THE CARBON CYCLE & THE GREENHOUSE EFFECT

Corresponding Readings in Primack, Richard B. Essentials of Conservation Biology.
Chapter 9: pages 239-248

Introduction

The carbon cycle provides an interesting illustration of the concepts of a biogeochemical cycle, and of mass balance. Human activity has unquestionably changed the dynamics of the carbon cycle; in particular, we have increased atmospheric concentrations of CO2. A highly probable consequence of increased CO2 in the atmosphere is global warming, perhaps the best-publicized environmental issue of our time, and the prime example of environmental change at the global scale. Global warming poses threats to biodiversity and ecosystems, as we shall see in the next lecture. We begin with an examination of the global carbon balance, and how human activity has altered it.

 The Carbon Cycle

The carbon cycle is simple. It is based upon carbon dioxide, which comprises only 0.03 % of earth's atmosphere. The carbon cycle involves the conversion of inorganic CO2 into organic compounds, and its subsequent re-mineralization to CO2. It is a biogeochemical cycle because it involves the cycling of a chemical element – carbon – between organic (CHO) and inorganic (CO2 and carbonate/bicarbonate compounds) states.

 All biogeochemical cycles can be envisioned as a series of stores and fluxes. 

Stores Fluxes

CO2 in atmosphere, ocean water Photosynthesis

Biomass carbon in soils, vegetation, animals Respiration

Fossil and sedimentary deposits Burial, sedimentation, chemical ppte

Sea bed carbonate deposits Erosion and mining

 Note that fluxes can be off-setting, such as photosynthesis and respiration. Photosynthesis (PS) evolved as a mechanism to transform solar energy into the energy of organic compounds, incorporating these rare CO2 molecules into carbon compounds of living organisms. Respiration releases this energy for metabolic work. Today, about 10 % of approx. 700 billion metric tons of CO2 in atmosphere are fixed into plant photosynthate annually.

 Important inorganic or abiotic reservoirs are CO₂ in the atmosphere and dissolved in the oceans. The main biotic or organic reservoirs are vegetation biomass and soil organic matter (e.g., peat). Atmospheric and biomass pools are small, and turn over rapidly. Other abiotic pools such as sediments, coal and oil are very large, and enter the cycle very slowly.

 The conventional wisdom used to be that PS and R balance exactly, and the system is roughly in balance at present. In the past, particularly during the Pennsylvanian and Carboniferous Periods, PS >> R, and huge deposits of organic matter resulted, which slowly compressed to form coal, oil, and gas. Burning these fossil fuels releases CO2, and there is broad consensus that increased atmospheric CO2 concentrations are the consequence. Daily readings from an observatory in Hawaii provide strong support. The annual cycle of atmospheric CO2 reveals high vales in April, after a fall, winter and spring of decomposition, and low values in September, after a summer of PS. This cycle fades towards the tropics, and is reversed south of the equator. The oscillations are nature’s signature, the upward trend is human influence.

 Although the Mauna Loa Observatory data are only from the 1950’s, other lines of evidence put the atmospheric CO2 concentration at ~ 260-275 ppm in 1850. Today it is ~ 360 ppm, and rising at roughly 1+ ppm annually. A doubling of the pre-industrial value – to the mid-500 ppm range – is expected before the close of the next century. This is because industrialization of developing countries, particularly China, which is rich in "dirty" coal, will raise the annual rate of increase.

 Sources, Sinks, and "Missing" Carbon

 Because of the simple chemical transformations of the carbon cycle, scientists can make careful calculations of carbon dioxide released, and carbon build-up in the atmosphere. Of the CO2 released each year, about 20 % cannot be accounted for. To ask the question, "where is the remainder?", examine the global carbon cycle, assume we have an accurate estimate of the amount of fossil fuels converted to CO2 (the source), and ask, "what are the possible sinks?". Only two possibilities seem likely: the oceans, and the forests. Attempting to account for all the missing carbon continues to stimulate much research (Science 277, p 315, 18 July 1997)..

 The oceans might seem the only sink large enough to account for this much CO₂ storage. Increased CO₂ in the atmosphere should stimulate algal photosynthesis, and as a result more CO2 should be fixed into plant biomass (causing more atmospheric CO2 to dissolve into the surface water). But mixing between surface and deep waters is slow. The thermocline prevents movement of dissolved CO₂ to the deep, vast bulk of the ocean. PS can only deplete surface water CO2. Without a mechanism to more rapidly transport carbon (eg, as algal cells) from surface water to ocean depths, oceanographers doubt that the oceans can account for so much "missing" CO2.

