Open Mind

CO2: They call it “life” …

November 10th, 2006 · 10 Comments

The Anthropogenic Global Warming hypothesis (AGW) is the belief that we (humankind) are responsible for earth’s climate change. The ultimate cause, says the hypothesis, is the addition of greenhouse gases to the atmosphere. These gases absorb infrared radiation, which is the principal way by which earth radiates energy to space. By blocking energy from leaving earth, greenhouse gases raise earth’s temperature.

The principal greenhouse gas which mankind has been adding to the atmosphere is carbon dioxide, or CO2. This raises such questions as, “Is CO2 concentration higher than it was in the past?” “If so, is the rise due to human activity?”

History of Atmospheric CO2 Concentration

We have the technology to measure the CO2 concentration in the atmosphere. In fact, it’s monitored regularly at numerous stations around the globe. Probably the best-known monitoring station is the Mauna Loa Atmospheric Observatory in Hawaii. Before proceeding further, I’d like to dispel a myth about this station. It is often objected that it’s folly to place an atmospheric observatory on top of a volcano! But in fact, the atmospheric observatory is not at the top of the volcano, but at its base.

CO2 is what is called a “well-mixed” gas in the atmosphere. This means that it is distributed pretty much uniformly throughout the atmosphere, so its concentration should be pretty much the same at all points of the globe. In fact that’s exactly what we observe; although there are tiny differences from place to place, overall the CO2 concentration is the same at all observing stations. So, we can look at measurements from Mauna Loa and reliably take them as representative of the atmosphere as a whole.

And what do we find? Since measurements have been recorded at Mauna Loa, CO2 has shown a consistent increase. CO2 concentration is measured in units called “ppmv,” meaning “parts per million by volume.” This just means that of every million molecules in the atmosphere, that many are molecues of CO2. Figure 1 shows the measured concentration of CO2 from Mauna Loa from the beginning of measurements (1958) to the present (click on the graph for a larger, clearer view).

mlo1.JPG

Figure 1. Monthly average CO2 concentration measured at Mauna Loa.

It’s abundantly clear that since 1958, CO2 concentration has risen a lot, from about 315 ppmv in 1958 to about 380 ppmv today. That’s a 21% increase in just 48 years! It’s also evident that in addition to this steady increase, CO2 concentration rises and falls in an annual cycle. The annual cycle is due to the respiration of land plants. Most of the land plants are in the northern hemisphere of our planet, because most of the land is in the northern hemisphere. During northern-hemisphere summer, plants grow, and one of the main building blocks of plant life (of all earthly life, for that matter) is carbon. Plants get their carbon from atmospheric CO2, so during northern-hemisphere summer land plants are extracting CO2 from the air, lowering its atmospheric concentration. During winter, plant matter decays, returning the carbon to the atmosphere as CO2. So, the annual rise and fall of atmospheric CO2 is a visible sign of the planet’s plant life “breathing.”

But what about that steady rise — 21% in just 48 years? Is this natural? We can get a clue about this by looking at atmospheric CO2 concentration long into the past. We can measure past CO2 concentration by studying samples of atmosphere from the past. And where, you wonder, can we get samples of past atmosphere? When ice accumulates in cold regions like Greenland or Antarctica, it traps bubbles of air — samples of the atmosphere. By drilling ice cores, we can extract those ancient bubbles of air, analyze them, and determine the atmospheric CO2 concentration in days long past.

One of the best records of CO2 for the last 1,000 years comes from the Law Dome ice core. If we combine the data from Law Dome with that from Mauna Loa, we get the graph in figure 2.



Figure 2. CO2 from Law Dome ice core and Mauna Load observatory.

As you can see, CO2 concentration was quite steady for the last 1,000 years, until the late 19th century. Before 1875, the concentration was not above 290 ppmv. In fact, it fluctuated slightly between 270 and 290 ppmv for nearly 900 years. Since then it has risen, slowly at first, then faster and faster, to 380 ppmv today — a 31% increase in less than a century and a half.

But can we be sure that this isn’t a natural fluctuation? Is it possible that every thousand years or so, CO2 increases or decreases by a large amount naturally? We have ice cores from Antarctica that stretch further back in time than just 1,000 years. One of these is the Vostok Ice Core, collected at Vostok Station in Antarctica. It gives us a record of CO2 concentration not just for the last 1,000 years, but for the last 420,000 years. The Vostok CO2 data are shown in figure 3.



Figure 3. CO2 data from Vostok ice core.

