Decarbonization Avenue C2V - CO2 to Value All about CO2

CO2 & climate change

With the entire world focussed on decarbonization, it appears as if everyone knows everything about CO2 emissions. But most of what is known are the basic data points in the context of CO2 emissions and climate change.

The intersection of CO2 and climate change is more nuanced - be it in the way CO2 influences climate or in the range of avenues possible to decarbonize.

We hope that the questions and answers in this section help you get a much better grip on the intersection of CO2 and climate change.

1. When was it discovered that CO2 was a greenhouse gas?

Who really discovered the greenhouse effect, and when, is a bit difficult to answer, as different scientists from different parts of the world had identified that CO2 and water vapour both absorbed and radiated heat.

However, one could say that the greenhouse phenomenon was formally described in 1896 by the world renowned Swedish scientist and Nobel Prize Winner Svante Arrhenius, who specifically described how CO2 influenced the climate.

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2. Is CO2 the most abundant greenhouse gas on earth?

Interestingly, it is not. Water vapour is the most abundant greenhouse gas on earth, and CO2 comes only second. It is also true that water vapour could account for alarge portion of the total greenhouse effect.

So, why isn't anyone bothered about water vapour while talking so much about CO2? It is because water vapour is more controlled by the greenhouse effect than is controlling it. Simply put, warmer the temperature, the higher the amount of water vapour.

Thus, assuming we are able to bring down the levels of CO2 in the atmosphere and as a result the global warming, the amount of water vapour would come down too, and so would its greenhouse effect. On the other hand, if the amount of CO2 increases further and results in further warming of the atmosphere, there is a further increase in the amount of water vapour, resulting in an even higher greenhouse effect.

3. What is the current level of CO2 concentration in the atmosphere?

In Oct 2021, the concentration was about 414 PPM. In Oct 2020, it was about 411 PPM.

The following points highlight the interesting path through which the above level was reached. 

4. What were the CO2 levels in 1800? 1900? 1950? 2000? 2010? 2015?

(All values in PPM, approximated to nearest integer)

(Note: CO2 data upto 1950 are estimates based on ice-core analysis. All values post 1950 are from real time analyses at Mauna Loa Observatory, USA)

5. How much CO2 does the world emit? How much did it emit in 2000? 1950? 1900? 1800? 

CO2 emissions, in billion tons

Here's an interesting fact: More than half of all the CO2 emissions since 1750 were emitted in the last 30 years. That is, for the time period 1750-2020, over 50% of CO2 emissions were emitted in just the last 10% of the time.

And another: About half the CO2 emitted since 1850 still remains in the atmosphere. Of the other half, some of it has partially dissolved in the world’s oceans. The terrestrial biosphere (land, soil & vegetation) is also a sink for CO2 but its CO2 capture assessment is more complex as this ecosystem also continuously emits CO2 from deforestation and other land use changes, which cancel their CO2 uptake.

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6. What is the CO2 level expected to be in our atmosphere in future?

This is a trillion dollar - you could even say a multi-trillion dollar - question. This is a difficult question to answer as the answer depends on what actions the world takes - and does not take - between now and the next few decades.

CO2 levels in the atmosphere depend not just upon the emissions from natural sources and industrial activities, they also depend upon the extent to which CO2 is being absorbed from the atmosphere. While the current "absorbers" of atmospheric CO2 belong mainly to the natural environment - plants & trees, oceans, rocks etc. - artificial capture of CO2 from atmospheric is also being explored.

All the above variables are quite dynamic, especially the human-made levels of emissions as well as the extent to which man-made direct-air CO2 capture would succeed.

Tremendous amounts of research and modelling are being undertaken by various research and government organizations worldwide to estimate the future CO2 levels as well as expected temperatures. While these are fairly complex exercises, most of these modelling are anchored around action pathways by the various countries around the world. For instance, an optimistic CO2 level prediction of 425 PPM by 2100 (from 414 PPM in 2021) is based on an assumption that countries stick to the 2015 Paris pledge for a 1.5 deg C pathway in which the total global CO2 emissions will be close to zero by 2100. For such a target to be achieved, exceptionally aggressive emissions reductions need to be started right away by all large CO2 emitting countries and these efforts continued and even accelerated over the next eighty years - an extremely challenging task. Under poor target implementation conditions, some models suggest a CO2 level as high as 800 PPM by 2100 - an unacceptably high level.

Under such circumstances, while it is almost impossible to put forth a "reliable" estimate for future atmospheric CO2 levels, here are some levels that one can expect if we were to continue with only somewhat more aggressive reductions in emissions than our current efforts:

The above estimates are mainly based on the average increases in CO2 levels over the past ten years.

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7. Who measures the atmospheric CO2 levels?

While many different climate and meteorological organizations worldwide measure CO2 levels, the most prominent estimates that are used worldwide are from two independent US-based CO2 monitoring programs - the National Oceanic & Atmospheric Administration (NOAA) and Scripps Institute of Oceanography. Both of them measure CO2 levels at the Mauna Loa Observatory in Hawaii, which is located about 3400 metres above sea level.

