Carbon dioxide in the atmosphere

Carbon dioxide (greenhouse gas) and its role in global warming

Introduction
This is the first of a series on climate change and global warming. Here we'll mostly be concerned with carbon dioxide (CO₂), the 5th most abundant gas in the atmosphere and (together with water vapor) probably the best known of the greenhouse gases.

It is generally accepted 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). The concern is that increasing amounts of CO₂ will strengthen the greenhouse effect and cause the earth's surface to warm.

The table above (from this source in an article about greenhouse gases in Wikipedia) shows the relative importance of water vapor and carbon dioxide in the greenhouse effect. Also note the important role of clouds. Clouds, of course, are not made up of gases. Rather they are composed of small drops of liquid water or crystals of ice.

There is still a fair amount of uncertainty about how much warming will occur, how the warming will vary by region, and about all the secondary effects that temperature changes may have. We will look at past and predicted future changes in temperature in more detail in the next part of this series.

It is important to remember that the greenhouse effect isn't all bad, it has a beneficial side. We'll 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 pretty cold and that's the average, there would be many locations on the earth 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.

Increasing atmospheric greenhouse gas concentrations might cause some additional warming (and we rely on computer models to predict or estimate how much warming there would be). This 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 would, at the very least, pose an environmental threat to coastal areas. Warming might change weather patterns and bring more precipitation to some areas and more frequent and more prolonged and severe drought to other places (like Arizona). Serious tropical diseases (such as malaria and dengue fever) might spread into areas where they're not currently found. 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 most of the discussion of the effects of global warming for later in the semester, here we will just concentrate on carbon dioxide.

Measurements of atmospheric CO₂ concentration
Let's first look at some of the experimental data that show atmospheric carbon dioxide concentration is increasing.

This is a sketch of the "Keeling" curve. The graph shows measurements of atmospheric CO₂ concentration that were begun (by a graduate student named Charles Keeling) in 1958 on top of the Mauna Loa volcano in Hawaii (the summit is over 13,000 ft above sea level and the air there is "clean" and not affected by nearby cities and other sources of pollutants). The carbon dioxide concentration was about 315 ppm when the measurements began and is now over 400 ppm. The units "ppm" stand for parts per million; 315 ppm means there are 315 CO₂ molecules mixed in with 1,000,000 (1 million) air molecules (equivalent to 0.0315% concentration).

The small wiggles (one wiggle per year) show that CO₂ concentration changes slightly during the course of a year (local concentrations also changes slightly during the course of a day). But overall the concentration has been increasing steadily over the past 50 plus years.

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 ( links at the bottom of the graph allow you to display CO₂ concentrations on time scales that range from one week to 800,000 years).

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

A side view of the Mauna Loa volcano
(source of this image)

Once scientists saw Keeling's data they began to wonder about how CO₂ concentrations might have been changing prior to 1958. But how could you now, in 2017 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 (drilling) 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, the bubbles are essentially sealed 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 must be 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.

The graph above comes from the Scripps Institute of Oceanography site.


This figure is from Climate Change 2007, IPCC 4th Assessment Report.	Source of this figure: Bullister, J.L. 2015. Atmospheric Histories (1765-2015) for CFC-11, CFC-12, CFC-113, CCl₄, SF₆ and N₂O. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, US Department of Energy, Oak Ridge, Tennessee.

The figure above at left indicates that atmospheric concentrations of two other greenhouse gases, methane and nitrous oxide, have been increasing in much that same way as carbon dioxide.

The graph at right shows atmospheric concentrations of some other more unusual materials. With the exception of nitrous oxide (N₂O), there aren't any natural sources of these compounds, they are added only by man. The concentrations of SF6, CCl , and the CFCs were zero up until the early to mid 1900s. If you examine the right graph carefully you will notice that the concentrations of the four CFC compounds have actually started to decrease. There's a good reason for this. In addition to being greenhouse gases, chlorofluorocarbons (CFCs) and carbon tetrachloride (CCl₄ ) also react with and destroy stratospheric ozone (the ozone layer). A series of international treaties, such as the Montreal Protocol, have successfully begun to phase out substances that are harmful to the ozone layer. As a result, atmospheric concentrations of CFCs and CCl₄ have started to decrease.

