I’m going to write a little bit more about a “climate misconception” to follow up on the blog I did a few weeks ago. This relates to how our cutting back on carbon production affects the amount of carbon dioxide in the atmosphere. In the earlier blog, I stressed the long lifetime that carbon dioxide has in the atmosphere as being an important reason why it will take a long time to reduce the amount of carbon dioxide in our atmosphere.

But I missed an important misconception: many think that simply keeping the amount of carbon dioxide we release the same will keep the carbon dioxide in the atmosphere the same. Similarly, many think that reducing the amount of carbon dioxide we release will reduce the amount of carbon dioxide in the atmosphere.

As recently discussed in a recent article in Science, there is a fundamental misconception. We forget that the total amount of carbon dioxide in the atmosphere relates to how much we release and how much nature (or, if you prefer, the Earth system) can absorb.

The article describes a question asked of students at the Massachusetts Institute of Technology – a very smart bunch of people. The question went something like this:

Suppose we put twice as much carbon dioxide into the atmosphere as the earth system can absorb. How will the carbon dioxide in the atmosphere change if we keep producing the same amount of carbon dioxide?

A surprisingly large number thought that the carbon dioxide in the atmosphere would stay the same if we didn’t change our habits at all.

Does that seem right to you? Perhaps it does, because we have seen reports of increased amounts of carbon dioxide put into the atmosphere by humans, and an increase of carbon dioxide in the atmosphere. So we might be led to think that if we keep putting in the same amount of carbon dioxide, then the amount of carbon dioxide in the air will stay the same. And we can reduce the amount of carbon dioxide in the air simply by reducing the amount we put into the air.

But let’s stop and think a minute. The question states that the earth system can absorb just half of what we are putting in.

Suppose we have a bathtub. We turn on the water faucet full blast and leave the drain open, so that the amount of water going into the bathtub is twice what is draining out.

Do you think that the level of water in the bathtub will remain the same? Or are you worried that the bathtub will overflow?

Let’s go through the bathtub problem step by step.

Start with an empty bathtub.

In one minute –10 liters of water comes out of the faucet, and we drain out five. (At least this is what happened when I did it.)

At the end of one minute, how much water is in the bathtub – five liters!

Figure 1. Putting water in a bathtub (well, a funny-looking bathtub) and draining it out.

And the end of the second minute, another 10 liters of water has come out of the faucet, and five have drained out.

How much water is in the bathtub? Five liters, plus the ten liters from the faucet, minus five liters that drained out – that’s 10 liters.

Figure 2. Filling up the same bathtub, after two and three minutes.

At the end of the third minute, another 10 liters of water has come out of the faucet, and five have drained out. How much water is in the bathtub now? Ten liters already there, plus the ten liters from the faucet, minus five liters that drained out … 15 liters!

I could continue on, but I think you have the idea – the bathtub is filling at the rate of 5 liters a minute.

Now let’s go back to the carbon dioxide? What do you think now? If you think that the amount of carbon dioxide in the atmosphere will increase with time, even if we release the same amount into the atmosphere, you have the right answer!

Of course, our atmosphere (and people) are much more complicated than that. Different parts of the earth system – trees, grasses, the ocean – take carbon dioxide out of the atmosphere in different ways. And there could be other natural sources of carbon dioxide. We have a good idea about how things work, and what possibilities there are, but there is still much we don’t know. Thinking about the bathtub again, the drain might clog up a little bit or drain better, or water could be flowing from a second faucet.

This blog was inspired by the Policy Forum, by John D. Sterman in the 24 October 2008 issue of Science.

Understanding something as complicated as climate change is really tough. So it’s easy to understand why people don’t always get things right. But it’s much easier to explain why the term “global warming” is misleading than it is to explain why some climate-change messages are only partially understood. So I put the “partial misconceptions” in a separate blog.

Partial Misconception: The greenhouse warming is due to carbon dioxide. Figure 4 shows that slightly over half of the warming near Earth’s surface is caused by carbon dioxide (CO2), with other gases – methane (CH4), Nitric oxide (N2O), halocarbons, and ozone in the lower atmosphere, accounting for the rest of the “forcing.” What is forcing? Forcing can be thought of as a “push” that warms (or cools) the Earth system.

