Since the start of the Industrial Revolution, the concentration of CO₂ in Earth’s atmosphere has risen from about 280 to about 417 parts per million. Most scientists studying the issue think most of the increase has come from burning fossil fuels. Some people, however, question that.

For instance, one person wrote to us:

From a basic calculation I have carried out, my estimate is that of the over 200 gigatons[gt]-C CO₂ increase of CO₂ in the atmosphere since the start of the Industrial Revolution (a value given corresponding to an increase in CO₂ from about 280 ppmv [parts per million by volume] in 1880 or so to a current value of over 400 ppmv) only about 40 gt-C CO₂ is due to burning of fossil fuels. …

The calculation that only 40 gt-C of CO₂ is from burning fossil fuels is made by multiplying the mass of fossil fuel burned annually by the mean residence time of CO₂ in the atmosphere. Less than 10 gt-C of fossil fuel is burned annually and the mean residence time of CO₂ in the atmosphere is about 4 years. Multiplying these numbers together gives 40 gt-C of CO₂ from fossil fuel burning.

Dr. Roy Spencer, a Cornwall Alliance Board Member and Senior Fellow and Principal Research Scientist in the Earth Systems Science Center of the University of Alabama at Huntsville, explains that there’s a missing component to this reasoning.

The calculation of how much anthropogenic CO₂ is in the atmosphere is probably correct. But that is not what we are interested in. The critical question that needs to be answered is, “How much of the increase in atmospheric CO₂ can be blamed on a human source?” That is not the same as the question, “How many of the CO₂ molecules in the atmosphere today came from an anthropogenic source?” The calculation above answers the second question but not the first.

An analogy can help us see the difference. Imagine a tub of water with a steady inflow from a faucet that exactly matches the outflow from a drain at the bottom. The water level in the tub will remain constant.

Now imagine you add a second faucet. Its added inflow is only a small fraction (say, 3%) of the inflow from the main, first faucet. With the added faucet, the level of water in the tub will rise over time, even though the number of water molecules put into the tub by the second, new faucet will be very small because most will already have gone down the drain.

This is analogous to the atmospheric CO₂ budget. The CO₂ exchange between the atmosphere and the surface is pretty rapid, so any given molecule added over the last four years is likely to have been absorbed somehow by the surface. But there is a long-term imbalance between outflow and inflow due to the new, anthropogenic source.

So, do fossil fuels (and a few other things, like cement making) really account for most of the increase in atmospheric CO₂ concentration? Yes.

Photo by Zbynek Burival on Unsplash.