Global warming is a very important issue that must be urgently addressed. The rise in average temperature is well documented and it seems to be inextricably linked to human activity. As indicated in the 2014 IPCC report “Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia” (Stocker, et al. 2013). This increase in global average temperature is associated with the accumulation of thermal energy in the atmosphere and its surroundings, the ocean and other water bodies, glaciers, etc.
The impact of human activities on the environment has been increasing and takes on many different forms. Human activities collect resources from the environment, which causes a significant disturbance in the natural equilibrium, but also causes emissions, both in terms of material emissions, including waste and greenhouse gases, but also energy emissions. For a long time, material disturbances were considered to be minor, in relation to the size of the natural environment, but this has changed, and material waste management is now a requirement for all human activities. Energy emissions have also been the object of interest, but mostly at the local level. The energy emissions correspond to different types of pollution, like sound pollution (Environmental Energy Agency 2020), light pollution (Falchi et al. 2019) and also thermal pollution (Vallero 2019). Energy emissions, which are often defined as “the discharge of heated water into bodies of water” (Posudin 2014) have been associated with environmental disturbances but mostly in terms of the heat released from thermoelectric power plants (Raptis et al. 2016), which can even disturb power production from other power plants (Miara et al. 2018). However, all these studies were only limited to the local impact of the heat released and are limited to the waste heat that is generated in the power plants and that is seen to add to the global warming effects (Liu et al. 2020).
In this paper, we intend to show that the heat emissions generated by human activity, which include but are not limited to energy production/consumption, are capable of having a major impact on global temperatures.
The commonly accepted description of the temperature in the atmosphere is based solely on a global energy budget, considering the energy that Earth receives from the surroundings (the Sun) as an input and the energy that is radiated back into outer space as the output (Archer 2012; Jacobson 2012). It is usually accepted that if a steady-state is to be attained, whereby the energy input and the energy output are the same, the temperature should remain fairly constant. This approach corresponds to a global energy balance and allows the analysis of the energy content of the Earth (and its atmosphere), but it is an incomplete description in terms of the understanding of the change of physical properties like the temperature. Actually, if a system is in a steady-state in terms of energy inputs and outputs, or even if it is an adiabatic condition with no heat exchange with its surroundings, the temperature may be altered due to the different ways the energy distributes itself on the various forms: heat, mechanic, chemical, nuclear, gravitational, etc. This is apparent in our daily life, for example when a gas heater is used to heat up a space.
However, according to the IPCC report, the anthropogenic forcing of climate is only attributed to radiative forcing, associated with the formation of atmospheric species that interfere with radiation absorption and emission (Stocker et al. 2013). The currently most widely accepted theory to explain global warming is the carbon dioxide theory, although alternative views have been put forward that question the weight of carbon dioxide emissions in global warming (Fleming 2018).
However, looking at the global energy balance just in terms of inputs and outputs is only valid, for the purpose of temperature calculation, if no chemical (or nuclear) reactions occur. Earth is a reacting system, where a series of chemical (or nuclear) changes occur, some natural, but more important, some induced by human activity, and most of these reactions involve significant amounts of energy being transformed from one type to another. In fact, chemical compounds have a specific amount of internal energy (U). When a chemical reaction occurs, part of the chemical energy, which can be described by the Gibbs terms in the overall internal energy of a system, will be transformed into (or from) thermal or mechanical energy.
$$U=TS-\text{P}\text{V} + \sum _{all chemical species}{\mu }_{i}{n}_{i}$$
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where T, S, P and V stand for temperature, entropy, pressure, and volume respectively, and \({\mu }_{i}{n}_{i}\) is the Gibbs term corresponding to the chemical internal potential of a species i.
At this stage, we should note that, although this discussion will be mainly centred on the energy associated with chemical reactions, as these are currently the major source of energy for human activities, the same applies to the energy associated with nuclear reactions, which are also of paramount importance for the energy supply of our society; a fission reaction will convert energy that is stored inside nuclei into heat when a heavy nucleus is split into two smaller ones. This is also relevant for the analysis of the Earth’s energy budget, and, in the following, whenever we discuss the energy involved in chemical reactions, we should note that the same applies to nuclear reactions or, in fact, to any other way of transforming different forms of energy into thermal energy.
Although thermochemistry is a regular subject in atmosphere thermodynamics textbooks (North and Erukhimova 2009), we have no knowledge of these issues being considered in terms of atmosphere warming evaluation, which is described, for example, by a “Layer Model with Greenhouse Effect” (Archer 2012). The implications of these thermochemical transformations will be our topic in the following discussion.
