Global Change in the Holocene

John Birks, Rick Battarbee, Anson Mackay, Frank Oldfield

• Publisher: Routledge
• Year: 2014
• Print ISBN: 9780340812143
• eBook ISBN: 9781134669974
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Geography
• Index: Geography

The Holocene spans the 11, 500 years since the end of the last Ice Age and has been a period of major global environmental change. However the rate of change has accelerated during the last hundred years, due largely to human impacts and this has led to a growing concern for the future of our environmental resources. Global Change in the Holocene demonstrates how reconstructing the record of past environmental change can provide us with essential knowledge about how our environment works and presents the reader with an informed viewpoint from which to project realistic future scenarios. The book brings together key techniques that are widely used in Holocene research, such as radiocarbon dating, dendrochronology and sediment analysis and offers a comprehensive analysis of various archives of environmental change including instrumental and documentary records, corals, lake sediments, glaciers and ice cores. This reference will be an informative and cutting-edge resource for all researchers in the fields of climate change, environmental science, geography, palaeoecology and archaeology.

Chapter 1
Introduction: The Holocene, a Special Time

Frank Oldfield

1.1 The Holocene in Temporal Perspective

For anyone raised in the tradition of field-based Quaternary studies in northwestern Europe, the transition from the end of glacial times to the beginning of the Holocene is one of the most notable and readily detectable of stratigraphic boundaries. Almost everywhere, it is marked by evidence for a dramatic shift in surface processes, denoting a major climate change that in turn triggered a whole sequence of responses in both abiotic and biotic ecosystem components. Evidence for the precise age, suddenness and synchroneity of the transition has gradually accumulated over the last 70 years until now, we have remarkably precise chronological control on its timing, the pace of change and the extraordinary spatial and temporal coherence of response over large areas of the globe. Indeed, were it not that colleagues dealing with contemporary transformations of the Earth System had placed their concerns so firmly under the heading of ‘Global Change’, that term could serve perfectly for the opening of the Holocene.

The isotopically-inferred temperature record in the GRISP/GISP ice cores from Central Greenland points up the sharp contrast between late Pleistocene and Holocene in terms both of mean values and of the amplitude of variability (Dansgaard et al., 1993). To a large extent, the shift to the Holocene appears to be a rapid switch in mode from low mean temperatures and extreme variability on all time-scales from decadal to millennial, to one of much higher mean values and lower variability. Thus, if the record from Central Greenland were the only template for our interpretation of Holocene environmental change, we would be considering a period of rather remarkable invariance relative to that which preceded it. But the empirical evidence from other parts of the world, as well as our knowledge of the changing patterns of, and interactions between, external forcings and feedbacks reveal this as a serious oversimplification.

The orbitally driven changes that appear to have triggered the Pleistocene–Holocene transition were relatively gradual. Moreover, orbitally driven changes in solar irradiance have continued throughout the Holocene and they have had different expressions at different latitudes. Only in the second half of the Holocene, roughly the last 6000 years, have they been broadly comparable to those prevailing today. Thus, at the opening of the Holocene, the effects of smoothly changing external forcing were mediated by internal system dynamics to generate a range of abrupt and synchronous changes in many parts of the world, but not all the responses were immediate and speedily accomplished. Ice takes time to melt and the great northern hemisphere polar ice did not disappear overnight. Nor did it simply wane smoothly and continuously everywhere. In consequence, eustatic sea-level too took several millennia to reach its mid-Holocene levels. Not only did many physical responses to Holocene warming and deglaciation take place over several thousands of years, biotic responses too were not completed instantaneously. Migration, soil development, competition and succession all played a part in modulating ecological responses to the major changes in the Earth System that marked the opening of the Holocene. We may therefore think of this latter shift as the beginning of a longer, complex period of transition as well as a sharp boundary between Earth System regimes. Depending on our research focus and on where and how we look, it was both.

Did these transitional changes during the first half of the Holocene play out against the backdrop of a global climate as relatively invariant as the Central Greenland temperature record suggests? Undoubtedly not. High latitude temperature variability may have been reduced, but there were still major changes, especially during the early Holocene. Elsewhere, at lower latitudes and notably in tropical regions, hydrological variability was extreme over the same period, with dramatic changes in lake level well documented in Africa and South America. To some extent, the climatic variability that is recorded during the first half of the Holocene may be attributed to the sequence of changes taking place in the wake of deglaciation and to the way in which the changes interacted with the prevalent patterns of orbital forcing, but these factors alone fail to account for all the changes observed. Changes in ocean currents and land biota also appear to have influenced climate, at least on a continental scale.

