Holocene Climate Variability (HOLIVAR)

The HOLIVAR programme seeks to bring together European scientists interested in climate variability over the last 11,500 years (the Holocene period). The scientists, principally include palaeoclimatologists, climate historians and climate modellers. The over-arching research questions of the programme concern how and why climate has varied naturally on different time-scales (annual to centennial) during the Holocene and how an understanding of past variability can be used to improve the performance of climate models. Although HOLIVAR is a free-standing programme it is also a contribution to the PAGES/CLIVAR intersection, a joint project sponsored by the International Geosphere Biosphere Programme (PAGES) and the World Climate Research Programme (CLIVAR). Holocene Climate Variability (HOLIVAR)

An ESF scientific programme Evidence for Holocene climate variability comes from several sources. Instrumental and documentary data are available for the recent past but natural archives of climate information, such as tree rings and peat bogs, are needed to extend the time-scales further into the past. However, the reconstruction of climate history from these proxy records depends upon a thorough understanding of all the processes involved in their formation and preservation, and to provide regional scale climate reconstructions it is necessary to combine the data from the different archives, a process that demands accurate dating and the use of multi-proxy databases. The HOLIVAR community is particularly interested in Holocene climate modelling, how the models can be further improved and how in turn they can be used to understand past climate change. A key issue is specifying the external climate forcings to be used in the models and the extent to which proxy methods can be used to identify past variability in forcings. Improved time-series of solar irradiance is especially needed. Data–model comparisons for the entire Holocene will not be possible for many years. However, progress can be made by focussing on time periods within the Holocene of known rapid change using models of intermediate complexity (EMICs) and on the last millennium in particular using The European Science fully coupled atmosphere-ocean general circulation Foundation acts models (AOGCMs). as a catalyst for the development The ultimate HOLIVAR objective is to engage with of science by bringing historians and archaeologists in studies of past climate- together leading scientists human society interactions both to examine the role of and funding agencies to debate, plan and climate change as a potential cause of changes in human implement pan-European activity in the past, and to gain an insight into how initiatives. humans might respond to climate change in the future. 1 (http://www.pages-igbp.org/) that is concerned with climate change along a pole to pole transect through Europe and Africa.

Scientific rationale

There is increasing evidence that greenhouse-gas forcing is becoming one of the dominant, though not the only process driving global warming. Consequently, future climate change will be the result of interactions between the effects of human-induced changes and the effects of natural variability. There is therefore an urgent need to understand these interac- Introduction tions, to document and identify the characteristics of natural climate variability The ESF HOLIVAR programme seeks to and to improve the performance of clima- bring together European scientists te models that seek to predict future interested in climate variability over the climate change. Holocene period, broadly the last 11,500 years of earth history. The scientists Natural variability, whether associated involved in the programme include with mechanisms external or internal to palaeoclimatologists, palynologists, the earth system, is expressed on inter- palaeolimnologists, glaciologists, annual, decadal, and century time-scales. geochemists, oceanographers, climate Data for only the most recent part of the historians, archaeologists and climate historical record can be derived from modellers. The over-arching research instrumental measurements and, therefore, questions are: there is a need to extend the instrumental record backwards in time using docu- . how and why has climate varied natural- mentary records and proxy records con- ly on different time-scales (annual, tained in naturally occurring archives centennial and millennial) over the such as lake and marine sediments, peat Holocene period? bogs, tree rings, speleothems and ice . how can an understanding of past varia- cores (Figure 1, cover page). bility improve the performance of cli- However, none of these archives alone is mate models, leading to an improved adequate to capture the temporal and prediction of future climate change? spatial variability needed for regional . how can climate models help to explain comparisons with climate model output. past climate change? Consequently the primary requirement is . how has climate variability and the to combine and harmonise palaeoclimate nature of human society interacted data from the different records and archives during the Holocene? and to develop the use of multi-proxy databases that enable data to be manipu- Although these are questions relevant for lated and translated into formats that the world as a whole, the HOLIVAR facilitate comparison with climate model programme focuses mainly on research simulations (Figure 2). in Europe, and is allied closely to the aims and objectives of the Past Global Changes (PAGES) PEP-III project

2 Data

Archives Methods Instrumental Pollen Functions Past responses Documentary Diatoms Age modelling Natural environment Tree rings Chironomids Climate reconstruction Ecosystems Speleothems Macrofossils Palaeo-model testing Human society Ice cores Isotopes Inter-site comparison Lake sediments Geochemistry Inter-archive comparison Marine sediments Geophysics Mapping Peat bogs FUTURE PREDICTIONS AND RESPONSES MULTI-PROXY SINGLE PROXY DATABASES DATABASE

Climate model Temperature Regional climate Precipitation Functions reconstruction Wind Age modelling Isotopes Transfer functions Carbon

Model Figure 2: Processes of data collection, storage, analysis, and their use in climate modelling.

