earth 103 - module 5

How far back in time do you have to go to find CO2 values equivalent to those today?

2.5 million years

runoff

Although most of the carbon loss from the soil reservoir occurs through respiration, some carbon is transported away by water running off over the soil surface. This runoff is eventually transported to the oceans by rivers. The actual magnitude of this flow is a bit uncertain, although it does appear to be quite small. The most recent estimates place it at 0.6 Gt C/yr.

Photosynthesis

Conversion of light energy from the sun into chemical energy.

One faucet represents the natural addition of water to the tub, while the other represents a new flow. The tub has a drain in it and it takes out 10% of the water in a given time interval (the value of k is 0.1 and the drain flow is just the amount in the tub times k). When you first open the model, the extra faucet is set to zero, and if you run the model you will see that it is in a steady state — the faucet and drain are equal, so the amount in the reservoir remains constant. What happens to the system if you increase the faucet flow using the upper knob to the right of the graph? If the faucet flow increases, then at first it will be greater than the drain and the amount in the tub will increase; but as faucet flow increases, so will the drain flow, and it will eventually become equal to the faucet flow and the system will return to steady state, but one with more water in the tub. Give it a try, and see what happens. At steady state, you can write a simple equation that says: F = k*W It turns out that the response time for this system is just 1/k. In this example model, k = 0.1 so the response time is 10 time units (the units depend on how the flows are given — liters per second or minute). This response time is not the length of time required for the system to get into a steady state — it is the time required to accomplish 63%, or the change to the new steady state. Why 63%? There is some math behind this choice, but in essence, it is just a convention that helps us avoid the problem of picking the time when steady state is achieved, which is difficult since it does so asymptotically. The figure below shows how this looks. In the graphs below, the blue line for the water tub is hidden beneath the red curve of the drain — these have different scales on the left hand side, but they are the same exact shape, so one plots over the other.

I changed the faucet here to 3, and the water in the tub rose until it reached a value of 30, where the drain value then became equal to the faucet value and the system reached a new steady state. As shown by the blue arrow above, this takes 10 time units to accomplish 63% of the change. So, this is a system that has a negative feedback (the drain) that drives it to a steady state, which means that regardless of the values of the faucet or the drain constant, k, the system will find a steady state. Now, look at what happens if we turn on a new faucet — add a new flow to a system that is in a steady state. In the figure below, you see what happens if we turn the extra faucet on for a bit and then turn it off.

plant respiration and litter fall

If we think of photosynthesis as the process of making fuel (carbohydrates), then respiration can be thought of as the process of burning that fuel — using it for maintenance and growth. This process can be described in the form of a reaction, just like photosynthesis. The chemical reaction is just the reverse of photosynthesis. Through respiration, plants (and animals) release water, carbon dioxide, and they use up oxygen. Do the carbon flows involved in respiration and photosynthesis balance each other as the equations seem to imply? The answer is no — otherwise, how could organisms grow? Experiments on a variety of plants indicate that the ratio of photosynthesis to respiration is generally about 2 to 1. When plants are young, and growing rapidly, but with not much biomass to maintain, this ratio is even higher; in older, larger plants, this ratio is lower since more carbon needs to go towards maintenance. Litter Fall Dead plant material enters the soil in two ways -- it falls on the surface as litter, and it is contributed below the surface from roots. The relative importance of these two pathways into the soil varies according to the plants in an ecosystem, but it appears that the two are commonly about equal, which may seem a bit surprising since loss of organic carbon from root systems is a process that we generally don't see. The flow of carbon associated with litter fall is roughly the difference between the photosynthetic uptake of carbon and the return of carbon through plant respiration. If this were not the case, then the size of the global land biota reservoir would be growing or declining, and although some regions are growing, others are shrinking, and they nearly balance out.

