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Overview of the Scientific Basis for the Greenhouse Effect and Geocycles

1.1. Greenhouse effect

During the planet's recent temperate period, the greenhouse effect has maintained the average temperature of the Earth at 15°C (at sea level), whereas without these gases, it would be just -18°C. The greenhouse effect is beneficial to life. Solar radiation is, depending on the wavelength, more or less absorbed by the atmosphere. Relatively little incident infrared radiation (known as "near" due to the proximity of its range of wavelength compared to that of visible radiation) is absorbed, and instead, this reaches the Earth's crust which then plays a part in warming (the infrared radiation absorbed by a material is converted into heat). Moreover, all bodies re-emit infrared radiation with a wavelength that depends on their temperature (the higher the temperature, the more energetic the radiation, meaning the shorter the wavelength). In this, we see that the infrared radiation that is re-emitted by the Earth (known as "far infrared") does not have the same spectral characteristic as that emitted by the sun. It turns out that certain atmospheric gases are high absorbers of this secondary infrared radiation, and then convert it into heat. This is the (simplified) principle of the greenhouse effect (see Figure 1.1).

In less than three centuries, due to human activities and, in particular, as a consequence of combustion of fossil energy, the concentration of this greenhouse gas carbon dioxide has increased by 40%. Although its current atmospheric concentration is only 0.04%, the disturbance has been sufficient to trigger a major change in the climate on the scale of the planet [LET 04]. The reason for this is that the climate system is complex, non-linear and subject to non-equilibrium thermodynamics, a foundation for positive feedback where self-amplification phenomena are able to make the system bifurcate to a new climate era [ROS 18].

Figure 1.1. Diagram of the greenhouse effect. For a color version of this figure, see www.iste.co.uk/rossignol/climatic.zip

Thus, certain consequences of the warming themselves cause an increase in the greenhouse effect:

• - the solubility of carbon dioxide decreases when the water temperature increases, causing desorption of CO2 from the oceans;
• - with atmospheric warming, the increase in vapor pressure of water, the main greenhouse gas, leads, in turn, to an increase in its concentration;
• - the evaporation of solid methane hydrates encased in permafrost and oceanic sediments releases a powerful greenhouse gas, methane;
• - along with the warming, the more intense mineralization of organic matter releases extra CO2;
• - the reduction in the surface area of snow cover leads to a decrease in the reflection of incident infrared, and extra absorption by the Earth's crust, which re-emits more infrared that can be absorbed by greenhouse gases;
• - etc.

Of course, there have always been climate fluctuations on the planet for various reasons:

• - cosmological (Milankovitch cycles, of which one process is the variation in the extension of the elliptical orbit of the Earth around the sun);
• - geological (orogenesis and exposure of calcium silicate which acts with the atmospheric carbon dioxide to create calcium and silica);
• - biogenic, with plant organism fossilization and the subsequent carbon sequestration (plants synthesize their matter with atmospheric carbon dioxide).

Currently, the cause of the warming is anthropogenic, but the problem is the abruptness of the change. The emissions due to our activities are massive and sudden, to such an extent that the ecosystems will not be able to easily readjust. They will be subject to a violent shock, and the disturbance will have destabilizing repercussions on the socioeconomic system.

1.2. The additional criteria of the emissions in the atmosphere

1.2.1. The carbon geocycle

Figure 1.2. Main cyclic processes involving carbon in oceanic environments. For a color version of this figure, see www.iste.co.uk/rossignol/climatic.zip

Figure 1.3. Main cyclic processes involving carbon in continental zones. For a color version of this figure, see www.iste.co.uk/rossignol/climatic.zip

Carbon circulates in the various planetary zones in various chemical forms (Figures 1.2 and 1.3). In particular, it transits through living organisms, of which it makes up a significant part. The retention time varies greatly from zone to zone: from a few minutes to a few millennia for living things; in the order of a century in the atmosphere in the form of carbon dioxide; in the order of hundreds of millions of years in the Earth's crust in fossil form.

