Carbon Budgets and Fluxes

The large carbon reservoirs on earth and the flux between reservoirs are not precisely known. This applies particularly to the carbon content in vegetation, soil and sediments, and in the deep ocean, and also to the net land-to-atmosphere and ocean-to-atmosphere flux. Best known is the emission of fossil fuel and the atmospheric increase. Budgets and fluxes are shown below in two graphic illustrations with different complexity. The fossil fuel emission of 6.4±0.4 GtC per year minus fluxes caused by sinks on land and in the oceans result in a net increase of atmospheric carbon by 3.2±0.1 GtC per year.

Carbon budgets in GtC in vegetation and soil, in the atmosphere and oceans, and carbon fluxes in GtC per year (shown by arrows) between these reservoirs. (GtC: Gigatons or 10⁹ tons of carbon)
Source: US Dept. of Energy, modified

Detailed view of budgets and fluxes in the global carbon cycle in the 1990ies. Pre-industrial or ‘natural’ values are given in black, anthropogenic values in red.
Source: IPCC

Scenario

Obviously, the oceans are a relevant sink for carbon dioxide which is emitted to the atmosphere as a result of combustion of oil and gas. Although the biomass in the oceans is much smaller than on land, biological productivity is almost the same. Hence, carbon fixation is in the same order! No wonder that attempts are underway to increase biological productivity in the oceans, hoping that an essential part of the carbon fixed to phytoplankton particles will sink to the deep ocean. If so, then the carbon will be buried there! Since currents are very slow in the ocean, this part of the greenhouse gas will be removed from the climate system for thousands of years.

We become a geo-engineer!

This sounds like a good idea: fertilizing the oceans shall increase the burial of carbon in the deep sea, thus reducing greenhouse gas concentrations in the atmosphere!

Questions

Which factors (e.g., light, temperature, salinity, transparency of the water, nutrients, ...) determine the biological productivity of phytoplankton in the oceans? Which quantities are most important, which ones are less relevant? Are there essential differences when compared with vegetation on land?
Which factors (e.g., grazing by zooplankton and fish, bacterial attack, lack of nutrients, ...) determine the loss of phytoplankton?
Assume that a phytoplankton bloom degrades because of circumstances mentioned in 2). Then the plankton particles start sinking slowly into the deep. Which processes can lead to a decay of particles during sinking, so that they will not reach the sea floor?
Please find arguments for the following hypotheses:
- Biological productivity in the ocean is limited by nutrients. Therefore, fertilization should lead to an increase of productivity and hence, to a higher uptake of carbon by phytoplankton which will be stored in the deep ocean
- Due to degradation processes which take place in the upper water column, increasing biological productivity will not result to an increase of carbon in the deep ocean. Instead, biomass will be rapidly re-mineralised. Organic carbon will be oxidised to carbon dioxide and subsequently released to the atmosphere.

Time needed:

Two school lessons and one exercise for homework

Proposed procedure:

First lesson in class: Students get together in groups of no more than five students and
- discuss the questions about carbon dioxide concentrations and temperature increase,
- decide which background information they may need to solve the problem, and
- assign a task to each member of their group which should be resolved by the time of the next lesson in school.

Homework: Students resolve the tasks that have been assigned to them by their group.
Second lesson in class: Students
- present what they have learned to their group
- discuss how their findings can help them to answer question 4, i.e. forecast of future atmospheric temperatures
- present their results in class using the information found in their enquiries.