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We develop a new data assimilative (DA) approach by combining two parallel frameworks: a parallel DA framework (PDAF) and a flexible model coupling framework (ESMF). The new DA system is built on the ESMF at the top level that drives the PDAF and any combination of earth system modeling (ESM) components, to allow maximum flexibility and easy implementation of data assimilation for fully coupled ESM applications. We demonstrate the new DA system using a 3D unstructured-grid ocean model as ESM in this paper. The new system is validated using a simple benchmark and applied to a realistic case of Kuroshio simulation around Taiwan. The new system is demonstrated to significantly improve the model skill for temperature, velocity and surface elevation before, during and after typhoon events. The flexibility and ease of implementation make the new system widely applicable for other coupled ESMs.

Marine litter is one of the most relevant pollution problems that our oceans are facing today. Marine litter in our oceans is a major threat to a sustainable planet. Here, we provide a comprehensive analysis of cutting-edge solutions developed globally to prevent, monitor and clean marine litter. Prevention in this research includes only innovative solutions to prevent litter entering oceans and seas rather than interventions such as waste reduction and recycling. On the basis of extensive search and data compilation, our analysis reveals that information is dispersed across platforms and is not easily accessible. In total, 177 solutions—the equivalent to

The activity of macrofauna on the sea floor is since long known to mediate deposition and erosion of sediment, but so far most studies addressed this effect at a local scale. In the present paper, the contribution of the observed macrofauna distribution (exemplified by a bivalve, the bean-like tellin Fabulina fabula, formerly known as Tellina fabula) on large-scale sediment transport in the southern North Sea is investigated by means of a model study. Macrofauna effects are considered with respect to the critical bed shear stress and erodibility, which are two important factors that control the resuspension rate. Simulation results for a typical winter month revealed for the first time that the suspended sediment concentration (SSC) is increased not only locally but beyond the inhabited zones. This alteration is not confined to near-bed zones but can be observed throughout the entire water column, especially during storm events. These effects are most prominent in the fine silt fraction, coarser and finer fractions are less affected. For a selected storm event in February 2010, we explain the counter-intuitive decrease in near-bed SSC in some areas with a high macrofauna abundance compared to a simulation excluding such macrofauna: A high macrofauna-induced entrainment rate leads to rapid exhaustion of available sediments at the bed in the model and consequently limits the near-bed SSC.

The increasing demand for renewable energy is projected to result in a 40-fold increase in offshore wind electricity in the European Union by 2030. Despite a great number of local impact studies for selected marine populations, the regional ecosystem impacts of offshore wind farm (OWF) structures are not yet well assessed nor understood. Our study investigates whether the accumulation of epifauna, dominated by the filter feeder Mytilus edulis (blue mussel), on turbine structures affects pelagic primary productivity and ecosystem functioning in the southern North Sea. We estimate the anthropogenically increased potential distribution based on the current projections of turbine locations and reported patterns of M. edulis settlement. This distribution is integrated through the Modular Coupling System for Shelves and Coasts to state-of-the-art hydrodynamic and ecosystem models. Our simulations reveal non-negligible potential changes in regional annual primary productivity of up to 8% within the OWF area, and induced maximal increases of the same magnitude in daily productivity also far from the wind farms. Our setup and modular coupling are effective tools for system scale studies of other environmental changes arising from large-scale offshore wind farming such as ocean physics and distributions of pelagic top predators.

