Earth’s Carbon Budget: The Carbon Cycle, Sources and Sinks
(Reading #6 for my course on Climate Change, Alan Holyoak, PhD)
1. Be able to explain that the global system has a carbon cycle including carbon sources and sinks.
2. Be able to describe the difference between biosphere carbon and fossil carbon.
3. Be able to explain what “The Big Experiment” is.
4. Be able to explain Henry’s Law and use it to decipher temperature and CO2 concentration curves from ice core data.
5. Be able to explain why atmospheric CO2 concentrations continue to increase, even though land and ocean carbon sinks appear to be able to accommodate increased amounts of CO2.
One thing we do in this course is look at climate change in the past and present, as well as projections for the future. Looking to recent history, humans have been running what has been referred to as “The Big Experiment” since the mid-1800s. And as it turns out, this experiment is affecting Earth’s climate. “The Big Experiment” began when we embarked on the Industrial Revolution, a pathway leading to mechanization in industry, agriculture, and transportation. This mechanization was accompanied by a shift away from using renewable energy sources like wood and waterpower to using increasing amounts of fossil fuels to run modern industry. As a result, the composition of the atmosphere is changing and consequently affecting the rate infrared radiation moves from the Earth back into space. In order to understand what is happening as a result of “The Big Experiment”, you need to understand the sources, movements, and storage of carbon. In other words, you need to understand what the Earth’s carbon cycle and carbon budget are, and how they work.
The Natural Carbon Cycle
The carbon cycle includes additions of carbon to the atmosphere and simultaneous loss of carbon from the atmosphere. Things that add carbon to the atmosphere are referred to as carbon sources, and things that remove carbon from the atmosphere are called carbon sinks. Table 1 lists the main carbon sources and sinks in Earth’s natural carbon cycle.
Table 1. Some natural carbon sources and sinks.
· Biological respiration (releases CO2)
· Decomposition of organic material (releases CO2)
· Swamp sediments (release CH4)
· Animal flatulence (releases CH4)
· CO2 released by the ocean
· Forest and grassland fires
· Release of carbon compounds by weathering of exposed rocks
· Photosynthesis (removes CO2 from the air)
· Organisms secreting CaCO3 shells or skeletons (removes CO2)
· CO2 absorbed by the ocean
· Carbon-containing compounds being trapped in lake and ocean sediments
The emission of carbon into the atmosphere from natural carbon sources and the loss of carbon from the atmosphere into natural carbon sinks has produced regular cycles in concentrations of greenhouse gases over the past one million years or so. During recent ice ages atmospheric CO2 concentrations hovered around 180ppm, and during interglacial warm periods CO2 concentrations increased to levels of about 270ppm. These upper and lower levels have been quite consistent because the factors affecting the carbon cycle were not changing in any major way.
The global carbon cycle, including natural and human-produced carbon sources and sinks, is shown in Fig. 1. In addition to natural sources and sinks, human have recently added anthropogenic carbon sources and sinks to the system. New sources include carbon emission from burning of fossil fuels, carbon release connected with changes in land use, and human-caused fires. The main anthropogenic carbon sink is the industrial production of materials that contain carbon.
Figure 1. The global carbon cycle. “N” refers to naturally occurring carbon pathways, “H” refers to human-produced pathways, and “N/H” refers to pathways that occur naturally and that are accentuated by human activities. (Image: NOAA, modified by ARH)
The carbon taken up by phytoplankton goes with them when they die and drift toward the sea floor, and as they are swept into deep waters with the global conveyor. Many plankton are also are eaten by other organisms before they reach the bottom. Of course, either they or their predators eventually die, and the carbon in their bodies ends up on the seafloor where they are buried by sediment. Over a long period of time, the slow but continual accumulation of carbon and sediment continues. In certain locations geologic processes including high pressure and temperatures produced layers of sediment rich in very high concentrations of carbon. This is the source of oil and natural gas.