The striking feature of these data is that there is an exponential rise in atmospheric CO2 (and methane, another greenhouse gas) that connects with the more recent Mauna Loa record to produce a rather frightening trend. Also shown in the above figure is the record of fossil fuel emissions from around the world, which show a very similar exponential trend. Notice that these two data sets show an exponential rise that seems to begin at about the same time. What does this mean? Does it mean that there is a cause-and-effect relationship between emissions of CO2 and atmospheric CO2 levels? Although we should remember that science cannot prove things to be true beyond all doubt, it is highly likely that there is a cause-and-effect relationship -- it would be an extremely bizarre coincidence if the observed rise in atmospheric CO2 and the emissions of CO2 were unrelated. How serious is our modification of the natural carbon cycle? Here, we need a slightly longer perspective from which to view our recent changes, so we return to the records from ice cores and look deeper and further back in time than we did in the figure we have been examining.

In addition to providing a record of the past concentration of CO2 in the atmosphere, as we learned in Module 1, the ice cores also give us a temperature record. By studying the ratios of stable isotopes of oxygen that make up the glacial ice, we can estimate the temperature (in the region of the ice) at the time the snow fell (glacial ice is formed by the compression of snow as it gets buried to greater and greater depths). From these data, shown in the figure below, we can see the natural variations in atmospheric CO2 and temperature that have occurred over the past 160,000 years (160 kyr).

carbon dioxide through time

In the late 1950s, Roger Revelle, an American oceanographer based at the Scripps Institution of Oceanography in La Jolla, California began to ring the alarm bells over the amount of CO2 being emitted into the atmosphere. Revelle was very concerned about the greenhouse effect from this emission and was cautious because the carbon cycle was not then well understood. So, he decided that it would be wise to begin monitoring atmospheric concentrations of CO2. In the late 1950s, Revelle and a colleague, Charles Keeling, began monitoring atmospheric CO2 at an observatory on Mauna Loa, on the big island of Hawaii. Mauna Loa was chosen because its elevation and location away from industrial centers made it as close to a global signal as any other location. The record from Mauna Loa, one of the most classic plots in all of science, shown in the figure below, is a dramatic sign of global change that captured the attention of the whole world because it shows that this "experiment" we are conducting is apparently having a significant effect on the global carbon cycle. The climatological consequences of this change are potentially of great importance to the future of the global population. The CO2 concentration recently crossed the 400 ppm mark for the first time in millions of years! In 2018, the yearly average was 412 ppm (check that number with the curve below!). As the Mauna Loa record and others like it from around the world accumulated, a diverse group of scientists began to appreciate Revelle's concern that we really did not know too much about the global carbon cycle that ultimately regulates how much of our CO2 emissions stay in the atmosphere. The importance of present-day changes in the carbon cycle, and the potential implications for climate change became much more apparent when scientists began to get results from studies of gas bubbles trapped in glacial ice. As we learned in Module 1, the bubbles are effectively samples of ancient atmospheres, and we can measure the concentration of CO2 and other trace gases like methane in these bubbles, and then by counting the annual layers preserved in glacial ice, we can date these atmospheric samples, providing a record of how CO2 changed over time in the past. The figure below shows the results of some of the ice core studies relevant for the recent past -- back to the year 900 A.D.

the marine carbon cycle

Processes of Carbon Flow in the Oceanic Realm Far less obvious to us than the terrestrial processes we just discussed, the cycling of carbon in the oceans is tremendously important to the global carbon cycle. For example, the oceans absorb a large portion of the CO2 emitted through anthropogenic activities. As with the terrestrial part of the global carbon cycle, we will explore here the various processes involved in transferring carbon in and out of the oceans. Below, we see a general depiction of the flows involved in the oceanic realm, along with the flow magnitudes.

