CLIMATE CHANGE : THE GREENHOUSE EFFECT - PART 1

Let start to analyze the solar radiation, which we have described in my previous article on the Albedo. First of all, let specify that every body emits electromagnetic radiation. The Earth emits electromagnetic radiation (the terrestrial radiation), the Sun emits electromagnetic radiation (the solar radiation), and so on. The body emitted radiation was studied and explained by Planck, Boltzmann, Wien and others famous physicist of the past (we are speaking of end of ‘800 and beginning of ‘900). Planck derived the formula for the radiation emitted by a black body, where, for the first time, he introduced the concept of quantum. That radiation does not depend from the body properties, but only from its temperature. Figure 1 shows the typical emitted black body radiation vs. its wavelength: the reader can see that when the temperature increases, the radiation wavelength decreases. Since the wavelength is inversely proportional to the radiation energy (I omit the mathematical formulas for the sake of simplicity, since also they can be found on textbooks or wikipedia), the radiations emitted at high energy have a lower wavelength, which corresponds to the ultraviolet spectrum (on the beach or on the mountains, indeed, we protect us from those kind of rays).

Figure 2 shows the incoming radiation measured by the satellites outside the atmosphere: it corresponds to the radiation emitted by a body of 5800 Kelvin degrees, the Sun temperature. Therefore, it is wrong to think (as I read in other contexts) that the electromagnetic radiation arriving on the Earth could derive from the solar system planets, like Jupiter or Saturn: those planets do not have the temperature to emit the radiation spectra measured by the satellites (for example, Jupiter has an average temperature of less than zero degree celsius).

The terrestrial radiation has been measured by satellites and corresponds to a radiation emitted by a body of 290 Kelvin degree (see Figure 3). It is thermal radiation, different from the radiation arriving from the Sun (shown on the left in the figure). The reader could think: “Understood. But what about the greenhouse effect?” If we consider only the solar radiation, the temperature of the Earth should be less than zero Celsius degree (this data was obtained by Fourier in the 1827!!): the temperature on the Earth, as all we know, is much higher. This difference is due to the atmosphere, which reflects the terrestrial radiation on the earth surface, contributing to the temperature increasing. This phenomenon, known also in the last two centuries, is called greenhouse effect (in analogy to what happens in a greenhouse, where the windows reflect the internal heat but are transparent to the visible radiation). The atmospheric gases contributing to this phenomenon are called greenhouse gas. The reader could ask:”What? Not all the gases in the atmosphere reflect the terrestrial radiation? Why?” Some gases, which absorb the terrestrial radiation, “change” their rotational and vibrational state. Those “changes”, according to the quantum mechanics laws, allow the emission of the thermal radiation which is responsible for the terrestrial warming, with an effect “rebound on the Earth”. Fortunately, not all the gases in the atmosphere have that characteristic. The oxygen and the nitrogen, which constitutes the 99% of the atmospheric gases, do not have it while the carbon dioxide, the methane, the ozone, the water vapor and other gases (like the Freon) have it.

We have explained why the greenhouse effect occurs and what the cause is. Let see how the greenhouse effect contributes to the global temperature increase. Figure 4 shows the solar and terrestrial radiation fluxes, measured by satellites.

I will omit the details, but a simple calculation of the incoming and outgoing radiation shows that the Earth warms while the atmosphere cools. Therefore, the greenhouse effect is responsible of the warming (and of the life) of the planet. Anyway, the reader could ask if, in these years, there has been a real planet warming, as reported by several journal and scientific newspaper. Let’s try to explain it with scientific data. The temperature has always been measured by several stations around the world and, from the end of ’70, also by satellites. The mean global temperature, as we can see in Figure 5, is increased, with a trend of 0.2 Celsius degrees per decade.

Is that increase due to the human activities emitting greenhouse gases? The answer seems easy: an increase of the greenhouse gases concentration implies a “rebound” on the Earth surface of a higher quantity of terrestrial radiation and, therefore, an increase of the temperature. However, there are also other questions: which greenhouse gases are the main responsible for the increase? Are they emitted by the human activities or follow a cyclical natural trend? Are there other effects which can increase the global temperature? Those are important questions, not only from a scientific perspective, but also for the economical and environmental policy to choice. I will discuss about them in the next post.