Instrumental and documentary Some are annually banded (Figure 3) climate records allowing calendar ages to be derived, Continuous instrumental records of others have no in-built seasonal or annual temperature, precipitation and other structures but can be dated, albeit with climate-related variables are available for less precision, using radiometric methods the last one to two centuries. They are such as radiocarbon dating. In this case insufficiently long to capture centennial- radiocarbon years can be converted into scale variability but, especially after calendar years using a tree-ring based homogenisation, are of fundamental value calibration or 14C wiggle-matching. in providing verification for model re- sults and as a means of calibration for The climatic information in these archi- proxy records. ves is stored in the form of physical, chemical and biological properties refer- Documentary sources cover a longer red to as proxy climate indicators or period, though they tend to be disconti- climate proxies (Figure 4). Each of the nuous in time and space, and are often major archives contains a range of climate difficult to quantify. They are particularly proxies (e.g. stable isotopes, diatoms), valuable, however, as sources of infor- and some proxies occur in more than one Figure 3: mation on extreme events, such as type of archive. The exact relationship Annually banded floods, droughts and severe frosts. archives: (from left to right) tree rings of Huon pine from Tasmania Extending instrumental records (E Cook); varves from using proxies from natural archives Lake Holzmaar, Natural archives such as tree rings, spe- Germany (B Zolitschka); leothems (cave stalagmites), lake and speleothems from Rana, Norway (J Kihle); coral marine sediments, peat bogs, and glacier from Papua New records preserve environmental respon- Guinea (S Tudhope); ses to climate change. ice from GISP2, Greenland (A Gow)

3 the accuracy and precision of dating techniques and to increase the robustness of the age models that are used in making independent comparisons between data from different sites. Institutionally combining data requires scientists to be willing to contribute data to common databases using a harmonised approach that allows database queries across data types to be processed swiftly and efficiently. HOLI- VAR stresses the need to develop a central, multi-proxy data system of this type Figure 4: Microfossils used as climate proxies: (clockwise from top left): pollen (Aster to bring proxy climate data together and linosyris) (H Halbritter); testate amoeba (Difflugia bacillariarum) (D Charman); diatom ultimately to enable regional climate (Cyclostephanos tholiformis) (C Sayer); ostracod (Sarscypridopsis aculeata) (R Jones); reconstruction through the Holocene. chironomid (Heterotrissocladius subpilosus) (S Brooks); foraminifera (Neogloboquadrina pachyderma) (UCL) Holocene climate modelling The earth’s climate is a system of immense between climate proxies and climatic complexity, incorporating numerous variables such as temperature, rainfall, or sources, sinks and transports of material wind can be indirect and complex. Re- and energy, with complex feedbacks construction of climate history from operating between different parts of the proxy records therefore depends on a system. Changes in the climate system thorough understanding of the processes over time can be simulated using models governing the formation of the proxy of varying types and complexity. These climate signals, their incorporation into include comprehensive coupled atmos- the archive, and the preservation of their phere-ocean general circulation models integrity over time. (AOGCMs), earth system models of intermediate complexity (EMICs) and Climate proxies can be calibrated by more simple energy balance models developing transfer functions which (EBMs). AOGCMs have the advantage describe the relationship between a cli- of high spatial resolution and include mate proxy and climate across the mo- representations of many key processes in dern-day environmental gradient and are the system but the disadvantage that long validated by comparing their recent time-scale simulations are expensive and record with historical climate data. Be- slow to run. However, simulations of the cause each climate proxy has its own last 250-500 years are becoming availa- particular strengths and weaknesses, the ble and the HOLIVAR community has a reliability of climate reconstructions can special interest in data-model compari- be further improved by integrating data sons over this time period that spans the from different proxies and across archi- intersection between instrumental, docu- ves. Consequently, one of the aims of mentary and proxy record availability and HOLIVAR is to encourage this multi- that covers a known period of relatively proxy, multi-archive approach (Figure 1). rapid climate change from the “Little Ice Age” to the recent period of warming. Combining data to provide regional The less complex EMICs have the ad- scale climate reconstruction vantage of speed and allow experiments Combining data from different archives and sensitivity testing to be carried out is not a trivial task. It requires both scien- over longer time periods on centennial to tific and institutional commitment. millennial time-scales (Figure 5). Scientifically the key issue is to improve HOLIVAR will in particular encourage