Now we see that CO2 concentration does fluctuate naturally, by quite a bit, on timescales of hundreds of thousands of years. The ups and downs of CO2 correlate with the ice ages; low CO2 concentrations occur during times of extensive glaciation, while high CO2 concentrations occur during interglacials. But the fluctuations don’t go nearly as high as present levels. Low levels (during extensive glaciation) are almost as low as 180 ppmv, while the highest level in the Vostok data is 299 ppmv some 324,000 years ago.

Recent ice core research has extended the record back nearly a million years into the past, with the same pattern: large fluctuations in step with the ice ages, from lows around 180 ppmv during glaciation and highs almost to 300 ppmv during interglacials. Before the time covered by ice core data, it’s much more difficult to estimate atmospheric CO2 concentration. Nonetheless, the available evidence is that CO2 concentration has been above 300 ppmv in the distant past, but not for the last 23 million years.

There is considerable evidence that CO2 has been in much higher concentration in the very distant past — hundreds of millions of years ago. But there is a tremendous uncertainty in those long-past estimates; you can find an interesting discussion on this topic here.

Source of CO2 Increase

What’s the source of all that CO2 increase over the last century and a quarter? An obvious suspect is the burning of fossil fuels. CO2 is one of the primary byproducts of fossil fuel burning, and the rise is CO2 coincides with the industrial revolution. In fact, if we take an inventory of CO2 emissions, and track the sources and sinks in the natural environment, the books balance — fossil fuel burning accounts for the CO2 increase precisely.

But we can be even more sure. Fossil fuels are plant matter that has been buried underground for millions of years. So, if the CO2 is from the burning of fossil fuels, it should be from long-dead plant matter. But how can we tell the difference between plant-matter CO2 and other forms?

Carbon comes in several different form, called “isotopes.” They differ primarily in atomic weight. Most carbon atoms weigh 12 atomic mass units (amu). But there are two other important isotopes: C13 (weighing 13 amu) and C14 (weighing 14 amu). C14 is radioactive, and decays with a half-life of about 5800 yr. So, nearly all the C14 in fossil-fuel CO2 has decayed (to nitrogen-14), hence this isotope is pretty much absent from fossil fuels. Also, when plants incorporate CO2 into their structure, they show a strong preference for “ordinary” C12 rather than the heavier isotope C13. So, fossil-fuel CO2 is made almost entirely of C12; it’s greatly depleted in C13 and C14.

This means that if CO2 enters the atmosphere from fossil fuel burning, the carbon should be almost entirely C12. So, if the rise in atmospheric CO2 is due to fossil fuel burning, then the atmospheric proportion of the heavier isotopes C13 and C14 should be decreasing. And that’s exactly what we observe. The proportion of C13 and C14 in atmospheric CO2 is decreasing, in exactly the way we would expect if the source of the additional CO2 is fossil-fuel combustion.

In fact, there’s simply no doubt. The source of the dramatic increase in atmospheric CO2 concentration is the burning of fossil fuels.

Effect of Increased CO2

CO2 really does absorb infrared radiation, acting as a “greenhouse gas” that raises global average temperature. But it has other effects too. Increased atmospheric CO2 tends to dissolve (slowly) in the oceans, and that alters the carbon chemistry of the oceans. In particular, it makes the oceans more acidic. Increased ocean acidity threatens to dissolve the calcium carbonate which many ocean life forms use to construct their bodies. This threatens the base of the marine food chain, threatening a collapse of marine ecosystems.

In fact, some people believe that the danger from ocean acidification is even greater than the danger from global warming; a collapse of marine ecosystems will certainly have a disastrous effect on earth’s life systems. But this is by no means certain. It seems to me, that we should not find out by experimenting with the only planet we have to live on.

Categories: Global Warming

10 responses so far ↓

  • Marvin Lange // Nov 30th 2006 at 10:50 pm

    This was disappointing, I wanted to see what the figures were for animal life contribution…including bacteria to human beings. there are about 6 billion people exhaling,
    the equivalent of about 10 billion in cattle, assuming 2 billion cattle. the ration being 1 for 3 in US. And cattle intuitively exhale a lot more co2 than humans. with about 5 times the weight.
    You could probably correlate the animal weight
    increase with CO2 over the years were such information available.

    just for starters.

    [Response: Not possible, for two reasons. First is the isotopic composition of carbon in atmospheric CO2: it matches precisely what we should see if CO2 increase is due to the burning of fossil fuels. Second, the carbon which makes up animal life (and which we exhale as CO2) comes from eating other animals and plants, and the carbon which makes up plant life comes from the atmosphere. So the “source” of that CO2 which you think is increasing atmospheric concentration, is the atmosphere itself.]

