In my last post on this subject, Understanding Carbon Dioxide – CO2 its Role in the Atmosphere, I alluded to some problems I had with understanding how CO2 works in climate change. I have decided to explore these here. To do this I am going to have to assert some facts as I understand them.
The Earth’s climate is powered by the sun. Energy from the sun is not received evenly. One half of the planet is always in darkness. Equatorial regions receive more energy in the form of heat than polar regions. 71% of the Earth’s surface is water, which absorbs heat at a different rate to land due to its lower density. The atmosphere absorbs and reemits a significant amount of the electromagnetic radiation, sending it back into space. Both the land and the seas also reemit heat radiation. The net flow of energy into and out of the planet is called the Earth’s energy budget. A radiative equilibrium is achieved when there is an equal flow of heat into and out of this energy budget. The global temperature remains relatively stable as a result. An increase or decrease in the amount of incoming or outgoing energy disturbs the radiative equilibrium and changes global temperatures accordingly.
The Earth’s Energy Budget
According to NASA 29% of the sun’s electromagnetic radiation is reflected back into space by the top of the atmosphere. Another 23% is absorbed by various layers of the atmosphere, leaving 48% to be absorbed by the surface. Together the atmosphere and the surface absorb 71% of electromagnetic radiation. To achieve a radiative equilibrium, they must radiate an equal amount of energy back into space. It is estimated that the atmosphere radiates 59% and the surface only 12%. Clearly, most solar heating of the planet occurs at the surface, but most radiative cooling occurs in the atmosphere. Nevertheless, they still equal the 71% required for radiative equilibrium.
The 48% of energy that is absorbed by the surface is returned by three processes: evaporation, convection, and infrared radiation. Evaporation accounts for 25% of returned energy. Water molecules absorb solar radiation, which increases their kinetic energy, making the molecules move, which in turn results in their heating up and changing from liquid to gas. The molecules that form the gas, or water vapour, possess more energy and as a result their temperature is higher than that of the liquid water, which cools as a result of this loss of energy. Conversely, this energy is transferred to other molecules in the atmosphere when the water molecules condense to form rain. The air around the forming rain cloud becomes warmer. This is known as Latent Heat and it plays an important role in thunderstorms.
Convection occurs when cooler air comes into contact with warmer ground surfaces. Heat is transferred from the hot area to the cold. The warmed air rises, taking the heat energy with it and allows cooler air to take its place. It is estimated that 5% of the solar energy that reaches the surface is returned in this way.
About 17% of energy is returned by infrared radiation. It is estimated that of this 12% eventually passes through the atmosphere and into space with 5% being absorbed into the atmosphere itself by H2O, methane, and CO2.
Influences that change the amount of energy entering or leaving the Earth’s climate system are known as climate forcings. Natural climate forcings include the sun, variations in the planet’s orbit and its axis of rotation, called Milankovith cycles, and volcanic eruptions. Human activity that is believed to create other climate forcings include particle pollution in the atmosphere, deforestation, and an increase in greenhouse gases from industrial processes or use of machinery such as cars.
The Role of CO2 as a Climate Forcing
Despite its presence in the atmosphere as a trace gas only CO2 is said to cause climate change because it absorbs infrared radiation with wavelengths longer than 12-13 micrometers. H2O, as water vapour absorbs infrared radiation at shorter wavelengths of around 10 micrometers. It is claimed that it is the absorption of infrared radiation in the longer wavelengths by CO2 that causes an imbalance in the Earth’s energy budget resulting in a rise in temperature. The actual affect of this is not instantaneous but may take decades to be noticeable.
Problems with these scenarios
I am a layman, not a qualified scientist, and I am trying to understand the whole science of climate change. Now it may be that I just do not understand the intricacies of the science itself but there are certain parts of what I have written above that do not make logical sense to me. First, CO2 as a climate forcing. It is a fact that CO2 absorbs infrared red radiation at wavelengths of 12-13 micrometers, but it is also true that CO2 reemits what it has absorbed almost instantly in every direction. It does not trap the infrared radiation, it actually releases it. Due to the scarcity of CO2 in the atmosphere, only 414ppm, the odds of the reemitted infrared radiation of a wavelength of 12-13 micrometers being absorbed by another CO2 molecule is statistically small. As the more numerous H2O does not absorb this wavelength then it is more likely that an equal amount will escape into space as will be radiated back to the planet’s surface. If it does encounter another CO2 molecule then it will be absorbed and reemitted in all directions once again. This process of absorption and reemission suggests that the amount of infrared radiation is actually reducing in each instance. The number of packets of radiation emitted must equal the sum total of radiation absorbed, each individual packet cannot be equal to the sum itself as that would mean that energy is being created and that would be contrary to the First Law of Thermodynamics. The actual process of how the absorption and reemission of longwave infrared radiation by CO2 leads to an increase in temperature in the atmosphere is unclear.
Second, the Earth’s energy budget. When I was reading information on this subject, I noticed that the radiative equilibrium appeared to be used as a necessary outcome of the energy budget rather than as a characteristic. What I mean by this is that it seemed from the language used that the radiative equilibrium was the result to which the processes of the energy budget were dedicated to achieving. That which goes in ‘must’ equal that which comes out. Now this might be true of a steady state system, one that either does not change or only changes a negligible amount over a length of time. The Earth’s climate does not match that definition, however. In its 4.6 billion-year history the Earth and everything about it has indeed changed. The radiative equilibrium has rarely been like it is today. The planet has been much hotter and much colder. If the energy budget existed to achieve the radiative equilibrium then that should not have happened, but climate change is a fact of the planet’s geological history. That suggest to me that it is incorrect to use radiative equilibrium as an imperative conclusion of the energy budget, it is just a characteristic, not a fixed end result.
Exact values for the flow of energy through the Earth’s energy budget do not exist because they are still a matter of research. NASA accepts that all estimates are only that and subject to uncertainty. This is a fact that is not always obvious when information regarding this subject is presented. Despite this lack of information propositions are made in respect of climate change as if everything is certain, understood, and proven.
I have an understanding of deep time through my interest in palaeontology. It is not a concept that everyone can come to terms with. 4.6 billion years of history is an awful lot to consider. My almost life-long love of dinosaurs and other prehistoric life educated me to the many ways in which the Earth has changed. Continents have appeared, broken apart, reformed, and moved across the surface of the planet. Temperatures have soared to unimaginable heights and fallen to equally destructive lows. From a human perspective it might seem that the climate has remained more or less the same but our life span, even as a species, is deceptively short compared to that of the planet on which we live. We evolved during an ice age and we are thriving in an interglacial period. The question is not will the climate change but rather to what to and how? At the end of this brief study of CO2 I remain sceptical about its role in the process because I have yet to find a simple explanation of what this process is and how it exactly works. That does not make me a climate change denier, just a CO2 sceptic. My mind is open, however, so if you know the answer to the CO2 riddle, then I would like to hear from you.