Looking at the atmospheric CO2 concentration measurements, e.g. from Mauna Loa, the table there says that the increase of CO2 concentration in the year 2012 was 2.66 ppm/yr.

  • How many percent of this 2.66 ppm/yr in 2012 is human caused (from fossil fuels, deforestation, etc.)?

  • How does this percentage change in the years 1960-2012 (if it changes)?

I wasn't able to find literature that would answer this in percent. The closest reference I was able to find is:

Ghosh, P., & Brand, W. a. (2003). Stable isotope ratio mass spectrometry in global climate change research. International Journal of Mass Spectrometry, 228(1), 1–33. doi:10.1016/S1387-3806(03)00289-6

which says:

Owing mainly to antropogenic activities including land use change and fossil fuel burning, the 13C/12C ratio of CO2 in the atmosphere has changed over the last 200 years by 1.5 parts per thousand (from about 0.0111073 to 0.0110906).

So 1.5 parts per thousand is 0.15%. Does it follow from this that human caused CO2 increase is on the order of 0.15% (in the last 200 years)? Update: it doesn't, as nicely answered by user6972 below.

Update/Clarification: In this question I am only asking about CO2 concentrations/emissions (how much is natural and how much is human caused), either measured or best estimate. The answer might be complicated, but I am specifically not asking about any possible influences on temperature (or climate change), that's a completely separate question, which I might ask later.

Further clarification: various estimates agree that the total human caused CO2 emissions are around 30 gigatons/year. The carbon cycle explains, among other things, how CO2 is circulated in the atmosphere. Is there a scientific consensus (based on evidence, not opinion) whether or not the 30 gigatons/year contribute majority (i.e. 50% or more) of the 2.66 ppm/yr increase in 2012 of the atmospheric CO2 concentrations?

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    If humans caused 1.5 ppt in 200 years, that means they averaged 7.5 ppm per year for those 200 years (1.5/1000/200), which is 2.8 times higher than the observed data point for 2012. So I'm not sure what you're looking at is comparable exactly -- it would appear that the 1.5ppt found by looking at C13/C12 is not the whole story.
    – tpg2114
    Commented Nov 25, 2013 at 21:37
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    @WetSavannaAnimalakaRodVance My point was that the average indicates an increase 3 times larger than that observed. Since the output is clearly not constant with time and exploded recently just as you say, there's no way we're talking about the same things/considering all the factors.
    – tpg2114
    Commented Nov 26, 2013 at 2:43
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    The quote above does not say anything about the amount of carbon dioxide in the atmosphere. It refers to a shift in the relative amounts of two stable isotopes of carbon in that carbon dioxide.
    – User58220
    Commented Nov 26, 2013 at 3:02
  • @User58220 That's the beginning point for a full answer to the OP's question then. You're spot on.
    – WetSavannaAnimal aka Rod Vance
    Commented Nov 26, 2013 at 6:34
  • @tpg2114 you're absolutely right - I think User38220 is onto what's wrong, though.
    – WetSavannaAnimal aka Rod Vance
    Commented Nov 26, 2013 at 6:41

1 Answer 1


Fossil fuels contain less carbon-13 than carbon-12 (and no carbon-14), compared with the atmosphere, because the fuels derive from plants, which preferentially take up the more common carbon-12. The ratio of carbon-13 to carbon-12 in the atmosphere and ocean surface waters is steadily falling, showing that more carbon-12 is entering the atmosphere. This ratio doesn't directly correspond to world ppm or even relate directly to the quantity released because they are just looking at the ratio.

This change in ratio is only tracked to show there is a correlation between human activity and CO2. The same can be said about the relative drop in carbon-14 as well.

The carbon cycle is complex and different sink absorption times and source release times make it hard to separate from the current ppm measurements. It is probably impossible to say 10% is from X, 15% is from Y and 75% is from Z.

What is well tracked is the industrial and human sources which have climbed. The question still out there is how well do the sinks work? What are their limits/tradeoffs? What new sources might arise in a warmer climate? It's possible a small amount of man-made CO2 could cause a larger release of natural CO2 and create a domino effect of warming.

In general the human related output of 29 gigatons of CO2 is tiny compared to the 750 gigatons moving through the carbon cycle each year, but it adds up because the land and ocean cannot absorb all of the extra CO2. About 40% of this additional CO2 is absorbed. The rest remains in the atmosphere, and as a consequence, atmospheric CO2 is at its highest level in 15 to 20 million years. A natural change of 100ppm normally takes 5,000 to 20,000 years. The recent increase of 100ppm has taken just 120 years.

