Chapter 4 looks at the concept of combatants and non-combatants, and its connected status, that of prisoner of war (POW). It examines who is entitled under IHL to combatant status, and examines those persons who have been denied combatant and POW status under IHL. Particular attention is paid to the status of resistance fighters, national liberation and guerrilla fighters, those participating in a levée en masse, and participants in non-international armed conflicts. The chapter outlines those categories of participant not entitled to combatant status such as spies, mercenaries, so-called unlawful combatants, and private military and security contractors. Chapter 4 also explores the current legal thinking regarding a contentious area of the law – that of civilians taking direct part in hostilities. The rules regarding POW status and the treatment of POWs are described. The chapter concludes by examining another developing area of the law: the power of detention in non-international armed conflicts.

Chapter 2 outlines the contemporary legal framework of IHL, examining the treaty and customary laws that govern conduct in armed conflict, and exploring the fundamental principles of the law. The distinction between the jus in bello and the jus ad bellum is explained, as well as some of the different terms used in IHL (Hague Law, Geneva Law, war vs armed conflict, etc). The main sources of IHL are explained – treaties and customary international humanitarian law. The chapter then explains the main principles governing IHL – distinction, discrimination, military necessity, proportionality, prohibition on unnecessary suffering, neutrality and humanity.

Chapter 5 looks at the detailed rules regarding the protection of the wounded, sick and shipwrecked members of the armed forces, and those who care for them – medical and religious personnel. It also looks at the law regarding what is perhaps the most recognisable emblem in the world – the Red Cross – and its affiliated emblems, the Red Crescent and Red Crystal.

Water is one of the most important, and fascinating, substances on the planet. It is, of course, essential for life, and so, of course, is the hydrological cycle. That cycle only exists because water can exist in all three phases in the atmosphere. Water vapour is the most variable substance in the Earth’s atmosphere, so the first step we need to take is to study how and why it varies. This comes down to saturation vapour pressure, which is strongly temperature-dependent. Clouds form when moist air is lifted, a very important meteorological process. This causes some of the water vapour to condense to form water droplets, while also releasing latent heat. It is this latent heat that is, ultimately, the source of power to all storms. We will conclude this chapter with the surprisingly complex but very important story of how individual cloud droplets form.

Oceans cover two-thirds of the Earth’s surface, and are the major lower boundary of the atmosphere. They are the overwhelming source of water vapour to the atmosphere, including a latent heat flux. The oceans circulate, just as the atmosphere does, transporting heat. We start by looking at the key properties of seawater, and the basic ocean circulation patterns; both the surface currents, and the deep-ocean circulation. Sea-surface temperatures exhibit subtle variations, and it is for this reason that the oceans are the major source of the interannual variability of regional climate. While the influences of these oceanic ‘modes of variability’ are mostly regional, some global-scale impacts are also known. The best known of these is ENSO: El Niño and the Southern Oscillation, which has climatic impacts around the Pacific and beyond. We also examine a number of the lesser-known modes that are now recognized as regionally important.

Temperature rises in the stratosphere due to the absorption of solar ultraviolet radiation by ozone. The ozone layer performs the vital role of protecting all terrestrial life from the damaging effects of UV radiation. In fact, terrestrial life could not appear until there was sufficient ozone to provide this protective shield. We will start by looking at the photochemical reactions which form the ozone layer, plus some catalytic reactions which reduce the amount of ozone. We will then briefly look at the major biological impacts of the UV radiation which the ozone layer largely, though not completely, filters out. The Ozone Hole will be discussed in the last two sections. The first will examine just what is the cause of the hole, and why it is largely confined to Antarctica. Finally, we look at the Montreal Protocol that has saved it, and us (fingers crossed).

This chapter asks two questions, although it is the second which is crucial. You may be tempted to scratch your head: after all, the theme of this book is Our Changing Climate, and we have referred to the rise in atmospheric CO2 content over the past century, and the rise in temperature over the past 75 years, multiple times. So, aren’t these self-answering questions? No. Firstly, scientists do not stop at self-answering questions: they delve deeper. But the key reason is that global average surface temperature is only the most reported evidence of a changing climate. In this chapter we will dive into AR6 in order to find many more indicators of a changing climate. We will also interrogate our CMIP6 simulations to see if we really do understand the science behind such changes. That is to say, how much of the change(s) can we attribute to human actions?

In Part I we looked at the chemistry of our atmosphere (with a brief excursion below the ocean surface). This is, of course, the air that we breathe, and its health is our health. At the same time, the atmosphere is a physical system, which must obey the relevant branches of physics: which, of course, means all of them, but in Part II we will focus on two.

We start by examining the current composition of the atmosphere, and then turn our attention to some of the most important chemical reactions that take place in the unpolluted atmosphere. In particular, we will introduce you to the hydroxyl radical, nature’s garbage collector. As well as the three well-known greenhouse gases, the IPCC refers to a wide range of other substances as Short-Lived Climate Forcers, including chemically reactive gases such as methane, ozone, nitrogen oxides, carbon monoxide, etc., and aerosols. The atmospheric fate of all these species needs to be understood. After that, we will examine the polluted atmosphere, particularly smog and acid rain. While this topic might not seem directly related to climate change, there are some useful lessons to be learned. We also include a short discussion on how we use isotope data to help narrow in on some of the more important processes in our environment.

What will the climate of the twenty-first century be like? If we knew the answer to that question, this chapter would be much simpler. But we don’t, because we have little or no idea what decisions humans, and in particular our leaders in politics, business, finance, technology and science, will make. In the absence of the necessary knowledge, we really only have two options: pack up and go home; or make some ‘educated’ guesses. So that – the educated guesses, known as scenarios – will form the first part of this chapter. After that we will take you through the conclusions that the IPCC has been able to draw, based on CMIP6 simulations of those educated guesses, focusing on the AR6 indicators of Chapter 18. We will also look at any implications for policy decisions our leaders may (or may not) make on our behalf.

In the first four parts of this book we focused on the first half of its title: the science. We looked at the chemistry of our environment, and how it has been changing. Then we looked at the atmosphere as a physical system, and the basic laws that govern it, and its circulation. In Part III we looked at radiant energy, the ultimate driver of climate, and some of the factors that have at least the potential to alter either the inflow, or the outflow, of that energy. Finally, in Part IV, we pulled the various pieces together, to see how well we understood the climate system. We know that we can build climate models, with various degrees of accuracy, while also understanding their limitations and uncertainties. This understanding is central to good science.