soil respiration, permafrost and runoff

Respiration (sometimes called decay) also occurs within the soil, as microorganisms consume the dead plant material. In terms of a chemical formula, this process is the same as described above for plant respiration (the reverse of photosynthesis). There is an unseen but fascinating universe of microbes living within the soil, and they are the key means by which nutrients such as carbon and nitrogen are cycled through the soil system. A great diversity of microorganisms live in the soil, and they are capable of consuming tremendous quantities of organic material. Much of the organic material added to the litter (the accumulated material at the surface of the soil) or within the root zone each year is almost completely consumed by microbes; thus there is a reservoir of carbon with a very fast turnover time — on the order of 1 to 3 years in many cases. The by-products of this microbial consumption are CO2, H2O, and a variety of other compounds, and are collectively known as humus (not the same as Hummus, the Mediterranean chickpea puree!). Humus is a much less palatable compound, as far as microbes are concerned, and is not decomposed very quickly. After it is produced at shallow levels within the soil, it generally moves downward and accumulates in regions of the soil with high clay content. Part of the reason it accumulates in the lower parts of the soil is that there tends to be less oxygen in that environment, and the lack of oxygen makes it even more difficult for microbes to work on this humus and decompose it further. But eventually, due to various processes (animals burrowing, people plowing, etc.) that stir the soil, this humus moves back up to where there is more oxygen, and then the microbes will eventually destroy the humus and release some more CO2. This humus then constitutes another, longer-lived reservoir of carbon in the soil. Carbon 14 (14C) dates on some of this soil humus give ages of several hundred to a thousand years old. Taken together, the fast and slow decomposition processes, both driven by microbes, lead to an average carbon residence time of around 20 to 30 years for most soils. The data used in our global carbon cycle model lead to a residence time of about 26 years for the global soil carbon reservoir. These microbes (considered in terms of their respiratory output) are very sensitive to the organic carbon content of the soil as well as the temperature and water content, respiring faster at higher carbon concentrations, higher temperatures and in moister conditions. Permafrost - an unknown In recent years, increasing attention has been directed at permafrost soil carbon since the polar regions are warming much faster than the rest of the globe. Permafrost is soil that has been frozen for at least two years. As the permafrost melts, carbon that was added to these soils by processes like litter fall will become available for soil microbes to respire and release to the atmosphere. In fact, it is almost surely happening already, but given that much of the permafrost is still frozen, we have probably not seen the real manifestation of this source of carbon. Estimates are variable, but a figure like 1000 to 1500 Gt of carbon reflects the current thinking; this is a huge amount of carbon and has the potential to significantly alter the future of atmospheric CO2 levels. As the permafrost begins to melt, some estimates are that it will contribute something in the range of 2-5 Gt C/yr, which is large compared to the human-related changes. Of course, some of this released carbon will be offset by new carbon sequestered into these formerly frozen soils, but initially, the system will not be in equilibrium and these regions can be expected to be a net source of CO2 to our atmosphere.

The mantle reservoir is huge and somewhat removed from the other reservoirs, thus we will not really bother with it. Among the other reservoirs, you can see that there is a huge range in the sizes. The ocean biota contain a very small amount of carbon relatively speaking, while sedimentary rocks contain a vast quantity (in the form of calcite — CaCO3 — that forms limestones, and coal, petroleum, etc.). Now, let's look at the system with the flows included, as arrows connecting the reservoirs:

The black arrows represent natural processes of carbon transfer, while the red arrows represent changes humans are responsible for. The magnitudes of the flows are shown in units of gigatons of carbon per year. The diagram as constructed here represents a steady state if we just consider the black arrows; the flows going into each reservoir are equal to the flows going out of the reservoir — in other words, there is a balance. We will step through this system, talking about the processes involved in the flows, but first, let's try to learn something from the numbers themselves in this diagram. First, just a bit more on the notion of steady state. The diagram below illustrates some simple systems, one of which is not in a steady state, and the others of which are.

