Carbon dioxide in the atmosphere

Carbon dioxide (CO₂) is the 5th most abundant gas in the atmosphere and, together with water vapor, probably the best known of the greenhouse gases.

Most everyone agrees that human activities are causing the atmospheric concentration of carbon dioxide to slowly increase. We'll look at some of the experimental evidence in a moment.

Not everyone agrees however about the effect that increasing amounts of CO₂ will have on the earth's energy budget and climate. The worry is that increasing greenhouse gas concentrations will strengthen the greenhouse effect and will cause the earth's surface to warm.

It is important to remember that the greenhouse effect isn't all bad, it has a beneficial side. You might refer to this as the natural greenhouse effect (i.e. one that has not been affected or influenced by human activities)

If the earth's atmosphere did not contain any greenhouse gases, the global annual average surface temperature would be about 0^o F. That's cold enough and that's the average, there would be many locations much colder than that. The presence of greenhouse gases raises this average temperature to about 60^o F and makes the earth a much more habitable place. So some warming is a good thing.

The concern is that increasing atmospheric greenhouse gas concentrations might cause some additional warming (and we rely on computer models to predict or estimate how much warming there will be). This alone might not sound like a bad thing. However even a small change in average temperature might melt polar ice and cause a rise in sea level which, at the very least, might pose an environmental threat to coastal areas. Warming might change weather patterns and bring more precipitation to some areas and prolonged drought to other places (like Arizona). Serious tropical diseases (such as malaria and dengue fever) might spread into more temperate areas. Plant and animal species might be forced to migrate in order to find a suitable environment; some might not be able to adapt quickly enough and could go extinct.

We will save much of that discussion for later in the semester, here we will just concentrate on carbon dioxide.

First some of the experimental data that show atmospheric carbon dioxide concentration is increasing.

This is a sketch of the "Keeling" curve. It shows measurements of CO₂ that were begun (by a graduate student named Charles Keeling) in 1958 on top of the Mauna Loa volcano in Hawaii (the air there is "clean" and not affected by nearby cities and other sources of pollutants). Carbon dioxide concentrations were about 315 ppm when the measurements began and just reached 400 ppm earlier this year (the figure above is a little out of date). to about 385 ppm between 1958 and the present day. The small wiggles (one wiggle per year) show that CO₂ concentration changes slightly during the course of a year (it probably also changes slightly during the course of a day).

You can find the latest measured atmospheric CO₂ concentration at Mauna Loa Observatory at an interesting, interactive site maintained by the Scripps Institution of Oceanography. Click on the "full record" box at the bottom of the graph to see the complete Keeling record.

The summit of Mauna Loa is left of center
on this image of the "big island" of Hawaii.
(source of this image)

(source of this image)

Once scientists saw this data they began to wonder about how CO₂ concentrations might have been changing prior to 1958. But how could you now, in 2013 say, go back and measure the amount of CO₂ in the atmosphere in the past? Scientists have found a very clever way of doing just that. It involves coring down into ice sheets that have been building up in Antarctica and Greenland for hundreds of thousands of years.

As layers of snow are piled on top of each other year after year, the snow at the bottom is compressed and eventually turns into a thin layer of solid ice. The ice contains small bubbles of air trapped in the snow, samples of the atmosphere at the time the snow originally fell. Scientists are able to date the ice layers and then take the air out of these bubbles and measure the carbon dioxide concentration. This can't be very easy, the layers are very thin, the bubbles are small and it is hard to avoid contamination.

(source of this image)

Using the ice core measurements scientists have determined that atmospheric CO₂ concentration was fairly constant at about 280 ppm between 0 AD and the mid-1700s when it started to increase. The start of rising CO₂ coincides with the beginning of the "Industrial Revolution." Combustion of fossil fuels needed to power factories began to add significant amounts of CO₂ to the atmosphere.

You can see a more accurately drawn version of this graph by going back to the Scripps Institute of Oceanography site and clicking on the "1700 - present" box. You can see the full 800,000 year long record of atmospheric CO₂ concentration (from Antarctic ice cores) by clicking on the "800,000 years" box. The current increase is the largest and most rapid in the ice core record.

This figure with actual data from Climate Change 2007, IPCC 4th Assessment Report, shows that concentrations of two other greenhouse gases, methane and nitrous oxide, have been increasing in much the same way that CO₂ has.

In order to understand why atmospheric carbon dioxide concentration is increasing, and before we look at what the earth's temperature has been doing during this period, we need to know how carbon dioxide is added to and removed from the atmosphere.

Carbon dioxide is added to the atmosphere naturally by respiration (animals breathe in oxygen and exhale carbon dioxide), decay, and volcanoes.