Inferred European Holocene population size exhibits large fluctuations, particularly around the onset of farming. We attempt to find explanations for these fluctuations by employing the concept of cycling, especially that of the Adaptive Cycle. We base our analysis on chronologically and chorologically highly resolved ceramic and site data from the Linear Pottery culture (Germ. Linearbandkermik ) of the early Neolithic of southwestern Central Europe. Typological seriation with dendrochronological anchor dates provides the age model for these data. Ceramic motifs are analysed with respect to the temporally changing diversity in decoration. The temporal sequence of major decoration motifs is interpreted as an indicator of social diversity: when stylistic diversity is low, social diversity is low and vice versa. The sequence of secondary decoration motifs is interpreted in terms of individual lineage emphasis: when this diversity is low, there is strong emphasis on individual lineage and vice versa. The diversity time series are complemented by a relative population size indicator derived from the count of occupational features. Diversity and population size share a shape that is typical for (part of) an Adaptive Cycle, and they differ in their positioning on the time axis — they are time-lagged. By relating the different curves to the (metaphorical) stages of the Adaptive Cycle, we find that these cycles progress at non-identical speed in different aspects of a social system. By relating the social dynamics to well-dated and highly resolved climate fluctuation records, we find evidence that severe climate excursions shaped the location of tipping points in the social system and that these social tipping points precede inferred population decline by several generations.

Wildfire occurrence is influenced by climate, vegetation and human activities. A key challenge for understanding fire-climate-vegetation interactions is to quantify the effect vegetation has in mediating fire regime. Here, we explore the relative importance of Holocene land cover and dominant functional forest type, and climate dynamics on biomass burned in temperate and boreo-nemoral regions of Central and Eastern Europe over the past 12 ka BP years. We used an extensive data set of Holocene pollen and sedimentary charcoal records, in combination with climate simulations and novel statistical modelling. Biomass burned was highest during the early Holocene and lowest during the mid Holocene in all three ecoregions, but diverged more markedly over the past 3–4 ka BP. Although the climate was an important driver of fire hazard during the warm and dry early Holocene, tree cover was consistently the strongest predictor of past biomass burning. In temperate forests, biomass burned was high at ~ 45 % tree cover and decreased strongly towards 60 % tree cover. In needleleaf dominated forests, biomass burned was highest at ~ 60–65 % tree cover and abruptly declined at > 65 % tree cover. Biomass burned also increased when arable lands and grasslands reached ~ 15–20 %, although this relationship was highly dynamic depending on land use intensity throughout ignition and fuel type and availability. Our observations cover the full range of Holocene climate variability and land cover changes and illustrates that percentages of land cover is a key predictor of the probability of fire occurrence over timescales of centuries to millennia. We suggest that long-term fire risk may be effectively reduced through land cover management, given that land cover has controlled fire regimes under the dynamic climates of the Holocene.

For more than a decade, scientists have argued about the warmth of the current interglaciation. Was the warmth of the preindustrial late Holocene natural in origin, the result of orbital changes that had not yet driven the system into a new glacial state? Or was it in considerable degree the result of humans intervening in the climate system through greenhouse gas emissions from early agriculture? Here we summarize new evidence that moves this debate forward by testing both hypotheses. By comparing late Holocene responses to those that occurred during previous interglaciations (in section 2), we assess whether the late Holocene responses look different (and thus anthropogenic) or similar (and thus natural). This comparison reveals anomalous (anthropogenic) signals. In section 3, we review paleoecological and archaeological syntheses that provide ground truth evidence on early anthropogenic releases of greenhouse gases. The available data document large early anthropogenic emissions consistent with the anthropogenic ice core anomalies, but more information is needed to constrain their size. A final section compares natural and anthropogenic interpretations of the C-13 trend in ice core CO2.

