Parts 1, 2, 3, 4 and 5 may be accessed here.
This post (Part 6) deals with a couple of complex concepts and may be more challenging.
Human activity is responsible for almost all the rise in atmospheric concentration of CO2 since 1990, and hence for the observed global warming. How we can make such a firm statement is the subject of a great scientific detective story. Essentially, from a sample of atmospheric air, we can tell what fraction of CO2 in it originated by burning fossil fuels and how much is due to plant and animal respiration. Amazing!
The slide shows the history of global carbon emissions by burning of fossil fuels. Currently we are sending 10 gigatons of carbon per year into the atmosphere and this is causing an increase in the atmospheric CO2 levels. Because we know how much fuel of each type is being burnt each year, it is possible to give a breakdown of contribution from different fossil fuel types. Coal is the most polluting fuel and the recent sharp increase in its use does not bode well for future carbon emissions.
Actual measurements of atmospheric CO2 have been carried out at various laboratories since 1958. Historic levels are derived from gases trapped in ice cores in Greenland and Antarctica. These observations suggest that 46% of the CO2 put into the atmosphere by human activity stays in the atmosphere, increasing its concentration. The rest is absorbed by the oceans and terrestrial biosphere.
The remaining CO2 will increase its atmospheric concentration by 2.1 ppm per year. (This can be calculated easily and the method is shown in the slide at the end of this blog).
In this part, I shall describe how we can actually identify the increase in atmospheric CO2 as due to the burning of fossil fuels.
Much of the following discussion is based on the work published by NOAA. I acknowledge Professor Jocelyn Turnbull's help for introducing me to NOAA publications on this subject.
Isotopes of Carbon: Carbon isotope studies can tell us where the atmospheric carbon originated from and the results point unequivocally to fossil fuels as the main source of increased CO2.
There are three isotopes of carbon that are found in nature - all isotopes of an element have the same chemical properties.
C-12 has a mass equal to 12 atomic mass units (1 amu = 1.66 x 10-27 kg).
C-13 has a mass equal to 13 amu, and
C-14 (also called radiocarbon) has a mass equal to 14 amu.
C-12 and C-13 are stable, but C-14 is radioactive and decays with a half-life of 5730 years (on decay, C-14 is converted into nitrogen; the amount of C-14 in a sample is reduced by half over 5730 years).
A sample of carbon nominally contains 98.93% C-12 and 1.07% C-13. C-14 makes up about 1 to 1.5 atoms per trillion (1012) carbon atoms. C-14 is being continually produced from N-14 in the atmosphere by cosmic rays at a more or less steady rate that determines its background concentration. The number of C-12 and C-13 atoms remains constant in time.
Different carbon reservoirs 'like' different isotopes; each reservoir contains different proportions of carbon isotopes - has its own isotopic fingerprint. By examining the isotopic mixture in the atmosphere, we can determine how much CO2 is coming from and going to each reservoir.
There are three main carbon reservoir that actively exchange CO2 (see Part 5 about Carbon Cycle).
The terrestrial reservoir consists mainly of plants, animals and soil. Most CO2 is exchanged between the atmosphere and the plants. CO2 diffuses through the stomata of the leaves and then photosynthesised by plants into carbohydrates. Both diffusion and photosynthesis prefer to uptake the lighter mass C-12 atoms in preference to the heavier C-13. Plant material is eaten by animals and also transferred to soil - therefore, carbon in the terrestrial pool is has less C-13 than in the atmosphere. It also has less C-14 incorporated due to the larger mass of C-14.
The top few hundred meters of ocean surface is actively involved in exchanging CO2 with the atmosphere. The lower depths store organic carbon and dissolved CO2 and mix their contents with the upper layers on a much longer timescales of centuries or more. The upper layer of the ocean exchange carbon di-oxide with the atmosphere. Ocean water has no preference for absorbing or outgassing C-12 over C-13. Oceans tend to have similar carbon isotope ratios as the atmosphere.
Fossil fuels are reservoirs which have held prehistoric carbon from millions of years ago. Human activity has brought fossil carbon in play and CO2 generated by burning fossil fuel is sent to the atmosphere in a one-way transaction. When plants and sea creatures got buried millions of years ago, they had some C-14 present. Dead plants do not uptake any fresh carbon. Over the long period since then, C-14 has decayed to almost zero levels (halving in amount every 5730 years) and is present in undetectable amounts in CO2 produced by burning them. Fossil fuels are derived from plants and they also have lower C-13 levels relative to C-12.