  • Steven // Jan 19th 2007 at 8:28 pm

    Misleading graph. Notice that the Vostok Ice Core data refers back to time period between 420000 and 200000 years ago. Current CO2 emission is at around 375 ppmv. Compared to the peak of the graph 300 ppmv, current CO2 emission is considerably high.

    [Response: I’m not sure what you mean. The graph of CO2 concentration from Vostok goes up to about 2300 years ago; the axis labels go from 420,000 yr ago to 20,000 (not 200,000) yr ago. And the data plotted are not emissions, they’re atmospheric concentration.]

  • bereans // Jan 27th 2007 at 3:43 am

    Hi OM!

    Enjoyed the post–ran into your comment over at Doug’s Darkworld.

    I am a global warming sketic–I believe that global warming, atmospheric changes, weather patterns (or lack thereof), magnet field fluctuations, etc., etc. are natural to this earth and have happened since the dawn of time. I also believe that we CAN put harmful things into our atmosphere and that we shouldn’t do it. But, what makes me a skeptic is the current contrived crisis that is completely political in nature.

    It all comes down to this–there are two kinds of people in this world. Those who believe that natural resources should be owned by individuals and those who think they should be owned by the collective (government). Socialism’s aim is to control all natural resources and as governments move closer toward socialism there is a resultant struggle for the control of natural resources. This has happened numerous times in history–(re: the Soviet Union, mainland Europe, China, etc., etc.) In order to gain control, the governments need the people to back them. Playing upon the fears of people gives the government the needed political power to increase regulation and eventually gain complete control of private industry. Globally, a degree temperature increase, a couple of well publicized hurricanes and trees budding in New York during winter is all of the sudden a reason to put the world in panic to advance as much political power and clout as one can. They are called opportunists and they are much smarter (I didn’t say intelligent) than the people the dupe into supporting their cause or advancing their agenda.

    Its kind of like religion in a way. (Religion comes from the latin “religare” which means to restrict or regulate). Of course its usually only the people at the top who know its false, but are all to willing to exploit the fundamental zealots they have converted to the fold.

    JMO

    Regards,

    -Jack

  • kennebecriver // Jan 28th 2007 at 12:57 am

    Jack — no offense, but you are scientifically illiterate.

  • bereans // Jan 31st 2007 at 2:41 am

    Hi Kennebec,

    I guess that is true–My engineering degree DID come from the University of Maine…

    -Jack

  • Andrew Dodds // Feb 1st 2007 at 10:13 am

    Jack -

    I suspect that the ’scientifically illiterate’ bit comes from that fact that you are clearly unaware of the entire science of climatology.

    And if you don’t like the nature of proposed solutions to AGW, then that is a direct result of the more right wing people simply refusing to admit that a problem exists; so who do you think is proposing any solutions?

  • Timothy // Feb 1st 2007 at 9:06 pm

    CO2 is only a very small percentage of the atmosphere, I think it is only about .03% and only about 5% of all greenhouse gases. Of this a small amount of CO2, my understanding is that an even smaller number is human caused. Although the amount of CO2 is going up, compared the amount of water vapor and the atmosphere on a whole it is still a very small amount. Do you know of any lab experiments that have shown inserting a small amount of CO2 into a biosphere would have this great an effect?

  • cytochrome_sea // Feb 4th 2007 at 11:53 am

    A couple minor nitpicks, “But in fact, the atmospheric observatory is not at the top of the volcano, but at its base” To be fair, seems a little misleading at ~11K’ elevation.

    also “Increased ocean acidity threatens to dissolve the calcium carbonate which many ocean life forms use to construct their bodies. This threatens the base of the marine food chain” Doesn’t sound quite right to me, I’d consider phytoplankton as the base of the chain. Of course the phytoplankton can also be a threat to the corals, etc… you were alluding to.

  • John Cross // Feb 4th 2007 at 4:07 pm

    Hi CC - I haven’t seen you around for a while. Just to add to your comment, I believe that some phytoplankton do use calcite in their tests.

    Regards,
    John

  • cytochrome_sea // Feb 5th 2007 at 9:16 pm

    Hey JC, I usually only tend to post when I’m fairly intoxicated and haven’t had the time to drink much lately ;) You’re quite right about the phytoplankton, I guess I had my head stuck on corals as they’re usually what I see discussed.

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