Carbon dioxide release and removal processes
In order to better understand why atmospheric carbon dioxide concentration is increasing we need to learn more about 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.

Living vegetation will remove CO₂ from the air by photosynthesis (the equation shown above). Killing or cutting down trees, deforestation, will reduce this CO₂ removal process. The dead tree will also decay and release CO₂ to the air. 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. Carbon sequestration (the capture/removal of CO₂ from the air and storage) is something that is being considered to lessen or prevent global warming.

The amount of CO₂ emitted during combustion depends on the particular fuel being burned. Natural gas is a relatively "clean" fuel and releases roughly half the amount of CO₂ per unit of energy generated as coal.

adapted from the U.S. Energy Information Administration (June 2017)

also from the U.S. Energy Information Administration (June 2017)

In recent years the use of coal has started to decrease in the United States and increasing amounts of energy are being generated by the combustion of natural gas. Largely because of the shift from coal to natural gas, emissions of in the United States have decreased by more than 10% since 2007 (http://www.washingtontimes.com/news/2016/apr/10/stephen-moore-how-fracking-reduces-greenhouse-gase/).

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 line below). The rate that CO₂ is removed from the air by 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). 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. A bank account behaves in the same kind of way. Assume you are depositing money into the account at a steady rate but spending varies during the year. The account balance will rise and fall depending on whether spending is greater or less than the amount being deposited.

The carbon cycle
Next we'll try to get a better feeling for the relative amounts of carbon dioxide moving into and out of the atmosphere. A simplified version of the carbon cycle is shown below (the next two figures are based on Fig. 6.1 in Climate Change 2013: The Physical Science Basis available at http://www.ipcc.ch/report/ar5/wg1/).

This first figure shows the movement of carbon into and out of the atmosphere before the beginning of the big industry. This is what we might have expected to see in the early 1700s.

Here are the main points to take from this figure:

1. The upward and downward arrows show "fluxes," the amounts of carbon moving into or out of the atmosphere. Over land, respiration and decay add 107.2 units* of carbon to the atmosphere every year. Photosynthesis (primarily) removes 108.9 units every year. The net effect is that 1.7 units of carbon are removed from the atmosphere every year over land.

2. Over the ocean more CO₂ is being added to the atmosphere (61.7 units) than is being removed (60 units). The excess 1.7 units of
CO₂ being added over the oceans balances the 1.7 units being removed over land (we've ignored the relatively small amounts of carbon dioxide released and removed by volcanoes and weathering).

3. The underlined number shown for the atmosphere, 589 units (278 ppm concentration), is the amount of carbon stored in the atmosphere. Because the addition and removal of CO₂ over land and ocean are in balance we wouldn't expect the atmospheric concentration to change much from year to year.

The next figure shows the carbon cycle as we think it is operating today.

What's changed. First the figure includes emissions of carbon dioxide by man. Compared to the natural processes, man's contribution are relatively small: about 7.8 units are added during combustion of fossil fuels (and during the manufacture of cement) and 1.1 units are added every year because of deforestation).

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 the situation over the oceans has reversed: the oceans are removing more carbon dioxide than they are releasing. 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).

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.

In the next 100 years or so, the 1500 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 a substantial amount of carbon to the air. Roughly an equal amount of carbon could be released if the permafrost were to melt. The big question is how will the atmospheric concentration change and what effects will that have on climate?

*Here are the units just in case you are interested:
Reservoirs - Gtons
Fluxes - Gtons/year
A Gton = 1 giga ton = 10¹² metric tons. (1 metric ton is 1000 kilograms or about 2200 pounds)

How much more will CO₂ concentration increase, what might we expect by the end of the century?
We'll have a look at some of the predictions. We'll try to not get into too many of the details. We just want to get an idea of how carbon dioxide concentrations might change (and what sort of temperature increase might occur).

