Chap 1 Holocene Climate Variability and Global Warming (p 1-6)

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Holocene climate variabilityand global warming

Richard W. Battarbee

Keywords

Holocene, natural climate variability, global warming

Introduction

This book addresses one of the key questions facing climate scientists today: how

important is natural variability in explaining global warming? The book aims to

place the past few decades of warming in the context of longer term climate vari-

ability and considers the causes of such variability on different time-scales through

the Holocene, the period of Earth history covering approximately the past 11 500

years that has elapsed since the last major Ice Age. In particular it reviews the evid-

ence for past climate change based on the analysis of data from naturally occurring

climate archives (such as tree rings, peat bogs, corals, and lake and marine sedi-

ments) and describes progress being made in developing the climate models

needed to simulate and explain past climate variability. It also considers how peo-

ple in the past have changed the environment and responded to climate change.

Over the past decade it has become increasingly clear that there is now a human

contribution to global warming (IPCC 2007). Antarctic ice-core records (e.g. Petit

1999; EPICA Community Members 2004) show that greenhouse-gas concentra-

tions are already higher than at any time in the past 750 000 years, temperatures in

the Northern Hemisphere are now on average probably higher than the previous

1000 years (Mann et al. 1998) and climate models can only simulate temperaturesaccurately over the past 150 years if greenhouse gases are included as a forcing

mechanism (Stott et al. 2001).

Evidence is also accumulating to suggest that changes in natural ecosystems

that can be unambiguously attributed to rising temperatures are also occurring. In

particular most mountain glaciers across the world are receding (Oerlemans 2005)

and unprecedented changes in the ecology of remote arctic lake ecosystems have

been recorded by lake sediments (Smol et al. 2005).

Natural Climate Variability and Global Warming:A Holocene Perspective Edited by Richard W. Battarbee and Heather A. Binney

2008 Blackwell Publishing. ISBN: 978-1-405-15905-0


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2 | Richard W. Battarbee

The evidence for human impact on the climate system is thought now to be so

compelling that Crutzen has argued that the recent period of Earth history dating

from the late 18th century increase in atmospheric CO2 should be given a new geo-

logic name, the Anthropocene (Crutzen and Stoermer 2000). Indeed Ruddiman

has even argued that human activity may have affected atmospheric greenhouse-

gas concentrations much earlier in the Holocene as a result of deforestation andland-cover change associated with early agriculture (Ruddiman 2003).

Yet despite the strength of the evidence for human-induced change, climate-

change sceptics still remain, arguing that the role of natural variability is being

underestimated. It can indeed be maintained that recent changes in climate, exem-

plified by ice-cover loss on lakes (Magnuson et al. 2000) or earlier spring flowering

(Menzel et al. 2006) are still within the long-term natural range of the climate

system, if viewed on centennial time-scales. In Europe, for example, historians

can point to the more northerly cultivation of vines in Medieval and Roman times

and in Africa major periods of very low lake-levels in previous centuries are well

documented (e.g. Verschuren 2004).

This debate, about the relative importance of natural variability and pollutantgreenhouse gases in explaining recent warming, is therefore still very much alive.

In this book we consider this issue in a Holocene perspective. We present evidence

for climate change on different time-scales using both paleoclimate reconstruc-

tions and modeling, and we include the results of recent research from both high-

and low-latitude environments.

Preview

The opening two chapters by John Birks and Frank Oldfield, respectively, provide a

comprehensive introductory context for the chapters that follow. John Birks traces

Holocene research back to its roots in the early 19th century and describes early

debates, principally in Scandinavia, about the interpretation of plant remains

preserved in peat bogs and their relevance to climate change. His account takes in

the development of pollen analysis and radiocarbon dating, the use of transfer

functions in an attempt to quantify past climate reconstruction from proxy

records and the pioneering work of COHMAP (Cooperative Holocene Mapping

Project) in paleoclimate modeling. He highlights the principal debates and develop-

ments in Holocene climate change research that have taken place in recent years

and points specifically to the importance of understanding the spatial as well astemporal component of natural climate variability.

Frank Oldfields chapter is concerned with the role of people in the Holocene.

He stresses the need to take into account a much longer history of interactions

between human activity and climate change than simply the very recent past.

