In the above Sections we restricted the analysis of a changing climate to the interaction with the different spheres involved. Central to these analyses stood the changing concentration of greenhouse gases in the atmosphere and the changing albedo by cloud cover, aerosols and snow cover. The dominant processes described, however, only involve five chemical compounds: carbon (greenhouse gases: CO2, CH4, CFC), nitrogen (greenhouse gas, aerosols, limiting nutrient for the photic zone), phosphorus (limiting nutrient for terrestrial biosphere), sulphur (aerosols) and water (greenhouse gas, different phases and albedo, water stress). Another method to understand changing climate is to quantify the flows into, the transformations within, and the removal from different reservoirs (e.g., spheres). This concept is central to the construction of a budget. The radiative budget has some similarities with this method. The connection of all reservoirs that conserve and transport a specific element is called a cycle. Hence, by definition, a cycle is closed, i.e., no material is lost.
A perfect example of how the above definitions of budget and flows must be interpreted is the question if tropical rainforests are very important in maintaining atmospheric oxygen levels. Or to put this question differently: are the tropical rainforests the "green lungs" of our atmosphere? Oxygen makes up for about 21% by volume or 23.1 % by weight of our atmosphere. Photosynthesis produces roughly 0.025 percent of the atmospheric oxygen mass annually. Currently 99.9 % of the oxygen produced via photosynthesis is used up by respiration and decomposition, converting organic carbon compounds back into CO2 and H2O by using O2. About 0.1% could be deposited in anaerobic compounds, a process called "carbon burial". Hence, carbon burial is the net oxygen source for the atmosphere. This source is balanced by a sink, the outflow of oxygen from the atmosphere (e.g., due to weathering of uplifted rocks). If oxygen production were to stop suddenly, then this sink, at its present speed, would take about 4 million years to remove all the oxygen from our atmosphere. The tropical rainforests are not the "lungs of the atmosphere" because the size of the atmospheric reservoir is very large compared to the turnover time of oxygen.
To understand how mankind's CO2 emissions affect the climate, we need to understand the carbon cycle. Large amounts of CO2 and other compounds containing carbon are continuously exchanged between the atmosphere, the oceans, and the biosphere (animals, plants, and soil). Relatively small stocks and large annual fluxes, carbon moves around this set of reservoirs on a time-scale of years (to centuries). These transfers are the fast wheels that spin within the overall biogeochemical cycle. They are ‘geared’ into that cycle by a series of much slower leaks (chemical weathering and sedimentation). Over the course of geological time, a net transfer along this route has locked up most of the Earth’s carbon, largely as calcium carbonate in sediments and rocks, but also as fossil fuels. We must understand these exchanges if we are to predict how CO2 emissions will affect future atmospheric concentrations of CO2.
Based on best available current knowledge of the sources and sinks of CO2, which is a mixture of observations and model-based estimates, it is not possible to obtain a balanced carbon budget. To balance the carbon budget, a rest term is introduced, I, which represents the missing sources and sinks. I might therefore be considered as an apparent net imbalance between the sources and sinks. Table 3 presents the observed global carbon balance over the decade 1980-1989 in terms of anthropogenic-induced perturbations to the natural carbon cycle as presented in the early 1990s.
|
Component |
1980-1989 (in GtC/yr.) |
based on |
|---|---|---|
|
1: Change in atmospheric mass of CO2 |
3.4 +/- 0.2 |
observations |
|
2: Uptake by the oceans |
2.0 +/- 0.8 |
model estimates |
|
3: Emissions from fossil fuel burning + cement manufacturing |
5.5 +/- 0.5 |
observations |
|
4: Emissions from land use changes (e.g., deforestation) |
1.6 +/- 1.0 |
observations |
|
Net imbalance (I= 1+2-3-4) |
-1.7 +/- 1.4 * |
The change of atmospheric mass of 3.4 GtC/yr is based on observation of changing concentrations of atmospheric gases containing carbon. The atmospheric reservoir of carbon is largely determined by CO2; and CH4 to a lesser extent. About 3.4GtC/yr is accumulated in the atmosphere which results in an increasing concentration of CO2.
This imbalance (or inferred sink) is known as the "missing carbon" problem. However, two of the entries in the table (ocean uptake and emissions from land use changes) are highly uncertain, and so too is the net imbalance; it could well be as little as 0.3 GtC/yr (or as large as 3.1GtC/yr). The geographical patterns of atmospheric CO2 increase suggest that there is an unexplained "sink" of CO2 somewhere in the northern hemisphere. Again based on model calculations it is very likely that an additional terrestrial sink could explain this imbalance. Therefore, the textbook states that the terrestrial biosphere has shifted from a net source to a net sink for carbon, after about 1940. In the IPCC Second Assessment this atmospheric sink of 1.2 GtC/yr is inferred from the other fluxes and accounted to carbon uptake due to nitrogen ferilisation, plus the range of other uptakes due to CO2 fertilisarion and climatic effects.
However, this conclusion is still very uncertain because not all aspects of the carbon cycle are well understood. Moreover, now that atmospheric CO2 has risen well above its natural level, many aspects of this complex carbon cycle are changing. This incomplete knowledge is a very important aspect because the exchanges of CO2 within the natural carbon cycle are many times greater than man-made CO2 emissions. For these reasons Edmonds (1992) made the following statement : "A serious implication of the apparent imbalance among known energy emissions, estimates of the C flux from land-use changes, and natural sinks is that model predictions of future atmospheric CO2 concentrations lack credibility".