Climate since AD 1500

Bradley, Jones, Raymond S. Bradley, Philip D. Jones

• Publisher: Routledge
• Year: 2003
• Topic: Physical Sciences
• eBook ISBN:
  9781134810352
• Subtopic: Geography
• Index: Geography

First Published in 2004. Climate Since A.D. 1500 presents a unique perspective on the 'Little Ice Age' and the climate of the twentieth century. Leading scientists explore historical documents, dendroclimatic data and ice core records from all over the world, presenting an invaluable compilation for all those concerned with past climate and the risks of man-made climatic change in the future. This revised edition includes a new chapter summarizing the wealth of literature on climatic change over the past few years and a new and expanded index.

1 Climate since A.D. 1500: Introduction

R. S. Bradley and P. D. Jones

1.1 The need for perspective

It is now common knowledge that today's climate is unlikely to prevail into the 21st century. An increase in “greenhouse gases”, as a result of human activity, is likely to perturb the world's energy balance, leading to higher surface temperatures and a redistribution of precipitation patterns (National Research Council 1982; Bolin et al. 1986; I.P.C.C., 1990). The magnitude of any climatic change, and its distribution both geographically and seasonally, is estimated by the use of computer models of the general circulation. Simulations of equilibrium climatic conditions with early twentieth century CO₂ levels are generally compared with those resulting from doubling of CO₂ to obtain the differences that might be expected in the future (Schlesinger 1984). More recently, transient climate models have been developed to simulate the gradually changing climate as CO₂ and the other greenhouse gases increase from year to year. The focus of all these experiments is, of course, to isolate the impact of human activities on climate over the next century or so. However, whatever the anthropogenic climatic effects may be in the future, they will be superimposed on a climatic system which also responds to “natural” forcing factors. Unless we improve our understanding of what these factors are, and how the climate system has responded to them in the past, there is little prospect of interpreting, or anticipating, future climatic changes. Therefore, in order to understand how climate may vary in the future we must understand how and why it has varied in the past. With such knowledge we may be able to place our contemporary climate in a longer term perspective and identify any underlying trends or periodicities in climate upon which future climatic changes might be superimposed. With such knowledge we may be able to isolate the causes of past climatic fluctuations, causes which may continue to operate in the future and influence the course of forthcoming climatic events (Bradley 1990).

In this book we focus on the most recent period of climate history, the last 500 years. This is an important interval for a number of reasons. Firstly, we can construct a fairly comprehensive picture of climatic variations during this period, and we can also document variations in potentially important forcing factors. There is thus the opportunity to develop and test hypotheses about how the climate system responds to these factors. Secondly, climatic variability on a decadal to century time-scale is of most relevance to concerns about future climate and the extent to which “natural” variability will amplify or subdue anthropogenic effects. Thirdly, in the last 500 years, world population has increased by a factor of 12, at least; our society has changed from one which produced local, or perhaps regional environmental impacts, to one which now produces environmental impacts of global extent. Understanding how human activity may already have altered climate locally, regionally and perhaps globally is of great significance as we enter a new century facing even more rapid increases in world population and continuing environmental degradation.

1.2 Climates of the past

Although the focus of this book is the last 500 years, it is appropriate to consider briefly how this period fits in with the longer term changes that we know have affected the earth's climate system. Twenty thousand years ago the world was experiencing a period of major continental glaciation, the most recent of a series of such events which have occurred with some regularity over the last 2 million years (Imbrie and Imbrie 1979). These events were brought about by small changes in the position of the earth relative to the sun and the consequent redistribution of solar radiation across the earth (Berger 1980). Changes in atmospheric composition and of atmospheric aerosol loading may also have played a role in the evolution of climate from glacial to interglacial periods and back again to glacial periods (Barnola et al. 1987; Genthon et al. 1987; Chappellaz et al. 1990). The last continental ice sheets in the Northern Hemisphere had largely disappeared by 6000–7000 years B.P. (Before Present) and this induced dramatic changes in the distribution of plants and animals and of the world's coastlines as the sea returned to higher levels.

There is evidence that some parts of the world experienced quite warm summers around 5000 to 6000 years ago, a period sometimes referred to, rather loosely, as the mid-Holocene Optimum (cf. Webb and Wigley 1985). Whether this was a globally extensive warm period, or if it was warm in other seasons is not yet known. However, it is thought that glaciers in many parts of the world reached their post-glacial minima around this time, a condition made all the more significant by the fact that glaciers subsequently expanded over the course of the next few thousand years. Such periods of glacier expansion are known collectively as Neoglacial episodes, times of renewed glacier activity (Porter and Denton 1967). It has been argued that such periods have a certain regularity in time, suggesting some periodic forcing, but the evidence is quite weak and the causes of these glacier advances, whether regional or global in extent, remains obscure. For our purposes, the important point is that the most recent of these neoglaciations occurred during the last 500 years, and there is abundant evidence that this most recent episode was the most significant of all the periods of glacier expansion that have occurred since the last Ice Age. The period since A.D. 1500 is thus of extraordinary scientific interest.