Using data from many different regions he shows that people, especially in the Old

World, have had a major impact on land-use and land-cover over many millennia.

He argues that the extent of land-cover change may have been sufficient to modify


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Holocene climate variability and global warming | 3

local and regional climate and that these changes in turn were responsible for causing

alterations in the hydrologic cycle and in soil erosion. The evidence for land-cover

change presented is not inconsistent with Ruddimans claim (see above) that early

agriculture may have been the cause of increased atmospheric greenhouse gas

concentrations over the past 8000 years and Oldfield stresses the need for further

paleoecological research to test this hypothesis. Finally he reviews evidence for theinteraction between climate change and human society in the past and calls for a

more balanced dialogue between the physical and social science communities in

debating this issue, calling on the need to develop models that couple biophysical

and social systems and that acknowledge the adaptive nature of human society.

Chapter 4 by Michel Crucifix is divided into two main sections. The first

describes the principles of climate modeling. He stresses the difficulty of modeling

an inherently complex and chaotic system and the need for models of different

kinds: conceptual, comprehensive and intermediate. He points out the importance

of specifying initial conditions and boundary conditions, describes some of the

problems of parameterization and the different ways in which equilibrium or

transient experiments are conducted. He also indicates how paleodata are usedby modelers, not only for direct comparison of output, but also, using data assimi-

lation techniques, for providing improved model parameterization. The second

section of the chapter is concerned with the results of two model applications. The

first asks the question how long will the Holocene last?, a question relevant to

the debate opened by Ruddiman (see above) about the role of human activity in the

early Holocene in increasing atmospheric concentrations of greenhouse gases.

Output from models of intermediate complexity do not rule out the Ruddiman

hypothesis in scenarios where early Holocene CO2 concentrations are allowed to

fall below 240 ppmv (parts per million by volume). On the other hand projections

forward from the present-day suggest that glacial conditions are not now likely to

return for approximately 50 000 years.

The final section of Crucifixs chapter focuses on ocean stability through the

Holocene. He argues that regional ocean instabilities, such as sudden coolings

related to the reduction in deep-ocean convection, could have occurred through-

out the Holocene in the North Atlantic by convective feedback related to inter-

actions with sea-ice and atmosphere dynamics.

Eystein Jansen and colleagues review data from the North Atlantic region that

indicate the Holocene climate optimum is recorded in many but not all marine

sediment cores and that different proxies from the same core have different

patterns. The differences are attributed to the seasonality of the insolation forcing

and the relationship of the proxy to surface ocean stratification. The results indi-cate that the thermal maximum is mainly caused by orbital forcing, enhanced by

sea-ice albedo feedbacks. The data also show that on shorter century to millennial

time-scales variability is an important aspect of the marine climate in the high-

latitude Atlantic Ocean, possibly increasing after the end of the thermal maximum.

There is little evidence for stationary cyclicity in this variability and the authors

conclude that the variability may be a response to long time-scale dynamics of the

climate system and not necessarily to a specific external forcing factor.


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Jrg Beer and Bas van Geel give an overview of the mechanisms causing natural

climate change on decadal to millennial time-scales focusing especially on solar

forcing. They argue that although the change in total solar irradiance over the

course of an 11-year Schwabe cycle is quite small, the variability of the solar radi-

ation is strongly wavelength dependent and that large changes in the spectral solar

irradiance strongly influence photochemistry in the upper atmosphere, and inparticular the ozone concentration, which may cause shifts in the tropospheric

circulation systems and therefore climate. As direct measurements of solar

irradiance are lacking prior to the advent of satellite technologies, evidence for

centennial-scale change needs to be derived from proxy records, especially from10Be from ice cores and 14C from tree rings.

In the second half of their chapter Beer and van Geel argue that there are a

rapidly growing number of examples of Holocene climate change that point to the

Sun as a major forcing factor. They present the well-known 850 bc event, equival-

ent to the Sub-borealSub-atlantic transition in the original Blytt and Sernander

scheme for the Holocene as a good example. This event is associated with increased

peat bog growth and lake-level increase in north-west Europe and with changinghusbandry and agricultural practices in south-central Siberia and central Africa.