The warming that results is actually larger then you might expect from an increase in these gases alone. This is because the warming surface and air leads to more water vapor, which is also a greenhouse gas. This leads us to the next partial misconception.

Figure 4. Effect of greenhouse gases and aerosols on surface air temperature warming, in terms of “forcings.” From 2007 report, Intergovernmental Panel on Climate Change.

Misconception: Carbon dioxide is the most important greenhouse gas. Certainly this is what you might expect from a first glance of Figure 4. But where is water vapor? I was taught as an Atmospheric Science graduate student that water vapor was the primary greenhouse gas, but carbon dioxide was also important. Modeling studies with various degrees of simplification confirm this first impression. A nice summary can be found on the RealClimate blog.

Why, then, do so many people say that carbon dioxide is the “most important greenhouse gas.” It’s probably because of figures like Figure 4. Note a very important adjective at the bottom which is often ignored, “anthropogenic,” meaning “made by humans.” Humans of course affect water vapor as well, but it cycles through very fast, and the amount of water vapor in the air is basically controlled by the temperature of the air and surface. In a climate model, water vapor continuously adjusts to the conditions within the model, while anthropogenic greenhouse gases in Figure 4 are adjusted by those who run the model.

Put another way, water vapor doesn’t appear in the “forcing” terms for climate models, because it is “internal” to the system. It changes as the result of a “feedback” within the model. Thus external inputs like solar radiation, changes in ground cover, and gases introduced into the atmosphere by human activity are counted as “forcing” but water vapor as not.

In short, we can say that carbon dioxide is the most important greenhouse gas whose amount people are directly altering. Not just in models, but in real life.

Partial Misconception: The warming climate means more exposure to dangerous diseases. I say “partial misconception” because there are multiple factors that change our exposure to disease. Many articles in scientific journals and newspapers discuss increased exposure to malaria, for example, in a warming climate. But that is not the whole story. For example, in the United States, malaria was a real threat over much of the country in the 1700s and the 1800s, and even into the early 20th century. However, public health efforts such as mosquito control and changes in peoples’ habits (for example, using window screens to keep out mosquitoes or staying indoors from dusk to dawn) have largely removed the malaria threat. Similarly, world travel spreads germs, such as the West Nile virus, around the world. This is not a new phenomenon. Europeans coming to the Americas brought small pox with them, leading to the tragic death of countless Native Americans. And populations moving into new areas can expose themselves to new germs.

However, we cannot ignore the fact that vectors for existing diseases will migrate with their preferred climate. Thus at some time in the future, some diseases will show up in areas where they haven’t been before; and in other areas where they have been suppressed.

Partial misconception: The warming climate means more birds will die. Again, there are many factors involved. There are stories of bird populations suffering because food supplies (for example caterpillars) are no longer available when the birds need them, because the two species are responding differently to climate change. However, songbird populations have also suffered because the scarcity of predators like wolves has led to an increase in the number of animals (like raccoons) who eat birds’ eggs. Similarly, pesticides have done serious harm to bird populations. This contributed to a ban on the use of the insecticide DDT in many countries. Finally, the West Nile virus has led to the deaths of many birds (although the magpies and crows, which fell victim to West Nile, seem to be recovering here in Boulder).

Once again, we cannot ignore the impact of climate change. If climate changes continue at the predicted rates, then the entire ecosystem will have to adjust to a new seasonal cycle. This will not be a smooth process: different plants and animals will respond in different ways. And, as in the case of the birds and caterpillars, the food supply will be interrupted at critical times.

Partial Misconception: If we cut back on our production of greenhouse gases, global warming will “go away.” This is true only over a very long period of time. It will take hundreds of years to decrease the carbon dioxide content back to pre-industrial levels through natural processes (the lifetime of carbon dioxide in the atmosphere is around 120 years). This does not mean we shouldn’t consider reducing carbon-dioxide emissions, because continuing the increase in carbon dioxide leads to even more warming than if we slow down the increase in carbon dioxide. One hopeful note is that not all greenhouse gases last as long as carbon dioxide, so reducing their release in the atmosphere might help on shorter time scales. Another hopeful note is that people are studying ways to take carbon dioxide out of the atmosphere, but this is the subject of another blog.