The temperature effects associated with chemical reactions are often addressed in the context of reaction engineering by using an Enthalpy balance since this thermodynamic potential is more convenient in situations where the mechanical energy associated with the term PV needs not be computed, as is the case of many chemical reactors. In the following, we address the issue of global warming using a similar approach by analysing the global enthalpy balance to the atmosphere. This approach will allow us to estimate temperature variations associated with the chemical transformations behind the production of energy.
Let us consider the entire Earth’s atmosphere and write an enthalpy balance to this system. The atmosphere is composed of gases that, upon temperature changes, will expand or contract. This involves work done against or by gravity (changing the PV term in the internal energy definition) which is not easily computed. Thus, in order to compute the temperature changes, we will consider an enthalpy balance, since enthalpy (H) is defined as
$$H=U+PV=TS + \sum _{all chemicalspecies}{\mu }_{i}{n}_{i}$$
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which is also an exact thermodynamic potential and subject to similar conservation laws, as the internal energy.
The enthalpy (\({H}_{atmosphere}\)) inventory of the Earth’s atmosphere system includes sensible heat, relevant phase change enthalpies, as well as all the chemical (and nuclear) potential energies:
$${H}_{atmosphere}=TS + \sum _{all chemicalspecies}{\mu }_{i}{n}_{i}$$
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The Earth’s atmosphere’s total enthalpy will change only due to heat exchanges with the surroundings, which include all the radiation coming from the sun and all the radiation that radiates back into space.
As the discussion is currently centred on the changes induced by human activity, we will assume that, in the absence of this activity, energy fluxes in and out of the atmosphere are the same, which can be seen as the pre-industrial situation.
During the industrial revolution, the major change that occurred was that human activity resorted extensively to the burning of fuels for its activities. This fact, although it did not change the influx or outflux of energy from the Earth, induced a very large release of thermal energy that was previously stored as chemical energy in the fuels that were used, initially coal but later oil, and gas and nuclear fuels. Note that the objective of the use of energy was not only to generate heat but to generate work, that powers the factories; however, all the work that is produced is eventually and inevitably degraded into thermal energy. Thus, even if the overall energy or enthalpy content on Earth does not change, the fact that chemical energy is being converted into heat, according to Eq. 3, is going to produce changes in Earth’s temperature. As we already said, there is a tendency to neglect these emissions as being insignificant in relation to the size of the environment and this is the assumption that we will analyse in the following.
It should also be pointed out that an increase in temperature will also result in an increase in radiation emissions by the earth, increasing energy output, although part of these emissions can, in fact, be retained by greenhouse gases in the atmosphere.
For argument’s sake let us consider, as a simplifying assumption, that the Earth’s atmosphere is in a balanced state, where the amount of energy that is received is exactly matched by the amount of energy that is radiated back to space.
$$\frac{d{H}_{atmosphere}}{dt}=0= \frac{dTS}{dt} + \sum _{all chemical species}{\mu }_{i} \frac{d{n}_{i}}{dt}$$
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As per the arguments above this would imply that the temperature would change if some of the energy contained in the chemical energy term is converted to thermal energy. Let us consider a simple example, which is very commonly used in our society to produce electricity, the combustion of coal:
$$C+ {O}_{2} ⟶{CO}_{2}+Heat$$
This reaction involves a decrease in enthalpy of the chemical system, associated with the change of its chemical composition, which is measured by the reaction enthalpy and corresponds to a decrease in the second term on the right-hand side of Eq. 3. If the overall enthalpy of Earth is to remain constant, as per the adiabatic assumption, this decrease will imply that the term TS, i.e., the content of thermal energy, increases thus inducing an increase in the temperature of the system. This reaction is the one that allows us to produce steam in coal power plants. Similar reactions, albeit involving other chemical species, allow us to have a hot bath when we burn natural gas to heat up water or power the engine in a vehicle. This is a clear example that the chemical reactions that are used to produce energy do change the temperature of the surroundings where they are taking place.
As already stated above, this also applies to nuclear energy, where heat is released by the change in the atomic composition of a system, hydropower energy, where potential energy is first converted into kinetic energy (of the water flowing), and ultimately into heat when this kinetic energy is used for other purposes, such as heating our houses. In fact, it can be argued that all the energy that is produced by human society is ultimately transformed into thermal energy – we use the energy that we produce to heat our buildings, which directly produces thermal energy, but we also use the energy to drive our transports, directly producing heat as a by-product in the engines, but also mechanical energy, in the form the kinetic energy of the vehicles. However, even this kinetic energy is ultimately converted into heat due to the resistance to the movement and braking.
In the following, we will assume that all energy that is produced in our society’s energy system will ultimately be converted into thermal energy and, thus, will contribute to the increase in temperature of the atmosphere.