Even during the second half of the Holocene, when orbitally driven external forcing was broadly similar to today, ice had melted to a minimum and eustatic sea-level had recovered, there is strong evidence for significant climate variability in all areas and on all time-scales. Such variability, as well as having had important effects on past hydrological regimes and ecosystems, is of outstanding interest at the present day. It is against this background variability that we must seek to detect and characterize the imprint of human-induced climate change resulting from ever increasing atmospheric greenhouse gas concentrations. Moreover, future climate change will be, in part, an expression of similar variability as it plays out in the future and interacts with the effects of any human-induced climate change.

The Holocene period thus emerges not as a bland, pastoral coda to the contrasted movements of a stirring Pleistocene symphony; rather we now see it as a period of continuous change, the documenting and understanding of which becomes increasingly urgent as our concerns for future climate change grow. All the foregoing serves to reinforce the importance of the Holocene as a major research challenge; but there is an additional element that may be of even greater importance, for it is during the Holocene, and especially the later part, that human activities have begun to reshape the nature of the Earth System not only through systemic impacts on the composition and concentrations of atmospheric trace gases, but through the cumulative effects of land clearance, deforestation, soil erosion, salinization, urbanization, loss of biodiversity and a myriad other impacts that have transformed our environment at an ever accelerating rate. These processes began many thousands of years ago at local and regional levels in long settled areas of the globe, but over the last two centuries and at an accelerating rate in the last few decades, the impacts have become global and the implications for rapidly increasing human populations a cause for growing anxiety. It follows from all the above that the themes of this book are of major relevance to our present-day environmental concerns (cf. Oldfield and Alverson, 2003).

1.2 The Demise of the 35-Year Mean

One of the cornerstones of climatology 50 years ago was the notion of the 35-year mean. This purported to encapsulate an adequate first approximation to the climate of a station or region. At the same time, it was recognized that climate had changed in the past, as witness the sequence of glaciations and the climate oscillations they implied. It is doubtful whether any conflict was perceived between these two perspectives as they were the concerns of quite different scholarly communities. Reconciling the notion of the 35-year mean with the realization that climate had changed was, in any case, quite easy if one took the view that past change entailed a switch between distinctive episodes, each of relative constancy. The ‘post-glacial’ period in northwest Europe for example, was one that could be divided into a suite of phases – Pre-Boreal, Boreal, Atlantic, Sub-Boreal and Sub-Atlantic. From around 500 BC, we had been in the cool, wet Sub-Atlantic phase and, by implication, the 35-year mean could serve to describe the climate regime typical of that period for any given location. It took the work of scholars like Gordon Manley (1974) and Hubert Lamb (1963) to bridge the gap between instrumental records and the longer time-scales of climate change. In so doing, they helped to show that climate variability was continuous on all time-scales, that short-term changes were nested within longer-term trends and that there was no such thing as a mean value that could serve for any time interval other than that for which it was calculated.

Put another way, change is the norm. This has important implications for almost every aspect of environmental science, for it shifts our perspective away from any static descriptor of a relatively constant state to an acknowledgement that for any place and over any time-scale there has been an envelope of variability which changes with the time-span which it represents. Characterizing and understanding the processes contributing to and modulating past variability on a wide range of time-scales constitutes a major scientific challenge, but one that is of vital interest at the present day and for the future.

1.3 Lessons from the Past

When the threat of future greenhouse warming was first identified and clearly stated (see summary in Oeschger, 2000), it was tempting to turn to the past for analogues. There had been warmer worlds in the past; what were they like? Could they provide a partial template for a possibly warmer world of the future? Quite quickly, this rather simple way of using hindsight was seen to be seriously flawed. We cannot hope to find analogues with any useful degree of realism by turning to periods when the external boundary conditions and the very configuration of the planet were different. Instead, palaeo-scientists began to interrogate the past record of environmental change with questions about processes, rates of change, long-term Earth System dynamics, non-linear responses to external forcing, feedback mechanisms involving the hydrosphere and biosphere and a myriad other similar issues (see e.g. Alverson et al., 2000, 2003).