Highlights: • 18 southern European charcoal records document 16,000 years of fire regime changes. • Since the Neolithic, land uses have artificially maintained high fire frequencies. • Human-induced decreases in forest cover led to a reduction in the biomass burned. • Human-modified landscapes affected ecological processes more meaningly than expected. Abstract: Variability in fire regime at the continental scale has primarily been attributed to climate change, often overshadowing the widely potential impact of human activities. However, human ignition modifies the rhythm of fire episodes occurrence (fire frequency), whereas land use alters vegetation composition and fuel load, and thus the amount of biomass burned. It is unclear, however, whether and how humans have exercised a significant influence over fire regimes at continental and millennial scales. Based on sedimentary charcoal records, we use new alternative estimate of fire frequency and biomass burned for the last 16000 years (here after 16 ky) that we evaluate with outputs from climate, vegetation, land use and population models. We find that pronounced regional-scale land use changes in southern Europe at the beginning of the Neolithic (8–6 ky), during the Bronze Age (5–4 ky) and the medieval period (1 ky) caused a doubling of fire frequency compared to the Holocene average (the last 11.5 ky). Despite anthropogenic influences, southern European biomass burned decreased from 7 ky, which is in line both with changes in orbital parameters leading climate cooling and also reductions in biomass availability because of land use. Our study underscores the role of elevation-dependent parameters, and particularly biomass and land management, as major drivers of fire regime variability. Results attest a determinant anthropogenic driving-force on fire regime and a decrease in fire-carbon emissions since 7 ky in Southern Europe.

The spread of farming into Europe some 9000–5000 years ago involved not only the advent of new plants and animals, but also of people, tools, technologies, and knowledge. While they all can be assumed to follow Fickian diffusion gradients, the mechanisms of spread can be quite different: when people migrate, there is mass balance in the number of people, but not in the knowledge and technologies brought along. Tools, plants and animals could also travel by trade, knowledge and technology by communication; there might even be local resistance to adoption of novelty. This chapter discusses these different diffusion mechanisms in the context of numerical trait- and agent-based socio-environmental modeling of the spread of farming.

Was the spread of agropastoralism from the Fertile Crescent throughout Europe influenced by extreme climate events, or was it independent of climate? We here generate idealized climate events using palaeoclimate records. In a mathematical model of regional sociocultural development, these events disturb the subsistence base of simulated forager and farmer societies. We evaluate the regional simulated transition timings and durations against a published large set of radiocarbon dates for western Eurasia; the model is able to realistically hindcast much of the inhomogeneous space-time evolution of regional Neolithic transitions. Our study shows that the consideration of climate events improves the simulation of typical lags between cultural complexes, but that the overall difference to a model without climate events is not significant. Climate events may not have been as important for early sociocultural dynamics as endogenous factors., Revised version submitted to the Journal of Archaeological Science, special issue on The World Reshaped: impacts of the Neolithic transition. 10 pages, 4 figures, 1 table + supplementary material

Despite the fact that Europe and Eastern North America both have similar woodland environments, the emergence of agriculture in these areas proceeded very differently varying in timing, speed, and mechanism. To analyze the different subsistence paths the Global Land Use and Technological Evolution Simulator was used, a numerical model for simulating demography, innovation, domestication, migration and trade within the geoenvironmental context. I demonstrate how Europe receives a large package of foreign domesticates and converts rapidly. In contrast, trajectories relating to Eastern North America exhibit a gradual transition in which hunting and gathering and agropastoralism coexist for a long period of time, and agriculture is integrated slowly into the existing subsistence scheme. I deduce from this a qualitative economic difference in the two regional transitions: limited population size in Europe, limited resources in Eastern North America.

Humans have altered the Earth’s land surface since the Paleolithic mainly by clearing woody vegetation first to improve hunting and gathering opportunities, and later to provide agricultural cropland. In the Holocene, agriculture was established on nearly all continents and led to widespread modification of terrestrial ecosystems. To quantify the role that humans played in the global carbon cycle over the Holocene, we developed a new, annually resolved inventory of anthropogenic land cover change from 8000 years ago to the beginning of large-scale industrialization (ad 1850). This inventory is based on a simple relationship between population and land use observed in several European countries over preindustrial time. Using this data set, and an alternative scenario based on the HYDE 3.1 land use data base, we forced the LPJ dynamic global vegetation model in a series of continuous simulations to evaluate the impacts of humans on terrestrial carbon storage during the preindustrial Holocene. Our model setup allowed us to quantify the importance of land degradation caused by repeated episodes of land use followed by abandonment. By 3 ka BP, cumulative carbon emissions caused by anthropogenic land cover change in our new scenario ranged between 84 and 102 Pg, translating to c. 7 ppm of atmospheric CO2. By ad 1850, emissions were 325–357 Pg in the new scenario, in contrast to 137–189 Pg when driven by HYDE. Regional events that resulted in local emissions or uptake of carbon were often balanced by contrasting patterns in other parts of the world. While we cannot close the carbon budget in the current study, simulated cumulative anthropogenic emissions over the preindustrial Holocene are consistent with the ice core record of atmospheric δ13CO2 and support the hypothesis that anthropogenic activities led to the stabilization of atmospheric CO2 concentrations at a level that made the world substantially warmer than it otherwise would be.