Determining Relative Carbon Isotope Values in a Sample:
One uses an isotope mass spectrometer (MS) or an accelerated mass spectrometer (AMS) for determing C-13/C-12 or C-14/C-12 concentration ratios. Spectrometers are good at telling us relative values but not so accurate for absolute values; and one makes a simultaneous measurement with a standard where the ratios are known accurately. Comparison with the standard provides absolute isotopic ratios for the sample - these are expressed as delta values - as explained in the slide below.
(The slide may be skipped without loss of continuity)
An Example: We can now show how the ∆ value of the atmosphere will change by injection of CO2 produced by burning fossil fuels. Suppose the CO2 level is 400 ppm and in a year there is net injection of 2.5 ppm CO2 from fossil fuels. This will change the ∆ value of the atmosphere to 38.5‰ from 45‰. From this example, we expect that ∆ (C-14) value will decrease by about 2.6% for every 1 ppm CO2 added to the atmosphere by burning fossil fuels.
Actual Measurements: CO2 takes about one year to fully mix in the atmosphere with other gases. In order to study the change in C-14 levels in the atmosphere, it is best to find a region where there is no local hot-spot of burning fossil fuels. In a well isolated region, away from populated areas we can monitor ∆ (C-14) values over a period of time and study any changes taking place - the changes will indicate the role of fossil fuels in the changing concentration of CO2.
The next slide shows the change in ∆ (C-14) values in an isolated region of the Rocky Mountains in Colorado:
If we make such measurements near a local industrial area then CO2 produced would not have time to mix in the atmosphere and measured ∆ (C-14) values will be lower than those obtained for an isolated region. This is shown in the next slide:
The above discussion shows that we can not only prove conclusively that majority of the added CO2 in the atmocphere is due to human activity of burning fossil fuels but we can also determine the exact amount of the added CO2 from fossil fuels.
δ(C-13) Values: We can also measure changes in δ(C-13) values that can be measured with very high precision with a mass spectrometer. Fossil fuels have a δ(C-13) of -28 ‰ while the atmosphere δ(C-13) = -8 ‰. Therefore, carbon di-oxide produced by burning fossil fuels will reduce the δ(C-13) value of atmospheric CO2. However, the terrestrial biosphere also exchanges CO2 with the atmosphere and will also decrease the δ(C-13) value. The analysis is more complex but we can separate the contribution of different sources by studying changes in δ(C-13). This discussion is outside the scope of this blog and I refer you to NOAA publication for a more detailed discussion.
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The slide shows the history of global carbon emissions by burning of fossil fuels. Currently we are sending 10 gigatons of carbon per year into the atmosphere and this is causing an increase in the atmospheric CO2 levels. Because we know how much fuel of each type is being burnt each year, it is possible to give a breakdown of contribution from different fossil fuel types. Coal is the most polluting fuel and the recent sharp increase in its use does not bode well for future carbon emissions.
The remaining CO2 will increase its atmospheric concentration by 2.1 ppm per year. (This can be calculated easily and the method is shown in the slide at the end of this blog).
In this part, I shall describe how we can actually identify the increase in atmospheric CO2 as due to the burning of fossil fuels.
Much of the following discussion is based on the work published by NOAA. I acknowledge Professor Jocelyn Turnbull's help for introducing me to NOAA publications on this subject.
Isotopes of Carbon: Carbon isotope studies can tell us where the atmospheric carbon originated from and the results point unequivocally to fossil fuels as the main source of increased CO2.
There are three isotopes of carbon that are found in nature - all isotopes of an element have the same chemical properties.
C-12 has a mass equal to 12 atomic mass units (1 amu = 1.66 x 10-27 kg).
C-13 has a mass equal to 13 amu, and
C-14 (also called radiocarbon) has a mass equal to 14 amu.
C-12 and C-13 are stable, but C-14 is radioactive and decays with a half-life of 5730 years (on decay, C-14 is converted into nitrogen; the amount of C-14 in a sample is reduced by half over 5730 years).