4 180 90W 0 90E 180 simulations of events that are well –5

–5 –10 –2 known from palaeoclimatic data. –2 –2 –1 60N –1 –2 60N –0.5 –1 –1 –1 –1 –0.5 –0.5 –0.5 Understanding climate forcing in the 30N 0 –0.5 30N 0 Holocene 0

0 Climate models, whether GCMs or EQ 0 0 0 EQ EMICs can be used to perform control 0 0 0 0 30S 0 30S runs that simulate the inherent variability 0

in the climate system or forced runs that 0 0 0.5 0.5 60S 60S –0.5 simulate the response of the system to 2 10.5 changes in external forcings. External 180 90W 0 90E 180 forcings, in turn, may be either natural,

e.g., changes in insolation from orbital –10 –5 –2 –1 –0.5 0 0.5 1 2 5 10 variations, solar activity or volcanic aerosols (Figure 6) or anthropogenic e.g. Figure 5: Annual mean surface temperature anomaly for the 8.2K event simulated by an EMIC (from Renssen, H,H Goosse, T Fichefet, and J M Campin, Geophysical Research changes in land cover, greenhouse gases Letters 28, 2001) or sulphate aerosols.

Whilst the timing and magnitude of some of the these forcings are reasonably Comparing model output with understood over the very recent period palaeoclimate data (last 100 years) uncertainty increases One of the key objectives of HOLIVAR with age and currently limits the is to encourage the comparison between usefulness of expensive GCM model simulations and palaeoclimate data, simulations over periods longer than the using data to test and improve model last few centuries. For Holocene performance on the one hand, while simulations improved time-series of solar using model experiments to investigate irradiance variations are especially the causes of past climate variability on needed. The extent to which such the other hand. Data-model comparisons information can be provided from the are not straightforward, however, because record of cosmogenic isotopes (such as the characteristics of model output and 10Be and 14C) in natural archives is a key palaeodata are very different, especially Figure 6: (A) Northern issue for the HOLIVAR community. with respect to scale and uncertainty. Hemisphere temperature Proxy records integrate climatic forcing over the past 1000 years attributable to information only locally over space, but documented volcanic 1.0

C) Vol.hl.alb 2.0 Sol.C14.Stv 2.0 eruptions (blue), solar

° often over longer time periods (seasons Sol.Be10 2.0 GHG 2.0 variability (magenta, green Sol.C14.Brd 2.0 Trop Aer 2.0 to decades, or more), as if climate history 0.5 and orange), anthropogenic has passed through a low-pass filter. greenhouse-gas emissions They also respond to non-climatic drivers, (red) and air pollution 0.0 making their climatic interpretation (brown), as calculated with an energy-balance climate

-0.5 imprecise. In contrast, numerical models model. (B) Comparison of the A simulate climate at larger spatial scales, Temperature Variations ( combined temperature effect not resolving variations at scales near to of these four natural and

C) or less than their resolution (typically anthropogenic climate-forcing ° All Be10 sm11 2.0 mechanisms (blue) with the 0.4 Mn (sm11) hundreds of kilometers). Matching the Jones.inst sm11 instrumental temperature two approaches consequently requires the record (black; AD 1860- 0.2 use of scaling techniques. In the inverse 1993) and an integrated 0.0 approach, climate is reconstructed from Northern Hemisphere temperature reconstruction -0.2 the proxy data, usually with some spatial based on natural archives with annual resolution and B aggregation to a scale appropriate for Temperature Variations ( age control (red; AD 1005- -0.4 comparison with the climate model 1000 1200 1400 1600 1800 2000 1993) (from Crowley, T J, Year simulation, whereas the forward Science 289, 2000)