In adopting this much more realistic research agenda, the main focus in palaeo-science has been on the late Quaternary period. Indeed, the record of the last four glacial cycles spanning the last 430,000 years from Vostok in Antarctica (Petit et al., 1999) has come to serve as an almost universal template for this type of research. The Holocene represents no more than the last 2.7 per cent of this time interval. What are the special qualities of the period that make it of compelling interest? What are the key questions we can address by using the record from the Holocene and what are the key issues that improved knowledge of the Holocene may help us to resolve?

1.4 The Special Interest of the Holocene

The realization that the isotopically inferred temperature record from Central Greenland was not a template for all aspects of Holocene climate everywhere has been noted above. Nevertheless, the contrast between late Pleistocene and Holocene variability in the ice core record has strongly influenced thinking in the research community. It has, for example, added special point to questions about climate variability in warm, interglacial intervals. Evidence for climate variability in the Eemian interglacial (Marine Isotope Stage 5e) has evoked a good deal of interest, but continuous, well dated, fine resolution records from the Eemian are rare. It is to the Holocene itself that we must turn for the bulk of our evidence for ‘warm climate’ variability. The paragraphs that follow seek to highlight some of those qualities of the Holocene that make the record of environmental change during the period of such special interest and value.

1.4.1 Boundary Conditions, External Forcing and Internal Feedbacks

As already hinted at above, the Holocene as a whole is the period for which we have the most information about climate variability during warm, interglacial times. Significant changes in temperature that were certainly synchronous between Greenland and Europe have been well documented for the early Holocene (Alley et al., 1997; von Grafenstein et al., 1998). Even more dramatic in human terms were the widescale changes in lake levels, plant cover and soil moisture that took place at lower latitudes and continued at least until around 4000 years ago (Gasse and Van Campo, 1994). Less dramatic, but nevertheless highly significant, changes in hydrology have also been recorded throughout the second half of the Holocene (see e.g. Verschuren et al., 2000).

The pattern of orbitally driven solar forcing changes relatively slowly and continuously, but over the last 6000 years, which is to say during the second half of the Holocene, it has not differed greatly from the pattern that prevailed during the centuries immediately before the human-induced increase in atmospheric greenhouse gas concentrations began. By the middle of the Holocene, other aspects of the Earth System that influence climate significantly – polar ice cap and sea-ice extent, sea-level and major terrestrial biomes, for example – had all achieved states within an envelope of variability not significantly different from that typical of the last millennium. Thus the main patterns of forcing and feedbacks that characterized the period immediately before human activities began to modify the atmosphere significantly were, broadly speaking, in place by the middle of the Holocene. Anything that we can learn about variability and environmental change since then thus has special relevance for understanding the processes operating now and in the most recent past.

As Bradley (pp. 10–19 in this volume) points out, solar irradiance reaching the outer edge of the earth’s atmosphere varies on many time-scales and is modulated by processes some of which are quite independent of orbital changes. The role of these shorter-term variations in solar activity as drivers of global climate has recently received increasing attention. In part, this is due to the fact that for the Holocene period it appears possible to reconstruct a detailed and well dated proxy record of variations in received solar irradiance by measuring deviations in the relationship between the decline in radiocarbon concentrations with age in tree-rings and true dendrochronological age (Stuiver et al., 1991). Where records of past climate variability have been sufficiently well and independently dated, this opens up the possibility of exploring the extent to which the climate changes recorded are coherent with inferred variations in solar activity. Our growing knowledge of the Holocene thus provides key information for testing hypotheses about climate forcing.

1.4.2 Modes of Variability

One of the ways in which climatologists make sense of climate variability on a global scale is by identifying and characterizing relatively distinct modes of variability. The El Niño Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) are well-known examples. Other modes currently recognized include a decadal oscillation in the North Pacific and an Arctic Oscillation that interacts with the NAO. One of the key findings of recent research on late Holocene records is that these modes of variability are remarkably protean. Over a period of decades and centuries, their amplitudes, frequencies and spatial domains change (see e.g. Cole and Cook, 1998; Markgraf and Diaz, 2001). This knowledge presents both a contemporary caution and a future challenge. In our present state of knowledge it reduces the confidence with which predictions of the long-term incidence and effects of these modes in the future can be made. At the same time, it challenges us to discover the factors responsible for the decadal- and century-scale variability. Only by understanding these and incorporating them in model simulations will there be any realistic chance of improving future predictability. Once more, the Holocene period is the crucial time interval for exploring these issues, though longer-term insights into the nature of ENSO variability, for example, also shed important light on the range of possible behaviours ENSO may assume (Tudhope et al., 2001).