Combustion of fossil fuels, a human activity also adds CO₂ to the atmosphere. Cutting down and killing a tree, deforestation, will keep it from removing CO₂ from the air by photosynthesis. The dead tree will also decay and release CO₂ to the air.

CO₂ is removed from the atmosphere by photosynthesis (the equation is shown above). CO₂ also dissolves in the oceans.

The ? means I'm not not aware of an anthropogenic process that removes significant amounts of carbon dioxide from the air. Though carbon sequestration (the capture/removal of CO₂ from the air and storage) is something that is being considered to lessen or prevent global warming.

We are now able to better understand the yearly variation in atmospheric CO₂ concentration (the "wiggles" on the Keeling Curve) and can figure out when the highest and lowest CO₂ concentrations should occur.

We will assume that the release of CO₂ to the air remains constant throughout the year (the straight brown line below). Photosynthesis (the green curve) will change. Photosynthesis is highest in the summer when plants are growing actively. It is lowest in the winter when many plants are dead or dormant.

Atmospheric CO₂ concentration will decrease as long as the rate of removal (photosynthesis) is greater than the rate of release (blue shaded portion above). Your bank account balance will drop as long as you spend more money than you deposit. The minimum occurs at the right end of the blue shaded portion where the removal and release curves cross.

Once the curves cross and the green photosynthesis curve drops below the brown curve (rate of release), more CO₂ is being released than removed and the CO₂ concentration will start to increase. The highest CO₂ concentration occurs once winter is over and the rate of photosynthesis increases and again becomes equal to the rate of release.

To really understand why human activities are causing atmospheric CO₂ concentration to increase we need to look at the actual amounts of CO₂ being added to and being removed from the atmosphere (like amounts of money moving into and out of a bank account and their effect on the account balance). A simplified version of the carbon cycle is shown below.

The carbon cycle shows movement of carbon into and out of the atmosphere or ocean.

Here are the main points to take from this figure:

1.    The underlined numbers show the amount of carbon stored in "reservoirs." For example 760 units* of carbon are stored in the atmosphere (predominantly in the form of CO₂, but also in small amounts of CH₄ (methane), CFCs and other gases; anything that contains carbon). Note that the atmosphere is a fairly small reservoir.

    The other numbers show "fluxes," the amount of carbon moving into or out of the atmosphere. Over land, respiration and decay add 120 units* of carbon to the atmosphere every year. Photosynthesis (primarily) removes 120 units every year.

2.    Note the natural processes (color coded blue and green) are pretty much in balance (over land: 120 units added to the atmosphere and 120 units removed, over the oceans: 90 units added balanced by 90 units of carbon removed from the atmosphere every year). If these were the only processes present, the atmospheric concentration (760 units) wouldn't change.

3.    Anthropogenic (man caused) emissions of carbon into the air are small compared to natural processes (orange in the figure). About 6.4 units are added during combustion of fossil fuels (and during the manufacture of cement) and 1.6 units are added every year because of deforestation (when trees are cut down they decay or are burned and add CO₂ to the air, also because they are dead they aren't able to remove CO₂ from the air by photosynthesis)

The rate at which carbon is added to the atmosphere by man is not balanced by an equal rate of removal: about half (4.6 of the 8) units added every year are removed (highlighted in yellow in the figure).

This small imbalance (8 - 4.6 = 3.6 units of carbon are added to the atmosphere every year) explains why atmospheric carbon dioxide concentrations are increasing with time. Note also that more carbon dioxide is added to the oceans every year than is removed. Addition of CO₂ to the oceans might increase the acidity of the ocean water which might might make it more difficult for coral and sea shells to form (shells and coral are made of calcium carbonate CaCO₃).

4.    In the next 100 years or so, the 7500 units or so of carbon stored in the fossil fuels reservoir (lower left hand corner of the figure) might be dug up or pumped out of the ground and burned. That would add 7500 units of carbon to the air. The big question is how will the atmospheric concentration change and what effects will that have on climate? Carbon dioxide can move into and out of the atmosphere fairly quickly, movement into and out of some of the other reservoirs is slow. Thus it is difficult to say precisely how and how quickly the picture will change when it is perturbed.

*don't worry about the units. But here they are just in case you are interested:
Reservoirs - Gtons
Fluxes - Gtons/year
A Gton = 10¹² metric tons. (1 metric ton is 1000 kilograms or about 2200 pounds)

So here's where we're at.
Atmospheric CO₂ concentration was fairly constant between 0 AD and the mid 1700s.
CO₂ concentration has been increasing since the mid 1700s.
The obvious question is what has the temperature of the earth been doing during this period? In particular has there been any warming associated with the increases in greenhouse gases that have occurred since the mid 1700s? The answer to that question will be the subject of a future 1S1P report topic.