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1 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: howimportant is natural variability in explaining global warming? The book aims toplace 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 throughthe Holocene, the period of Earth history covering approximately the past 11 500years 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 occurringclimate archives (such as tree rings, peat bogs, corals, and lake and marine sedi-ments) and describes progress being made in developing the climate modelsneeded 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 humancontribution to global warming (IPCC 2007). Antarctic ice-core records (e.g. Petit1999; EPICA Community Members 2004) show that greenhouse-gas concentra-tions are already higher than at any time in the past 750 000 years, temperatures inthe Northern Hemisphere are now on average probably higher than the previous1000 years (Mann et al. 1998) and climate models can only simulate temperaturesaccurately over the past 150 years if greenhouse gases are included as a forcingmechanism (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. Inparticular most mountain glaciers across the world are receding (Oerlemans 2005)and unprecedented changes in the ecology of remote arctic lake ecosystems havebeen recorded by lake sediments (Smol et al. 2005).

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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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The evidence for human impact on the climate system is thought now to be socompelling that Crutzen has argued that the recent period of Earth history datingfrom the late 18th century increase in atmospheric CO2 should be given a new geo-logic name, the Anthropocene (Crutzen and Stoermer 2000). Indeed Ruddimanhas 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 beingunderestimated. 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 timesand in Africa major periods of very low lake-levels in previous centuries are welldocumented (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 evidencefor 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 acomprehensive introductory context for the chapters that follow. John Birks tracesHolocene research back to its roots in the early 19th century and describes earlydebates, principally in Scandinavia, about the interpretation of plant remains preserved in peat bogs and their relevance to climate change. His account takes inthe development of pollen analysis and radiocarbon dating, the use of transferfunctions in an attempt to quantify past climate reconstruction from proxyrecords and the pioneering work of COHMAP (Cooperative Holocene MappingProject) in paleoclimate modeling. He highlights the principal debates and develop-ments in Holocene climate change research that have taken place in recent yearsand points specifically to the importance of understanding the spatial as well astemporal component of natural climate variability.

Frank Oldfield’s chapter is concerned with the role of people in the Holocene.He stresses the need to take into account a much longer history of interactionsbetween human activity and climate change than simply the very recent past.Using data from many different regions he shows that people, especially in the OldWorld, 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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local and regional climate and that these changes in turn were responsible for causingalterations in the hydrologic cycle and in soil erosion. The evidence for land-coverchange presented is not inconsistent with Ruddiman’s claim (see above) that earlyagriculture may have been the cause of increased atmospheric greenhouse gas concentrations over the past 8000 years and Oldfield stresses the need for furtherpaleoecological research to test this hypothesis. Finally he reviews evidence for theinteraction between climate change and human society in the past and calls for amore balanced dialogue between the physical and social science communities indebating this issue, calling on the need to develop models that couple biophysicaland social systems and that acknowledge the adaptive nature of human society.

Chapter 4 by Michel Crucifix is divided into two main sections. The firstdescribes the principles of climate modeling. He stresses the difficulty of modelingan inherently complex and chaotic system and the need for models of differentkinds: conceptual, comprehensive and intermediate. He points out the importanceof specifying initial conditions and boundary conditions, describes some of theproblems of parameterization and the different ways in which equilibrium or transient experiments are conducted. He also indicates how paleodata are used by modelers, not only for direct comparison of output, but also, using data assimi-lation techniques, for providing improved model parameterization. The secondsection of the chapter is concerned with the results of two model applications. Thefirst 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 theearly Holocene in increasing atmospheric concentrations of greenhouse gases.Output from models of intermediate complexity do not rule out the Ruddimanhypothesis in scenarios where early Holocene CO2 concentrations are allowed tofall below 240 ppmv (parts per million by volume). On the other hand projectionsforward from the present-day suggest that glacial conditions are not now likely toreturn for approximately 50 000 years.

The final section of Crucifix’s chapter focuses on ocean stability through theHolocene. He argues that regional ocean instabilities, such as sudden coolingsrelated 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 thatindicate the Holocene “climate optimum” is recorded in many but not all marinesediment cores and that different proxies from the same core have different patterns. The differences are attributed to the seasonality of the insolation forcingand the relationship of the proxy to surface ocean stratification. The results indi-cate that the thermal maximum is mainly caused by orbital forcing, enhanced bysea-ice albedo feedbacks. The data also show that on shorter century to millennialtime-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 authorsconclude that the variability may be a response to long time-scale dynamics of theclimate system and not necessarily to a specific external forcing factor.

