Freshman Seminar:
Introduction to the Science of Climate Change
Prof. Jordi Miralda-Escudé, F 9:30

Lecture 4: The present carbon cycle.


Introduction: review of anthropogenic emissions.

We saw in the last lecture that the concentration of greenhouse gases has been increasing over the last couple centuries because of various human activities. This causes a radiative forcing, which at present is 2.4 Watts per square meter. Of this radiative forcing, carbon dioxide is responsible for 60%.

The question we will examine in this lecture is: what happens to the greenhouse gases after they are emitted to the atmosphere? Do they all stay there? What was happening before human activities started, and where did the naturally emitted greenhouse gases go? Because carbon dioxide is the most important anthropogenic greenhouse gas, we will focus on it and only briefly mention a few things for methane at the end.


The natural carbon cycle: fluxes and reservoirs

Before human activities were important, carbon dioxide was always emitted to the atmosphere by volcanoes, and large quantities were being exchanged between the atmosphere, the vegetation and soils in the land, and the ocean. Today all these exchanges continue but human emissions from fossil fuels have been added to perturb the cycle. We will see that human emissions have greatly altered the natural cycle and have caused the increase of carbon dioxide in the atmosphere; however, the natural cycle is at present absorbing a large fraction of the carbon dioxide emitted by humans (between a half and two thirds of it), so that the increase in carbon dioxide has been greatly mitigated up to the present time by nature. The concepts of carbon reservoirs and fluxes are very important to understand the carbon cycle. Reservoirs are the total amount of carbon that is present in a component of the Earth that participates in the carbon cycle. These reservoirs are conveniently measured in the units of Gigatons of carbon, or GtC, which means a billion tons of carbon. The important reservoirs are:

There are large fluxes of carbon moving between the atmosphere, vegetation and oceans every year:

A description of the carbon cycle with this plot is available at The Hadley Center page on carbon cycle .
In the graph you can see that about 60 GtC are exchanged back and forth between the vegetation and soils and the atmosphere, and 90 GtC between the ocean and atmosphere, every year. These large fluxes are near equilibrium as plants and plankton grow and die. Local rates of absorption or emission in different places in the ocean depend on many variable factors like temperature and alkalinity, but when globally averaged the absorption and emission of carbon are in equilibrium and there is only a small net rate of absorption into vegetation, soils and the ocean.

At present, human carbon emissions are 6.5 GtC from fossil fuel burning (plus a small part due to cement production), plus about 1.5 GtC from deforestation. This is a total of 8 GtC emitted every year. However, the increase in the carbon in the atmosphere is only about 3.5 GtC per year, so there are about 4.5 GtC that are somehow being removed every year from the atmosphere. Measurements indicate that the global ocean is currently absorbing 2 GtC every year. Vegetation and soils are thought to absorb around 1.5 GtC in places where forests are regrowing or are stable. Perhaps another 1 GtC is being dissolved in water in the soil and carried by rivers to the ocean. The net amounts being absorbed by vegetation and soils and carried by rivers are uncertain and there is ongoing experiments and discussion among scientists as to how exactly all this carbon is being taken away from the atmosphere.

Why is there a net absorption of carbon dioxide by natural processes in the Earth system at the present time? The reason is probably that as carbon dioxide concentrations increase in the atmosphere, the equilibrium in the exchange with ocean and soils is altered. Carbon dioxide tends to diffuse more easily into the ocean, plants grow to larger sizes and more carbon can be accumulated in the soil. Some scientists also think that the fertilizers used in agriculture result in more vigorous plant growth across the planet (because of fertilizer runoff from fields into forests).

Before the industrial revolution, the carbon dioxide in the atmosphere was stable, with a total reservoir size of 570 GtC. The only emissions before human activities became significant were from volcanoes or other outgassing of carbon dioxide from geologically active regions. These emit only 0.2 GtC every year (about 3% of the present human emissions). Vegetation and soils were not causing any net absorption in the past, and the ocean was absorbing 0.2 GtC per year. Several mechanisms carry carbon from the surface to the deep ocean; for example, small animals with shells that contain carbonates sink to the ocean floor when they die. This causes an important flux of carbon to the deep ocean, and as a result the deep ocean has a higher concentration of carbon dioxide than the surface ocean. Most of this carbon dioxide flux is balanced by an upwards diffusion from the deep to the surface ocean, but a small fraction of 0.2 GtC per year is buried in sediments in the sea floor. These sediments are eventually buried deeper into the interior of the Earth as sea floor is subducted under continental plates, therefore returning the carbon that is emitted from volcanoes.

At the present time, because the carbon dioxide concentration in the atmosphere has increased, there is a much larger net flux of carbon dioxide into the ocean. But it takes about 100 years for the equilibrium of ocean and atmosphere to be reached again: for example, at present the ocean absorbs 2 GtC per year when the atmosphere has increased its carbon content by nearly 200 GtC relative to pre-industrial times, so it would take roughly 100 years to absorb all that carbon.

The carbon cycle can be separated into two parts that differ in timescale. The short-timescale cycle, occurring over hundreds of years, involves exchange between the atmosphere, ocean and soil. Over a much longer timescale, volcanoes inject buried carbon into the atmosphere-ocean-soil system, and the carbon is removed by deposition in sea-floor sediments and subduction; this can change the total carbon reservoir of the atmosphere-ocean-soil only over timescales of hundreds of thousands of years.


Methane:

Methane has a short lifetime in the atmosphere of about 12 years. Methane is destroyed in the upper layers of the stratosphere by reacting with the OH molecule. The OH molecule is made when ultraviolet light from the Sun splits a water vapor molecule. The concentration of methane in the atmosphere is simply determined by the sources of methane over the previous 12 years. Therefore, if the sources of methane are either increased or decreased, the concentration in the atmosphere will respond rapidly.


Summary:

  • The natural carbon cycle consists of exchange of carbon between atmosphere, vegetation and soils, and ocean. Most of the carbon is contained in the ocean. The exchanges tend to maintain the concentrations in the ocean and atmosphere in equilibrium, responding on a timescale of 100 years.
  • There is also an exchange with carbon buried in carbonate rocks, with volcanoes emitting carbon dioxide to the atmosphere and sediments containing carbon being buried in the sea-floor, acting only over much longer timescales.
  • At the present time, humans are emitting about 8 GtC per year to the atmosphere. The reservoir of carbon in the atmosphere increases by only 3.5 GtC per year, because the rest is being absorbed by the ocean and soils.
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