Part 2 of 5: What Chemistry Tells Us

This is the second installment of a five-part series by Bill Chameides on How We Know Humans Cause Global Warming.

1. A 175-year-old Puzzle
2. What Chemistry Tells Us
3. Causes of Past Climate Change
4. The Medieval Warming Period
5. The Only Explanation Left


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.

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9 Comments

  1. odie
    Posted June 29, 2007 at 6:59 am | Permalink

    WHY is it that everyone talks only about CO2-emissions? Why doesn't anyone acknowledge the fact that it's deforestation that is the biggest problem?? Forests, be they Amazonian or temperate, are our lungs, remember? They can (or could) absorb great loads of CO2 and produce O2. It's incredible – the Phoenicians and the Romans destroyed the Mediterranean forests and turned the lush country to dry eroded land – and here we are doing the same to the whole planet. For beef and biofuel and paper… Too delicate, is it?

  2. Posted June 29, 2007 at 5:17 pm | Permalink

    I don't know that deforestation is the "biggest" problem, but it's certainly a significant problem. We've talked about it in several places in the blog already:

    Slicing the greenhouse gas pie: What gases?
    Slicing the greenhouse gas pie: Where from?
    Agricultural Offsets
    Comment from Annie Petsonk on Deforestation

    Also, Annie Petsonk (our International Council – she works on the deforestation problem) will be writing a more extensive post about it in the next few weeks. It's already in the works – stay tuned!

  3. tania ramirez
    Posted July 3, 2007 at 2:24 pm | Permalink

    So ? What can the ordinary people do? Housewives,teachers,students etc. Advices should be done in a public way for the benefit of the environment

  4. Posted July 5, 2007 at 11:32 am | Permalink

    Our fightglobalwarming.com Web site has a whole section on things you can do to stop global warming. Link is here.

  5. AdrianVance
    Posted August 5, 2007 at 1:15 am | Permalink

    CO2 is a trace gas in the atmosphere. Water vapor absorbs four times as much IR in the 1 to 16 micron range of "heat waves" and has 200 times as many molecules. Thus, it has 800 times the effect of CO2 which is insignificant to all but those who want to tax carbon and bring America down.

    Of the 186 gigatons of CO2 produced on Earth every year, man makes six of it and the USA makes only 20% of that or 0.64% of the total. Cutting our energy use and CO2 by 50% per Kyoto will only reduce the CO2 production by 0.32% from what it would be otherwise, but it would ruin America.

  6. Posted August 9, 2007 at 1:08 pm | Permalink

    Here are links to previous posts that address your arguments:

    The Water Vapor Fallacy.

    Four-Part Series on Action Needed to Stop Global Warming (explains how much reduction in carbon emissions is needed, and how getting there will not cripple the economy).

  7. Dave
    Posted October 6, 2007 at 6:21 pm | Permalink

    Re: This key passage above: "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?" (The following graph shows oxygen concentration decreasing, as is apparently expected from the human-combustion theory)

    If warm air always holds less oxygen (I think this is true), then the oxygen levels are expected to be dropping anyway, whether or not human combustion is the cause of the warming. The author should demonstrate that the reduction in concentration of oxygen is not simply the reduction that is expected from the warmer atmospheric temperatures alone.

  8. Posted October 10, 2007 at 3:51 pm | Permalink

    Dave, warmer air does not hold less oxygen. Warmer water holds less oxygen – maybe you're thinking of that.

  9. Posted February 26, 2010 at 6:24 am | Permalink

    Warmer water holds less oxygen – maybe you're thinking of that.

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