The curve above at left is from a Summary for Policymakers report published by the Intergovernmental Panel on Climate Change in 2001 (the complete collection of reports is available at https://www.ipcc.ch/ipccreports/tar/). To produce the figure at left a range of assumptions were made about future population growth, economic development around the world, the level of cooperation among different countries, and the pace at which we transition from fossil fuels to alternate sources of energy (see an article about the Special Report on Emissions Scenarios in Wikipedia for more details). The A1 and A2 curves have been circled because they seem to be worst case scenarios. If the A1 fossil fuel intensive curve turns out the be the case, the IPCC report predicts a 3 - 5.5 C increase in temperature by the year 2100.

The A1T and B1 curves seem to be best case scenarios. The A1T curve assumes an emphasis on increasing uses of alternative sources of energy. We might expect a 1 to about 2.5 C increase in temperature with this scenario.

The graph at right comes from the most recent IPCC report published in 2013-2014 (view the entire collection of reports at https://www.ipcc.ch/report/ar5/). In this case rather than guessing about economic development and population growth around the world, the IPCC simply made several assumptions about how much and how quickly CO₂ would be emitted during the next century (you can read more about these so-called Representative Concentration Pathways in Wikipedia for additional information).

The green curve is clearly the best case scenario. However, because it assume that CO₂ emissions will peak between 2010-2020 it can probably be ruled out. We are most of the way through that decade and while US emissions of CO₂ have begun to decrease but that is not the case globally. Note also that even if the emissions were to peak between 2020-2020, the atmospheric concentration doesn't peak and begin to drop until about 2040. The blue curve (RCP 4.5) might be something to shoot for. Let's call that the best case situation. It assumes that emissions peak around 2040 and atmospheric CO₂ concentration begins to level off by the end of the 21st century. We might expect a temperature change of about 1 - 2.5 C.

The red curve where emissions continue to increase throughout the century is clearly the worst case scenario. Global average temperature is predicted to increase by roughly 2.5 to 5 C in that case.

Is there anything we can do as individuals to slow or reverse the buildup of atmospheric CO₂?
This question is addressed at the end of a recent book, The Thinking Person's Guide to Climate Change by Robert Henson (publ. by the American Meteorological Society in 2014). We'll look briefly at some of what is mentioned there and will limit ourselves to home energy use and transportation. We won't consider public policy or energy use at work or school.

The generation of electricity used in your home and energy used to warm you home account for between 15 - 20% of greenhouse gas emissions. Here are some of Henson's suggestions:

Replacing tungsten bulbs with compact fluorescent (CFL) bulbs or LED bulbs. Roughly 90% of the energy used by tungsten bulbs generates heat and invisible infrared light rather than visible light. Dispose of CFL and LED bulbs properly as they contain hazardous materials.

Lower thermostat settings during the winter and raise them during the summer to reduce heating and cooling costs. Using programmable thermostats so that different settings can be used during the day when you might be at home and during the night.

Using low-flow shower heads to reduce the use of hot water (and use of water in general - an important consideration in a desert location like Tucson). Lower the temperature setting on your hot water heater. Wash laundry in cold water and dry your clothes outdoors.

Refrigerators can use a lot of energy because they are on all the time. Newer models are better insulated and more efficient than older ones. Try to locate your refrigerator away from sources of heat.

Many modern electronic devices (televisions, computers, DVRs, cable boxes, etc) are never really completely off, they remain in standby mode and in some cases may account for 10% of home energy use. Often it wouldn't be convenient to turn them off completely as they might require reprogramming. During long absences however (and during lightning storms) you might consider unplugging them.

Globally transportation accounts for about 15% of greenhouse emissions. It might be considerably higher in the US. Here are some suggestions in that area.

Consider using mass transit or another form of transportation (such as a bicycle) instead of your personal vehicle.

Keep your vehicle tuned up and be sure the tires are properly inflated (also important for safety reasons). Accelerate and stop gradually and travel at reasonable speeds. This is especially important on the highway as small increases in speed result in larger increases in air resistance and increases in fuel consumption. At highway speeds it is probably better to use the car's air conditioning rather than opening the windows which result in more air resistance. At lower speed the car will consume less fuel with the AC off.

Consider reducing air travel by consolidating trips and taking fewer but longer vacations.