In the US, CO2 levels are also measured by the Barrow Atmospheric Baseline Observatory, established in 1973, and located on the northernmost point of the United States. 

In Australia, Cape Grim is a key international CO2 level measurement monitoring facility, operated by the Bureau of Meteorology, and one of three key sites identified by the World Meteorological Association for long-term carbon dioxide measurements. Measurements have been made here since 1976.

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8. How is the atmospheric CO2 level measured?

The NOAA Global Observation Laboratory provides details of how the CO2 levels are being measured since 2019: “In April of 2019, a new CO2 analyzer was installed at Mauna Loa that uses a technique called Cavity Ring-Down Spectroscopy (CRDS). (Prior to this date, an analyzer was used based on infrared absorption.) CRDS is based on the measurement of the rate of absorption, rather than the magnitude of absorption, of light circulating in an optical cavity. The beam from a laser enters an optical cavity consisting of two or more highly reflective mirrors. The laser beam is reflected back and forth inside the cavity, the so-called ‘ring-down cavity’. The laser is then turned off and the light intensity inside the cavity steadily leaks out and decays to zero in an exponential fashion. A detector measures the intensity of the transmitted light as a function of time. The decay time is called the cavity ring-down time. By comparing the ring down times when the laser is at a wavelength that the CO2 molecule does not absorb, to the ring down time when the laser is at a wavelength that the CO2 molecule does absorb, the amount of CO2 can be calculated” Source: NOAA 

The observatory goes on to further inform what they actually measure: “Most people assume that we measure the “concentration” of CO2 in air, and in communicating with the general public we frequently use that word because it is familiar. The quantity we actually determine is accurately described by the chemical term “mole fraction”, defined as the number of carbon dioxide molecules in a given number of molecules of air, after removal of water vapor.”

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9. How much CO2 needs to be taken out of the atmosphere for earth to return to safe zone?  

Let's first review why just emissions reduction alone may not be enough and we may need to take CO2 out of the atmosphere.

Experts from IPCC and other organizations report that. owing to global warming, the earth is fast going towards a 1.5 deg C increase in temperature over that of pre-industrial times. This increase is not owing to our current emissions alone, it is also owing to the cumulative levels of CO2 already in the atmosphere.

As a result, even under aggressive CO2 emission reduction scenarios, we are likely to overshoot the 1.5 deg C target in a few years. While that would be bad enough, some modelling studies suggest that even a 2 degree temperature increase in a few decades is not out of question even with aggressive emissions reduction. That level of temperature increase could prove catastophic.

If emissions reductions alone may not be able to bring the temperature increase and global warming under control, what is the solution?

Taking out the CO2 currently present in the atmosphere so that we significantly bring down the cumulative levels.

The earth's atmosphere contains about 850 billion tons of CO2 currently. How much CO2 would need to be taken out of the atmosphere for us to get back to safety?    

Expert studies suggest we need to pull out 100-1000 giga tons (billion tons) of CO2 from the atmosphere by 2100 to meet targets. That's a wide range, but that appears to be the best estimate we have as of 2021.

Assuming we continue emitting about 30 billion tons of CO2 every year until 2030, and somehow drastically reduce the emissions to about 15 billion tons on average between 2030 and 2050 and even further down to an average of an average annual 10 billion tons between 2050 and 2100, without any CO2 capture from air, the total cumulative CO2 in the atmosphere by 2100 would be about 1950 billion tons. Thus, in the aggressive air capture scenarion, we are talking about taking out a little over 50% of the CO2 present in the atmosphere between now and 2100.

Taking out 1000 billion tons estimate would mean pulling out about 12 billion tons out of the atmospehere, on average, between 2020 and 2100. Some  approaches suggest that we may start at a lower level and remove about 10 gigatonnes of CO2 per year by 2050, and by the end of the century that number will need to double to 20 gigatonnes per year, in total pulling out about 1000 billion tons by end of the century.

We are talking about very large amounts of CO2 to be pulled out of the atmosphere. Given that only very few direct air cabon projects - and those too on pretty small scales - are operating as of mid 2021, it is not clear whether it would be technically and economically feasible to pull out billions of tons of CO2. Most experts thus rightly warn that we should not consider the direct air capture avenue to be some sort of a silver bullet, and should continue focussing aggressively on emissions reductions.

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10. Why does so little CO2 concentration in the atmosphere create so much trouble?  

CO2 comprises just 0.04% of our atmosphere, by volume. That's a very small proportion indeed.

Not only is CO2 present in only small proportions in the atmosphere, it also absorbs less heat per molecule than greenhouse gases such as methane or nitrous oxide. That is, per unit mass, other gases have greenhouse potentials that are over a hundred times that of CO2.

But yet, CO2 is considered to have the highest global warming potential. The reasons are as follows.