EXERCISE 1.1.-

Wine-making involves alcohol fermentation of must (squashed grapes). During the fermentation process, driven by yeast, sugar is transformed into alcohol and carbon dioxide is released:

C6H12O6 2 C2H5OH + 2 CO2

Glucose Ethanol + Carbon dioxide

The fermentation releases around 12 kg of carbon dioxide per hectoliter of must.

Motivated by sales strategy choices with the objective of producing better quality wine, a vineyard operator aims to reduce the yield of their vines without increasing the surface area. An intern from an agricultural college points out that by reducing their production in this way, the company will reduce their emissions, but the operator disagrees with this. Who is right?

(Answer in section 1.3)

EXERCISE 1.2.-

How much time does it take for us to generate, by breathing, the equivalent of the climate impact of a 5CV car which travels 1 km, emitting 234 g of CO2?

Data:

Breathing rate: 15 cycles/min

Exhaled volume at rest: 0.5 l/cycle

Level of CO2 in the exhaled air: 5%

Level of CO2 in the inhaled air: 0.04%

Volumetric mass of CO2: 1.87 kg.m-3

(Answer in section 1.3)

Carbon migrates in the geosphere on a cyclic pathway, temporarily retained in "reservoirs" [DEN 07, p. 511]. Several cycles affect the carbon. At our human scale, some extend over very long time periods, such as for the carbon cycle of fossilized carbon, or short time periods, such as in the case of a biogenic cycle (cycle related to living things). The length of the cycle is the deciding factor for the current climate, when the carbon goes back through the atmospheric zone in dioxide form (Figure 1.4). Thus, destocking of fossilized carbon enriches the current atmosphere, in which levels of this gas have for a long time been depleted due to the trapping effect of fossilization of living things for long geological periods (long cycle).

However, current carbon dioxide emissions due, for example, to plant decomposition (short cycle) do not contribute to modification of the current atmospheric concentration, because an equivalent quantity of carbon dioxide had been extracted from the atmosphere by these plants just before this, during photosynthesis. Since retention time of carbon dioxide in the atmosphere is a century, all complete cycles involving this gas and which are shorter than this period can be considered to have no effect on climate change. However, despite being short, if the cycle is unbalanced, the climate impact does indeed come into play. The inbalance can be a result of an emission of carbon dioxide that is greater than the quantity previously extracted from the atmosphere (for example, combustion of wood from a forest that has not regenerated) or even when the carbon produced by photosynthesis is re-emitted in a chemical form other than CO2 (e.g. methane (CH4)), due to the fact that its warming capacity is higher than that of carbon dioxide.

Box 1.1. Three examples that illustrate the carbon cycle: the long cycle of fossilized carbon, which pollutes, and the short cycle of biogenic carbon, with or without an additional greenhouse effect that depends on the chemical nature of the gas

Over the course of past geological eras, different biogeochemical processes have absorbed atmospheric carbon dioxide and have transferred it to other zones, such as the Earth's crust. Thus, during the Carboniferous era, plant fossilization contributed to this. During the combustion of coal, carbon dioxide is liberated and modifies the current atmospheric composition (Figure 1.4, part a).

In the case of a forest floor where there is a balance between the contribution of organic matter via photosynthesis, and mineralization processes carried out by decomposing microorganisms with an associated release of carbon dioxide, the current composition of the atmosphere is not modified (Figure 1.4, part b).

In the case of anaerobic fermentation in hydromorphic soils (marshes, rice paddies, etc.), carbon is released in a chemical form (methane) with a Global Warming Potential (GWP) that is 28 times higher than that of the carbon dioxide extracted by photosynthesis at a previous stage in the cycle. The process therefore has an impact on the climate, although it occurs during the short carbon cycle (Figure 1.4, part c).

IMPORTANT.-...