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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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What is causing the increase in atmospheric CO2?

What the science says...

There are many lines of evidence that clearly show that atmospheric CO2 has increased to the highest levels seen in 800,000 years due to human behavior. Observational evidence indicates that this increase is caused by humans because the rise in CO2 levels is consistent with recent industrial trends and emissions are largely linked to the burning fossil fuels.

Climate Myth...

CO2 increase is natural, not human-caused

"[A]tmospheric CO2 increase that we observe is a product of temperature increase, and not the other way around, meaning it is a product of natural variation..." (Anthony Watts)

CO2 Levels are Higher Than They’ve Been in 800,000 Years

Atmospheric CO2 has increased by more than 100 parts per million since the Industrial Revolution in the mid-1700s, when humans began burning fossil fuels like coal and oil. Human activity has increased CO2 to levels not seen in the past 800,000 years.

CO2 over 800,000 years

Figure 1: Atmospheric CO2 concentrations in parts per million (PPM). Levels have peaked throughout time but we’ve seen a steep increase of 100 PPM since the industrial revolution, Climate.Gov 

To understand atmospheric CO2 levels, we must look to the carbon cycle. The carbon cycle allows us to track the CO2 being emitted into the atmosphere and absorbed by the planet.

Bathtub Simulation

Figure 2: Bathtub Simulation, Climate Interactive

A common analogy for this is a sink or bathtub. When you turn on a faucet, water flows into the basin and out through a drain. As the amount of water in the basin increases, more pressure is put on the drain, and therefore more water releases from the drain. The carbon cycle works in a similar way. As more CO2 (water from a faucet) is released into the atmosphere (the basin), more CO2 is naturally absorbed by the planet (the drain). In addition to the atmosphere, the planet has three main carbon reservoirs which include the solid earth, the land surface, and the ocean. Natural glacial and interglacial cycles over the last 800,000 years have worked to move CO2 between the atmosphere and these reservoirs, specifically the ocean reservoir.

Figure 1 shows that CO2 levels were lower during the glacial periods (180 PPM) and higher during warmer interglacial periods (280 PPM). Since the Industrial Revolution, there is a visible difference in CO2 level measurements in the “basin” (aka the atmosphere). This could be caused by either an increase in emissions from the “faucet” or decreased net removals in the “drain.” Below we will take an in-depth look at the scientific evidence that net removals are actually increasing, and therefore the recent atmospheric CO2 increase must be due to an increase in emissions.

The Climate Myth Ignores Evidence of Human’s Role in Increasing CO2

Take a close look at the myth and you’ll see that the argument being made implies that humans aren’t the cause of recent CO2 increases because the increase is natural. Instead, the myth suggests that global warming is the cause of the CO2 increase and further, that global warming is occurring naturally. This commits the fallacy of “slothful induction” or ignoring relevant information to come to a conclusion. In this case, the myth ignores multiple sources of observational evidence that burning fossil fuels has caused increased atmospheric CO2 levels.

Ocean are Becoming More Acidic

We know nature is not the source of growing CO2 levels in the atmosphere because land and ocean CO2 storage has increased. One way that we know the planet’s “drain” is absorbing increased levels of CO2, is by looking at our oceans.

The oceans store the largest amount of the Earth's carbon, so if the atmospheric CO2 increase were "natural", it would likely be coming from the oceans.  But we know the CO2 increase is not coming from the oceans because the pH of the oceans is dropping (a.k.a. ocean acidification).

When CO2 is absorbed into a solution, it binds with a water molecule to form a molecule of carbonic acid (H2CO3)

Carbonic acid [H2CO3] has a rather strong acidifying effect in that 95% of it turns into bicarbonate [HCO3-].  This loss of an H+ ion causes the ocean pH to decrease.

In short, CO2 could be coming from the ocean, but falling pH shows that it’s not. Instead, our oceans are absorbing more carbon than they are releasing.

Isotopic Signature Shows Increased Fossil Fuels Emissions in Atmosphere

Isotopic evidence points to fossil fuels as the source of CO2 emissions. Carbon is composed of three different isotopes: carbon-12, 13, and 14.  Carbon-12 is by far the most common, while carbon-13 is about 1% of the total, and carbon-14 accounts for only about 1 in 1 trillion carbon atoms in the atmosphere.