5 Nigardsbreen Climate-human society interactions

16 in the Holocene Studies of past climate-human society 14 interaction can contribute strongly to 12 debates on the collapse of past civilisa- 10 tions and, although the nature of human society is today very different, they can km 8 provide useful insights into how humans 6 might respond to climate change in the 4 future (Figure 8). There are many exam- 2 ples throughout the world of rapid popu-

0 lation expansion and decline during the 109876543210 latter part of the Holocene, and in most Ky BP cases the role of climate, directly or indi- Figure 7: Glacier length (in km) for Nigardsbreen as rectly, has been a central topic of debate. a function of time (in kyr BP) showing a phase of rapid expansion and high variability during the mid- Explanations for civilisation collapse Holocene. Length of records are generated using a often range from the purely deterministic process-based glacier model coupled to the climate with climate as the main driver to those model ECBilt, which is driven by orbital forcing (from that favour human-induced over-exploi- Weber, S L, and J Oerlemans, The Holocene, 2003) tation of resources, environmental degra- dation or the development of dysfunctional approach involves the comparison of societies, irrespective of climatic influen- actual proxy data with simulated proxy ces. Of key importance in these debates time series generated by process-based or is the availability and quality of the empirical models that are driven by palaeo-environmental data needed to climate model output that has been assess the relationships between environ- downscaled to the proxy site (Figure 7). mental and societal change. HOLIVAR seeks to encourage the Consequently HOLIVAR seeks to encou- development of both approaches. The rage the palaeoclimate research commu- inverse approach can be facilitated by nity to improve further our understanding harmonising and combining proxy data at of climate variability and its impacts the regional scale using multi-proxy data across archaeologically important re- systems for sharing and manipulating gions, and to engage in these debates with data, and forward modelling has major historians and archaeologists. future potential as it allows the exploration of the processes that lie behind the climate-proxy relationship.

Data–model comparisons for the entire Holocene will not be possible for some years as an extended period of time is needed for data compilation and synthe- sis to take place. However, progress can be made by focussing jointly on time periods within the Holocene of known rapid change and for which data are Figure 8: Multi-phase rock art from the Ennedi, NE available, such as the 8.2 ky BP cooling Chad. Rock paintings of domesticated cattle in a event, the period of aridification in low currently arid region illustrating humid conditions during the early and middle Holocene. Late latitudes in the mid Holocene, the Sub- Holocene drier conditions are indicated by boreal/Sub-atlantic transition (about superposed camel drawings less than 200 years 2750 BP), and the last millennium. old. (S Kroepelin, Heinrich-Barth-Institute, Cologne)

6 Activities

Open Science Meetings . Holocene climate forcing, scheduled The HOLIVAR programme began in for 2004 2001 with an international conference . Climate-human society interactions co-sponsored by IGBP-PAGES in Aix- during the Holocene, scheduled for 2005 en-Provence on Past Climate Variability through Europe and Africa. The procee- Training courses dings of this meeting will be published in Training courses that include all the 2003 by Kluwer. The programme will HOLIVAR themes and that stress and end in 2005 with a second international exemplify the need to integrate climate meeting on Holocene climate variability: change science are planned. The course data, models and impacts. scheduled for title is Quantitative Holocene climate September 2005 in London. reconstruction and data-model comparisons. They will be offered both in 2003 Workshops and 2004 and are designed to bring HOLIVAR has scheduled a series of together students with both modelling annual workshops as follows: and data interests. The first course will . Combining proxies, April 2002 in take place in London, 23rd June – 4th July Lammi (Finland); organised by Matti 2003. Saarnisto and Antti Ojala . Data-model comparisons, June 2002 External links in Louvain-la-Neuve (Belgium); organised HOLIVAR is concerned with scientific by Hans Renssen and Tim Osborn issues identified as being of international . Holocene dating, chronologies, and importance for climate change studies by age modelling, April 2003 in Utrecht the PEPIII research community in IGBP- (The Netherlands); organised by André PAGES. It is also a contribution to The Lotter PAGES/CLIVAR Intersection which is a . Database meeting, scheduled for shared research agenda between WCRP 2003/4 and IGBP.

