This is the second installment of a five-part series by Bill Chameides on How We Know Humans Cause Global Warming.
There's no question that carbon dioxide (CO2) is a greenhouse gas that warms the planet, and that CO2 has been increasing. The graph below shows it vividly (the measurements were taken at Mauna Loa, Hawaii).
But how do we know that the increase is due to people burning fossil fuels? One way is by analyzing the chemistry of our atmosphere, since human activities leave an unmistakable "fingerprint".
Anything that burns consumes oxygen. If the extra CO2 in the atmosphere is coming from burning fossil fuels, then the concentration of oxygen should be decreasing. Is it?
Until the late 1980s, we didn't have the instruments to measure this. But now we do and, lo and behold, oxygen concentrations are indeed decreasing. You can see this in the graph below, from Figure 2.3(a) in Chapter 2 [PDF] of the IPCC report. The pink and blue lines show oxygen concentrations. (The zig-zag is due to seasonal shifts in plant activity.)
Proof that the rise is due to burning fossil fuels is evident in another chemical analysis – the make-up of the CO2 itself.
What makes any given atom a specific element is the number of protons in its nucleus. A carbon atom contains six protons. But carbon can have different numbers of neutrons. These are the "isotopes" of carbon. About 99 percent of all the carbon on the earth is carbon-12 – that is, it has six protons and six neutrons. About 1 percent is carbon-13, with seven neutrons instead of six.
How does this help in determining the source of CO2 increase? The ratio of carbon-13 to carbon-12 in atmospheric CO2 is larger than the ratio in fossil fuel. If atmospheric CO2 is increasing due to burning fossil fuels, then the ratio of carbon-13 to carbon-12 should be decreasing. And that is exactly what is happening.
In the graph below (Figure 2.3(b) in Chapter 2 [PDF] of the IPCC report), the black line shows increasing greenhouse gas emissions, and the red line shows the decreasing ratio of carbon-13 to carbon-12. (The red scale is reversed, so the line goes up as the ratio declines.)
The changing isotopic ratio is like a fingerprint or DNA match that definitively identifies fossil fuels as the main source of the rise in atmospheric CO2.
In the next article in this series, I'll talk about the causes of past warming, and how we know that the cause of today's warming is different.