A sample of carbon nominally contains 98.93% C-12 and 1.07% C-13. C-14 makes up about 1 to 1.5 atoms per trillion (1012) carbon atoms. C-14 is being continually produced from N-14 in the atmosphere by cosmic rays at a more or less steady rate that determines its background concentration. The number of C-12 and C-13 atoms remains constant in time.
Different carbon reservoirs 'like' different isotopes; each reservoir contains different proportions of carbon isotopes - has its own isotopic fingerprint. By examining the isotopic mixture in the atmosphere, we can determine how much CO2 is coming from and going to each reservoir.
There are three main carbon reservoir that actively exchange CO2 (see Part 5 about Carbon Cycle).
The terrestrial reservoir consists mainly of plants, animals and soil. Most CO2 is exchanged between the atmosphere and the plants. CO2 diffuses through the stomata of the leaves and then photosynthesised by plants into carbohydrates. Both diffusion and photosynthesis prefer to uptake the lighter mass C-12 atoms in preference to the heavier C-13. Plant material is eaten by animals and also transferred to soil - therefore, carbon in the terrestrial pool is has less C-13 than in the atmosphere. It also has less C-14 incorporated due to the larger mass of C-14.
The top few hundred meters of ocean surface is actively involved in exchanging CO2 with the atmosphere. The lower depths store organic carbon and dissolved CO2 and mix their contents with the upper layers on a much longer timescales of centuries or more. The upper layer of the ocean exchange carbon di-oxide with the atmosphere. Ocean water has no preference for absorbing or outgassing C-12 over C-13. Oceans tend to have similar carbon isotope ratios as the atmosphere.
Fossil fuels are reservoirs which have held prehistoric carbon from millions of years ago. Human activity has brought fossil carbon in play and CO2 generated by burning fossil fuel is sent to the atmosphere in a one-way transaction. When plants and sea creatures got buried millions of years ago, they had some C-14 present. Dead plants do not uptake any fresh carbon. Over the long period since then, C-14 has decayed to almost zero levels (halving in amount every 5730 years) and is present in undetectable amounts in CO2 produced by burning them. Fossil fuels are derived from plants and they also have lower C-13 levels relative to C-12.
Determining Relative Carbon Isotope Values in a Sample:
One uses an isotope mass spectrometer (MS) or an accelerated mass spectrometer (AMS) for determing C-13/C-12 or C-14/C-12 concentration ratios. Spectrometers are good at telling us relative values but not so accurate for absolute values; and one makes a simultaneous measurement with a standard where the ratios are known accurately. Comparison with the standard provides absolute isotopic ratios for the sample - these are expressed as delta values - as explained in the slide below.
(The slide may be skipped without loss of continuity)
An Example: We can now show how the ∆ value of the atmosphere will change by injection of CO2 produced by burning fossil fuels. Suppose the CO2 level is 400 ppm and in a year there is net injection of 2.5 ppm CO2 from fossil fuels. This will change the ∆ value of the atmosphere to 38.5‰ from 45‰. From this example, we expect that ∆ (C-14) value will decrease by about 2.6% for every 1 ppm CO2 added to the atmosphere by burning fossil fuels.
Actual Measurements: CO2 takes about one year to fully mix in the atmosphere with other gases. In order to study the change in C-14 levels in the atmosphere, it is best to find a region where there is no local hot-spot of burning fossil fuels. In a well isolated region, away from populated areas we can monitor ∆ (C-14) values over a period of time and study any changes taking place - the changes will indicate the role of fossil fuels in the changing concentration of CO2.
The next slide shows the change in ∆ (C-14) values in an isolated region of the Rocky Mountains in Colorado:
The above discussion shows that we can not only prove conclusively that majority of the added CO2 in the atmocphere is due to human activity of burning fossil fuels but we can also determine the exact amount of the added CO2 from fossil fuels.
δ(C-13) Values: We can also measure changes in δ(C-13) values that can be measured with very high precision with a mass spectrometer. Fossil fuels have a δ(C-13) of -28 ‰ while the atmosphere δ(C-13) = -8 ‰. Therefore, carbon di-oxide produced by burning fossil fuels will reduce the δ(C-13) value of atmospheric CO2. However, the terrestrial biosphere also exchanges CO2 with the atmosphere and will also decrease the δ(C-13) value. The analysis is more complex but we can separate the contribution of different sources by studying changes in δ(C-13). This discussion is outside the scope of this blog and I refer you to NOAA publication for a more detailed discussion.
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