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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.
We turn now to the third of the three branches of science that underpin our changing climate: radiant energy – the energy carried by light and other electromagnetic waves – and the ways it interacts with the gases and particles in our atmosphere. In this chapter we will outline the necessary science, and especially the laws of thermal emission. It turns out that we are bathed in electromagnetic radiation that we can’t see. We will then show how these laws lead to the counter-intuitive outcome that the ‘temperature’ of our planet is actually well below freezing! Fortunately, however, our Earth has a ‘good’ atmosphere, containing some very important gases – greenhouse gases – which act as a blanket, keeping the surface at a suitable temperature for liquid water, and hence for life. A simple model will help you gain a basic understanding of the greenhouse effect.
For centuries, humans have been using the atmosphere as a dumping ground, starting with our first use of fire. After the Earth came out of the Last Glacial Maximum ~11,000 years ago, things began to ramp up, as people congregated in settled communities. The arrival of the Bronze Age, about 5,000 years ago, added the waste products (and their odours) from the smelting of copper and other ores to the (local) atmospheric burden. The first voices of complaint were probably heard in the ancient societies of the Nile and Mesopotamia. The Greeks and Romans expected their leaders to keep the air in their towns clean.
Thermodynamics is the science that governs most of the basic, or ‘static’, properties of a system such as a gas. In this chapter we will be focusing on thermodynamics of the dry atmosphere. Water vapour is sufficiently important to deserve a separate chapter. A thermodynamic system is characterised by a small number of state variables, primarily density, pressure, and temperature. After defining these, we will see how they vary as we move up through the atmosphere. Two key equations – the hydrostatic equation and the ideal gas equation – interconnect these variables and their profiles. The First Law of Thermodynamics tells us how the atmosphere responds to an input of heat, as well as a change in pressure, which may come about with elevation (vertical motion). This is the key to the atmosphere’s vertical stability, and (ultimately) to cloud formation.
The Earth’s radiation balances are being altered by a number of changes in the composition of the atmosphere, and as a consequence the climate system is being ‘forced’, almost certainly in the direction of higher temperatures. In earlier chapters we examined the physics of the processes involved. What are likely to be the effects of such changes? The only way to answer questions such as this is to model the Climate System in sufficient detail. Firstly, we need to model the atmosphere, something we have been doing for half a century to forecast the weather. Because of the significant exchanges of both heat and water between the ocean and the atmosphere, it is clearly necessary to couple an ocean model. Ice sheets are likely to be affected by warming, as they are one of the key feedback processes just mentioned. The land surface also has significant interactions on various timescales.
We experience weather in the air around us, and that implies climate: the statistics of weather. But if we are to understand climate, and especially how it changes, we need to examine all its interactions with the planet. In the previous chapter we looked at the role of sea-surface temperature. If we are to go further, we need to look at other components of our planet and their atmospheric interactions. When we see the range of interconnections, we realise that we are dealing with a complex system, with multiple interactions between components. Whenever we see such a system, we must realise the potential for one interaction between components to end up impacting back on another: this is feedback. If we are to understand climate, we must understand these feedbacks. In the last sections we will illustrate one of the most important of these processes with a ‘relatively simple’ model.
Over the past century, average temperatures have risen a little over 1°C. This may not seem like much: after all, temperatures vary from one day to the next by much more than that, and we take it in our stride. However, over the past couple of decades we have become more aware of the rising incidence of what we call extreme events: heatwaves, droughts, wildfires, floods, severe storms. These are the signs of the times; signs, perhaps, that Mother Nature is not happy. Or is this all simply part of the natural unpredictability of the world we live in? In this chapter we look at recent extremes, along with the recent branch of climate science, Event Attribution, where we endeavour to assess any human contribution to these events. We illustrate both the nature of extreme events, and our growing understanding, with several detailed case studies.
Apart from a tiny amount of energy generated in the core by radioactive decay, and tidal energy from the gravitational interaction of the Moon, the Earth generates no energy of its own. Yet this is a planet full of life, both biological, and physical – assuming we may stretch the language, and call atmospheric circulation, and the hydrological cycle, forms of ‘physical life’. (James Lovelock’s Gaia hypothesis treats the entire planet as a living organism.)