How do we know that the recent increase in CO2 is Human Caused

INTRODUCTION

It has been clear for a long time that humanity has been actively burning fossil fuels (coal, natural gas, and oil) and clearing land. These processes have been occurring for the last two centuries. These activities add carbon dioxide (CO₂) to the atmosphere. Nineteenth-century science showed that increasing CO₂ would warm the climate. See this link for details of the Nineteenth-century science.

MEASURING ATMOSPHERIC CARBON DIOXIDE LEVELS

It was not until 1958 that scientists started direct measurement of CO₂ in the atmosphere when Charles David Keeling, started measuring atmospheric CO₂ levels at Mauna Loa Observatory in Hawaii, and a number of other places including Antartica. It became obvious within a few years that CO₂ was increasing in the atmosphere. The graph that he produced has become known as the Keeling Curve. The latest version of the Keeling Curve (as if the time of writing this post) is displayed below. Note the measurements are in parts per million (ppm).

Figure 1: The Keeling Curve
Source: https://scripps.ucsd.edu/programs/keelingcurve/wp-content/plugins/sio-bluemoon/graphs/mlo_full_record.png

The Keeling Curve only extends back to 1958, but it is possible to determine the CO₂ concentration in earlier times from ice cores. For information about ice cores click on this link. Ice cores contain bubbles of air from the time that the ice was formed. To check that the bubbles of air in ice cores faithfully reflected the air at the time of ice formation, scientists compared ice core bubble measurements with atmospheric measurements during the period of overlap of the two air sources. The graphic below shows the results of that analysis - the air in ice bubbles is very similar to that of air at the time of the ice formation.

Figure 2: Validating Ice Core CO₂ Measurements
Source: Source: https://www.climatescience.cam.ac.uk/docs/icecoreco2.pdf

Sections of ice cores can be dated by the following methods:

• Counting the annual layers based on visual appearance (summer layers are often darker due to more dust content or ice that has melted and then refrozen)

• Measuring the concentration of particular radioactive isotopes with a known half life such as tritium,

• Chemically matching volcanic layers with known eruptions that have been previously dated.

Using these methods for dating ice cores and analysing the air bubbles trapped in the ice it is possible to determine the CO₂ levels in the past. The graphic below shows the CO₂ level for the last 10,000 years. The values up to 1958 were determined from ice cores, post 1958 from direct atmospheric measurements.

Figure 3: CO₂ Levels Over the Last 10,000 Years
Source: https://scripps.ucsd.edu/programs/keelingcurve/wp-content/plugins/sio-bluemoon/graphs/co2_10k.png

Note that for all of this period with the exception of the last 200 years the CO₂ level was steady, only varying between 260 ppm to 280 ppm, a variation of only 20 ppm. In the last two centuries, though, it has increased from 280 ppm to 408 ppm, ie by 128 ppm. The last 200 years are clearly very different to the rest of the record.

The amazing anomaly of the last two centuries is even more obvious when the last 800,000 years are considered, as shown in the graphic below.

Figure 4: CO₂ Levels Over the Last 800,000 Years
Source: Source: https://scripps.ucsd.edu/programs/keelingcurve/wp-content/plugins/sio-bluemoon/graphs/co2_800k.png

Over the last 800,000 years the CO₂ level has varied between 150 ppm and 300 ppm. Our current level of CO₂ is over 100 ppm higher that any other level in the past 800,000 years! The level of CO₂ in the atmosphere is continuing to rise.

MEASURING HUMAN CAUSED CARBON DIOXIDE EMISSIONS - SOURCES AND SINKS

That the dramatic increase in CO₂ coincides with rapid industrialization and land clearing strongly suggests a human origin of the extra CO₂, but serious study of historical human carbon emissions did not begin until the early 1980s with the work of Brian Mitchell. For many references on the process of compiling data sets on human carbon emissions see this link and this one.

Figure 5: Human Carbon Emissions
Source: Source: http://cdiac.ess-dive.lbl.gov/trends/emis/glo_2010.html

Human Carbon Emissions shown in Figure 5 are very similar to The Keeling Curve since 1700 as shown in Figure 6, providing strong evidence that the CO₂ increase in the last few centuries is strongly influenced by human CO₂ emissions.

