This is an excerpt from Global Climate Justice: Theory and Practice. You can download the book free of charge from E-International Relations.

Climate change is happening, and its impacts are increasingly felt by all communities, especially the vulnerable ones that do not have enough resources to adapt to the changes. Global warming, sea-ice melting and sea- level rise have all been accelerating, and with them the frequency and intensity of extreme weather events. Human activities bear most of the responsibility for what we have been observing since the Industrial Revolution. Greenhouse gas emissions must be cut dramatically by about 5-8% every year starting now if we want to achieve net-zero emissions by 2040–2050, and limit global average warming to below 2°C above the pre-industrial level as agreed in Paris in 2015. Failure to act swiftly and effectively will cause even more severe impacts, as indicated by the climate projections generated using state-of-the-art Earth-system models.

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This chapter, firstly, presents the evidence for climate change and explains the role of human beings in causing it. Secondly, it explains the concept of an Earth-system model and discusses its role as a tool for understanding the past climate evolution and generating projections of the future climate. Thirdly, it discusses the per capita greenhouse gas emissions of different countries and economic sectors. Finally, it presents climate projections to illustrate how Earth’s climate could evolve under different emissions scenarios.

The aim of this chapter is to provide a solid, objective and quantitative background to climate change. In Section 2 we will review the greenhouse effect, a key phenomenon that has made Earth a planet where complex lifeforms could evolve, and what are the main greenhouse gases. Understanding the greenhouse effect, the main gases that cause it and its role in the development of life on Earth is essential to be able to recognize what has been happening to Earth’s climate in the 100 years following the Industrial Revolution. Sections 3 and 4 analyse the observed trends in greenhouse gas emissions over time and discuss the observed climate variations and their impacts.

This first part of the chapter will provide a sound understanding of what has been happening to the Earth climate. Section 5 explains what an Earth-system model is and how it can be used to estimate future climate scenarios and their probabilities of occurrence. It discusses how these scenarios are generated and the role that emission assumptions play in determining the future climate. Section 6 presents an analysis of which countries have contributed mostly to the stock of greenhouse gases, and whether there is a clear link between greenhouse gas accumulation in the atmosphere and global warming. Section 7 discusses the quasi-linear relationship between accumulated emissions and average global warming. It also discusses how we must constrain the emissions if we want to limit the global warming below 1.5°C or 2°C degrees. Finally, Section 8 summarizes the key points discussed in this chapter, and discusses the global warming that we could experience in 2050 under four possible emissions scenarios: one with a continuous future increase of greenhouse gas emissions of 1% per year, and three with a continuous future decrease of greenhouse gas emissions by 1%, 3% or 5%.

2. The Greenhouse Effect

The Earth’s temperature would be about 30°C colder if not for the atmosphere’s greenhouse effect. The problem we face today is that too strong a greenhouse effect has been warming the planet in an unprecedented way. And with the warming, we have been experiencing an increased frequency and intensity of extreme weather events and sea-level rise, which have had negative impacts on tens of millions of people who do not have always have sufficient resources to adapt to climate change.

Before discussing climate change and the actions we can take to control it, let us briefly review the greenhouse effect by considering the Earth as a very simple system in equilibrium. The Earth is a system that receives heat in the form of solar radiation and emits heat as a black body. If we apply the first law of thermodynamics (which says that energy must be conserved) to this simple model of the Earth, then there must be an equilibrium between the absorbed and the emitted energy.

The Earth, as any physical ‘body’ that has a temperature above absolute zero (–273.15°C), emits radiation with a certain characteristic (frequency, wavelength, spectrum). The sun, a much warmer body than the Earth, also emits radiation. We can apply the Stefan-Boltzmann law to compute the energy emitted per square meter by a black-body with a temperature T. Here, the term ‘black body’ means that we assume that the Earth and the sun behave as idealized, opaque and non-reflective bodies. The Stefan-Boltzmann law relates the emitted radiation per square meter to the black body’s temperature.

Thanks to the presence of the atmosphere, which absorbs most of the long-wave radiation emitted by Earth’s surface, and re-emits radiation as a black body, the Earth has an average surface temperature of 304°K, which is about 31°C. This temperature (31°C) is about 50°C higher than the temperature the Earth would have without a greenhouse effect.