CO2 produced from burning fossil fuels or burning forests has a different isotopic composition than CO2 in the atmosphere. This is because plants have a preference for the lighter isotope (carbon-12); thus they have lower carbon-13 to 12 ratios. Since fossil fuels are ultimately derived from ancient plants, plants and fossil fuels all have roughly the same carbon-13 to 12 ratio – about 2% lower than that of the atmosphere. As CO2 from these materials is released into, and mixes with, the atmosphere, the average carbon-13 to 12 ratio of the atmosphere decreases.

Reconstructions of atmospheric carbon isotope ratios from various proxies, such as tree rings and ice cores, have determined that the carbon-13 to 12 ratios in the atmosphere are the lowest today than they’ve been in the last 10,000 years. Furthermore, the carbon-13 to 12 ratios begin to decline dramatically just as the CO2 starts to increase — around 1850 AD. This is exactly what we expect if the increased CO2 is in fact due to fossil fuel burning beginning in the Industrial Revolution. These isotopic observations confirm that the increase in atmospheric CO2 comes from plant-based carbon, not from the oceans or volcanoes.  

Ice Core Measurements - Isotopic Signature

Figure 3: Ice core carbon isotope measurements of atmospheric CO2

Some argue that the carbon-13 ratio isn't unique to fossil fuels.  However, because the carbon-14 ratio has also decreased significantly (Figure 4), we know it's from old (fossil fuel) sources, not modern sources. The carbon-14 ratio decreases as more fossil fuel emissions dilute the atmospheric CO2 signature. This is not new science either, it's something we've known for over half a century (Revelle and Suess 1957), and there have been many studies confirming these results.  For example, Levin & Hesshaimer (2000):  

Tree Ring CO2 Signatures

Figure 4: Temporal change of carbon-14 ratio in tree rings grown at the Pacific coast (Levin & Hesshaimer 2000)

Atmospheric O2 is Decreasing

We also know that burning fossil fuels has led to the recent increases in CO2 levels because of observed decreases in atmospheric oxygen levels. Burning carbon requires oxygen, so when we burn an atom of carbon, the required oxygen becomes part of the CO2 molecule.  If the CO2 increase is caused by burning carbon (fossil fuels), we would expect atmospheric O2 levels to decrease at the same rate.  And that's indeed what we observe (Figure 5). O2 concentration is changing exactly as we would expect from a fossil-fuel driven CO2 increase.

O2 Concentrations

Figure 5: Atmospheric Oxygen Concentration observed from Cape Grim, Tasmania

Correct Causation: Human Emissions Contribute to Atmospheric CO2 Increases

So let’s review. One climate myth argues that atmospheric CO2 increases are the result of natural global temperature changes. However, as described above, scientists have found multiple lines of evidence that show recent CO2 level increases are the result of burning fossil fuels. Between ocean acidification, isotopic signatures of the atmosphere, and trends in O2 levels, the evidence is abundantly clear that burning fossil fuels have resulted in an increase of atmospheric CO2, not seen in 800,000 years. The correct causation is an important distinction to make. Rather than global warming causing the increase in atmospheric CO2, our use of fossil fuels are actually the source of the increase. Moreover, we know that it is this human-caused increase in atmospheric CO2 that is warming our planet.

Last updated on 16 November 2020 by eharrin5 . View Archives

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Related videos

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


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Comments 26 to 33 out of 33:

  1. I'm having a little trouble getting some math to come out right.  CO2 concentration is rising by 2 ppmv/year.  The net flux is given as 15 gigatons/year.

    For the surface area of the earth I take a radius of 6.4e6 meters to get an area of 171e12 m2.  I convert 14.7 lb/in2 atmospheric pressure at sea level to a metric value of 10.35e3 kg/m2 or 10.35 ton/m2.  When I multiply those together, it comes out to 1.771e15 tons for the total weight of the atmosphere.  One ppm of that would weigh 1.771e9 tons or 1.771 gigatons.

    The atmosphere is primarily molecular nitrogen.  Each molecule has a weight of 28.  The weight for CO2 is 44.  This brings the weight of one ppm by volume up to 2.78 gigatons.  The increase of concentration given in the article, 2ppmv/year would represent a weight of 5.56 gigatons.  This is low by a factor of almost three from the value of 15 given in the article.

    What am I missing?

  2. @26

    Area wrong.  Should be pi times 4 , not pi times 4/3 .

  3. Eclectic

    Thank-you.  It has been a few years.  I was confusing the area and volume formulas.