A final section that is a little off-topic

Is CO₂ a pollutant, is the CO₂ concentration ever high enough to be dangerous?
Before we leave this topic a little more information about carbon dioxide. Up to this point we've been interested in CO₂ because of its role in the atmospheric greenhouse effect. I generally don't consider CO₂ to be an air pollutant because the atmospheric concentration is small and its not a toxic gas. Under certain circumstances however CO₂ can build to unhealthy even deadly levels.

Here is a brief summary of the physiological conditions that can occur because of exposure to elevated CO₂ levels (adapted from http://en.wikipedia.org/wiki/Carbon_dioxide). For comparison, keep in mind the atmospheric concentration of carbon dioxide is about 400 ppm (0.04%).

_{concentration}	physiological symptoms
1% (10,000 ppm)	some people start to experience drowsiness.
2%	mildly narcotic, increases blood pressure and pulse rate, and decreases hearing
5%	shortness of breath, dizziness, confusion, anxiety, headache
8%	dimmed sight, sweating, muscular tremors, loss of consciousness after 5 to 10 minutes exposure

The Occupational Health and Safety Administration (OSHA) has set a permissible 8 hour working day exposure limit of 5000 ppm (0.5 %). CO₂ concentration is sometimes measured/monitored in work areas to insure that ventilation systems are adequate and working properly.

Submarines (and spacecraft) are one place where carbon dioxide levels could potentially build to dangerous levels and "scrubbers" are used to remove CO₂ from the air and keep CO₂ levels within acceptable levels (generally less than 8000 ppm) You may remember that one of the problems faced by the Apollo 13 crew was jury-rigging a scrubber to keep the carbon dioxide inside their spacecraft within acceptable levels.

Carbon dioxide poisoning ("blackdamp") is one of several hazards that miners face when working underground. And this is, as best I can tell, the reason miners used to carry a caged canary into the mine with them. Birds are more sensitive to carbon dioxide than humans and the canary would stop singing and fall off its perch if CO₂ levels were too high. Carbon dioxide is also involved in a rare type of natural disaster called a lake overturn or limnic eruption. What happens here is that cold water at the bottom of a lake containing dissolved CO₂ is suddenly forced to the lake surface and releases its CO₂ (like the bubbles coming from a carbonated beverage when opened). Carbon dioxide is heavier than air and is odorless. People and animals near the lake maybe unaware of the release and buildup of CO₂ and could suffocate. According the Wikipedia article cited above, events like this have apparently only been observed twice: in Cameroon at Lake Monoun in 1984 (causing the death of 37 people living nearby) and in 1986 at nearby Lake Nyos where around 1200 people were killed.

Finally brief mention of a fairly recent archaeological discovery: Pluto's Gate to Hell (the god of the underworld was named Pluto by the Romans and Hades by the Greeks). Pluto's Gate was considered in classical times to be the entrance to the underworld (one of many perhaps) and was discovered in 2013 by Italian archaeologists at the ancient city of Hierapolis in southwestern Turkey.


The site as it appears now (source of this photograph)	The site as it might have appeared in ancient times. This photograph, credited to Francesco D'Andria, the Italian archaeologist that announced the discovery in March, 2013, is found in a news report from the National Geographic Society.

The "gate" was built on top of a cavern and, in ancient times, a mist of deadly vapors could be seen coming from the cave. Here's a quote from the Slate article where I first read about the discovery: "Two millennia ago, visitors to Pluto's Gate could buy small birds or other animals (the sale of which supported the temple) and test out the toxic air that blew out of the mysterious cavern. Only the priests, high and hallucinating on the fumes, could stand on the steps by the opening to hell. They would sometimes lead sacrificial bulls inside, later pulling out their dead bodies in front of an awed crowd.

As the Greek geographer, philosopher, and prolific traveler Strabo, who lived from 64/63 B.C. to 24 A.D., so enticingly described it: 'This space is full of a vapor so misty and dense that one can scarcely see the ground. Any animal that passes inside meets instant death. I threw in sparrows and they immediately breathed their last and fell.' Can you guess what such a deadly gas might be escaping from Pluto's Gate? The answer is carbon dioxide.