We should point out that 31°C is higher than the observed average surface temperature, which is about 15°C. This difference is due to the fact that we have assumed that the Earth behaves as a simple system with a very simple atmosphere. In other words, we have over-simplified the Earth and did not consider the complexity of the Earth system. Thus, we should not be surprised if the completed and observed values are different. Despite this, this simple model highlights the important role that an atmosphere has on the Earth’s surface temperature. To make the model slightly more complex, we could assume that the atmosphere is compounded by few different layers instead of only one, with each layer characterized by a different temperature and find a solution for the Earth surface’s temperature that is closer to the observed one.

The atmosphere allows most short-wave solar radiation to reach Earth’s surface, while the amount of the long-wave radiation emitted by the Earth that it absorbs depends on its chemical composition. The principal greenhouse gases are water vapour, carbon dioxide, methane, nitrous oxide and fluorinated gases (Table 1). Note that their concentrations are very small, ranging from about 0.3% (in mass fraction with respect to dry air) for water vapour to much smaller values for carbon dioxide and methane.

There are two key differences between water vapour and the other gases. First, for any air mass, the water vapour concentration strongly depends on the pressure and the temperature of the air. If a mass of air that contains water vapour is cooled and/or compressed, water vapour would condense and precipitate; water continues to cycle, and as a consequence, the total concentration of water vapour in the atmosphere changes very little. Indeed, the time that a molecule of H2O spends in the atmosphere is about 10 days. In the last 100 years, human beings have increased the concentration of carbon dioxide and methane in the atmosphere by more than 50%, from about 270 parts per million (ppm) at the end of the nineteenth century, to about 415 ppm today.

A further, important point to make is that the time that a molecule of CO2 spends in the atmosphere is between 300 and 1,000 years, and the time that a molecule of CH4 spends in the atmosphere is about 10 years. This means that greenhouse gases continue to exercise their effect long after they have been injected in the atmosphere. Because of these two differences, the continuous injection of large quantities of greenhouse gases in the atmosphere has and will for many tens and hundreds of years influence the Earth’s climate.

3. Observed Trends in Greenhouse Gases

The concentration of the greenhouse gases has been increasing since the start of the Industrial Revolution. Figure 1.4 shows the annual mean concentrations of CO2 and CH4, computed using data collected at the Mauna Loa Observatory (Hawaii, United States).

Let us start by looking at CO2. The atmospheric concentration of CO2 has exceeded 400 ppm in 2015. It is worth stressing that the Earth has not seen such a high concentration in the past 2.5 million years (see, e.g., Wallace and Hobbs 2006; Hartmann 2016; IPCC 2021). Note also that the increase follows a roughly exponential curve, and that the annual percent increase has been growing nearly continuously. In fact, as shown in Figure 1.5, while in the 1960s and 1970s the annual percent increase in CO2 was between 0.2-0.4%, in the last two decades it has been above 0.5%. Figure 1.5 also shows that the last two decades have seen a clear positive trend, with the growth rate increasing by about 0.01 every year, from about 0.47% in 2000 to about 0.65% in 2020.

CH4 concentrations have also been increasing from about 1,645 ppb in 1985 (data before 1985 were not available) to 1,879 ppb in 2020 (Figure 1.4). Figure 1.5 shows that the annual growth rate decreased between 1985 and 2000, but since then values have been growing at a fast rate. The last two decades have seen a clear positive trend, with the growth rate increasing by about 0.03 every year, from about 0% in 2000 to 0.60% in 2020.

4. Observed Climate Change

As the concentration of the greenhouse gases increased, the atmosphere has been absorbing more long-wave radiation emitted by the Earth, and thus has been warming. A warmer atmosphere has been emitting more long-wave radiation towards the Earth’s surface, which thus has also been warming. This cause-and-effect mechanism between increasing emissions of greenhouse gases and the global average temperature is a direct consequence of the greenhouse effect that was discussed in Section 1.

Figure 1.6 shows the anomaly of the global annual mean temperature with respect to the pre-industrial value. In other words, for each year, the figure shows the difference between the global annual mean temperature of that year and the global annual-mean temperature of the period 1850-1900. The solid line shows the annual values, while the straight line (which has a slope of ~0.02°C/year) shows the long-term warming trend. Note that superimposed over this linear warming trend of ~0.2°C per decade are natural oscillations of about 0.1-0.2°C. These natural oscillations are due to natural variability, for example linked to large volcanic eruptions or heat exchanged between the Earth’s oceans and atmosphere. In this latter case, the........

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