Question

•The persistent presence of O2 in the atmosphere is intimately connected with the removal of CO2 from the atm as part of the inorganic C cycle.

•What is the chemistry behind it?

•How is the geospheric cycle involved in this?

•We referred to this not as a simple cycle, but as a feedback loop. How does this feedback loop work?

Answer 1

Answer to part 1 :

There are three important carbon cycles in the earth system:

• the short term organic carbon cycle, with emphasis on the interactions between the atmosphere and the biosphere: it has terrestrial (land) and marine (ocean) components
• the long term organic carbon cycle, with emphasis on the formation and destruction of fossil fuels and other sediments containing organic carbon
• the long term inorganic carbon cycle with emphasis on calcium carbonate (CaCO₃, limestone), by far the largest of the carbon reservoirs. This cycle is linked to thecarbonate-silicate cycle, supplying the calcium ions necessary for the formation of limestone.

In the the short term organic carbon cycle, the transfer rates are large but the biospheric reservoir is relatively small, whereas in the long term cycles the reverse is true.

All three of these cycles are linked together as part of the global carbon cycle, but we examine them separately because they control atmospheric levels of CO₂ on different timescales ranging from months (short term organic carbon cycle), to tens of millions of years (long term inorganic carbon cycle). Dividing the carbon cycle in these individual pieces help us represent this system in simple terms.

The short term organic carbon cycle
The photosynthesis reaction removes carbon atoms from the atmosphere and incorporates them into the living tissue of green plants. It requires energy from sunlight. The chemical reaction,

CO₂ + H₂O --> CH₂O + O₂

The respiration (also burning and decay) reaction undoes the work of photosynthesis, thereby returning carbon atoms to the atmosphere:

CH₂O + O₂ --> CO₂+ H₂O

In contrast to photosynthesis, respiration involve a release of energy.

Anaerobic (without oxygen) decomposition, such as occurs deep in soils, has a slightly different reaction

2CH₂O --> CO₂+ NH₄

producing methane instead of water.

The terrestrial biosphere is much more massive than the marine biosphere, largely because of the presence of trees. Soils also contain a large amount of organic material. The influence of the land biosphere is evident in Fig. 8-4. Some portion of the organic matter (CH₂O) is eroded from land to the sea.

The marine biosphere operates like a 'biological pump'. In the sunlit uppermost 100 meters of the ocean, photosynthesis serves as a source of oxygen and a sink for carbon dioxide and nutrients like phosphorous. Fecal pellets (waists) and dying marine organisms decay as they sink. Their organic content (C-H and C-C bonds) decomposes in the upper (1 km or so) ocean, consuming dissolved oxygen and giving off (dissolved) carbon dioxide. Hence, the upper ocean has much higher carbon dioxide concentrations and lower oxygen concentrations than the waters below 1km. Remember this is ON AVERAGE because the marine biosphere is active only in those limited regions of the ocean where upwelling is bringing up nutrients from below.  

The long term organic carbon cycle
Only a tiny fraction of the organic material that is generated by photosynthesis each year escapes the decay process by being buried and ultimately incorporated into fossil fuel deposits or sediments containing organic material. Through this slow process, carbon from both terrestrial and marine biosphere reservoirs enters into the long term organic carbon cycle. The rate is so slow as to be virtually unmeasurable. Weathering of these same sentiments releases carbon back into the other reservoirs.

Human society is burning fossil fuels at a rate many orders of magnitude faster than they were created. The fossil fuel reservoir in Fig. 8.3 is 4700/760 = 6 times larger than the atmospheric reservoir, so if it were all added to the atmospheric reservoir (by the burning of fossil fuels) without any of it being taken up by the other reservoirs, the atmospheric concentration of carbon dioxide would increase by a factor of 6.5. This, of course, is an upper limit and not an actual prediction.

Answer to part 2:

All living things are made of carbon. Carbon is also a part of the ocean, air, and even rocks. Because the Earth is a dynamic place, carbon does not stay still. It is on the move!

In the atmosphere, carbon is attached to some oxygen in a gas called carbon dioxide.

Plants use carbon dioxide and sunlight to make their own food and grow. The carbon becomes part of the plant. Plants that die and are buried may turn into fossil fuels made of carbon like coal and oil over millions of years. When humans burn fossil fuels, most of the carbon quickly enters the atmosphere as carbon dioxide.

Carbon dioxide is a greenhouse gas and traps heat in the atmosphere. Without it and other greenhouse gases, Earth would be a frozen world. But humans have burned so much fuel that there is about 30% more carbon dioxide in the air today than there was about 150 years ago, and Earth is becoming a warmer place. In fact, ice cores show us that there is now more carbon dioxide in the atmosphere than there has been in the last 420,000 years.

Answer to part 3:

A number of processes in the carbon cycle can produce feedbacks. Lack of knowledge about feedbacks is a major factor limiting our ability to estimate future atmospheric CO2 concentrations. For example, an increase in atmospheric CO2 that leads to an increase in the earth’s surface temperature may be a positive feedback in some regions because warmer temperatures increase soil respiration, thereby further increasing atmospheric CO2. However, warmer temperatures can also extend the growing season of plants, and thus the length of time over which they draw CO2 out of the atmosphere. This effect would reduce the amount of atmospheric CO2, which then could reduce the amount of warming. Thus increased atmospheric CO2 has both positive and negative feedbacks, and it is hard to discern which would prevail globally.