The above two reasons result in CO2 having the highest global warming potential of all the greenhouse gases excepting water vapour, viz., Methane, Nitrous oxides, CFCs and other minor greenhouse gases.

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11. How long does CO2 stay in the atmosphere?

The lifetime in the air of CO2, the most significant man-made greenhouse gas, is quite challenging to determine, because there are several processes that remove carbon dioxide from the atmosphere.

A good portion of the CO2 released into the air dissolves into the ocean over a period of 20–200 years. The rest is removed by slower processes that take up to several hundreds of thousands of years - these comprise processes such as chemical weathering and rock formation.

That is, once in the atmosphere, portions of carbon dioxide can continue to affect climate for thousands of years.

From a greenhouse gas and climate change context, we can consider the average duration of stay of CO2 emissions to be in the range 300-1000 years.

Let's look at how long the other greenhouse gases stay in the atmosphere. Methane persists for about 12 years. Thus, although methane has a much higher greenhouse potential than CO2, its effect is relatively short-lived. Nitrous oxide exists in the atmosphere for around 114 years. Compounds containing chlorine and/or fluorine (CFCs, HCFCs, HFCs, PFCs) each has its own lifetime in the atmosphere – from less than a year to many thousands of years. Water vapour is a potent greenhouse gas too, but it does not accumulate in the atmosphere, and has a very short atmospheric lifetime, of the order of hours to days, because it is rapidly removed as rain and snow.

12. Do human and animal breathing add to global warming and climate change?


Human beings do exhale a lot of CO2 - almost three billion tons of carbon dioxide annually - but the carbon we exhale is the same carbon that was “inhaled” from the atmosphere by the plants we consume, or the carbon present in the animals which in turn get their food from plants.

Thus, every atom of carbon in the exhaled carbon dioxide from humans comes from food that had been recently produced - directly or indirectly - by photosynthesis. 

13.  What is the CO2 concentration in the exhaust gases of thermal power plants?

5-15% by volume, depending on whether it is a coal power plant or natural gas power plant.

The rest of the flue gas components are N2  - about 65-75%, O2 - 3-5% and H2O - 10-20%.

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14. What is the CO2 concentration in the exhaust gases from vehicles?  

Following are the components of the internal combustion engine exhaust (% by volume, approximate estimates and could vary depending on type of IC engine - diesel engine, gasoline engine etc.)

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15. Why is CO2 a greenhouse but oxygen and nitrogen are not?  

Oxygen and nitrogen together constitute 99% of our atmosphere, and just imagine if either of them is a greenhouse gas.

Thankfully, neither oxygen nor nitrogen is a greenhouse gas. They are not greenhouse gases because they are transparent to infrared light.

Some key properties of a chemical compound make it a greenhouse gas. For instance, symmetrical molecules do not produce greenhouse effect.

On the other hand, all molecules with three or more atoms are  good absorbers of energy from the infrared spectrum of sunlight and thus potential greenhouse gases. But even for those molecules that meet the criteria of symmetry and number of atoms, the greenhouse potential will vary depending on the wavelengths of the sunlight that the molecules absorb, the strength of the absorption and the lifetime of the molecule in the atmosphere.

CO2 qualifies on most of the above criteria, while gases such as nitrogen and oxygen do not. Thus, even though these two gases are far more abundant in the atmosphere than CO2, they produce no greenhouse effect, while CO2, which is present in very small proportions, does.

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16. How much will it cost to capture CO2 directly from air?  

Estimates from commercially operating faciilties suggest it is currently about $500/ton of CO2 captured (2021 estimates).

If you are wondering why it costs so much to capture CO2, it is mainly because of the low concentration of CO2 in air (0.04% by volume). This requires special sorbents to capture the CO2.

Let's consider the process that is being used by Climeworks, a pioneer in direct air CO2 capture. It's method is called solid direct air capture. Fans suck in air that is passed over a special solid sorbent filter that traps CO2. The choice of filter could be a key element in the entire design as it needs to attract CO2 efficiently enough to separate it out of the air. When the filter is fully saturated, the process is stopped and the filter is heated to about 100 deg C, which releases the carbon dioxide.

That's a lot of work - and a lot of expense.

With scale however, experts expect the cost of such processes to reach the $100-150/ton mark.

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17. Why can't we simply plant billions of trees in forests and capture large amounts of CO2?

Forests are an important component in the decarbonization toolkit. But trees can’t absorb enough CO2 to stop climate change on their own, no matter how many we plant.

Forests have metabolisms just like us, and as they approach maturity, forests reach an equilibrium where they are carbon neutral. That is, trees are not a dedicated, permanent sink of carbon from the atmosphere. In mature forests, the uptake of CO2 by photosynthesis is balanced by the release of CO2 back to the atmosphere, through decay of wood and leaves, consumption by insects and animals, and respiration by the trees themselves.

Most experts conservatively estimate that large afforestation efforts along with dramatically reduced deforestation could capture an additional 3 billion tons of CO2, about 10% of all CO2 emissions - significant, but not enough.

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