The beginning of this event is coincident with a significant increase in the atmo-

spheric production of14C. By extension they argue that as the amplification mech-

anisms for changing solar activity are not well understood, and therefore cannot

yet be sufficiently quantified in climate models, solar forcing of climate change

may be more important than has been suggested to date. They argue that if the

Little Ice Age and the subsequent warming were mainly driven by changes in

solar activity this component of natural forcing may well play an important role in

estimating future trends in climate.

In Chapter 7, Hugues Goosse, Michael Mann, and Hans Renssen present a

strong defence of the hockey-stick curve of Northern Hemisphere temperature

trends for the past 1000 years, pointing out that since the first curve was presented

(Mann et al. 1998) there are now several additional independent analyses covering

the same period. All are essentially in agreement in showing anomalously high

temperatures over the past few decades. Goosse et al. also show from datamodel

comparisons how natural (especially volcanic) forcing could explain many features

of pre-19th climate variability, including the regional patterns of change associated

with the North Atlantic Oscillation (NAO) and El Nio.

Goosse et al. present simulations of the past 1000 years that explore the separate

and combined role of internal and forced variability. They present model results

that show how temperature differences between regions could be due to spatialresponses to particular forcings and/or to internal variability. They also show how

progress could be made in datamodel comparisons by using paleoproxy data to

select the best realization in an ensemble. In this way a climate reconstruction

could be derived that was consistent with the paleorecord, model physics, and

the forcings.

Dirk Verschuren and Dan Charman stress the difficulty of relating past

hydrologic variability on decadal to century time-scales to external forcing. Using


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Holocene climate variability and global warming | 5

proxy evidence from Europe and Africa, however, they argue that a number of

periods of cooler and wetter conditions inferred from peatland and lake-level

changes in Europe correspond to periods of reduced solar activity. In Africa there is

evidence for substantial spatial variation across the continent, but some evidence,

especially in eastern Equatorial Africa, for an inverse relationship between solar

activity and moisture.Martin Claussen presents evidence that rapid climate change, capable of affect-

ing early civilizations, occurred in North Africa during the Holocene, and that the

climate at 5500 years BP was especially unstable. Earth system models are now

capable of simulating rapid swings between arid and wet phases in the past but may

not yet be able to reliably predict future transitions.

Finally, Ray Bradley provides a perspective on Holocene climate change and

presents an array of evidence to demonstrate the relevance of understanding past

climate in order to provide insights for the future. He stresses the importance of

reconstructing the history of climate forcing and the need to understand the causes

and consequences of rapid changes especially AUPs (abrupt, unprecedented and

persistent climate anomalies), for which there are many examples, but mainlydroughts, in the paleorecord.

Acknowledgments

The chapters in this book are based on the keynote lectures delivered at the

University College London (UCL) Open Science Meeting in June 2006. That

meeting was financed by the European Science Foundation (ESF), with co-funding

from the International GeosphereBiosphere Programme and Past Global

Changes (IGBPPAGES), who sponsored bursaries for young scientists fromDeveloping Countries. I also thank UCL who provided conference facilities and

Heather Binney and Mike Hughes who were the principal organizers. The produc-

tion of this book has further benefited from the help of many expert reviewers,

from Cathy Jenks who carried out the technical editing and from Heather Binney,

my co-editor, without whom little would have been possible.

Finally I would like to thank all those who have contributed to the success of

HOLIVAR (Holocene Climate Variability) over the past few years: to the Steering

Committee, the organizers of the workshops and training courses, the tutors on

the training courses, the participants in the workshops and training courses, the

UCL-based administrators, Andrew McGovern, Heather Binney and Cath Rose,

and the support team in the ESF, especially Joanne Goetz.

References

Crutzen P.J. & Stoermer E.F. (2000) The Anthropocene. Global Change News-

letter, 41, 1213.


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EPICA Community Members (2004) Eight glacial cycles from an Antarctic ice

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Magnuson J.J., Robertson D.M., Benson B.J., et al. (2000) Historical trends in lakeand river ice cover in the Northern Hemisphere. Science, 289, 17431746.

Mann M.E., Bradley R.S. & Hughes M.K. (1998) Global-scale temperature patterns

and climate forcing over the past six centuries.Nature, 392, 779787.

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