On land, plants also carry out photosynthesis and take up CO2 that they use to build their bodies. When plants die the remains of their bodies add carbon to the soil. Over long time periods the slow accumulation of this carbon continues, and eventually resulted in pockets in the Earth’s sediments where it was subjected to intense pressure and converted into coal and natural gas.
The amount of energy stored in these fossil fuels is extremely large. For example, one US gallon of gasoline has energy content of about 132 million Joules. This may not mean that much, but let’s put this information into context with something that is easier to imagine. Elite world-class bicyclists, when riding at full speed, can produce an average of 400 Watts of energy per hour. In other words, they produce 400 Joules per second of work over a one-hour period. At that rate, how many hours would one of these bicyclists have to ride before they generated the same amount of energy that is in one gallon of gasoline? If you do the math you will see that they would have to go at full speed for about 92 hours to expend the same amount of energy that is stored in one gallon of gasoline. Wow!
“The Big Experiment”
Prior to the early 1800s humans relied mainly on renewable energy, e.g., water power, wind power, and burning renewable fuels like wood and animal waste (e.g., buffalo chips) to meet our energy needs. These are renewable energy sources because even if they are temporarily depleted they can regenerate themselves, given enough time (new trees can grow, etc.). Between 1800 and 1850, however, we embarked on a new phase of industrial and technological development we now call the Industrial Revolution. This is when we started to shift away from using mainly renewable energy to using non-renewable energy – mainly fossil fuels. Since then our reliance on fossil fuels has grown continuously, following population growth and industrial activity.
When we started using fossil fuels we had no idea that this would affect the climate, but as it turns out, it does. When we burn renewable energy sources like wood or biofuels the carbon they release was already part of the biosphere, and therefore does not change the net total amount of carbon in the biosphere. When we burn fossil fuels, however, we are emitting carbon into the biosphere that was removed from the biosphere long ago and trapped in rocks. So when this carbon enters the atmosphere it produces a net increase in total carbon in the biosphere. This is “The Big Experiment”. In other words, is the global climate affected when we disrupt historically stable levels of carbon and other greenhouse gases in the biosphere?
As mentioned above, fossil fuels are extremely concentrated forms of carbon and energy. This makes them effective and attractive sources of energy for driving our industrial world. Fossil fuels, of course, provide us with a myriad of benefits. At the same time though, the use of fossil fuels has its downside. It is important to understand that a very long time – millions or even hundreds of millions of years, is required for the Earth to accumulate and produce new reserves of fossil fuels. When we mine or burn them we are using energy harvested from the sun by photosynthesis over eons.
Figure 2 shows the amount of various fossil fuels produced during past geologic periods. By human life spans, it requires a LONG time to form them. The authors of the study who produced this graph also did calculations comparing geologic rates of fossil fuel formation to our present rate of use of these substances for energy. The result is compelling. They state, “The amount of fuel we burn in one year required 175,000 years to sequester.”
Figure 2. The geologic age and amount of each class of fossil fuel formed. Most of the heavy oil was formed between 140-23 mya (million years ago). Most of the light oil or “sweet crude” was formed between 196 and a few million years ago. Natural gas formed at about the same time as sweet crude, with an additional pulse of production 300-230 mya, during the Permian, when our largest coal reserves were formed. (Image: Patzek and Pimental, 2005.)
These data tell us some important things. First, fossil fuels are effectively non-renewable, and once we take as much as is technologically possible, there will be no more we can get. Some estimates state that we have a minimum of 300 years of coal remaining, and who knows how many years of oil exactly? Decades, certainly, but how many? Estimates vary depending on the sources you consult. Second, these data show how rapidly we are sending carbon into the atmosphere, considering how long it took to form. The global carbon budget is being altered, resulting in increasing CO2 concentrations in the atmosphere.