So, when you read or hear about the effects of people on the environment, or try to figure out what you can do to help the environment, please remember that we affect our environments in many ways. Similarly, actions we take to help our environment can improve our environment in many ways. But responding to climate change will remain a challenge for years to come.

The amount of CO2 given off by industry in a year

Figure 1 is a diagram of the carbon cycle from the GLOBE Carbon Cycle Project, based at the University of New Hampshire. This diagram shows where the carbon is, and where it is going. So, for example, industry produces about 6 petagrams of carbon a year. What is a petagram? A petagram is written 1,000,000,000,000,000 grams, which can be written 1 times 10^15.

In order to compare the value of CO2 production for human respiration to the “flux” or exchange terms in the diagram (in red), we have to (a) convert it to a flux for carbon rather than CO2 and (b) compute a total for the entire world population for a year.

So, we take

0.9 kg per person per day, times
6,700,000,000 people in the world, times
365 days in a year (neglecting leap years), to get
220,000,000,000 or 2.2 x 10^11 kg or 2.2 x 10^14 grams, or

0.22 petagrams

To convert this to carbon, we multiply by 12/44, the fraction of CO2 that is carbon, to obtain 0.06 petagrams a year.

From Figure 1, that’s about 1% of most of the exchange terms, and about one-hundredth the carbon released by burning fossil fuels globally. And less than one-thousandth the amount of carbon uptake by plants.

What does this really mean? It was pointed out to me by Richard Wolfson, a professor of physics at Middlebury College [who wrote the book Energy, Environment, and Climate (W.W. Norton, 2008), cited a few blogs ago.], that we get our energy from plants, or animals that eat plants, or animals that eat animals that eat plants, so one could argue that we are “carbon-neutral” in the sense that we are part of the natural system, with the plants taking back the carbon we emit. Only in a sense, however – as Wolfson notes, our food production is not carbon-neutral: we produce carbon dioxide in growing the food and transporting the food, not to mention keeping it warm or cold, and, usually, cooking it.

Figure 1. The carbon cycle. The numbers in blue in represent the amount of carbon stored (e.g., 38,000 petagrams of Carbon in the ocean). The numbers in red represent “fluxes” – carbon flowing from one part of the earth system to another. Figure © GLOBE Carbon Cycle.

CO2 from Space

Figure 2 is a snapshot of the global distribution of CO2 at 8 kilometers (5 miles) above the surface. This is high enough so that there is a lot of mixing by the winds, but you can see a pattern anyway. And the pattern is associated with the sources and sinks of carbon in Figure 1.

Figure 2. July 2003 average CO2 from the Atmospheric Infrared Sounder (AIRS) on the Aqua Satellite. From http://www-airs.jpl.nasa.gov/Products/CarbonDioxide/. Preliminary data.

For example, the higher values are associated with the industrial parts of the world. The high values in the north Atlantic are downstream from the United States and Canada. The lowest values are over the high-latitude oceans in the Southern Hemisphere and over Antarctica.

Figure 3. CO2 from AIRS. From http://www-airs.jpl.nasa.gov/Products/CarbonDioxide. Preliminary data. The curve shows carbon dioxide decreasing in the Northern Hemisphere spring and summer, when vegetation is growing and leafing out, and increasing in fall and winter, when respiration dominates.

The “snapshot” in Figure 2 is from the Atmosphere Infrared Sounder (AIRS) on the NASA/Aqua satellite. These data can also be used to look at trends in the global average CO2. Like the well-known surface-based curve from Mauna Loa, there is an upward trend, and you can clearly see the effect of the seasons. If you compare this figure to the curves in the blog Land Use and CO2, posted 7 September 2007, you will find the curves quite similar, with CO2 decreasing during the Northern Hemisphere spring and summer. As noted there, this decrease in carbon dioxide is associated with photosynthesis. During photosynthesis the plants take carbon dioxide out of the atmosphere and use it to grow and leaf out. It is not surprising that photosynthesis is the largest term in Figure 1.

The blog about carbon dioxide (CO2) produced by our bodies during respiration created so much discussion that I decided to work harder to put the numbers into context.

Last time, we calculated an average adult human breathes out between 0.7 and 0.9 kg of carbon dioxide each day. This is based on lots of assumptions, with people of all ages and nationalities counted as processing 0.5 liters of air, 16 times an hour, for 24 hours.