  4. The arguments presented are helpful and fairly comprehensive, but I was surprised the author, dana1981, did not address what, in my view, is the most important scientific publication on this issue: “The phase relation between atmospheric carbon dioxide and global temperature” by Ole Humlum, Kjell Stordahl and Jan-Erik Solheim in Global and Planetary Change 100: 51-69, 2013. These authors showed, using published temperature time series from multiple sources and global CO2 and anthropogenic CO2 data that, for the years 1980 to 2011:

    1. There was a good temporal correlation between global CO2 and ocean temp, land temp, global temp and lower troposphere temp BUT the global CO2 FOLLOWED the ocean temp, then the land temp, then the lower troposphere temp, in that order, with lags of 9-12 months.

    2. In contrast, there was poor temporal correlation between anthropogenic CO2 emissions and both global CO2 and temperature.

    3. While anthropogenic CO2 was emitted overwhelmingly from the northern hemisphere, the time sequence of ocean temperature variation commenced in the Southern Hemisphere, reasonably close to the equator, then spread north and south to the poles, always preceding the global CO2 time sequence.

    These carefully determined temporal sequences and correlations, based squarely on the published temperature and CO2 data, clearly indicate a causal sequence in which global temperature changes PRECEDE global CO2 changes by 9-12 months, commencing with changes in the ocean surface temperature, then the land temperature, then the lower troposphere temperature. These observations are the complete OPPOSITE of what should be expected if anthropogenic CO2 emissions were driving both the global CO2 levels and then causing a secondary increase in temperatures.

    So, while I appreciate the energy balance and other arguments advanced above, causality requires a demonstrated temporal sequence of changes that the data I describe here simply do not support. I would be very interested in your explanation for these observations.


    [TD] Humlum is wrong. Type "Humlum" into the Search field at the top left of this (or any) SkepticalScience page.

  5. mkrichew:Having difficulty understanding how 2.4 ppm change annually ( or 19 billion tons )  atmospheric CO2 translates to delta Catm = 18 billion tons.

  6. mkrichew @30,

    I would say it is a bit lax to substitute 18 for 19 within the OP but given the situation the OP describes, it makes zero difference to the argument presented. The "19 billion tons" figure in the OP is described as "roughly" the ΔCatm required to give a +2.4ppm(v) increase which is given as the rate of CO2 increase "recently."

    We could be more precise and say that a +2.4ppm increase would require ΔCatm = 18.7 Gt(CO2), but given the wobbles caused by ENSO to the annual increase in atmospheric CO2, it is impossible to be that precise about it. The OP was written in 2012 and the source of the MLO CO2 data cited ESRL give a value for the 2012 annual MLO CO2 increase as +2.61ppm = 20.4Gt(CO2) although if the average of the 12-month increases through 2012 is used to calculate a value the result is +2.20ppm. Or if the ESRL Global data is used instead of MLO data, ΔCatm  for 2012 is given by ESRL as +2.39ppm while tha average of the months yields +2.00ppm =15.6Gt(CO2). Or an alternative source of the value would be the Global Carbon Project's 2012 ΔCatm of 5.07Gt(C) = 18.6Gt(CO2) (altough note the 2012 LOC emissions are a long way from zero which is the assumption made in the the OP).

  7. Thank you for your kind response. As you may have guessed I am the author of the Mike Krichew Theory of What Causes Ice Ages which I wrote sometime after Al Gore released his documentary "An Inconvenient Truth" and conservative elements responded as President Trump did, suggesting a conspiracy. At the time I suggested a comet tail reflecting sunlight might account for the increased insolation that would warm the oceans causing an increase in the atmospheric CO2 levels which would warm the atmosphere and further warm the oceans. At the time I was not much of a believer in the Milankovitch cycles theory. However, the other day it occurred to me that if the earth is indeed an oblate speroid or ellipsoid in shape then it may be possible for the earth to present different size cross-sectional areas to the sun during the cycle. This would result in different insolations. Someone with a talent for mathematics might show the different cross-sectional areas if it has not already been done. Someone else with an interest in celestial science might calculate where the minimums and maximums of cross-section occur and plot them on the slightly sinusoidal graph of climate change over time. If there is any correlation, it should then be possible to calculate and model the increased insolation that occurs during the cycle. If this has already been done, a reference would be nice.


    [TD] Please copy and paste your comment into a relevant thread you find by typing Milankovich into the Search field at the top left of the page--for example, this one.

  8. I have responded to mkrichew in the appropriate place.

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