Figure 6: The Keeling Curve Since 1700
Source: https://scripps.ucsd.edu/programs/keelingcurve/wp-content/plugins/sio-bluemoon/graphs/co2_800k_zoom.png

If you carefully compare Figure 5 and Figure 6, it looks like human CO₂ emissions are increasing more quickly than the increase in the amount of CO₂ in the atmosphere. It is possible that this is a false impression, with the graphs using different scaling and units. Figure 7 shows human CO₂ emissions and and CO₂ levels in the atmosphere on the same graph using the same scaling and units. Clearly human emissions of CO₂ are increasing at a faster rate than CO₂ accumulation in the atmosphere.

Figure 7: CO₂ Emissions Vs CO₂ Levels
Source: Source: https://radioviceonline.com/wp-content/uploads/2009/11/knorr2009_co2_sequestration.pdf Figure 1. (The only difference between this graphic and Figure 1 from the Knorr paper is the addition of the labels to make it clearer. In the Knorr paper the various lines were described in the description of the figure.)

The atmosphere only takes up about 46% of human carbon emissions, as shown in Figure 7. This is clear evidence that all of the increase in CO₂ in the atmosphere in the last two centuries comes from human sources. So where does the rest of the CO₂ emitted by humans go to. There are only two possible locations, the oceans and the biosphere. As figure 8 shows this is clearly the case.

Figure 8: Anthropogenic (human caused) Sources and Sinks in 2005
Source: http://www.ldeo.columbia.edu/~spk/Research/AnthropogenicCarbon/anthroco2.html

Here is the authors' commentary on Figure 8:

The cartoon ... attempts to summarize our current knowledge of the sources and sinks of anthropogenic CO2. There are two principal sources. The largest is the burning of fossil fuels which emits on the order of 8 PgC/y. (A petagram (Pg) is 1 billion metric tons.) The second largest, at around 1.5 PgC/y, is changes in land use, primarily deforestation in the tropics to make way for agriculture. Estimates for this source are relatively very uncertain however. Together, these two sources contributed somewhere between 370 to 610 PgC between the start of the industrial period (nominally taken as 1765) and 2005. So what happened to this CO2? Less than 50% of it, or 215 PgC, currently resides in the atmosphere so the balance must have been taken up by the ocean or the terrestrial biosphere. It is believed that the ocean sequesters 20 to 35% of manmade CO2 emissions and thus plays a critical role in mitigating the effects of this perturbation to the climate system. However, considerable uncertainties remain as to the distribution of anthropogenic CO2 in the ocean, its rate of uptake over the industrial era, and the relative roles of the ocean and terrestrial biosphere in taking up manmade CO2.

Irrespective of the uncertainties in the exact values it is clear that all of the extra CO₂ in the atmosphere over the past 200 years has come from human sources, as more than half of the human CO₂ have been sequestered in the ocean or biosphere.

CARBON ISOTOPES

Atoms consist of a positively charged nucleus surrounded by a cloud to negatively charged electrons. The nucleus consists of positively charged protons and neutral neutrons. The number of protons determines the element - for instance hydrogen has one proton, carbon has 6 and oxygen has 8. A neutral atom has the same number of electrons as protons. The number of neutrons in an atomic nucleus can vary - producing different isotopes of the element. For instance carbon can have 6 neutrons, 7 neutrons or 8 neutrons making Carbon-12, Carbon-13 or Carbon-14 respectively. See diagram below:

Figure 9. Isotopes of Carbon

In the graphic above the average concentraton of Carbon-13 is 1.1%, but this can vary in different substances. The amount of Carbon-13 in a particular sample is measured as the ratio between Carbon-13 and Carbon-12, which is called delta 13C, and is written as δ¹³C. It is measured in parts per thousand which has the symbol ‰. Values of δ¹³C are determined in relation to a standard called Pee Dee Belemnite (PDB), sometimes called VPDB (for "Vienna PDB"). For details click on this link. The standard material has a high concentration of Carbon-13 and was established with a δ¹³C value of zero; consequently most substances have a lower (ie negative) value. The more negative the value the less Carbon-13 in the substance. A substance with a δ¹³C‰ value of -28 will have less Carbon-13 than a substance with a value of -10. The table below shows delta values for Carbon-13 and Carbon-14 for some important CO₂ pools.

