How Do We Know We're Responsible for all the Postindustrial Increase in CO2 Concentrations?
Sometimes I still hear objections to AGW that claim that humans could not be causing the atmosphere to warm because our emissions constitute only a tiny fraction of the CO2 that's in the atmosphere. There are several forms of this claim, but most have to do with the fact that human CO2 emissions are a small fraction of total emissions every year, which of course is true. What they don't tell you is that natural sinks remove what natural sources add each year, and a little more, making the natural carbon cycle a net sink. So while human emissions are small compared to natural, they are responsible for flipping the carbon cycle from being a net sink to a net source of CO2. Human activity is responsible for virtually all the increase in CO2 above preindustrial levels.
The graph below shows various components of the carbon cycle with averages for 2011-2020. Here the natural carbon cycle removes about 5.9 GtC annually, while human emissions from fossil fuels and land use change contributed 10.6 GtC annually. Even though natural sources dwarf anthropogenic sources, natural sinks remove what they add, so virtually all the increase in CO2 is from human sources.
I don’t know of any expert who disputes that the rise in CO2 concentration over the past 150 years is almost entirely due to human activities, since there are five independent lines of evidence supporting that conclusion.Notice he says both that he agrees that human activities are responsible for all the increase in CO2, but he says he knows of no expert who disputes it. This is a point of climate science that even Koonin would agree is settled. And he gives 5 lines of reasoning for this. I've used the full quote in another post, and to preserve the length of this post, I'll not duplicate the whole quote here. But what I offer below is 5 lines of evidence, largely patterned after and modified from Koonin (but I think improved) with more details explaining the force of these lines of evidence.
1. The Global Carbon Budget
The 2021 Global Carbon Budget[2] keeps track of all natural and anthropogenic sinks and sources, and while there are uncertainties in each of these estimates, the entire budget from 1750 balances to 10 GtC. For a sense of how small that number is, the cumulative total of all human carbon emissions is 700 GtC, so about 1.4% of all human emissions. We can construct a time series of human carbon emissions (from fossil fuels, industry, and land use) and how they have grown since 1750, and we can also calculate the expected increase in CO2 from those carbon emissions. If we compare that time series, to the increase in CO2 concentrations, they match incredibly well. CO2 concentrations increase in lockstep agreement with human carbon emissions.
The graph below shows cumulative carbon emissions from the Global Carbon Budget through 2021 with GtC emissions converted to GtCO2 by multiplying by 3.67 GtCO2/GtC. Since about 44% of our emissions remains in the atmosphere, I multiplied cumulative GtCO2 by 0.44 for each year. This provides the amount of cumulative GtCO2 in the atmosphere from human activity. I then converted that to ppm by dividing by 7.81 ppm/CtCO2. These values are added to the initial concentration of CO2 in 1750 to see how well they track with the increase in CO2 concentrations.
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| Comparison of CO2 Emissions and Concentrations |
This evidence, with the knowledge we have of the carbon cycler, is conclusive. However, there are at least 4 other lines of evidence that confirm what we know to be true from our accounting of emissions from human activity.
2. Paleoclimate Proxies for CO2
Ice core samples taken in Antarctica can be examined for CO2 content, since air bubbles preserved in these ice cores preserve for us information about the atmospheric composition of past climates. In Antarctica, CO2 concentrations ranged from about 170 ppm to about 300 ppm over the last 800,000 years.[3] The low concentrations of CO2 correlate with glacial periods and the high concentrations of CO2 correlate with interglacial periods. Preindustrial levels were at about 280 ppm, but we didn't exceed 300 ppm (the highest CO2 concentrations in the Quaternary) until about 1910. Now, 112 years later, CO2 levels have risen by another 116 ppm CO2. This short amount of time for such a large increase is unheard of over the entire Quaternary, and there is only one thing that has changed significantly that could be responsible for this increase - human emissions of CO2.[4]
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| Variability in CO2 in Antarctica |
Some may object that this ignores uncertainties in the measurements. After all, the deeper the ice core, the more compressed the ice becomes, and the more uncertainty there is in the date of any particular CO2 measurement. And of course, this is true. The difference between the "ice age" and the "proxy age" for any reading could be as much as 150 years, so we can't be extremely precise about the timing of these cycles. However, even with a resolution of 150 years we can still see that CO2 levels do not vary by anywhere near 100 ppm in any 150 year increment. Even the most rapid increases in CO2 the earth emerges from a glacial period do not compare to the rates we're currently experiencing. And natural process have not pushed CO2 above 300 ppm during the entire Quaternary. The last time CO2 was 400 ppm was about 3 million years ago.
3. The Northern vs Southern Hemispheres
CO2 concentrations have consistently been higher in the Northern Hemisphere where most human emissions have occurred. They are lower in the Southern Hemisphere because there is less land area and fewer people emitting lower concentrations of greenhouse gases. In fact, the NH leads concentrations in the SH by about 2 years, since more anthropogenic CO2 emissions come from the NH, and it takes time for CO2 to circulate throughout the atmosphere.[5]
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| Emissions by Latitude |
During the glacial cycles of the Quaternary, warming was initiated by orbital cycles, and CO2 increases were triggered by the warming of the oceans. CO2 is less soluble in warmer water, so as the oceans warmed, they degas CO2. CO2 increased by about 100 ppm as global temperatures increased by about 5 or 6 C. If a similar mechanism were causing the current increase in CO2 levels, we'd expect two things. First, the oceans would be a net source of CO2, but they aren't. They are a net sink, absorbing about 25% of our emissions.[8] Second, since there is less land and more ocean in the SH, concentrations in SH should be higher than the NH with the SH leading the NH in concentrations. We see the exact opposite.
