Climate Change

Past, Present, and Future

Marie-Antoinette Mélières, Chloé Maréchal

• Publisher: Wiley-Blackwell
• Year: 2015
• ISBN:
  9781118708491
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Meteorology & Climatology

This book is designed for first- and second-year university students (and their instructors) in earth science, environmental science, and physical geography degree programmes worldwide. The summaries at the end of each section constitute essential reading for policy makers and planners. It provides a simple but masterly account, with a minimum of equations, of how the Earth's climate system works, of the physical processes that have given rise to the long sequence of glacial and interglacial periods of the Quaternary, and that will continue to cause the climate to evolve. Its straightforward and elegant description, with an abundance of well chosen illustrations, focuses on different time scales, and includes the most recent research in climate science by the United Nations Intergovernmental Panel on Climate Change (IPCC). It shows how it is human behaviour that will determine whether or not the present century is a turning point to a new climate, unprecedented on Earth in the last several million years.

PART I
THE CLIMATE ENGINE OF THE EARTH: ENERGY

Chapter 1
Why are there many different climates on Earth?

At any one place, the climate is defined essentially by the prevailing temperature and by the rainfall. These two quantities, both in their annual averages and in their seasonal variations, are distributed unevenly over the surface of the planet. The result is a mosaic of extremely varied climates. Why is this? What are the factors that produce such a wide range of temperatures that water exists in abundance in all its three phases (gas, liquid and solid), and that our planet occupies a unique place in the solar system? What factors govern the distribution of temperature and rainfall? The primary driving force is the annual amount of energy arriving at the surface of the Earth, and its seasonal distribution. The guiding principle of this energy distribution will become apparent as we introduce the various relevant parameters.

1. First, we enquire into the source of energy that continuously supplies the Earth’s surface and sets the operating range of temperature. This is the Sun alone, all the other sources of energy being incomparably weaker. The flow of energy is defined primarily by the Sun’s radiation and is a function of its activity. The amount of solar energy received on the Earth also depends on the distance between the Earth and the Sun. The position of the Earth in the solar system is thus the first key factor that, unlike our neighbouring planets, enables it to host life in abundance.
2. The second characteristic of planet Earth is its atmosphere, which by its composition modifies the flow of energy arriving at the surface. The greenhouse gases (GHGs) in our atmosphere play a leading role in this flow, increasing the energy available at the surface of the planet and raising its average temperature.
3. Since the Earth is practically spherical, the solar flux falling on its surface is spread very unevenly over the different latitudes. At higher latitudes, the Sun’s rays become increasingly tilted with respect to the surface and, on moving from the Equator to the poles, less and less energy is received per square metre (Part I, Note 1). This property defines the first major characteristic of climates on Earth: temperature decreases from the Equator to the polar regions.
4. The temperature difference between the Equator and the poles is nonetheless attenuated by the universal principle that heat propagates from hot regions to cold regions. Heat is transferred from the tropics towards higher latitudes by three transport mechanisms: atmospheric circulation, ocean circulation and the water cycle.
5. Owing to the tilt of the axis of rotation of the Earth in the ecliptic (the plane in which the Earth moves around the Sun during the year), the slope of the Sun’s rays, and hence the energy delivered to each point on the Earth, oscillates throughout the year. This gives rise to the different seasons, as described in Box 1.1.
6. Finally, since the orbit of the Earth around the Sun is slightly elliptical rather than perfectly circular, the Earth–Sun distance varies over the course of the year. This is accompanied by variations in the amount of energy received during the year, but these variations are much smaller than those that give rise to the seasons. Over thousands of years, however, their slow changes have a major impact.

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BOX 1.1 THE SEASONS

In the course of a year, the Earth travels in an almost circular orbit around the Sun, in a plane called the ecliptic plane (Fig. B1.1). The axis of rotation of the Earth (the polar axis) is at present tilted at 23°27’ with respect to the normal to this plane. At any given place, therefore, the angle of the Sun’s rays at zenith (i.e. the angle of the rays with respect to the normal to the Earth’s surface) varies throughout the year, with accompanying changes in the amount of sunlight each day. At latitude 45°N, for example, this angle varies between 21°33’ (summer solstice) and 68°27’ (winter solstice). At the Equator, it varies in the range ± 23°27’. The amount of solar energy received, also called the solar irradiance (in W/m²), varies during the year, bringing in its train the succession of seasons. In each hemisphere, the year is marked by four dates that define the beginning of each season: the two solstices, summer and winter, when the irradiance is at a maximum (summer solstice) and then at a minimum (winter solstice) at latitudes situated between the tropics and the poles, and the two equinoxes (spring and autumn) when the day and night have the same duration.

Fig. B1.1 The position of the Earth at different seasons. The 23°27’ tilt (obliquity) of the Earth’s axis with respect to the normal to the ecliptic plane defines the Tropic of Cancer (23°27’N) and the Tropic of Capricorn (23°27’S). At those latitudes, the rays from the Sun at zenith fall perpendicularly on the surface of the Earth at the June and December solstices, respectively. The obliquity also defines the polar circles (66.33°N and 66.33°S). Between the pole and the polar circle, the day lasts for 24 hours at summer solstice.

At the equinox of 20 March – the spring equinox in the Northern Hemisphere (NH) and autumn in the Southern Hemisphere (SH) – the Sun’s rays strike the Earth vertically at the Equator and tangentially at the poles. Day and night have the same length (12 hours) over the whole planet, from the North Pole to the South Pole. Then, in the NH, for example, between 20 March and 21 June, the daily irradiance gradually increases in latitudes situated above the Tropic of Cancer (23°27’N), reaching a maximum on 21 June, the longest day of the year. On this date, the Sun’s rays at solar zenith fall vertically on the surface of the Earth at the Tropic of Cancer, and the irradiance is at a maximum. This is the summer solstice in the NH, which marks the beginning of summer. North of the Polar Circle (67°33’N), daylight lasts for 24 hours. Then, after 21 June, from the Tropic of Cancer to the North Pole, the daily irradiance decreases, together with the length of the day. At the 22 September equinox (the autumn equinox in the NH), day and night are once again of the same length over the whole planet and, at solar midday, the Sun’s rays fall perpendicularly on the Earth’s surface at the Equator. After that, the irradiance continues to decrease further until 21 December (the NH winter solstice), on which date the length of the day is at a minimum. Beyond the NH polar circle, the night lasts for 24 hours. In the SH, the situation is reversed.

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These different mechanisms, which are listed in Table 1.1, generate differences in temperature and average rainfall that enable a great variety of climates to flourish on Earth. From frozen regions or scorching deserts to very wet zones, which may be either cool or warm, each zone is a home to suitably adapted forms of life.

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Table 1.1 SUCCESSIVE STEPS IN THE DISTRIBUTION OF AVAILABLE ENERGY AT GROUND LEVEL ON THE EARTH’S SURFACE. CONTROLLING FACTORS ARE SHOWN IN RED. (a) THE AVERAGE ENERGY AVAILABL...

Table of contents

• Cover
• Title page
• Copyright
• Foreword
• Acknowledgements
• About the companion website
• Introduction
• PART I: THE CLIMATE ENGINE OF THE EARTH: ENERGY
• PART II: MORE ON THE ENERGY BALANCE OF THE PLANET
• PART III: THE DIFFERENT CAUSES OF CLIMATE CHANGE
• PART IV: LEARNING FROM THE PAST …
• PART V: CLIMATE CHANGE IN RECENT YEARS
• PART VI: CLIMATE IN THE 21ST CENTURY: DIFFERENT SCENARIOS
• Conclusion
• References
• Index
• End User License Agreement