1. Prospero: ‘. . .ye that on the sands with printless foot

2. Do chase the ebbing Neptune and do fly him

3. When he comes back’

4. Shakespeare, The Tempest

Sea levels are always changing, for many reasons. Some changes are rapid while others take place very slowly. The changes can be local, or extend globally. This book is about the science of these changes.

In this first chapter we outline what constitutes sea-level science. A brief account of the development of scientific ideas is followed by an outline of how sea levels are affected by a wide range of physical forces and processes. Finally we give some basic definitions, and discuss the fundamental statistics of sea levels as time series.

Background

Living by the sea has many benefits. Statistics show that about half the global population lives within 100 km of the sea. Most of the world’s largest cities are on or near the ocean. Ninety per cent of all global trade is carried by sea. The coast offers possibilities of both trade and travel, and increasingly of water-based recreation. Natural geological processes have often conspired to create flat and fertile land near to the present sea level, to which people are drawn or driven to settle.

In Chapters 2 and 9 we looked at different tidal levels and their use as zero or Datum Levels [1]. Tidal datums are also used to define shorelines, adopted as the state, national and international boundaries shown on maps. Generally the important levels are some form of High Tide, or some form of Low Tide, depending on locally adopted definitions. The determination of these levels, and their projection to mapped shorelines may require long records of sea-level measurements, often a complete nodal cycle of 18.61 years. Inevitably, knowledge of tidal principles has an important role to play in the development and interpretation of legal rules, but the technical aspects of defining tidal boundaries have sometimes been underemphasised [2].

The three components of a robust boundary definition are:

• clear and measurable tidal level parameters,

• a means of converting these levels into map coordinates,

• a legal system for interpreting and enforcing the boundaries.

To these we might add a process for adjusting to tidal and coastal changes over the long-term.

Introduction

Geodesy is a branch of science that is concerned with the Earth’s time-dependent geometric shape, gravitational field and rotation. Over 70 per cent of our planet is covered by ocean, and its shape is largely defined by the mean sea surface, so geodesy and sea-level science are intimately connected. A number of classic texts on geodesy are available [1, 2], and also excellent reviews of modern geodetic techniques [3]. In this chapter, we confine ourselves to a description of some of the main developments in geodesy that relate to understanding the spatial variations in the sea surface. In the next chapter we discuss its temporal variations.

The International Terrestrial Reference Frame

A principal goal of geodesy is to assign coordinates to points on the Earth’s surface as a function of time. Such a time series could be, for example, the height of sea level at a particular location (latitude, longitude) in the ocean, or the height of a benchmark on land. It is important to know something of the geodetic framework within which many of the sea- and land-level measurements described later in this chapter are made.

Introduction

This chapter contains our personal selection of topics on how the tides and sea-level changes have played important roles in the development of life, and in human history. The populations of coastal zones are increasing dramatically, and tides, storm surges and long-term sea-level changes will continue to play important roles in our lives. Impacts of sea-level change will take place alongside those due to many other climate and environmental stresses.

It is our belief that, if these impacts are to be avoided, or managed, as far as possible, then we have to learn more about the space and time scales of sea-level change and the various reasons for them. Also we have to communicate a greater appreciation of how tides and sea levels have contributed to the coastal environment in general, providing many communities with food sources and places to live and thrive, contributions that continue in many and varied ways today.