The Global Carbon Budget
You are already familiar with the concept of a natural budget from our earlier discussion on Earth’s energy budget – the balance between energy coming in and leaving the Earth. Similarly, the atmosphere has a carbon budget – the balance between the rate of carbon added to the atmosphere and the rate carbon is lost rom the atmosphere. This budget describes the sources and fates of carbon as it moves through the environment. The Global Carbon Project calculates anthropogenic (human-produced) contributions to Earth’s carbon budge each year. Figure 3 compares annual anthropogenic carbon emissions from 1990-2000 and 2000-2009.
Figure 3. The amounts of carbon moved by the pathways indicated. The blue numbers indicate the amount of carbon moved in gigatons of carbon per year between 1990-2000. The red numbers indicate the amount of carbon moved during 2000-2009. (Image: The Global Carbon Project.)
Humans are responsible for two significant sources of anthropogenic carbon emission into the atmosphere that cannot be accounted for by natural processes. First is the emission of carbon from burning fossil fuels. This also includes the production of cement that produces a smaller, but still significant amount of carbon emission. The second source is carbon released into the atmosphere as a result of land-use changes. For example, when we remove vegetation, especially forests, often the residual plant material is burned which released carbon directly into the atmosphere. Plus, deforestation removes a community that previously removed CO2 through photosynthesis, and we typically replace forests with a different environment that either absorbs less CO2 or none at all. By so doing we have compromised or removed an important carbon sink.
It is important to note that less than half of the annual carbon emissions stay in the atmosphere. Where does the rest go? Terrestrial ecosystems and the ocean absorb it. Vegetation on land surfaces removes carbon through photosynthesis and stores it in plants and ultimately the soil. This is most pronounced in forests. There is also photosynthesis in oceans, and seawater absorbs CO2 directly as well. The amount of CO2 that can be dissolved in water is related to temperature by a well-known expression called Henry’s Law.
OK, this is important, so pay attention! Henry’s Law states that as water temperature increases, less CO2 is able to stay in solution, and as water temperature decreases, more CO2 can stay in solution. This means that cold seawater can absorb more CO2 than warm seawater. You are probably already familiar with this principle. Think about a soda. When you open a soda that is cold less CO2 fizzes out than when it is warm. You model Henry’s Law if you want to. Get two cans of the same kind of soda. Cool one can in a bowl containing water and lots of ice cubes. At the same time, warm the other can of soda in a pan of water (don’t use a microwave, that could be a disaster!). Oh, don’t heat the can too much…it could explode. When you have one cold can and one warm can, open them side-by-side and observe the difference. OK, back to the carbon story.
The sizes of the two main carbon sinks – land and oceans – are not known exactly, so the numbers shown in Fig. 3 are informed estimates. Our best data show that land and oceans together absorb 53% of anthropogenic CO2 emitted into the atmosphere. This means that 47% of anthropogenic carbon emissions stays in the atmosphere! This is important, because the rate of increase of CO2 in the atmosphere would be much greater without these sinks. This observation obviously leads to a pressing question: Will these sinks continue to take up additional emissions of anthropogenic carbon from the atmosphere indefinitely?
First, consider the global distribution of the main carbon sources and sinks (Fig. 4). The main carbon sources are in the middle latitudes, mainly in the northern hemisphere. These latitudes align with the regions of major industrial economies. Note the carbon sinks in the Amazon rainforest in South America, tropical Africa, and the tropical and polar oceans.
Figure 4. The global distribution of carbon sources and carbon sinks. (Image: NASA.)
Changes in Atmospheric CO2 Concentration
How much has CO2 concentration in the atmosphere changed over time? Let’s consider several time scales as we ponder this question. First, let’s look at the past 800,000 years. We will consider paleoclimatology in more detail in the next reading, but all data indicate that for the past 800,000 years the main variations in atmospheric temperatures and CO2 concentrations were associated with natural cycles of ice ages and interglacial periods driven by Milankovitch Cycles that we have already discussed.