Was the spread of agropastoralism from the Eurasian founder regions dominated by demic or by cultural diffusion? This study employs a mathematical model of regional sociocultural development that includes different diffusion processes, local innovation and societal adaptation. Simulations hindcast the emergence and expansion of agropastoral life style in 294 regions of Eurasia and North Africa. Different scenarios for demic and diffusive exchange processes between adjacent regions are contrasted and the spatiotemporal pattern of diffusive events is evaluated. This study supports from a modeling perspective the hypothesis that there is no simple or exclusive demic or cultural diffusion, but that in most regions of Eurasia a combination of demic and cultural processes were important. Furthermore, we demonstrate the strong spatial and temporal variability in the balance of spread processes. Each region shows sometimes more demic, and at other times more cultural diffusion. Only few, possibly environmentally marginal, areas show a dominance of demic diffusion. This study affirms that diffusion processes should be investigated in a diachronic fashion and not from a time-integrated perspective., 9 pages, 3 figures, revised version submitted to Documenta Prehistoric

Here, we present a community perspective on how to explore, exploit and evolve the diversity in aquatic ecosystem models. These models play an important role in understanding the functioning of aquatic ecosystems, filling in observation gaps and developing effective strategies for water quality management. In this spirit, numerous models have been developed since the 1970s. We set off to explore model diversity by making an inventory among 42 aquatic ecosystem modellers, by categorizing the resulting set of models and by analysing them for diversity. We then focus on how to exploit model diversity by comparing and combining different aspects of existing models. Finally, we discuss how model diversity came about in the past and could evolve in the future. Throughout our study, we use analogies from biodiversity research to analyse and interpret model diversity. We recommend to make models publicly available through open-source policies, to standardize documentation and technical implementation of models, and to compare models through ensemble modelling and interdisciplinary approaches. We end with our perspective on how the field of aquatic ecosystem modelling might develop in the next 5-10 years. To strive for clarity and to improve readability for non-modellers, we include a glossary.

During the Holocene strong gradients in the distribution of technologyincluding subsistence ways emerged on a global scale.These patterns were further amplified in historic times and are stillvisible through worldwide differences in national wealth.In order to evaluate major factors responsible for the shift fromforaging to food production we here employ quantitative methods bydeveloping a deterministic but simple model. After compiling existing maps of potential vegetation at 5000 BP theinhabited world is split into 197 regions with homogeneous environmentalconditions. Suitable variables for the macro-economic and culturaldevelopment in the Neolithic period are found to be farming to hunting-gatheringratio, number of agricultural economies and a technological development index.The model explicitly describes economic adaptation, growth and migrationof human populations together with the spread of their cultural characteristics; it accounts for over-exploitation of natural resources, crowdingmortality and the climate variability on a millennium scale.In a thorough model validation region specific trajectories are compared toarchaeological evidence revealing a high correspondence. Major parts of the knownsequence of Neolithic centers including the timing differences are robustlyreproduced. A series of known problems in prehistory is discussedcomprising the lag between domestication and full scale farming, the off-levelingof the technological boost following the transition, the emergence ofdistinct migration waves and sensitivity to climate fluctuations.Not mere population pressure but continuous innovation and competition betweensubsistence strategies is identified as a prime mover of agricultural development.The results suggest that few aspects of biogeography may have determined theobserved continental gradients in the number of domesticable species ultimatelyleading to an increasing differentiation in technology and demography.

