The greenhouse effect

The dayly average intensity of solar radiation that reaches the earth is 1372 watts per square metre. The Earth spins on its axis and distributes on average over the year this amount of energy (equivalent to 343 watts/m2) on every square meter of its surface and in the high layers of the atmosphere. Thirty per cent of this flow of energy is reflected directly in space. The remainder is absorbed and, after having contributed to the climate, is re-radiated in the form of infrared energy. Among the factors affecting the climate of our planet, the most important is certainly the slope of the earth’s orbit respect to the Ecliptic plane. Because of this in a certain period the northern hemisphere is more exposed to the sun. Six months later, the southern hemisphere is more exposed to the sun.
The atmosphere is composed primarily of nitrogen, oxygen and in lesser quantities from other gases. The light coming from the sun crosses it easily and that part not reflected by ice or clouds illuminates and warms the surface.
The heat is accumulated, subsequently reissued from Earth in the form of infrared rays which are partly reflected back toward Earth and in part to space.
The gases that contribute further to reflect in part to the Earth energy re-radiated by the Earth itself are the water vapor, ozone, chlorofluorocarbons, nitrogen oxides, methane and especially carbon dioxide, namely greenhouse gases. Without the presence of these gases throughout which the infrared energy would be reflected in space, the average temperature of the planet would be around -20 degrees centigrade, impossible for life.
Instead, the presence of greenhouse gases into the atmosphere allow to intercept about 70 percent of infrared radiation re-emitted from Earth, causing rising temperatures average around 14 degrees centigrade.
To understand the nature and significance of the greenhouse effect is necessary to examine in more detail what is the energy balance in the global climate system. The main source of energy, but also unique, given the negligible influence of other sources of energy from space or from underground, is the Sun. The sun emits a flow of energy form of electromagnetic radiation, equal to about 64 million of watt/m2 (watts per square meter) at a temperature of about 56 thousand ° C. This energy is dispersed evenly in space in all directions. On Earth, which is about 150 million km from the sun, only a fraction extremely small of theinitial flow reaches, which amounted to about 0.002 per thousand. However, this flow of energy would reach fully to the ground if the Earth was flat and was exactly perpendicular to sunlight and, moreover, had not come any means, as the atmosphere, which mitigates and disseminates the solar radiation. Instead, the Earth is spherical and sunlight come, perpendicular or slightly tilted, at low latitudes (zone intertropicale), but gradually more and more inclined to high latitudes, and this causes an uneven distribution of solar flux between equatorial zones and Polar zones. In addition, the Earth rotates on itself and rotates around the sun, and this causes a further uneven distribution of solar radiation between day and night and between one season and another.
All these fluctuations are, however, cyclical and are repeated regularly with daily and annual basis.
If we take as a reference the calendar year, the average annual solar energy that would reach the surface, if there were no atmosphere, would amount at 343 watt/m2. This energy flow is the maximum average value of solar energy per unit area, which our planet can theoretically have.
The flow of energy actually used by our planet is, however, less because the Earth is not a surface "black" (it not totally absorb the incoming solar radiation) but has areas composed of soil, rocks, seas and oceans, who have a certain reflectivity (called albedo). The average albedo of the earth is equal to 30%. Therefore, the flow of average energy of solar radiation actually used by our planet as a source of energy is 240 watt/m2.
This energy we have would enable our planet, if there were no atmosphere, to reach a maximum mean temperature of 20 ° C below zero.
The temperature ranges around this average would, however, very high between polar and equatorial zones between night and day and between summer and winter.
Once the solar radiation has come on the Earth's surface, it is absorbed by soil and seas, which are increasing their warm temperature. As it gradually increases the temperature, the Earth's surface emits energy in the form of heat and, in other words, as infrared radiation, until it establishes a balance between flow of solar energy incident and flow of Earth's energy outgoing to the space.
