About the First Assessment Report’s CO2 projections

A previous article looked at the differences between the forcing projections of the IPCC’s First Assessment Report and reality. This article will do the same, but instead of forcing I’ll deal with the airborne fraction of CO2. The share of CO2 emissions that remains in the atmosphere is one of the key determinants of climate change.

This article was published on March 19th, 2020. So far it hasn’t been updated other than to fix typos.

For data on real-world CO2 emissions I turn to the Global Carbon Project. For some reason, the Global Carbon Project makes its estimates available through this other website; the specific file I’m discussing is this one, which has data up to 2018. Cumulative emissions from 1991 on are 261.43 gigatons of carbon (GtC), whereas growth of atmospheric CO2 was equivalent to 114.62GtC. Divide one number by the other and you conclude that the airborne fraction for the period was 43.8%. The real-world airborne fraction of the future is of course unknown, but one can still deduce something interesting from the data available.

For the IPCC, emissions are given in Figure A.2(a). I’m going to focus on Scenario A, also called Business-as-usual. Since the increase in emissions under this scenario is almost exactly linear, cumulative emissions over 1991–2100 will be the average of the whole period, multiplied by 110. A more exact accounting would result if one digitized each dot along the line, but for now the rough approach described previously will suffice.

I digitize two points: 1991 and 2100. It turns out emissions according to the IPCC’s chart are 6.80 GtC in the former year and 22.54 GtC in the latter; the average of 14.67 GtC gives cumulative emissions, for the 1991–2100 period, of 1,613.7 GtC.

As for concentrations, I look at Figure 5, which shows CO2 in parts per million (ppm). I haven’t digitized the chart, but the end point falls on the 825ppm mark — or close enough not to matter.

Additionally, Table 2.5 shows the exact concentration baseline, i.e. the 1990 value: 353.93 ppm.

Putting these two figures together, one concludes that the First Assessment Report expected its Business-as-usual scenario to result in a concentration increase of 471.03 ppm. One ppm is equivalent to 2.13 gigatons of carbon, which means the IPCC expected CO2 concentrations to increase by an amount equivalent to 1,003.29 GtC. Dividing this amount by the previously-calculated emissions gives an airborne fraction of 62.2%. The calculations I did in previous articles show that the expected airborne fraction is very stable, with minimal or no growth over time; for the 1991–2018 period the airborne fraction in the IPCC’s projection is already about 61%.

The airborne fraction that the IPCC projected has been, until now, far higher than that of the real world. But could it be correct in the long run? Let’s do a thought experiment.

Remember that, according to the Global Carbon Project, the 1991–2018 period saw global CO2 emissions of 261.43 GtC. For real-world emissions to match the IPCC’s Business-as-usual scenario over the whole 1990–2100 period, emissions from 2019 to 2100 would have to total 1,352.27 GtC. That is 16.49 GtC per year, which sounds implausibly high. To get an idea of how unlikely this is, emissions in 2018 were 11.49GtC, so the average for the rest of the century would have to be 43% higher. But let’s assume it happens.

Now recall that the IPCC expected CO2 concentrations to increase by 1,003.29 GtC. Per the Global Carbon Project, in the real world between 1990 and 2018 they increased by 114.62GtC. So, for the IPCC’s projection to come about, the period 2018–2100 would have to see concentration growth of 888.67GtC, or 10.84GtC/year.

In other words: for the IPCC’s projected airborne fraction to happen, the real-world airborne fraction between 2019 and 2100 would have to be 65.7%. That is 50% more than the actual fraction we’ve seen so far. To call it unlikely would be an understatement.

Although exact numbers for 2019 emissions are not yet in, data from NOAA reveals that CO2 concentrations grew by 2.47 ppm. Assuming emissions of approximately 11.5GtC, this would put the airborne fraction around the 45% mark, in line with the trend since 1990.