To understand the two-way interaction between past societies and Holocene climate, we conduct a series of integrated model- and data-based studies. The climate-culture feedback is investigated using a coupled Earth System Civilization Model, including a new methodology to incorporate proxy information into an Earth System Model. Our study reconstructs the transition to agriculture for Western Eurasia in the paleoclimatic context; it shows that migration is not a necessary prerequisite for this transition, which is a yet unresolved problem in European archeology. Climate variability and extreme events had no significant impact, which reflects societal resilience. Also, our simulation studies indicate a considerable range of global and regional carbon emissions by deforestation. In conclusion, we find on the one hand a lower sensitivity of past societies to changes in Holocene climate than frequently suggested, on the other hand a possibly larger influence of those societies on regional and global climate.

The relationship between humans and their environment underwent a radical change during the last 10,000 years: from mobile and small groups of foragers to sedentary extensive cultivators and on to high-density intensive agriculture-based modern society; these transitions fundamentally transformed the formerly predominantly passive human user of the environment into an active component of the Earth system. The most striking impacts of these global transitions have only become visible and measurable during the last 150 years (Crutzen, Nature 415:23, 2002; Crutzen and Stoermer, IGBP Newsletter 41(1):17–18, 2000). Prior to this time frame, the use of forest resources for metal smelting in early Roman times and the extensive medieval agricultural system had already changed the landscape (Barker, Nature 473:163–164, 2011; Kaplan et al., Quaternary Science Reviews 28(27/28):3016–3034, 2009); the global climate effects of these early extensive cultivation and harvesting practices are still under debate (Kaplan et al., The Holocene 21(5):775–791, 2011; Lemmen, Géomorphologie: relief, processus, environnement 2009(4):303–312, 2010; Ruddiman, Climatic Change 61(3):261–293, 2003; Stocker et al., Biogeosciences 8:69–88, 2011).

The Indus Valley Civilization (IVC) was one of the first great civilizations in prehistory. This bronze age civilization flourished from the end of the fourth millennium BC. It disintegrated during the second millennium BC; despite much research effort, this decline is not well understood. Less research has been devoted to the emergence of the IVC, which shows continuous cultural precursors since at least the seventh millennium BC. To understand the decline, we believe it is necessary to investigate the rise of the IVC, i.e., the establishment of agriculture and livestock, dense populations and technological developments 7000--3000 BC. Although much archaeological information is available, our capability to investigate the system is hindered by poorly resolved chronology, and by a lack of field work in the intermediate areas between the Indus valley and Mesopotamia. We thus employ a complementary numerical simulation to develop a consistent picture of technology, agropastoralism and population developments in the IVC domain. Results from this Global Land Use and technological Evolution Simulator show that there is (1) fair agreement between the simulated timing of the agricultural transition and radiocarbon dates from early agricultural sites, but the transition is simulated first in India then Pakistan; (2) an independent agropastoralism developing on the Indian subcontinent; and (3) a positive relationship between archeological artifact richness and simulated population density which remains to be quantified.