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Jürg Beer and Bas van Geel give an overview of the mechanisms causing naturalclimate change on decadal to millennial time-scales focusing especially on solarforcing. They argue that although the change in total solar irradiance over thecourse 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 solarirradiance strongly influence photochemistry in the upper atmosphere, and in particular 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 arapidly growing number of examples of Holocene climate change that point to theSun as a major forcing factor. They present the well-known 850 bc event, equival-ent to the Sub-boreal–Sub-atlantic transition in the original Blytt and Sernanderscheme for the Holocene as a good example. This event is associated with increasedpeat 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 of 14C. By extension they argue that as the amplification mech-anisms for changing solar activity are not well understood, and therefore cannotyet be sufficiently quantified in climate models, solar forcing of climate changemay be more important than has been suggested to date. They argue that if theLittle 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 inestimating future trends in climate.

In Chapter 7, Hugues Goosse, Michael Mann, and Hans Renssen present astrong defence of the “hockey-stick” curve of Northern Hemisphere temperaturetrends 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 coveringthe same period. All are essentially in agreement in showing anomalously hightemperatures over the past few decades. Goosse et al. also show from data–modelcomparisons how natural (especially volcanic) forcing could explain many featuresof pre-19th climate variability, including the regional patterns of change associatedwith the North Atlantic Oscillation (NAO) and El Niño.

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 resultsthat show how temperature differences between regions could be due to spatialresponses to particular forcings and/or to internal variability. They also show howprogress could be made in data–model comparisons by using paleoproxy data toselect the best realization in an ensemble. In this way a climate reconstructioncould 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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proxy evidence from Europe and Africa, however, they argue that a number ofperiods of cooler and wetter conditions inferred from peatland and lake-levelchanges in Europe correspond to periods of reduced solar activity. In Africa there isevidence for substantial spatial variation across the continent, but some evidence,especially in eastern Equatorial Africa, for an inverse relationship between solaractivity 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 theclimate at 5500 years BP was especially unstable. Earth system models are nowcapable of simulating rapid swings between arid and wet phases in the past but maynot yet be able to reliably predict future transitions.

Finally, Ray Bradley provides a perspective on Holocene climate change andpresents an array of evidence to demonstrate the relevance of understanding pastclimate in order to provide insights for the future. He stresses the importance ofreconstructing the history of climate forcing and the need to understand the causesand consequences of rapid changes especially AUPs (abrupt, unprecedented andpersistent 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 theUniversity College London (UCL) Open Science Meeting in June 2006. That meeting was financed by the European Science Foundation (ESF), with co-fundingfrom the International Geosphere–Biosphere Programme and Past GlobalChanges (IGBP–PAGES), who sponsored bursaries for young scientists fromDeveloping Countries. I also thank UCL who provided conference facilities andHeather 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 ofHOLIVAR (Holocene Climate Variability) over the past few years: to the SteeringCommittee, the organizers of the workshops and training courses, the tutors onthe training courses, the participants in the workshops and training courses, theUCL-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, 12–13.

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EPICA Community Members (2004) Eight glacial cycles from an Antarctic icecore. Nature, 429, 623–628.

IPCC (2007) Climate Change 2007 – The Physical Science Basis Working Group I.Contribution to the Fourth Assessment Report of the Intergovernmental Panelon Climate Change, Cambridge University Press.

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, 1743–1746.

Mann M.E., Bradley R.S. & Hughes M.K. (1998) Global-scale temperature patternsand climate forcing over the past six centuries. Nature, 392, 779–787.

Menzel A., Sparks T.H., Estrella N., et al. (2006) European phenological responseto climate change matches the warming pattern. Global Change Biology, 12,1969–1976.

Oerlemans J. (2005) Extracting a climate signal from 169 glacier records. Science,308, 675–677.

Petit J.R., Jouzel J., Raynaud D., et al. (1999) Climate and atmospheric history ofthe past 420 000 years from the Vostok ice core, Antarctica. Nature, 399,429–436.

Ruddiman W.F. (2003) The anthropogenic greenhouse era began thousands ofyears ago. Climatic Change, 61, 261–293.

Smol J.P., Wolfe A.P., Birks H.J.B., et al. (2005) Climate-driven regime shifts in thebiological communities of arctic lakes. Proceedings of the National Academy ofSciences, 102, 4397–4402.

Stott P.A., Tett S.F.B., Jones G.S., Ingram W.J. & Mitchell J.F.B. (2001) Attributionof twentieth century temperature change to natural and anthropogenic causes.Climate Dynamics, 17, 1–21.

Verschuren D. (2004) Decadal and century-scale climate variability in tropicalAfrica during the past 2000 years. In: Past Climate Variability through Europeand Africa (Eds R.W. Battarbee, F. Gasse & C.E. Stickley), pp. 139–158.Springer-Verlag, Berlin.

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