Scientists have plotted temperature and CO2 concentration data from Antarctic ice cores going back 800,000 years (Fig. 5). They discovered that global temperature changes are probably triggered by Milankovitch Cycles, which happen first, followed by changes in atmospheric CO2 levels. Some people have erroneously tried to use this observation to support a hypothesis that CO2 levels are not a significant factor in climate change. Sorry, but that is not correct. CO2 plays a large role in Earth’s climate. So, why do CO2 levels sometimes lag behind temperature change curves? This is where knowing about the high specific heat of water and Henry’s Law gives us a clearer picture. How? Read on.
Figure 5. Temperature, CO2 (ppm), and methane (ppb) profiles from Antarctic ice core data. Note that the CO2 profile tends to show a slight lag time following shifts in the temperature profile. See explanation in the reading for clarification. (Image: the Environmental Defense Council.)
When the Earth undergoes a global temperature shift, the atmosphere and land change temperature much more quickly than oceans do. This is because of the high specific heat of water. Recall that this is the physical property of water that explains that water has to absorb a large amount of energy to change its temperature just a little (much more than either earth or air). And, when you consider the huge volume of water in the oceans, it takes a MASSIVE amount of energy to change the ocean temperature even a little bit. But, eventually sufficient energy can be absorbed, and the ocean will warm. Recall that Henry’s Law states that cool water can hold more CO2 in solution than warm water. The lag time between warming of the atmosphere and warming of the ocean is what you should expect due to the high specific heat of water and Henry’s Law. That is, the ocean will heat more slowly than land, but once it warms enough it will release additional CO2. The same thing happens in reverse when the Earth enters an ice age. It takes a period of time for the oceans to cool enough to absorb enough CO2 to make a significant difference in atmospheric concentrations of that gas. This is why gradual cooling typically follows rapid warming.
If you look again at Fig. 5 you will see that it shows temperature, CO2, and methane profiles, but if you look at the far right end of the graphs you will see CO2 and methane curves shooting off of the top of the chart. The concentration of CO2 reached a high of 397ppm in May 2012 (Fig. 6; NOAA). Note the consistent range of CO2 between 180-280ppm over most of the past 800,000 years, and the very rapid rise of CO2 in the last century.
Figure 6. Concentrations of atmospheric CO2 at the Moana Loa Observatory, Hawaii, 1958-2012. (Data: NOAA)
In 1958, the International Geophysical Year, scientists started taking measurements of the atmosphere, including CO2, at an observatory on the top of Mauna Loa on the Big Island of Hawaii and at a monitoring station in the Antarctic. These are excellent sites for monitoring the atmosphere. They are the most geographically isolated places on Earth, and are far away from centers of industry or other significant sources of anthropogenic CO2 emission. It is important to note the obvious trend showing significant increases in total atmospheric CO2. Though Fig. 6 does not show this, there is an annual cycle of decreasing and increasing CO2 levels. The decreasing part of the jagged annual cycle of CO2 flux occurs when the northern hemisphere is entering springtime. This is when the deciduous forests of North America, Europe, and Asia produce leaves and grow. When they do this they take up large amounts of CO2. Later, during the northern hemisphere fall and winter, deciduous forests drop their leaves, these leaves decompose, and much of the carbon stored in them is returned to the atmosphere. This produces an annual pulse in atmospheric CO2 levels. There is also a slight delay in the atmospheric data compared to when these forests do this, because it takes some time for the atmosphere to mix.
Other Greenhouse Gases
There are, of course, greenhouse gases other than CO2 that affect climate. Some of these include CH4 (methane), N2O (nitrous oxide), and CFCs (chlorofluorocarbons), as well as water vapor. The concentrations of these gases, except water vapor, are shown in Fig. 7. Methane, CO2, and N2O levels show increasing concentrations over the past 30 years, but many kinds of CFCs have leveled off and some are actually in decline. You may be interested to know that CFCs and other similar compounds are known to be the primary culprits in stratospheric ozone depletion. Because of this, industrial nations signed an agreement in 1986, the Montreal Protocol. The Montreal Protocol mandated a complete ban on the production and use of CFCs. That agreement went into effect in 1989. Some CFCs are long-lived molecules (as long as 100 years), so it is not surprising that their levels remain high, even more than 20 years after the Montreal Protocol went into effect. But, since CFCs are no longer being emitted into the atmosphere in large amounts, their overall concentrations have leveled off. The other gases, however, continue to be emitted, and their concentrations continue to rise.