The predictive models of the temperate cycle based on greenhouse gases and energy input/outputs can be tested against observations to see what might contribute to the temperature rise. models

Edit: In one of the links I provided was from the IPCC. Thousands of scientists and other experts contribute (on a voluntary basis, without payment from the IPCC) to writing and reviewing reports, which are reviewed by representatives from all the governments, with a Summary for Policymakers being subject to line-by-line approval by all participating governments. Typically this involves the governments of more than 120 countries.

In their report linked above (which you must read completely if you're serious about this subject) they summarize the increase of greenhouse gases:

For about a thousand years before the Industrial Revolution, the amount of greenhouse gases in the atmosphere remained relatively constant. Since then, the concentration of various greenhouse gases has increased. The amount of carbon dioxide, for example, has increased by more than 30% since pre-industrial times and is still increasing at an unprecedented rate of on average 0.4% per year, mainly due to the combustion of fossil fuels and deforestation. We know that this increase is anthropogenic because the changing isotopic composition of the atmospheric CO2 betrays the fossil origin of the increase. The concentration of other natural radiatively active atmospheric components, such as methane and nitrous oxide, is increasing as well due to agricultural, industrial and other activities. The concentration of the nitrogen oxides (NO and NO2) and of carbon monoxide (CO) are also increasing. Although these gases are not greenhouse gases, they play a role in the atmospheric chemistry and have led to an increase in tropospheric ozone, a greenhouse gas, by 40% since pre-industrial times (Chapter 4).

The idea is that over decades we can measure our human based output and then measure the increase in the sinks vs atmosphere. Then we can estimate how much contributes to the overall atmosphere. Since sinks can sources all have different carbon time cycles you can not say "this year x% of the ppm increase is anthropomorphic". You can say that over the decades we see that x% produced leads to y% absorbed and a corresponding z% increase in the atmosphere. But as the sinks saturate and temperatures change they change too, so it is not a rule but a dynamic system which we try to model to understand it. We model these complex systems so that we can answer the types of abstract questions you are asking. "What does this measured ppm increase mean exactly? What are the possible contributors? What happens yearly? What happens monthly? Why is the average temperature increasing by x? What happens if XX process doubles, or halves?"

Given that CO2 has such a major role in the natural greenhouse effect, it makes intuitive sense that changes in its concentration because of human activities might significantly enhance the greenhouse effect. However, calculating the impact of a change in CO2 is very different from calculating the current role with respect to water vapor and clouds. This is because both of these other substances depend on temperatures and atmospheric circulation in ways that CO2 does not. For instance, as temperature rises, the maximum sustainable water vapor concentration increases by about 7% per degree Celsius. Clouds too depend on temperature, pressure, convection and water vapor amounts. So a change in CO2 that affects the greenhouse effect will also change the water vapor and the clouds. Thus, the total greenhouse effect after a change in CO2 needs to account for the consequent changes in the other components as well. If, for instance, CO2 concentrations are doubled, then the absorption would increase by 4 W/m2, but once the water vapor and clouds react, the absorption increases by almost 20 W/m2 — demonstrating that (in the GISS climate model, at least) the "feedbacks" are amplifying the effects of the initial radiative forcing from CO2 alone. Past climate data suggests that this is what happens in the real world as well. (from nasa)

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    Models are not measurements... but are usually based on another indicators and some on a science and/or fiction. As original question went specifically to measurement.. there is just an estimation.
    – Dee
    Commented Nov 26, 2013 at 11:33
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    @Dee as I said it is likely impossible to tell which gigaton of CO2 came from where. We have to depend on models of sources/sinks and test them against the measurements of the whole system. Trends like C-13/C-12 ratios and C-14 dropping are clues to the origin of the CO2 increases.
    – user6972
    Commented Nov 26, 2013 at 19:30
  • @user6972, thanks a lot for the thorough answer! It sheds a light on the paper I cited in the question. I have updated the question (see the last paragraph). Do you know the answer to that? You wrote, resp. cited "We know that this increase is anthropogenic", but does this mean 100% of the increase, more than 50% or less than 50%?
    – Ondřej Čertík
    Commented Nov 26, 2013 at 20:21
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    @OndřejČertík You could easily argue that the increase is 100% anthropogenic based on historical data showing the system was close to equilibrium prior to the industrial age of increased greenhouse gases and resulting warmer temperatures.
    – user6972
    Commented Nov 27, 2013 at 3:13
  • @user6972 Disagree. The world was at 'equilibrium' but does naturally warm and cool (ice ages, etc) over time, the equilibrium never lasts. However there is no evidence of a warming period in anywhere near the time span we're currently experiencing.
    – Meep
    Commented Nov 27, 2013 at 21:27

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