Let’s compare this rough estimate to some other numbers.

The amount of carbon dioxide given off by an automobile in a mile (1.6 kilometers)

I’ve heard a number quoted for this one, but thought it would be good to estimate to find out how close I was, and then I will convert the number to metric units. We start from some facts.

• Density of gasoline is 0.71-0.77 grams per cubic centimeter (that’s 0.71-0.77 kg per liter)
• Gasoline is 85% carbon by mass

So there is approximately 0.74 times 0.85 = 0.63 kg carbon per liter.

This converts to 0.63 kg C x 3.79 liter/gallon or 2.39 kg C per gallon (C=Carbon).

If our car drives 20 miles on one gallon of gas (this is clearly not a very efficient car!), the car burns 2.39 kg per gallon x 1 gallon per 20 miles, or 0.12 kg of carbon per mile.

This is equivalent to 0.12 x 44 divided by 12 = 0.44 kg per mile, or 0.96 pounds (~1) pound of carbon dioxide per mile. Or, in metric units, 0.28 kg per kilometer.

And, driving this car for two miles (3.2 km) produces 0.88 kg carbon dioxide – as much as we produce by breathing all day! (What if the car could travel twice as far per gallon?)

Carbon dioxide released by going from Point A to Point B.

I’m going to suppose someone wants to travel two miles or 3.2 kilometers. That’s a distance many of us would be willing to walk (and about the distance between where I live and where I work).

That means:

If we walk three miles per hour, it would take us 40 minutes to reach Point B walking 3 miles an hour.

If we ride a bicycle at 8 miles (12.8 kilometers) per hour on average, it would take 15 minutes to get to Point B.
The Web is full of charts listing the number of Calories (kCal, abbreviated kCal) used in different types of exercise. I’ll select the following values. For a 155-pound (70 kg) person:

• Walking at 3 miles per hour (4.8 km/hr) burns 250 kCal
• Riding a bicycle at 8 miles per hour (12.8 kilometers per hour) burns 280 kCal

Which means the number kCal burned going from Point A to Point B is:

• 167 kCal walking for 40 minutes compared to 56 kCal for 40 minutes at rest
• 70 kCal riding a bicycle for 15 minutes compared to 21 kCal for 15 minutes at rest

The “at rest” numbers are based on the previous blog, where we used energy production to estimate carbon dioxide output. We assumed a human produced 2000 kCal of energy (equal to the amount eaten) and found that roughly equivalent to 0.7 kilograms of carbon dioxide a day. (0.9 kg a day could be used as well. We used 0.7 simply because that was the number associated with the 2000 kCal.

The carbon dioxide we produce by going two miles on foot or on a bicycle is then, if we count the total:

• 0.7 kg CO2 per 2000 kCal times 167 kCal: 0.058 kg CO2 walking
• 0.7 kg CO2 per 2000 kCal times 70 kCal: 0.025 kg CO2 biking

But the “extra cost” of traveling the distance should be the difference between the “exercising” number and the “at rest” number, namely:

• 0.7 kg per 2000 kCal times (167-56) kCal = 0.039 kg of extra CO2 walking
• 0.7 kg per 2000 kCal times (70-21) kCal = 0.017 kg of extra CO2 riding a bike

Thus: traveling the 2 miles (3.2 kilometers) produces this amount of CO2 above what was produced by respiration at rest:

Traveling 2 miles (3.2 kilometers)

By car: 0.88 kg CO2
Walking: 0.039 kg CO2
Riding a bike: 0.017 kg CO2

While the numbers aren’t exact, the large factor – 20 or more, is probably close. Walking or riding a bicycle does reduce the production of CO2 relative to driving. And – these modes of transportation provide healthful exercise as well! If we have to drive, putting more people in the car reduces the impact of driving. And, driving a car that uses half as much gasoline per unit distance would also help.

This blog was inspired by activities at the 2008 GLOBE Learning Expedition (GLE) in South Africa. As part of their field activities, the students visited the Global Atmosphere Watch station (GAWS) at Cape Point, where carbon dioxide and several other trace gases are measured from the top of a 30-m tower. The carbon dioxide record goes back to 1978, showing a rise comparable to that seen in the Northern Hemisphere.