Funding

ESF scientific programmes are principally gemeinschaft, Germany; Nederlandse financed by the Foundation’s Member Organisatie voor Wetenschappelijk Organisations on an à la carte basis. Onderzoek, Netherlands; Norges HOLIVAR is supported by: Forskningsråd, Norway; Vetenskapsrådet, Sweden; Schweizerischer Nationalfonds Österreichische Akademie der zur Förderung der wissenschaftlichen Wissenschaften, Austria; Fonds zur Forschung/Fonds National Suisse de la Förderung der wissenschaftlichen Recherche Scientifique, Switzerland; Forschung, Austria; Fonds National de la Natural Environment Research Council, Recherche Scientifique, Belgium; Fonds United Kingdom. voor Wetenschappelijk Onderzoek - Vlaanderen, Belgium; Suomen Akatemia/ Finlands Akademi, Finland; Centre National de la Recherche Scientifique, France; Deutsche Forschungs-

7 HOLIVAR Steering Committee

Rick Battarbee (Chair) Eystein Jansen Dirk Verschuren Environmental Change Bjerknes Centre for Climate Research Department of Biology Research Centre University of Bergen Ghent University Department of Geography Allégaten 55 Ledeganckstraat 35 University College London 5007 Bergen 9000 Ghent 26 Bedford Way Norway Belgium London WC1H 0AP Tel: +47 55 583 491 Tel: +32 9 264 5262 United Kingdom Fax: +47 55 584 330 Fax: +32 9 264 5343 Tel: +44 20 7679 7582 Email: [email protected] Email: [email protected] Fax: +44 20 7679 7565 Email: [email protected] Ingemar Renberg Bernd Zolitschka Department of Ecology and Institute of Geography Jürg Beer Environmental Science University of Bremen EAWAG Umeå University Celsiusstrasse FVG-M Ueberlandstrasse 133 901 87 Umeå 28359 Bremen Postfach 611 Sweden Germany 8600 Dübendorf Tel: +46 90 786 6029 Tel: +49 421 218 2158 Switzerland Fax: +46 90 786 6705 Fax: +49 421 218 9658 Tel: +41 1 823 5111 Email: [email protected] Email: [email protected] Fax: +41 1 823 5210 Email: [email protected] Matti Saarnisto Geological Survey of Finland Svenje Mehlert André Berger Betonimiehenkuja 4 ESF Scientific Secretary Institut d’Astronomie et de PO Box 96 Joanne Dalton Goetz Géophysique G. Lemaître 02151 Espoo ESF Administrative Assistant Université Catholique de Louvain Finland European Science Foundation 2 Chemin du Cyclotron Tel: +358 20 550 2184 1 quai Lezay-Marnésia 1348 Louvain-la-Neuve Fax: +358 205 5012 67080 Strasbourg cedex Belgium Email: [email protected] France Tel: +32 10 473303 Fax: +32 10 474722 Christoph Spötl www.esf.org Email: [email protected] Institute of Geology and Paleontology Tel: +33 (0)3 88 76 71 22 University of Innsbruck Fax: +33 (0)3 88 37 05 32 Keith Briffa Innrain 52 E-mail: [email protected] Climatic Research Unit 6020 Innsbruck University of East Anglia Austria Norwich NR4 7TJ Tel: +43 512 507 5593 For the latest information on this United Kingdom Fax: +43 512 507 2914 programme consult the HOLIVAR Tel: +44 1603 593 909 Email: [email protected] home page: Fax: +44 1603 507 784 www.esf.org/holivar Email: [email protected] Bas van Geel Faculty of Science Françoise Gasse University of Amsterdam CEREGE PO Box 94062 Figure 1 (cover): Europôle Méditerranéen de l’Arbois 1090 GB Amsterdam Naturally occurring archives of BP 80 The Netherlands palaeoclimatic proxy record 13545 Aix-en-Provence cedex 04 Tel: +31 20 525 7664 (clockwise from top left): France Fax: +31 20 525 7832 Raised bog profile, Bargerveen, Tel: +33 4 42 97 15 70 Email: [email protected] the Netherlands (B van Geel); Fax: +33 4 42 97 15 95 Lochnagar, Scotland (M Hughes); Email: [email protected] Quelccaya ice cap, Peru (L Thompson); Crag Cave,

Ireland (A Baker); Pinus sylvestris d’ordre: 031041

° (K Briffa); Ocean Drilling Program’s drillship JOIDES Resolution, (ODP)

March 2003