Why do we blame the problem of global warming and climate change mainly on the increase in atmospheric CO2 concentration?



By Jordi Sardans, researcher in CREAF

One of the most serious challenges for the future of humanity is, without a doubt, to face and mitigate global change. A comprehensive review based on the most basic laws of chemistry and science in general should allow us to understand why the atmosphere is warming and absorbing more and more energy. This energy, mainly absorbed in the form of heat by certain gases in the atmosphere, is largely transformed into movements of the air masses, resulting in increasingly stronger squalls and anticyclones. This is the cause of extreme phenomena such as heat and cold waves, as well as droughts and episodes of increasingly intense rains and storms.

This hardening of climatic conditions associated with global warming, is felt in all the media and in the scientific community say that it is based on the increase of concentrations in the atmosphere of one of the gases that form it: carbon dioxide (CO2). But it is never explained what this gas has that the other gases do not have, such as the two major gases in the atmosphere: oxygen (O2) and nitrogen (N2), which give it the leading role in global warming and climate change in general. On the other hand, the lack of knowledge and understanding of what is happening may increase when we are told that other more unknown gases such as methane (CH4) or nitrogen oxides such as N2O or NO2 also contribute to atmospheric warming.

First of all, it is necessary to clarify why gases such as CO2, CH4 and N2O or NO2 contribute to warming the atmosphere and others such as O2 and N2 do not. Without needing to have too much knowledge of chemistry, the first thing we can easily grasp is that the gases of the CO2 group have different letters in their chemical formula. What does this mean? Well, they are compounds (molecules) made up of different atoms with different chemical characteristics. For example, each CO2 molecule is made up of one carbon atom and two oxygen atoms. If we take into account that molecules are formed because atoms come together to share electrons, forming what we call chemical bonds, in the case of CO2, the carbon atom (C) shares electrons on both sides with an oxygen, as we can see in Figure 1. This situation is repeated in a similar and equivalent way in the case of CH4, N2O or NO2, where in CH4, C forms bonds with hydrogens (H), and in nitrogen oxides, N forms bonds with oxygen. On the contrary, molecules formed by equal atoms such as O2 and N2 form chemical bonds between equal atoms. It is, therefore, this fact, whether the bonds are between different atoms or between equal atoms, that makes the difference between greenhouse gases such as CO2 and those that do not have a greenhouse effect, such as O2, so that the former can absorb the heat energy that reaches us from the Sun and the latter cannot.

Figure 1. Molecular structure of CO2.

Why do some heat and others do not? Well, because when the bonds are between different atoms, a little or a lot, there is always one of the two that, because it has certain characteristics such as the number of positive charges of the nucleus different from the other, attracts the electrons (negative charges) that form the bonds more strongly than the other atom that forms the corresponding chemical bond. It is this asymmetry in the distribution of charges that allows these molecules formed by different atoms, such as CO2, to absorb the infrared radiation coming from the Sun, heating up, which molecules formed by the same atoms, such as O2, cannot do. To go a little deeper into this characteristic of the molecules formed by more than one atom, it is necessary to say that the electrons of their bonds have some levels of vibration (internal movements of the atoms of the molecule for elongations, contractions, torsions and vibrations of the bonds that hold them together). These vibration levels have energy differences of the same magnitude as those of infrared radiation and can therefore absorb them and thus vibrate and move more and more. What does this mean? Well, they heat up, these vibrations and movements cause them to collide with other molecules in the atmosphere such as O2 and N2 which, although they do not directly absorb the infrared radiation due to these collisions, they also move and vibrate more and, therefore, also end up heating up. Therefore, the atmosphere heats up from the solar radiation that reaches us on Earth because the gases formed by more than one atom can absorb the infrared.

Thus, the more gases made up of more than one type of atom, the more the atmosphere warms up when it is traversed by infrared radiation. And why is the atmosphere traversed by infrared radiation? Very simple: because of direct and indirect effects due to the solar radiations that reach us day by day. Solar radiation carries a considerable amount of infrared radiation when it reaches us from space, but also the materials on the Earth's surface, by absorbing the Sun's energy, emit part of it back into space in the form of infrared, passing through the atmosphere from the bottom up. Thus, the atmosphere is traversed day and night by infrared radiation.