Figure 10. Delta values for Carbon-13 and Carbon-14
Source: Source: https://www.esrl.noaa.gov/gmd/outreach/isotopes/mixing.html

It is clear from Figure 10, that Fossil Fuels are significantly more depleted in Carbon-13 than the atmosphere (ie -28 compared to -8). If a large amount of CO₂ from Fossil Fuel burning is added to the atmosphere then the δ¹³C‰ should decline. That this has happened is clear from the graphic below:

Figure 11. CO₂ Concentration Vs Carbon-13 Concentration
Source: Source: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/jgrd.50668

For the 800 years between 1000 AD and 1800 AD The values of atmospheric CO₂ and δ¹³C‰ were steady, but when at about 1800 CO₂ levels began to increase δ¹³C‰ values began to decline. This can only be explained if the increase of CO₂ was from Fossil Fuel burning or the clearing of forests - ie from Human Activities.

As Fossil Fuels are also highly depleted in the radioactive isotope of Carbon, Carbon-14, that isotope should decline in the atmosphere as CO₂ is increased, if the increase in CO₂ is caused by fossil fuel burning. This is indeed the case. See this link, particularly Figure 1C. Delta Carbon-14 decreased until the atmospheric atomic bomb tests in the 1950s. The additional Carbon-14 put into the atmosphere by the bomb tests has been used as a tracer to analyse Carbon Cycle processes.

OXYGEN DECLINE

As carbon is burned it combines with Oxygen to produce Carbon dioxide - CO₂. This process should remove some oxygen from the atmosphere. This is what is found to have happened.

In the brief video below Jeff Severinghaus, a professor of geosciences from Scripps Institution of Oceanography, University of California, discusses the graphic in Figure 12.

Video 1: Oxygen decline as a result of carbon burning

That the biosphere is growing slightly should not be surprising as it is a net carbon sink - see Figures 7 and 8.

Figure 12. Oxygen decline.
Sources: http://www.ipcc.ch/ipccreports/tar/wg1/108.htm https://www.skepticalscience.com/anthrocarbon-brief.html

CHANGES IN OCEANIC CHEMISTRY

Figures 7,8,10 and 11, provide evidence that the oceans are a net carbon sink, ie that CO₂ is being added to the oceans. Another piece of evidence for this comes from changes in ocean chemistry, particularly changes in the Acidity / Alkalinity of the oceans measured with a value called pH.

The video below explains the chemistry, as well as changes that have been measured and implications for life in the ocean.

Video 2: Ocean chemistry

This process is often called Ocean Acidification. Oceans are currently Alkaline (basic) but as video 2 explains, increases in CO₂ will reduce the pH of the ocean moving it in a more acidic direction.

It is clear that ocean pH is declining. Here is a graph showing data from three oceanic areas - Bermuda (Caribbean), Canary Islands (Tropical Atlantic) and Hawaii (Pacific) showing increasing CO₂ in the oceans and decreasing pH.

Figure 13. Oceanic CO₂ and pH levels at three sites.
Source: https://www.epa.gov/climate-indicators/climate-change-indicators-ocean-acidity

Note that there is significant seasonal variability in the data but the long term trend is unmistakable.

Similar changes in pH have been measured in other oceans including the North Sea , Arctic, NE Atlantic and North Atlantic

The pH scale is logarithmic so the decline is significantly larger than appears in the graphs. For the pre-industrial value of oceanic pH see this link, section 2.6 and Table 1. Pre-industrial pH level 8.18 current level 8.07, a decline of 0.11 pH units. This does not sound a large difference but remember that pH is a logarithmic scale.

Figure 14. The pH scale.
Source: https://pmel.noaa.gov/co2/story/A+primer+on+pH

The scale measures the number of H⁺ ions in a litre of water, (measured in moles). A change of 1 in the scale changes the number of H⁺ ions by 10 - for instance, changing from 9 to 8 will increase the number of H⁺ ions by 10. The way to calculate a percentage change is by using the actual values of H⁺ ions. Figure 14 and the associated source explains this.