In the first example, A, more (10 units per time) is subtracted from the reservoir than is added (5 units per time), and so over time, the amount in the reservoir will decline — it will not remain constant, or steady. In each time step, it will lose 5 units of whatever the material is. In the other two (B,C), the amount added is the same as the amount subtracted, so these reservoirs will be in a steady state. If you look at example C, you see that the sum of inflows (5+5) is equal to the outflow (10). When a system is in a steady state, we can say something about the average time something will spend in the reservoir — this is called the residence time for the reservoir. Here is a simple example — if there are 40,000 students at Penn State, and 10,000 students enter each year and 10,000 students graduate each year, then the system is in a steady state. The residence time is the total number of students divided by either the number entering or graduating each year — this give 4 years as the average residence time. Here is a figure to explain the idea further:

The concept of residence time is a useful one in studying any kind of system because it tells us something about how quickly material is moving through a system, and more important, it tells us how quickly some part of the system, or the system as a whole, can respond to changes. If something has a short residence time, it can respond quickly to changes, whereas if it has a long residence time, it responds very slowly. Mathematically, the residence time is the same as something called the response time which as the name implies, is a measure of how much time it takes for the system to respond to a change. The word "respond" in this context means "return to a steady state." Although the residence time and the response time are often the same value, they represent different ideas. As we said earlier, the residence time is a measure of the average length of time something spends in a reservoir — like the average length of time a carbon atom spends in the atmosphere. The response time, instead, is a measure of how quickly something returns to steady state after some disturbance that knocks it out of steady state. So, response time is only meaningful in cases where a system has a tendency to remain in a steady state. Now, let's turn to the carbon cycle and consider some of the flows in and out of the reservoirs. What is the residence time for the atmosphere? To get this, we take the amount in the reservoir (750 GT) and divide it by the sum of the inflows or the outflows. Let's take the outflows: 100 GT C/yr for photosynthesis + 90 GT C/yr going into the oceans. The residence time is thus: 750 GT/190 GTyr-1 = 3.9 years This is a pretty short residence time. Now, let's look at the deep ocean (which is the vast majority of the oceans) — its residence time is: 38000 GT/10 GTyr-1 = 3,800 years We use 10 for the inflow/outflow value because we use the net of the water flux into and out of the deep ocean. The result, 3,800 years, is much longer than the atmosphere, and what this means is that the carbon cycle has some parts that respond quickly, but other parts that respond very slowly, and the very slow parts tend to put a damper on how quickly the other parts can change. In other words, if we suddenly inject carbon dioxide into the atmosphere, you might think that the short residence time of the atmosphere means that the excess CO2 can be removed very quickly, but because these reservoirs are linked together, it turns out that the deep ocean must return to its steady state before the atmosphere can get back to its steady state.

Overview of the Carbon Cycle from a Systems Perspective

The global carbon cycle is a whole system of processes that transfers carbon in various forms through the Earth's different parts. The carbon that is in the atmosphere in the form of CO2 and CH4 (methane) doesn't stay in the atmosphere for long — it moves from there to other places and takes different forms. Plants use the CO2 from the atmosphere in photosynthesis to make carbohydrates and other organic molecules, and from there it may return to the atmosphere as CO2 or it may enter the soil as still different compounds that contain carbon. Some carbon is deposited in sedimentary rocks from the oceans, and much later, this carbon may be released to the atmosphere. So, carbon moves around — it flows — from place to place. Because CO2 is such an important greenhouse gas, the way the carbon cycle works is central to the operation of the global climate system. Later in this module, we will work with a computer model of the carbon cycle to do experiments that will help us understand how it works, but it will help to begin with an overview of the carbon cycle from the systems perspective. What is meant by a systems perspective? It just means that we focus on the places where carbon resides (the reservoirs, in systems terminology), how it moves from reservoir to reservoir, how much of it moves from place to place, and what controls those movements. This same perspective is behind the simple climate model we worked on in Module 3. First, let's consider the main reservoirs of carbon. These can be seen in the diagram below, where each box represents a different reservoir, and remember that in each of them, the carbon may be in very different forms. Note a GT or gigaton is a billion metric tons or 1015 grams which is a whole lot of carbon!

In fact, looking at this much longer span of time enables us to clearly see that the present CO2 concentration of the atmosphere is unprecedented in the last several hundreds of thousands of years. As geoscientists, we are interested in more than just the last few hundred kiloyears, and so we look back into the past using sediment cores retrieved from the deep sea. Geochemists studying these sediments have been able to reconstruct the approximate concentration of CO2 in the atmosphere and the sea surface temperature (SST).