Standing for much of two days with groups of students at the base of the weather tower at the GAWS site at Cape Point, I found myself wondering how much we were contributing to the carbon dioxide in the atmosphere. I returned home, resolving to estimate how much carbon dioxide an average human gives off in a given day simply by breathing.

Figure 1a. 30-meter tall Global Atmosphere Watch Station (GAWS) tower from a distance. It is located almost at the southern tip of Africa.

Figure 1b Close-up of GAWS tower. The air is pumped in from the top of the tower into the laboratory building, when it is analyzed for the fraction of carbon dioxide and other trace gases.

I will estimate this in two ways. First, based on how many Calories a “typical” human consumes. And secondly, based on how much carbon dioxide is released with each breath.

Based on how much we eat

I start with some rather gross assumptions:

1. The average human eats 2000 Calories (kiloCalories) of food a day
2. 100% of this food is processed, with all the carbon returning to the atmosphere
3. All of the food eaten is in the form of sugars with carbon:hydrogen:oxygen ratios of 1:2:1.

And some information:
Atomic weight of carbon: 12
Atomic weight of hydrogen: 1
Atomic weight of oxygen: 16
Molecular weight of carbon dioxide (2 x 16 + 12 = 44)

This means that:
By mass, the sugars are 40% carbon
By mass, carbon dioxide is 27% carbon

Sugar provides 4 kiloCalories of energy per gram, meaning that our human eats 500 grams of sugar each day. 40% of this or 200 grams is carbon. Assuming all this carbon is released as part of carbon dioxide, our human releases 733 grams of carbon dioxide (200 grams x 44/12).

So, let’s just call our estimate 700 grams of carbon dioxide a day, recognizing that the number is an approximate one.

There are a number of reasons this is probably an overestimate. Our human wouldn’t eat all sugar. He/she would eat some fat as well, which has 9 kiloCalories per gram. We are assuming our human to be in steady state – so that net uptake by the body would be zero. But our human would release carbon in other forms (feces, dried skin, shed hair, etc.) So there would be some solid waste as well as gas – but over long term, there would be some carbon dioxide released from that.

Based on carbon dioxide released through breathing (respiration)

Let’s try another way to estimate the amount of carbon dioxide our human releases. But this time we focus on breathing. Again, some facts:

A human adult breathes 15 times a minute, on average (Reference 1). While I am writing this, my respiration rate is 16 breaths per minute, so this number seems reasonable. And, just for fun, I’ll use my respiration rate.

Each breath exchanges 500 cubic centimeters of air (Reference 2)

Assuming an air density of 1 kilogram per cubic meter, we can find out how many kilograms of air are exchanged for each breath:

500 cm x cm x cm x 0.01 m/cm x 0.01 m/cm x 0.01 m/cm
= 0.0005 cubic meters

0.0005 cubic meters x 1 kilogram per cubic meter
= 0.0005 kilograms of air per breath.

We now use this to estimate the kilograms of air processed each day, which is

0.0005 kilograms per breath x 16 breaths per minute x 1440 minutes per day
= 11.52 kilograms per day “processed” by breathing

To find out how much carbon dioxide is put into the atmosphere, we compare the amount of carbon dioxide (0.038% by volume) inhaled to the amount (4.6-5.9% by volume exhaled, Reference 3.), from the same web site. But first we need to allow that “by volume” means (using carbon dioxide as an example)

0.038 carbon dioxide molecules per 100 air molecules, or
3.8 carbon dioxide molecules per 10000 air molecules.

From above, we know that the molecular weight for carbon dioxide is about 44. The molecular weight for moist air is about 28, which means that the air we inhale contains about

3.8 x 44 divided by 28 x 10000 = or 0.0006 grams carbon dioxide per gram of air

The number “.0006″ is really a fraction – which I am labeling in grams per gram. It could just as easily be pound per pound.

Similarly, the fractional amount of carbon dioxide exhaled, by mass is, assuming 5% by volume:

0.05 x 44 divided by 28 x 100 or 0.0786

So the net fractional change in carbon dioxide for each breath is

0.0786 – 0.0006 or 0.0.078

Now we convert this to a mass by multiplying the fraction times the mass per breath, namely:

11.52 kilograms of air exchanged each day x 0.078 fractional increase in carbon dioxide,

= 0.9 kilograms of carbon dioxide for each day per human.