 The forests seem an improbable sink for the missing carbon. Substantial amounts of carbon are stored in plants and in the soil as humus and peat. It is released if forests are cut and burned. It is believed that forests have been reduced for the last 1000 to 2000 years. Today, especially in the tropics, forest harvesting and burning go on at a high rate. After deforestation, the remaining humus may be lost more rapidly also. By some recent estimates, the annual release of CO₂ from plants and humus, due to forest fires, cutting for fuel, and slash & burn agriculture, is about equal to the annual release from burning fossil fuels. It could be up to 3x as great. Rather than being a sink for the atmospheric CO₂, the terrestrial biota might be a major source. 

The case of the missing carbon is still unresolved. However, new estimates implicate resurgent forests of the temperate zone and high latitudes as a possible sink. In addition, it appears that storage of carbon as organic matter in soils is even more important than its storage in above-ground vegetation. Thus forest management, and reversing deforestation, become potential weapons in combating the rise in atmospheric CO2. By one estimate, US forest regeneration over the last 40 years has stored enough carbon to offset 25% of US CO2 emissions. Tropical forests appear to have roughly twice the potential to store carbon, but all evidence to date continues to point to tropical forests as a source, not a sink, of atmospheric CO2. 

CO2 and Climate Change

The evidence that burning fossil fuels causes a rise in atmospheric CO2 is very strong. What about the evidence that climate change also is a consequence? We shall very briefly examine this complex issue by asking three questions.

1. Should increased atmospheric CO2 result in a warmer climate? There is strong evidence to answer, "yes". CO₂ in the atmosphere acts like glass: it admits visible direct light, and prevents the escape of infrared wave length light into which incoming light waves are scattered by the earth's surface. Potentially this "greenhouse effect" would warm the atmosphere. Long-term correlations between atmospheric CO2 and global temperatures also strongly indicate that high CO2 levels and high global temperatures are correlated. (Oxygen isotopes in ice cores give temperature estimates, bubbles of ancient air from ice core bubbles give CO2 estimates.)
2. Is the earth getting warmer? Estimates of annual earth surface temperatures over about 100 years show a rise of roughly 0.5 – 0.6 C. Estimates of the retreat of glaciers and melting of lake ice also support a warming trend. Biological evidence is accumulating of earlier arriving bird migrants in springtime, of longer growing seasons, and of species extending their ranges northward. The case is getting very strong. Based on past periods of earth history, only a 3 C mean temperature rise is necessary to melt the ice caps and flood coastal cities 
3. Is rising CO2 the cause? This may be the hardest piece to answer with confidence. The average temperature of the earth has varied greatly over earth history. Possible causes include: (a) changes in solar radiation, (b) changes in land/ocean distribution due to tectonic movement, (c) changes in atmospheric concentrations of CO2, CH4, and other compounds, (d) changes in reflectivity of earth’s surface (e) changes in earth’s orbit around the sun, (f) catastrophic events such as volcanic activity, meteor impacts, etc.  

Predicted Future Climates

General circulation models (GCMs) predict future climate scenarios. Further work is needed to improve their realism. For example, they operate at a fairly coarse scale (500 km grid), are better at predicting temperature than rainfall, and have some problems with the representation of clouds (a potential negative feedback or climate stabilizer). Nevertheless, the several models agree in general on a number of predictions.

1. A global temperature rise of 1.5 to 4.5 C is predicted
2. A greater warming effect is expected at higher latitudes on land, while sea surface temperatures should warm more uniformly
3. Warming will be more evident in winter than in summer, particularly at high latitudes
4. Large areas will become wetter, other large areas will become drier, and storms are likely to become more intense

What will be the effects of these changes on species and ecosystems?

Transparencies: 1. Global carbon cycle 2. Stores and fluxes 3. CO2 records from Mauna Loa Observatory since 1958 4. 160,000-year correlation of CO2 and temperature 5. 100-year record of increasing surface temperature 6. Scope of natural temperature variation, and causes 7. Sunspot cycle and temperature correlation 8. Predictions of General Circulation Models 9. Planet earth

Links:

CO2 Emissions and the "missing carbon" problem: this page is part of the Information Unit on Climate Change's fact sheets about all the different components of climate change.

The Carbon Cycle: this site provides a good diagram of the carbon cycle and provides more information on the ocean as a source.

Why climate change and global warming are not the same thing: often confused by many people, this article gives a clear definition to both terms.

The Paleoclimate Record and Climate Models: this is a lecutre from the NRE 111 course home page that should give a good understanding of the topic. Check the entire web site for interesting links and info.