Topic Posting – Chemistry 105, College of Marin, California
May 11, 2006
The ‘greenhouse effect’ has become synonymous with the idea of human-induced global warming of the planet, but in reality were it not for this effect, there would be no life here on Earth. Without the very sparse amount of carbon dioxide (CO2) and the great quantity of water vapor molecules in our atmosphere, the average temperature would be a very chilly minus -18C, and Earth would spin around the Sun in permanent deep freeze. The greenhouse effect is therefore utterly vital for life and we may feel deep appreciation for it. We alter it at our peril, however, and this is what is now beginning to grip the minds of many people, worldwide. If this natural warming effect becomes more pronounced as a result of uncontrolled emissions of CO2, NO2 (nitrous oxide) and CH4 (methane), as it has already done over the last 150 years of industrial activity, we face the possibility of widespread death and devastation across the entire planet due to climate change. Some predict that such disaster will occur as soon as the end of this century, if we do not take firm action to prevent global warming. Clearly we have a major issue before us.
In chapter 17 of Conceptual Chemistry: Understanding our World of Atoms and Molecules, dealing with the chemistry of our atmosphere, the author John Suchocki introduces the reader to the greenhouse effect. How it actually works, however, is not explained. We are told simply that certain gaseous molecules ‘absorb’ infrared radiation. Following my curiosity and concern about global warming, I decided to explore how the warming actually happens with respect to carbon dioxide in the atmosphere. I was led into a fascinating story of the interaction of matter with radiant energy, in the real life world of planet Earth.
CO2 is present as a small but critical quantity of our atmosphere. Of a sample containing a million atmospheric ‘parts’, it forms just 0.03% of the total volume. It is also present in the oceans, freshwaters, and in the soils of the land. We do not find it there for wholly arbitrary and chance reasons, however. It is there primarily as a result of Life itself and its presence belies a very long-lived planet on which photosynthesis has been active for nearly four billion years, with the gases of carbon and oxygen cycling back and forth between organism and the environment in a way that keeps everything alive.
As a molecule, carbon dioxide originates both from biological processes and from the geological properties of our planet. Volcanoes produce CO2 when they erupt and today, (in a vastly greater quantity) it is also formed and emitted by forest fires and by human burning of formerly living, carbon-based things such as wood, coal, and oil. Prior to the human industrial era and over immense periods of time, a balance was struck, in which the level of CO2 in the environment became “just right” for the support of life beyond the microbial size.
Our bodies collect 0.04 % of C02 molecules per volume of breath we inhale, and when we breathe out we exhale 4.5 % per volume. The difference derives from what is involved in digesting and absorbing the energy content of our food, itself substantially formed with carbon. If, by chance, you were to breathe in air that contained between 6 and 10% carbon dioxide per volume, however, you would become very ill and, if breathed long enough you would most likely die. A single breath of air containing 30% of carbon dioxide would put you into a coma and almost certain death. So the tiny quantity of atmospheric CO2 is, indeed, no accident. The entire system of gaseous exchange between living beings and the environment is known as the Carbon Cycle. And this cycle, formed by life (in the main) is substantially responsible for Earth being warm enough to sustain life.
So, how does such a tiny quantity of carbon dioxide in the atmosphere act to create a warm planet? Our planet circles the Sun at a distance of 93 million miles, nearly all of which space is extremely cold—yet how balmy and comfortable it is out here! What is it that makes CO2 so powerful a force in forming a natural “greenhouse” around our heads?
To answer this, we have to examine what carbon dioxide looks like and then consider how it behaves when it interacts with photons of energy (‘light’). We go to a basic level of reality here, so hold on tight…
Carbon dioxide is a three-atom molecule that forms a linear structure comprising one carbon atom centered between two oxygen atoms, hence the ‘dioxide’ part of the name. Through the covalent double bonds that hold the atoms together, the linear shape is naturally created. These bonds are called ‘double’ because they involve two electrons being shared between the carbon and the oxygen. In this case, the sharing of two electrons is duplicated because carbon can bond with a second oxygen atom, in an identical fashion. So the total number of electrons being shared is four, which is exactly what the carbon atom needs to complete its outer shell of electrons and become stable. And it is also exactly what the larger oxygen atoms need to ‘complete’ themselves, as well. Carbon dioxide, like water, is a molecule of atomic contentment.
But do not imagine that stability means this molecule is static and rigid. Far from it! In practice, the sharing of electrons between the three atomic nuclei of the molecule means that the bonds are more like coiled springs, able to bend, rotate and flex in four different directions and this means that the molecule has available to it a small range of different vibrational ‘styles’. One of these (the asymmetric stretch, as it is called) is of critical importance for creating the greenhouse effect.