These are five reasons why climate scientists generally regard this issue as "settled," even among those who are quick to point to aspects of climate science that they do not believe to be settled. The evidence is overwhelming and there simply is no evidence to the contrary that undermines the conclusions from the above lines of evidence.
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However, this decrease in emissions was not nearly enough to halt or even significantly slow the increase in CO2 concentrations. In 2020, we had 9.5 GtC emissions from fossil fuels and 0.88 GtC from land use change. That's a total of 10.38 GtC. After converting to GtCO2, accounting for 44% staying in the atmosphere, and converting to ppm, that means we'd expect a 2.15 ppm increase in CO2 from 2020 emissions alone. The actual increase in CO2 concentrations was 2.58 ppm.[4] The 0.43 ppm difference is small considering the impact from natural variability from ENSO and other natural fluxes of CO2. These natural fluxes introduce variability into the annual increase in CO2 concentrations, but since they are basically noise in the signal, they cancel each other out over time. Almost certainly CO2 levels did not increase by as much as they would have without the 5% drop in emissions, but the drop in emissions was simply not large enough to cause a detectable change in the amount of expected increase in CO2 concentrations.
4. Decreasing Oxygen Levels
The natural carbon cycle and living processes recycle CO2 in the atmosphere, so they affect oxygen levels in the atmosphere very little. We breathe in more O2 and exhale more CO2, but the carbon that is added to the oxygen we breathe in comes directly from the food we eat, and that carbon originated in the atmosphere. Plants photosynthesize CO2 and water to form sugars, but they also respire, returning CO2 to the atmosphere, mostly at night. The difference between photosynthesis and respiration is contained in the biomass of plants, and when the plant dies it decomposes and returns the carbon to the atmosphere where it becomes CO2 again.
However, when plants die and are buried, they can sometimes become fossilized before decomposing, and the processes that allow this to happen sequester carbon from the atmosphere. Because this carbon is not returned to the atmosphere, it doesn't combine with oxygen, and O2 levels in the atmosphere can increase. All our reservoirs of fossil fuels (coal, oil, gas) contain sequestered carbon, mostly from the Devonian to the Carboniferous. When it gets burned, it combines with oxygen in the atmosphere, converting some O2 molecules in the atmosphere to CO2. So if CO2 levels are increasing due to the human emissions of fossil fuels, you'd expect O2 levels to decrease slightly, and this is exactly what we observe. Because O2 concentrations are so much greater than CO2 levels, these emissions do not decrease atmospheric CO2 levels that much; there's no danger of us asphyxiating ourselves through our carbon emissions. But it is confirming evidence that our carbon emissions are the cause of the increase in CO2 concentrations.[6]
5. Carbon Isotopes
The ratio of carbon isotopes in the atmosphere show the increase is due to fossil fuels. The sources of Carbon emissions can be identified by the carbons isotopes - 12C and 13C are stable, and 14C is not. Living organisms prefer 12C to 13C while volcanic carbon prefers 13C to 12C; the difference is slight but measurable. And because 14C is not stable, older sources of carbon, like fossil fuels and volcanic eruptions, will be 14C-depleted, but plant biomass is not. From this we can say several things that we can compare to observed changes in atmospheric composition.
- Volcanic eruptions introduce a “heavier” 13C/12C ratio and no 14C.
- The burning/decomposition of plants introduce a “lighter” 13C/12C but has 14C.
- The burning of fossil fuels, since they come from plants will share the "lighter" 13C/12C ratio, but because they are hundreds of millions of years old, have no 14C.
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| 13C/12C Ratios are Becoming Lighter Due to CO2 Emissions |
Now admittedly, the 14C situation is complicated by nuclear testing beginning in 1950, which radically altered the amount of 14C in the atmosphere, but careful simulations of 14C amounts with no nuclear testing and no fossil fuel emissions show that observed changes in 14C are consistent with what we would expect from the increase in fossil fuel emissions.
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| Trends in 14C since 1940 |
These are five reasons why climate scientists generally regard this issue as "settled," even among those who are quick to point to aspects of climate science that they do not believe to be settled. The evidence is overwhelming and there simply is no evidence to the contrary that undermines the conclusions from the above lines of evidence.
But What About COVID?
The most significant objection I currently hear that I haven't already addressed to some extent above has to do with the fact that our emissions decreased due to the global coronavirus pandemic while CO2 concentrations increased. And it's true that there was about a 5.4% decrease in carbon emissions during 2020.
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[1] Koonin, Steven E.; Koonin, Steven E.. Unsettled. BenBella Books, Inc.. Kindle Edition. p. 65.
[2] 2021 Global Carbon Budget. https://www.icos-cp.eu/science-and-impact/global-carbon-budget/2021
[3] The Keeling Curve. https://keelingcurve.ucsd.edu/
[4] Rebecca Lindsay. Climate Change: Atmospheric Carbon Dioxide. https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide
[5] Akira Tomizuka. Why Is Atmospheric Carbon Dioxide Concentration Higher in the Northern Hemisphere. Environmental Science 26.4 (2013): 374-387. www.jstage.jst.go.jp/article/sesj/26/4/26_374/_pdf
[6] Iris Crawford and Andrew Babbin. How will future warming and CO2 emissions affect oxygen concentrations? Ask MIT Climate. https://climate.mit.edu/ask-mit/how-will-future-warming-and-co2-emissions-affect-oxygen-concentrations
[7] Graven, H., Keeling, R. F., & Rogelj, J. (2020). Changes to carbon isotopes in atmospheric CO2 over the industrial era and into the future. Global Biogeochemical Cycles, 34, e2019GB006170. https://doi.org/10.1029/2019GB006170
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