Figure 7. Concentrations of CO2 (top left), methane (bottom left), nitrous oxide (top right), and CFC-12 and CFC-13 (bottom right) between 1978-2009. (Image: NASA)
So, what about water vapor? Water vapor is an effective greenhouse gas, but it is different in one respect than other kinds of greenhouse gases: water vapor cannot accumulate in the atmosphere the way other greenhouse gases do. Water evaporates and enters the atmosphere as water vapor. It accumulates until the atmosphere reaches 100% humidity. The atmosphere then drops the water vapor as precipitation, and the cycle begins again.
The direct emissions of carbon from fossil fuel use and indirect emissions due to land use change have been tracked over time. The fossil fuel emissions have grown rapidly over the past decade (Fig. 8). The brief downturn in fossil fuel emissions is believed to be associated with the recent economic recession. The total amount of annual carbon emissions from land use accounts for about 10% of all anthropogenic carbon emissions. Interestingly, land use emissions appear to be tapering off slightly, though this does not yet represent a significant decline (Fig. 9).
Figure 9. CO2 emissions from burning fossil fuels (upper line) and release into the atmosphere due to land-use change (lower line) 1960-2009. (Image courtesy of the Global Carbon Project.)
Currently, the total annual anthropogenic emission of CO2 is just over 10 petagrams or 10 gigatons of carbon per year (a petagram is 1015 grams of carbon, and a gigaton is 109 metric tons or 1012 kilograms). Just in case you are not that familiar with the metric system, one kilogram is 2.2 pounds, so one metric ton is about 2200 pounds. No matter how you slice it, these are BIG numbers!
Next let’s look at terrestrial carbon sinks. Global deforestation has significantly reduced total area covered by forests, one of the most important terrestrial carbon sinks. Figure 10 compares the best estimates of original forest cover and the remaining forest cover on Earth. Much of this deforestation has happened in recent years.
Figure 10. Original total global forest cover (top), and forest cover in 2005 (bottom). Green indicates forest cover, and tan indicates non-forested areas. (Image: The World Resources Institute.)
Now that we have some idea about sources and sinks of CO2 on the global scale, let’s look at regional differences of carbon emissions from burning fossil fuels and land use changes (Fig. 11). Land use in North America is providing a small carbon sink. Increases in eastern forest cover over recent decades are probably responsible for this, but emissions from industry are huge. Europe is a large emitter, with only a small amount of emission from land-use. Asia is the biggest contributor in both categories. Asia is cutting down its tropical forests and increasing industrial activity, relying mainly on coal. Countries like China, India, and Indonesia are all emerging industrial nations. South America and Africa have larger land use emissions than industrial – this is due mainly to deforestation. China has now surpassed the USA as the world’s largest single-nation carbon emitter. This is related to their rapid economic development, and abundant supplies of coal upon which they rely for their industrial energy.
If, however, emissions are considered on a national basis, China, the USA, India, Russia, Japan, and Germany are at the top emitters (Fig. 12). But, when we divide total emissions by population size, four countries stand out above the rest in per capita emissions: the USA, Saudi Arabia, Australia, and Canada. Why is this the case? Saudi Arabia is an oil-producing nation, and emits a lot of carbon as a by-product of that industry. Australia and Canada both have relatively small populations, highly developed economies, and industries that require energy to keep them running. Of the top four, only the USA has both a high per capita and large national emissions. This means that people in the USA have the largest carbon footprint of any nation in the world.
Figure 11. All bars extending upward indicate emissions of CO2. Red bars show emissions from industry (mainly fossil fuel use), and green bars show emissions due to land-use changes. (Image: The Global Carbon Project.)