1.4.3 Continuity and Overlap with the Present Day

Many of the archives and proxies that form the toolkit of the palaeo-scientists can bring the Holocene record of variability right up to the present day. Tree-rings are still being formed, lake sediments continue to accumulate, corals and speleothems still grow. This allows Holocene research to reap multiple benefits. The insights gained contribute to our understanding of present-day ecosystems and environmental processes that have been in part conditioned by their antecedents. By bringing records of climate change from the centuries well before significant human impact right through to the short period of instrumental records (Jones and Thompson, pp. 140–158 in this volume), palaeoclimatology makes a crucial contribution to resolving the questions of detection and attribution raised by global warming in recent decades. The same kind of temporal overlap allows direct comparison between recent instrumental records of the amplitude, duration and recurrence intervals of extreme events and their longer-term history (Page et al., 1994; Knox, 2000).

The above examples stress only one facet of the importance of continuity and overlap, for the points made would count for little were it not possible to translate proxy records of environmental change into inferences sufficiently quantitative to permit comparison with direct measurements. This requires calibration (Birks, pp. 107–123 in this volume) and, in this regard, the period of overlap between past proxy records and present-day measurements is crucial. Calibration, whether achieved by comparing directly measured sequences with proxy records covering the same time interval, or by linking proxies to a spatial array of contemporary measurements spanning a range of variability, is at its most robust for situations where the processes, biological communities or geochemical signatures in which the proxy signals reside lie within or as close as possible to the variability encompassed by the calibration process. For time intervals in which past biological communities lack present-day analogues, or abiotic proxies have values that can only be matched by significant extrapolation of a calibration function, the inferences become less secure and the statistical uncertainties much greater. Once more, the Holocene, and especially the late Holocene, have important advantages. As we move further back in time, confidence in quantitative reconstructions of climate often decline quite steeply (see e.g. Bigler et al., 2002).

1.4.4 Chronology

Not onl...

Table of contents

• Cover
• Title
• Copyright
• CONTENTS
• List of Contributors
• Acknowledgements
• Preface
• Chapter 1 Introduction: the Holocene, A Special Time
• Chapter 2 Climate Forcing During the Holocene
• Chapter 3 An Introduction to Climate Modelling of the Holocene
• Chapter 4 Holocene Climate and Human Populations: An Archaeological Approach
• Chapter 5 Climatic Change and the Origin of Agriculture in the Near East
• Chapter 6 Radiocarbon Dating and Environmental Radiocarbon Studies
• Chapter 7 Dendrochronology and the Reconstruction of Fine-Resolution Environmental Change in the Holocene
• Chapter 8 Dating Based on Freshwater- and Marine-Laminated Sediments
• Chapter 9 Quantitative Palaeoenvironmental Reconstructions from Holocene Biological Data
• Chapter 10 Stable-Isotopes in Lakes and Lake Sediment Archives
• Chapter 11 Instrumental Records
• Chapter 12 Documentary Records
• Chapter 13 Holocene Coral Records: Windows on Tropical Climate Variability
• Chapter 14 Evidence of Holocene Climate Variability in Marine Sediments
• Chapter 15 Holocene Palaeoclimate Records from Peatlands
• Chapter 16 Lacustrine Perspectives on Holocene Climate
• Chapter 17 Reconstructing Holocene Climate Records from Speleothems
• Chapter 18 Glaciers as Indicators of Holocene Climate Change
• Chapter 19 Holocene Ice-Core Climate History: a Multi-variable Approach
• Chapter 20 Approaches to Holocene Climate Reconstruction Using Diatoms
• Chapter 21 Non-Marine Ostracod Records of Holocene Environmental Change
• Chapter 22 Chironomid Analysis to Interpret and Quantify Holocene Climate Change
• Chapter 23 Reconstructing Holocene Climates from Pollen and Plant Macrofossils
• Chapter 24 Biomarkers as Proxies of Climate Change
• Chapter 25 Multi-Proxy Climatic Reconstructions
• Chapter 26 Holocene Climates of the Lowland Tropical Forests
• Chapter 27 The Holocene and Middle Latitude Arid Areas
• Chapter 28 Unravelling Climatic Influences on Late Holocene Sea-Level Variability
• Chapter 29 Simulation of Holocene Climate Change Using Climate-System Models
• References
• Index
• Plates