Again, we made assumptions to make things simple. Our human wasn’t exercising. Our human was an adult. And our human was exchanging a typical amount of air. Recognizing that the number is a crude estimate, I will again round the number to one significant figure, so that we have 0.9 kilograms of carbon dioxide released each day per human.

Isn’t it exciting that we came up with roughly the same answer! For comparison, Wickipedia (http://en.wikipedia.org/wiki/Breathing) quotes an estimate of 900 grams of carbon dioxide a day by the United States Department of Agriculture (USDA).

Here are some questions to think about:

The respiration rate I used was for an average adult. When I measured my respiration, I was sitting, so I’m thinking this is for an average adult at rest. How would these numbers be changed for someone who was exercising? Children breathe faster (Reference 3) but have smaller lungs. How would each of these factors affect the result? Finally, if you wanted a more accurate number, how would you change the calculations?

Comparison to carbon dioxide uptake by plants

How does that compare to some other things?

Prairie near Mandan, ND during the growing season (24 Apr – 26 October) 1996-1999, (reference 4)
1.85 grams CO2 per square meter taken from the atmosphere on average
(Meaning that 380 square meters of land would cancel out the effect of our human) – but remember – this in only during the growing season!

A generic tree (reference 5)

This tree (I’m assuming this is a big one) is said to take up 21.8 kilograms of carbon dioxide a year. For a year, our human produces about 365 x 0.7 kilograms a year, or 255 kilograms. So we’d need 10 of these threes to cancel the carbon dioxide we exhale. This site unfortunately does not quote a source.

Pine forest in Finland (Reference 6)

During the period of measurement, this forest took up
2.4 grams carbon dioxide per square meter per day during July/August, and
1.7 grams carbon dioxide per square meter per day during September

In “human units”, taking 0.7 kg/day, this means we’d need
290 square meters to offset our exhaled carbon dioxide in July and August, and 410 square meters to offset our exhaled carbon dioxide in September.

So – we are part of the carbon cycle, too! At Cape Point, we were breathing out carbon dioxide, but the atmosphere sampled was 30 meters above us – so we probably did not affect the measurements there. But I hear stories from scientists who are measuring carbon dioxide uptake about how they avoid contaminating their measurements. Some of the things they do – push their cars when they get close to the instruments instead of driving them, and leaving their dogs inside the car instead of letting them wander around the site. For more about the carbon cycle, visit the carbon cycle pages on the GLOBE web site.

References

1. p. 151, Berkow, , R., et al., 1997: The Merck Manuel for Medical Information: Home Edition. Merck & Co, publishers, 1509 pp.

2. p. 44, Kapit, W., et al., 1987: The Physiology Coloring Book. HarperCollins. 154 pp.

3. The five percent was decided on based on several references. The Argonne National Laboratory “Ask a Scientist” (http://www.newton.dep.anl.gov/askasci/zoo00/zoo00065.htm) lists 5.3 per cent by volume for “alveolar air” in response to a question about how much CO2 is exhaled. This is slightly lower than the range of values for arterial blood gases derived from p. 907, Taylor, C., C. Lillis, and P. LeMone, 1989: Fundamentals of Nursing. J. B. Lippincott Company, Philadelphia. 1356 pp. On the other hand, http://en.wikipedia.org/wiki/Breath writes exhaled air has 4-5% carbon dioxide by volume, with the BBC listing 4%.

4. Frank, A.B., and A. Dugas, 2001: Carbon dioxide fluxes over a northern, semiarid. mixed-grass prairie. Agricultural and Forest Meteorology. 108, 317-326,

5. http://www.coloradotrees.org/benefits.htm#10

6. U. Rannik et al, 2002 fluxes of carbon dioxide and water vapour over Scots pine forest and clearing. Agricultural and Forest Meteorology, 111, 187-202

Acknowledgments. I talked about this blog a great deal with colleagues. I am indebted to Jimy Dudhia and Greg Holland for contributing useful ideas and information. Also, our sincere thanks to the staff at the Cape Point GAWS station for sharing their facility with the students at the GLE.