Figure 12. National total and per capita carbon emissions for the top 20 emitting nations in the world. (Image: The Global Carbon Project.)
The concentration of CO2 in the atmosphere has continued its rapid rise over the past few years. Figure 13 shows the monthly CO2 concentrations from Mauna Loa in red and the updated monthly averages in black. This graph looks only at a 12-month average. Also shown in Fig. 13 is the average rate of increase for each year from 2000-2009. You need to remember that these values do not show the amount of CO2 in the atmosphere, just the rate of change in the amount of CO2 there.
Figure 13. Annual increases in atmospheric CO2 (top left graph), decadal rates of CO2 accumulation in the atmosphere from 1970-2009 (bottom left box), and annual growth rate of atmospheric CO2 2000-2009 (left box). (Image courtesy of the Global Carbon Project.)
What are the main carbon sinks, and how big are they? In other words, how much carbon are they able to remove from Earth’s atmosphere in relation to carbon emissions?
Figure 14 shows the annual averages of carbon flow for 2000-2009 during which an average of about 8.8 PgC was emitted per year. In 2010, however, total anthropogenic emissions rose to 10 PgC/yr. The more worrisome thing is that it appears that land and ocean sinks are not able to accommodate as much CO2 as they used to, so a larger percentage of the CO2 is remaining in the atmosphere.
Figure 14. Global sources and sinks of anthropogenic carbon in 2010 (Image: The Global Carbon Project.)
The oceans and land together currently provide a sink for only about half of the anthropogenic carbon emitted each year. This important, since the observed increases in atmospheric CO2 concentrations would have been much greater without these sinks. Figure 15 shows changes in annual carbon emissions and the amount remaining in the atmosphere over the past 50 years. The amount staying in the atmosphere shows variability from year to year. Why is this? This variability is tied to natural variations in the amount and rate of photosynthesis and respiration, as well as ocean temperatures. The conclusion supported by these data is that emissions have grown rapidly, but the amount of carbon remaining in the atmosphere has increased much more slowly, thanks to the capacity of global carbon sinks so far.
Figure 15. Total annual carbon emissions 1960-2009 (above), and amount staying in the atmosphere (below). (Image courtesy of the Global Carbon Project.)
When data over the last 50 years are examined we can see changes in the proportion of CO2 emissions staying in the atmosphere (Fig. 16). The trend line statistically fitted to the data shows that the proportion of CO2 remaining in the atmosphere is gradually increasing, e.g., 5% more CO2 is remained in the atmosphere in 2009 than was observed in 1960. This suggests that carbon sinks are no longer keeping pace with carbon emissions.
Figure 16. Fraction of CO2 remaining in the atmosphere, 1960-2009. (Image courtesy of the PNAS.)
Figure 17 shows graphs for land and ocean sinks over time. Negative values on that graph indicate carbon being removed from the atmosphere. Annual variations in temperature and the amount and timing of precipitation can have a significant affect on photosynthesis of large ecosystems – like entire forests or grasslands – in a given year, so the value for the land varies greatly. Because of this it is sometimes difficult to see a readily identifiable trend for the global land sink, though a slight trend toward absorbing more carbon may be appearing. Ocean results, on the other hand, show a clear trend. It shows that increasing amounts of carbon are being removed from the atmosphere each year. Yet, if the size of the combined sinks is growing larger, why is the fraction of CO2 in the atmosphere still increasing? The short answer is that while the land and ocean sinks are taking up larger amounts of carbon than they used to, this is not enough to keep up with the amount of carbon being emitted into the atmosphere. The total amount of CO2 remaining in the atmosphere is therefore rising. While this is worrisome, just imagine what the atmosphere would be like without those sinks!
Figure 17. Natural carbon sinks – land (top) and oceans (below). Negative values represent carbon leaving the atmosphere. (Image courtesy of PNAS.)
Hipps, LE. 2010. Personal communication and readings produced by Dr. Hipps. Professor of Atmospheric Science, Department of Plants, Soils, and Climate. Utah State University.