship in detail on a day by day basis;

    Another significant indicator is the dip in 2008 which stopped China's output for two months in the mauna loa graph.

    Thank you; the Inspector.

  17. the Inspector @41,

    I'm not at all convinced by your 'China' theory or even that there is a convincing 'dip' in MLO CO2 levels (or perhaps best described as "less of a February CO2 rise relative to consecutive months").

    But I would draw your attention to a definite error in your argument. The reason the graphics are showing sky-high levels over China and very low levels elsewhere is because this is CO (carbon monoxide) being plotted in parts per billion. It is not showing CO2 (carbon dioxide).

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