How Much CO2 have Human Activities Added to the Atmosphere Since the Industrial Revolution?

Little Big Econ State Forest

One objection I frequently hear to climate science is that the human contribution to the increase in CO2 concentrations is small, so reducing our carbon emissions will have little to no effect on global temperatures. This claim is completely misguided, and calculations using estimates of human carbon emissions can settle this pretty quickly. The following calculations are through 2018, with CO2 levels at 410 ppm, about 130 ppm higher than pre-industrial 280 pm. We can provide a rough estimate of how much of the 410 ppm is human by looking at empirical data regarding anthropogenic CO2 emissions  and a couple conversion factors:[1]

3.67GtC = 1 GtCO2 

1 ppm CO2 = 7.81 GtCO2.

According to the global carbon budget through 2018, humans have emitted 441 GtC through fossil fuel use and 203 GtC through land use, for a total of 604 GtC anthropogenic emissions.[2] That translates to 2217 Gt CO2 emissions. About 50% of that amount stays in the atmosphere. So:

2217 GtCO2 * 0.50 = 1108 GtCO2.

We can then convert that number to ppm:

1108 GtCO2 * 1 ppm CO2 / 7.81 GtCO2 = or 142 ppm in the atmosphere as a result of human activity.

The increase from 180 to 410 ppm is 130 ppm, so the anthropogenic contribution exceeds the actual increase in concentrations, which is consistent with what we know about  the carbon cycle. The carbon cycle is nearly balanced but currently is roughly a net sink. Human activity has tipped the scales of the carbon cycle to a net source. In other  words, we would be losing a little bit of CO2 each year, but because of human activity it's accumulating in the atmosphere.

This can be confirmed by comparing this with stats of actual CO2 concentrations on NOAA’s website. I totaled the amount of emissions from human sources from the global carbon budget from 2000 to 2018 and calculated a 192 GtC increase in CO2 from human emissions. At 50% remaining in the atmosphere,  approximate 45 ppm contribution in CO2. If you look at NOAA data for CO2 concentrations during that time period,[3] you can see that CO2 rose about 39 ppm (concentrations rose from 369.71 ppm to 408.72 ppm). In other words, virtually 100% of the rise in CO2 between 2000 and 2018 can be attributed to human activity. Furthermore, this is not a point of contention among scientists and other experts. This is not an issue about which climate scientists debate. It's acknowledged on pretty much all sides that human activity is responsible for all the increase in CO2 since the industrial revolution. For instance, Steven Koonin, who criticizes climate scientists for claiming too much of the science is "settled," seems to have no problems with saying this particular point has been settled.

"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. One is the timing of the rise—concentrations in air samples over the past ten thousand years varied between 260 and 280 ppm before a sharp uptick began in the mid-nineteenth century. The second is that the size of the rise is in the ballpark of what we’d expect from the CO2 emitted by burning fossil fuels. A third is that the rise in the Northern Hemisphere leads that in the South by about two years—most fossil fuels are burned in the north, which has more land and people—and that lead is increasing as emissions grow.

A fourth, more subtle confirmation comes from carbon isotopes—the relatively rare carbon atoms that are about 8 percent heavier than ordinary carbon atoms. About 1.1 percent of the earth’s carbon is the isotope 13C; the rest is the lighter isotope 12C. But the proportion of 12C and 13C isn’t the same in all forms of carbon. In particular, the chemical reactions of life have a very slight preference for 12C, so that the carbon in living things (as opposed to mineral carbon in the earth’s crust) is “light”; that is, it has a slightly lesser proportion of 13C. Since the carbon in the atmosphere’s CO2 has become progressively “lighter” over the decades, we can infer that it arises from the burning of fossil fuels, which, after all, were once living things. Finally, measurements over the past three decades have shown a tiny but detectable and steady decrease in the atmosphere’s oxygen concentration. The decrease is too small to raise any concern at all about our ability to breathe, but it roughly matches what’s been needed to turn the fossil carbon into CO2."[4]

There is much more we could unpack here, and I may do a post on carbon isotope ratios in the future, but I think it's fair to say that the science is "settled" that humans are responsible for virtually all the increase in CO2 since the Industrial Revolution.

References:

[1] https://www.co2.earth/global-co2-emissions

[2] https://www.icos-cp.eu/global-carbon-budget-2019

[3] https://gml.noaa.gov/webdata/ccgg/trends/co2/co2_annmean_mlo.txt

[4] Koonin, Steven E.; Koonin, Steven E.. Unsettled (pp. 65-66). BenBella Books, Inc.. Kindle Edition.