When light from the Sun reaches the Earth, some of it is visible to our eyes (visible light) and some of it is not. There are three main types of radiation that arrive which we cannot see: ultraviolet (in limited quantity as most of it is absorbed by high altitude ozone molecules), infrared and radio waves. It is the infrared that concerns us here, for it is this lower energy radiation than visible light that interacts with carbon dioxide molecules moving around in our atmosphere. Much of the infrared radiation from the Sun is absorbed by the atmosphere before it reaches the ground. Unlike visible light, which comprises a very narrow band of wavelength frequencies (approx 400 – 750 nanometers) the infrared band of radiation covers a considerable range of wavelengths, which represent different energy levels of photons of light. It covers three main divisions, called Near, Mid and Far Infrared, extending from around 10 microns to 1000 microns of wavelength. (1000 microns is a millimeter)
Of this range, according to the way it vibrates as a structure, CO2 is responsive to two main wavelengths of infrared. At these two wavelengths, the collective electronic structure of the molecule, in one of the four kinds of vibration it is capable of, ‘speaks the same language’ as the photons. At the moment of contact, the molecule is altered: it becomes excited to a precisely allowed higher energy level. The electrons move immediately, without any intermediate steps, to this higher state of energy. The photons are thereby ‘absorbed’. They cease to exist. Their energy has been transferred into one specific state of excitement that is available to the molecular structure. In this regard, we see that molecules respond in exactly the same ‘quantized’ way as individual atoms do. And just as atoms do, when electrons move to a higher energy orbit, there is an emission of a photon of energy as the electrons drop back to their ground state. In my mind’s eye, I see the carbon dioxide receiving and then throwing back out a tiny ‘ball’ of energy. It is dynamic and elegant. The molecule and the radiation are in resonance and energy is flowing.
A key reality of the Greenhouse Effect is that the Earth, like any other object here or out in space, is itself producing infrared radiation. Rocks, ice, oceans, living organisms of all kinds—all are producing infrared radiation. It is this terrestrial radiation which is particularly significant for forming a blanket of warmth around the planet. It comes as a result of absorption of visible light by other kinds of molecules forming the surface of the Earth, (or, like us, living on the surface) which, in turn, emit some photons of energy. These are no longer in the visible light range but are in the infrared range.
Thus, with infrared saturating the planet both from the Sun and from the planet’s reactions to visible light radiation, carbon dioxide (and other greenhouse gases) act to ‘keep the heat turned up’. All the molecules of gas in our atmosphere are in constant motion and collisions with one another, but when a CO2 gains speed from the absorption of infrared radiation, it increases the speed of any molecules of nitrogen and oxygen that it randomly knocks into—and speed of molecules forming the atmosphere, in general, not just CO2, means heat. If carbon dioxide and water vapor could not absorb photons of infrared radiation, there would be nothing to stop the loss of heat from the Earth’s atmosphere and surface. Heat from the Sun would exit back out to space and we would not be here at all. The Earth would be frozen solid.
We may therefore see the Earth as a gigantic cooking stove; it is sending out heat which but for water vapor, CO2 and the other greenhouse gases, would escape into space immediately. These days, however, the infrared photons encounter more molecules of CO2 than there used to be. The levels have gone up from around 280 parts per million, prior to the industrial revolution, to around 380 parts per million today. It is this interaction between more carbon dioxide and the same amount of infrared energy that is creating a stronger greenhouse effect. It is adding more energy to the overall system, causing our planet to heat up.
Clearly, energy absorption is not a cul-de-sac or one-way street. This is what I have learned. We are seeing the way that heat is retained and re-circulated over and over in one little spot of space occupied by our planet in the overall context of the solar system. As this radiation increases, it affects absolutely everything else: water temperature, air temperature, land temperature, the temperature of individual organisms and of whole eco-systems. When we speak of global warming therefore, we are speaking of the entirety of existence here on Earth. Climate change is not just a bit more or a bit less rain here and there. It is a wholesale rearrangement of the total energy ‘budget’ of the planet. This is inevitably comprehensive, for the planet exists and functions as a Whole. It is not in parts and pieces, as humans have created through their ideas (and feelings) of nationality, ethnicity, religions—and economies. So, if we want to have a long-term future, we have something to address here in the way we create and use energy. It concerns our sense of identity.
At the end of the day, what price our notions of being American or being British or being Arab or being Indian or being Chinese? Would we sacrifice our children’s future for these ideas and traditions? I would like to think we would not, but right now, this is the core question facing each and every one of us. In other words, just as the planet functions as whole unit, a whole system, so we humans have got to move swiftly to function in an identical way. Instead of identifying with our particular histories and cultures, or with our corporations, we must now make it a higher priority to identify with the Earth, from which we came and with which we can forge a long-term future, providing we recognize with total clarity, on which side our bread is buttered.
Carbon Dioxide may be our teacher about how to form a more unified and balanced world order.
©Caroline Webb 2006. All rights reserved. No use of any kind without permission. Contact me