We have seen before that if there were no atmosphere, the average temperature of our planet would be almost 20 ° C below zero. Instead, thanks to the presence of the atmosphere, the average temperature reaches almost 14 ° C above zero. Then, the earth's atmosphere with its content of greenhouse gases raise global average of over 34 ° C.
But the most important thing to emphasise is that, while the temperature of 20 ° C below zero would reaches with a stream of solar energy of 240 watt/m2, as we have seen before, the temperature of 14 ° C above zero that occurs with the presence of the atmosphere, vice versa is reached with a stream of solar energy lower, namely with about 170 watt/m2, a value that is half the maximum available (343 watt/m2).
The reason for that, on the ground, in the presence of atmosphere, the surface get a flow less energy compared to the absence of atmosphere, comes from loss of energy that solar radiation suffer while entering in the increasingly dense layers of the atmosphere. The losses (102 watt/m2) are both caused by the reflection of sunlight to the space by the clouds, and by its dispersion and spread into the atmosphere of the solar radiation, part of which returns to space.
The cause for that, on the ground, with an energy flow slower compared to case of the absence of atmosphere, temperatures of over 34 ° C are reached, depends on the ability to store heat in the atmosphere: this capacity is called "greenhouse effect"…
The atmosphere is a mixture of gases that owns a characteristic property (with clean air and clear from clouds): it is transparent to radiation from the Sun, but it is opaque to infrared radiation reflected from Earth, because of the presence of certain gases such as the water vapor and carbon dioxide that are strong absorbers and emitting of infrared radiation. But when carbon dioxide differs from the rates naturally present in the atmosphere, things are more complicated. Let's see how.
The greenhouse effect involves the Earth, with the land, oceans and atmosphere. Among them there is a balance heat and thus energy which also involves the space: the amount of energy absorbed should be equal to that re-emitted. The thermodynamics not escape. So it is.
The increased concentration of carbon dioxide caused by human activity traps more infrared energy that increases the temperature of the atmosphere. This increase leads to greater evaporation of water from the oceans. The water vapor is, in turn, a powerful greenhouse gas that contributes to retain the infrared radiation facilitating overheating from Earth. The increase heat leads to the dissolution of the polar caps and, consequently, reduces the surface reflecting the light radiation from the sun causing a further rise in temperature of the planet.
At the same time also increases the condensation of water vapor that turns into clouds that reflect light from the Sun that decreases the radiation absorbed by the planet, and thus temperature.
But the melting ice cancel out the effect of the gulf current, concomitant mild European climate, causing a small ice age in part of central and northern Russia. They form so again a wide air ice that reflect the light radiation in space.
The thermodynamic equilibrium is re-estabilshed forever...
In about two hundred years from start of the industrial age, we have increased by almost 40 per cent carbon dioxide in the atmosphere and, more time passes, more we globalize industrial activities and more it increases.
The increase of carbon dioxide creates a problem, a big and serious problem: intercepted infrared energy is greater and this causes the increase of average temperature of the planet.
Already today the modest increase in temperature that was recorded is reflected on climate change with consequences sometimes dramatic.
The increased energy of the atmosphere active extreme increasingly disruptive phenomena because the natural climate balance is, in part, already upset.
The polar caps are melting and the Ecotone, located at the southern Central Massif of France is shifting (the Ecotono is a boundary between two or more ecological zones where the vegetation changes from a distance of a few metres, in this case vegetation typical of temperate areas to that of Mediterranean scrub).
The Tropical Convergence Zone is always less stable and monsoon that governs rains are increasingly less predictable in duration, intensity and areas where they act. Another problem is El Nino. The Southern Oscillation that it activates, which is a swing of the values of atmospheric pressure between the eastern and equatorial Pacific, is increasingly unpredictably in intensity and duration.
So, working to switch from oil to other sources of energy and study and propose solutions that can give us forecasts on climate change is fundamental to reduce our vulnerability from predictable extreme events that may occur.