There is a very strong relation between the rise in CO2 in the atmosphere which we saw in the previous post and the consumption of fossil fuels.
You may recall the slight surge in CO2 in the atmosphere after 1900 and the subsequent flattening until 1950, followed by a strong surge that has continued until today. Well, here is the same pattern in the CO2 emissions from the burning of fossils fuels and other industrial sources.
If you try to link the emissions directly to the growth in atmospheric CO2 you fail. There are many "sinks" which remove CO2 from the atmosphere. One that is often overlooked is that higher CO2 levels in the air stimulate plant growth, so the more we emit, the more the plants mop up.Trying to follow the dynamics of all the sinks has proved difficult, so we do not have a really good chemical balance between what is emitted and what turns up in the air.
Fortunately isotopes come to our aid. There are two primary plant chemistries, called C3 and C4. C3 plants are ancient, and they tend to prefer the 12C carbon isotope to the 13C. Plants with a C4 chemistry are comparatively recent arrivals, and they are not so picky about their isotopic diet. Fossil fuels primarily come from a time before C4 chemistry was around, so they are richer in 12C than today's biomass. Injecting into the air 12C-rich CO2 from fossil fuels should therefore cause the 13C in the air to drop, which is precisely what is observed:
So the evidence that fossil fuel burning is the underlying cause of the increase in the CO2 in the atmosphere is really quite conclusive. But does it have any effect? We will consider that in the next post.