What more do we need to add to understand the whole equation of climate change and its causes? We now know that the atmosphere warms up more or less if it has more or less gases made up of more than one different atom. Let's look at what is happening with these gases. It is known for certain that in large numbers we can say that in the middle of the nineteenth century, when the industrial revolution began, in the troposphere, the lower layer of the atmosphere where life and climate develop, there were about 280 parts per million (ppm) of CO2 and now we are at 420 ppm and rising. This is due to the fact that humans, with our activity, have mobilized a very high percentage of fossil fuels, sedimentary reserves trapped in the Earth's crust, coming from the remains of wood (coal) or from organisms, especially marine organisms (oil and natural gas). This mobilization has happened in almost less than 200 years. Therefore, since there has been more or less stable life on the continents, the carbon cycle has never undergone such a big change in so few years. Many deniers rely on saying that we had a few thousand ppm of CO2 at certain times, and this is true. What they don't say is that changes of plus or minus 1 ppm per year (if we average the last 140 years), which is thousands of times faster than there has ever been, where natural changes in CO2 concentrations of 1 ppm were occurring in time windows of many thousands if not millions of years. Moreover, in the last twenty years the changes have been more than 2 ppm per year.

Therefore, the atmosphere is warming at an unprecedented rate and consequently the climate is warming as well, at a rate that is far faster than the rate at which life on the planet is evolving and can respond by adapting. In addition, other gases that absorb infrared radiation and, as we have seen, contribute to warming the atmosphere, such as methane, have also increased due to human activities, such as the increase in rice fields or herds of cattle, especially bovids. Or nitrogen oxides, also and above all from the combustion of petroleum products.

Finally, it is worth remembering that if all the heat absorbed were not transformed into movement, the atmosphere would not stop heating up to temperatures that would make it unfeasible to sustain biological systems as we know them today, including our own species. Most of the heat that the atmosphere increasingly traps as greenhouse gases increase is converted into motion of air masses at the planetary level. The movement is transmitted from the level of molecule to molecule to the movement of large air masses at the planetary level. And why does this happen? Well, because where the solar radiation arrives with greater intensity, it now heats up much faster than where it arrives with less intensity. At the equator, where solar radiation arrives with the maximum number of hours and with the maximum intensity, there is a faster warming than in areas closer to the poles. This causes large-scale air movements, evaporation of water (i.e. the formation of anticyclones and squalls) to be more frequent and/or more intense. This is the origin of increasingly intense heat and cold waves, as well as more frequent droughts and stronger storms. All this is the consequence of the energy captured in the form of heat that is converted into the movement of air masses, either in the form of anticyclones or squalls.

Obviously, this will not affect the entire planet equally. The large areas of formation of squalls and anticyclones are distributed over the Earth according to their latitudinal location. Since the Earth revolves around the Sun at an angle of inclination, the areas near the equator always receive the maximum insolation, i.e., the maximum amount of energy from the Sun, including infrared. In temperate zones, on the other hand, we experience a summer that coincides when the Earth is oriented towards the northern or southern hemisphere. This causes, for example, the equatorial zone to be a place where the air warms more rapidly, becomes lighter and has a tendency to rise, absorbing humidity and thus forming squalls. The air masses on either side tend to fill the space left by the rising air, creating high pressure zones or anticyclones in the tropics.

Notice in Figure 2, where all of this is associated with the formation of air movement cells. For example, the northern tropic cell shows how the air rises from the equator level to the upper layers of the troposphere, travels towards the northern tropic zone, descends to the surface and returns towards the equator to fill the air that rises to this region, as this is where the air warms the most.

Figure 2. Schematic representation of the great world zones of squalls and anticyclones formation.

In the Mediterranean, we are very close to the Tropic of Capricorn, especially in summer in the northern hemisphere, which means that we have a high probability of being affected by anticyclones, which will be increasingly stronger, aggravating the episodes of sunny weather without rain. In winter, on the other hand, the air movement cells move from north to south, placing the Mediterranean between the areas of tropical anticyclones and the formation of squalls in the region of the polar circle. This makes the climate more variable in terms of the presence of squalls and anticyclones.

However, most of the year we are closer to the area of anticyclone formation, and therefore, in the context of climate change, it is expected that we will continue to receive more anticyclones than squalls. However, the anticyclones that form closer to us will be larger, with more energy, and it will be more difficult for more distant squalls to displace them, thus making precipitation more difficult. In any case, sooner or later, a sufficiently strong squall will form between the anticyclones and bring rainfall. Due to its strong intensity, the rains will be more abundant. This implies that we will receive more rainfall, but poorly distributed, with episodes of heavy storms and a greater proportion of torrential rain in relation to the total, although the total amount of water falling may be equal or less, limiting its use and increasing the capacity to cause damage in case of heavy rains.

Categorías: Sustainability