To find atmospheric CO2 levels equivalent to the present, we have to go back 2.5 million years. This means that, to the extent that the state of the carbon cycle is closely linked to the condition of the global climate, we are pushing the system toward a climate that has not occurred any time within the last several million years -- not something to be taken lightly. The farther back in time we go, the more difficult it is to figure out how CO2 concentrations have changed, but that has not stopped some from attempting: One thing that is clear is that further back in time, CO2 levels have been much, much higher, and the average global temperatures have also been much higher. Why does the CO2 concentration change so much? This is a big question whose answer involves many factors, but consider two that are relevant to what we'll learn about in this module. Photosynthesis only started in the Silurian (S on the timescale in the figure above), and photosynthesis is a major sink or absorber of atmospheric CO2. Sea level was much higher during the two big peaks in CO2 — this leaves less room for photosynthesis and it also decreases the planet's albedo, making it warmer. A warmer ocean cannot absorb atmospheric CO2 and instead, it releases it to the atmosphere. In conclusion, from this brief look at the record of fossil fuel emissions and atmospheric CO2 concentrations, it is clear that we have cause for concern about the effects of the global CO2 "experiment". Because of this concern, there is a tremendous effort underway to better understand the global carbon cycle. In the remainder of this module, we will explore the global carbon cycle by first examining the components and processes involved and then by constructing and experimenting with a variety of models. The models will be relevant to the dynamics of the carbon cycle over a period of several hundred years -- these will enable us to explore a variety of questions about how the system will behave in our lifetimes and a bit beyond.

a simple analogy

We're going to take a step backwards for a second and think about a much simpler system, but one that has some things in common with the global carbon cycle. First, however, we need to recognize that the natural carbon cycle is something that always varies a bit, but it has some important feedbacks in it that tend to make it stable or steady. And then, along come humans, burning an impressive amount of fossil fuel and creating a new flow in the carbon cycle. To get a sense of how a system with a tendency to remain in a steady state might respond to a new flow, we turn to a simpler model. Our simple model is a water tub with a drain and two faucets.

The system gets thrown out of its steady state temporarily, but returns to the original steady state when the extra faucet is turned back off. Note that there is a lag time here — the water tub peaks about 5 time units after the faucet peaks (this is the sum of the two faucets). If we were to decrease k, then the response time would lengthen, and the lag time would also lengthen. Think of this spike in the extra faucet as being equivalent to a short-term addition of CO2 into the atmosphere. But what if we turn the extra faucet on and then leave it on for some time at a steady rate? This might be equivalent to us adding CO2 to the atmosphere and then keeping those emissions constant (sometimes referred to as the "stabilization" of emissions). We can simulate this scenario with our simple model, and the results are shown below.

What you see is that the system responds by increasing the water in the tub until a new steady state is reached. The length of time needed to achieve the new steady state is determined by the response time of the system, which again, is governed by the magnitude of the drain constant, k. In the real carbon cycle, this response time is measured in tens of thousands of years. For example, remember that the residence time of the deep ocean is about 3,800 years. The response time of the whole carbon cycle must be much longer than this because CO2 emissions are cycled through more than just the deep ocean. Unfortunately you can't add up the residence time of the individual reservoirs to get the response time of the whole carbon cycle which is really what we want to know, the system is much more complex than this. The natural carbon cycle will find a new steady state (it will "stabilize") in response to our carbon emissions, but it will take many thousands of years to do so. In the meantime, the system will continue to change as it makes this adjustment.

why does input of co2 into the ocean lower seawater pH?

because after several chemical reactions, H+ increases

Photosynthesis formula

co2 + h20 = sugar + o2

The residence time of carbon in the deep ocean is longer than in the atmosphere because there is many times more carbon residing in the deep ocean

correct

terrestrial carbon cycle

picked up from the atmosphere by plants (photosynthesis), enters the ecosystem, released back into the atmosphere (respiration, burning, volcanic activity, etc.), recovered in the from of coal, oil, or natural gas