The decline in pH since pre-industrial times is about 30%.

VOLCANOES

Some people make the claim that the increase in CO₂ comes from Volcanoes. For instance, Ian Plimer in his book "Heaven and Earth" on page 413 claims: "Volcanoes produce more CO2 than the world's cars and industries combined."

In an article found at this link Plimer makes the following claim:

"Over the past 250 years, humans have added just one part of CO2 in 10,000 to the atmosphere. One volcanic cough can do this in a day."

So how do human carbon emissions compare with volcanic carbon emissions?

Here is a link to an article by Terry Gerlach estimating volcanic carbon emissions. Gerlach,s conclusion is quoted below:

Global estimates of the annual present- day CO2 output of the Earth’s degassing subaerial and submarine volcanoes range from 0.13 to 0.44 billion metric tons (gigatons) per year [Gerlach, 1991; Allard, 1992; Varekamp etal., 1992; Sano and Wil-liams, 1996; Marty and Tolstikhin, 1998]...

Burton, Sawyer and Granieri at this link in a more recent paper than Gerlach, give a figure of 540 Mt/yr ie 0.54 billion tons. If carbon emitted from the Earth from non-volcanic sources are included the figure rises to 840 Mt/yr ie 0.84 billion tons. It is possible that estimates of volcanic will rise with more research, but it is very unlikely that volcanic emissions will rival human ones. Immediately after stating the 840 mt.yr figure Burton et note: "The global subaerial CO2 flux we report is higher than previous estimates, but remains insignificant relative to anthropogenic emissions, which are two orders of magnitude greater at 35,000 Mt/yr (Friedlingstein et al. 2010)."

Gerlach also notes that there are other reasons for rejecting the claim that volcanic CO₂ emissions rival human ones. He discusses these in a section called "Problematic Implications" where he concludes:

In short, the belief that volcanic CO2 exceeds anthropogenic CO2 implies either unbelievable volumes of magma production or unbelievable concentrations of magmatic CO2. These dilemmas and their related problematic implications corroborate the observational evidence that volcanoes emit far less CO2 than human activities.

Another reason why volcanic emissions cannot rival human ones relates to the ratio of carbon isotopes. Figure 11 shows that the δ¹³C‰ has declined in line with CO₂ in the last two hundred years. The best explanation for this is the addition of large amounts of low Carbon-13 CO₂ source - ie Fossil Fuels.

Figure 15. Volcanic CO₂ emissions and volcanic δ¹³C‰ values
Source: Source: https://www.repository.cam.ac.uk/bitstream/handle/1810/266762/Mason_final_combined.pdf?sequence=1&isAllowed=y Table S3

Note in Figure 15 volcanic gasses have a range of -0.7 to -7.0 δ¹³C‰. This means that volcanic CO₂ has more Carbon-13 than the atmosphere which has a value -8 δ¹³C‰ (see Figure 10). This should mean that volcanic outgassing would increase the value of δ¹³C‰ rather than reducing it as is observed in Figure 11.

SUMMARY CONCLUSION

• The amount of atmospheric CO₂ has dramatically increased in the last 200 years. This is very unusual (probably unique) in the last 800,000 years. See Figures 1, 3 and 4.
• The increase in CO₂ in the last 200 years coincided with a dramatic increase in human caused CO₂. See figure 5.
• Only 46% of this human caused CO₂ went into the atmosphere. See Figure 7.
• The rest went into the oceans and biosphere. See Figure 8.
• The increase in CO₂ in the oceans has changed ocean chemistry, reducing oceanic pH. See Figures 13 and 14 and Video 2
• The increase in CO₂ coincided with a significant change in carbon isotopes that can only be explained by human caused CO₂ increase. See Figure 11.
• As fossil fuels are burned the Oxygen in the atmosphere should decline. This decline has been measured. See Figure 12 and Video 1.
• Other sources of the increased CO₂ have been ruled out. The ocean is clearly a sink not a source of CO₂. Measurements of volcanic emissions and volcanic gas carbon isotope ratios preclude volcanoes as a source of the increased CO₂