Although there is voluminous indirect evidence that climatic conditions in the past 500 years were often quite different from our contemporary experience (e.g. von Rudloff 1967; Lamb 1982; Grove 1988) the precise nature of these differences, and what caused them, remains elusive. Certainly, there were a number of cold intervals which had dramatic environmental consequences. In almost every glacierised mountain region of the world, glaciers grew and advanced down-valley, often to positions as extensive as at any time since the last Ice Age, more than 10,000 years ago. These changes in alpine glaciers are so characteristic of the period that it is often referred to as “the Little Ice Age”. However, among those who use this term there is little consensus on when it began, or when it ended.

Furthermore, the term suggests a period of uniformly cold conditions which obscures the fact that relatively warm intervals did occur (see Chapter 33). It also focuses attention on those parts of the world where snow and ice are common phenomena; how climate changed in tropical and sub-tropical regions during the last 500 years is far less well-documented. Only by careful paleoclimatic reconstruction can we hope to unravel the sequence of events which occurred during this “Little Ice Age” and to understand how extensive the changes really were geographically. With this information, it may then be possible to isolate the causes of such climatic variations.

1.3 The world in A.D. 1500

The beginning of the 16th century was somewhat of a watershed in the history of civilisation. Between 1492, when Columbus reached the Caribbean, and 1532, when Pisarro arrived in Peru, the map of the known world had irrevocably changed. Vasco de Gama rounded the southern tip of Africa in 1497 and reached as far as Calicut in India, opening up an entirely new trading route between Europe and the East. By 1519, Magellan's expedition had circumnavigated the globe. The stage was set for the emergence of colonial states, ruled from small but powerful countries, geographically isolated from their remote territories. Only Australasia and the extreme polar regions were to remain beyond the reach of explorers for a further 200 years or more.

Over the course of the next 500 years extraordinary changes in society took place. These occurred against a background of environmental changes, which may have played a critical role in some of the events which occurred. However, until we can document climatic variations of the last 500 years, the extent of such influences will remain controversial.

1.4 Sources of high resolution data for paleoclimatic reconstruction

Meteorological measurements (from which we can assess climatic conditions) are only available for relatively short periods (generally a century or less) from most parts of the world. Although observations have been maintained in a few locations for two centuries or more (see Jones and Bradley, Chapter 13) to obtain a broader picture of past climatic variations we must rely on additional non-instrumental records, from which climatic conditions can be deduced. Such records of climatically sensitive natural phenomena are surrogate or proxy measures of past climate; they contain climatic information which must be extracted and separated from the non-climatic matrix in which it is embedded. The analyst must isolate the climatic signal from the extraneous noise. As more detailed and geographically extensive records are built up, the possibility of identifying causes and mechanisms of climatic variation is increased and so the prospects of understanding future climate are enhanced.

Although there are numerous approaches to the reconstruction of past climates (Bradley 1985) only a few types of evidence have the potential of providing a record which can be resolved to the annual or seasonal level (Table 1.1). Of these, quantitative estimates of past

Table 1.1 Sources of high resolution paleoclimatic data

Type of record	Main distribution	Potential Information:
Historical Documents	All continents	Almost all aspects of climate.
Tree rings	Continental areas*, excluding desert and tundra regions	Temperature precipitation pressure patterns, drought runoff.
Ice cores	Polar and high mountain regions	Temperature, precipitation atmospheric aerosols, atmospheric composition.
Varved sediments	Continents and some coastal basins	Temperature, precipitation solar radiation.
Corals	Tropical oceans	Sea surface temperatures, adjacent continental rainfall.

* Studies of tree growth in tropical areas have not yet provided useful paleoclimatic reconstructions

climate from studies of varved sediments, and corals, have not yet been widely carried out. These natural archives have great potential for paleoclimatology, but a number of problems have yet to be resolved. Two approaches to climatic reconstruction have the potential of providing wide geographic coverage: the analysis of documentary records (discussed in Section A) and of tree growth (dendroclimatic) indices (Section B). These can be supplemented in high latitude and high altitude regions by the analysis of ice cores (Section C). Figure 1.1 shows the distribution of records discussed in the various chapters of this book. By combining these different approaches, the aim is to construct a picture of past climatic variations on larger and larger spatial scales in which the whole is greater than the sum of its individual parts. At the same time, we need records of those phenomena which are thought to have played a role in causing climatic variations of the past. The most likely candidates for climate forcing on this time scale include explosive volcanic eruptions, solar activity variations and El Nino-Southern Oscillation (ENSO) events of varying magnitude. As with the paleoclimatic record itself, proxy records can be used to reconstruct the history of these climatic forcing factors. Such records are discussed in Section D.

Figure 1.1 Approximate location of studies discussed in text. Numbers refer to chapters in this volume.

1.5 Methods of paleoclimatic reconstruction

1.5.1 Documentary records

Historical data can be grouped into three major categories. First, there are observations of weather phenomena per...

Table of contents

• Cover Page
• Half Title page
• Titles of Related Interest
• Title Page
• Copyright Page
• Contents
• Acknowledgements
• Contributing Authors
• Preface to the revised edition
• Preface to the first edition
• 1 Climate since A.D. 1500: Introduction
• Section A: Documentary Evidence
• Section B: Dendroclimatic Evidence
• Section C: Ice Core Evidence
• Section D: Forcing Factors
• Section E: Summary
• Subject Index
• Name Index