The role of Pre- and Protohistoric anthropogenic land cover changes needs to be quantified i) to establish a baseline for comparison with current human impact on the environment and ii) to separate it from naturally occurring changes in our environment. Results are presented from the simple, adaptation-driven, spatially explicit Global Land Use and technological Evolution Simulator (GLUES) for pre-Bronze age demographic, technological and economic change. Using scaling parameters from the History Database of the Global Environment as well as GLUES-simulated population density and subsistence style, the land requirement for growing crops is estimated. The intrusion of cropland into potentially forested areas is translated into carbon loss due to deforestation with the dynamic global vegetation model VECODE. The land demand in important Prehistoric growth areas - converted from mostly forested areas - led to large-scale regional (country size) deforestation of up to 11% of the potential forest. In total, 29 Gt carbon were lost from global forests between 10 000 BC and 2000 BC and were replaced by crops; this value is consistent with other estimates of Prehistoric deforestation. The generation of realistic (agri-)cultural development trajectories at a regional resolution is a major strength of GLUES. Most of the pre-Bronze age deforestation is simulated in a broad farming belt from Central Europe via India to China. Regional carbon loss is, e.g., 5 Gt in Europe and the Mediterranean, 6 Gt on the Indian subcontinent, 18 Gt in East and Southeast Asia, or 2.3 Gt in subsaharan Africa. Le rôle de l’Homme dans l’évolution des états de surface de la Terre au cours de la Pré- et Protohistoire doit être estimé afin de répondre à deux objectifs : 1) établir une base de données qui permette de comparer les différentes actions anthropiques sur l’environnement et 2) bien distinguer les causes « naturelles » des causes d’origine anthropique dans l’évolution des dynamiques environnementales. L’utilisation d’un logiciel intitulé « Global Land Use and technological Evolution Simulator (GLUES) » a permis de modéliser les changements démographiques, technologiques et économiques antérieurs à l’Âge du Bronze. La démarche a consisté à estimer le besoin en terres pour répondre à la croissance continue de mises en culture en se basant sur les données de la « History Database of the Global Environment » et sur la modélisation des densités de population et des types de subsistance estimées d’après GLUES. Les défrichements forestiers se sont traduits par une perte de carbone estimée grâce au modèle VECODE. Le besoin en terres dans les foyers d’expansion préhistoriques - développés principalement dans des secteurs forestiers - a conduit à des déforestations massives, représentant jusqu’à 11 % de l’ensemble des forêts potentielles. Sur la période 10 000-2000 av. J.-C., ce sont au total environ 29 Gt de carbone qui ont été perdues en raison de la substitution des champs cultivés aux forêts. GLUES a permis la visualisation d’axes d’expansion agricole à une échelle régionale. La plupart des défrichements observés avant l’Age du Bronze ont affecté un vaste territoire qui s’étend depuis l’Europe centrale jusqu’à la Chine, en passant par l’Inde. Les valeurs régionales de perte en carbone sont estimées à 5 Gt en Europe et en Méditerranée, à 6 Gt en Inde, à 18 Gt en Asie du sud et du sud-est et à 2,3 Gt pour l’Afrique subsaharienne.

Farming and herding were introduced to Europe from the Near East and Anatolia; there are, however, considerable arguments about the mechanisms of this transition. Were it people who moved and outplaced the indigenous hunter- gatherer groups or admixed with them? Or was it just material and information that moved-the Neolithic Package-consisting of domesticated plants and animals and the knowledge of its use? The latter process is commonly referred to as cultural diffusion and the former as demic diffusion. Despite continuous and partly combined efforts by archaeologists, anthropologists, linguists, paleontologists and geneticists a final resolution of the debate has not yet been reached. In the present contribution we interpret results from the Global Land Use and technological Evolution Simulator (GLUES), a mathematical model for regional sociocultural development embedded in the western Eurasian geoenvironmental context during the Holocene. We demonstrate that the model is able to realistically hindcast the expansion speed and the inhomogeneous space-time evolution of the transition to agropastoralism in Europe. GLUES, in contrast to models that do not resolve endogenous sociocultural dynamics, also describes and explains how and why the Neolithic advanced in stages. In the model analysis, we uncouple the mechanisms of migration and information exchange. We find that (1) an indigenous form of agropastoralism could well have arisen in certain Mediterranean landscapes, but not in Northern and Central Europe, where it depended on imported technology and material, (2) both demic diffusion by migration or cultural diffusion by trade may explain the western European transition equally well, (3) [...], Comment: Accepted Author Manuscript version accepted for publication in Journal of Archaeological Science. A definitive version will be subsequently published in the Journal of Archaological Science

The major objectives of this paper are: (1) to review the pros and cons of the scenarios of past anthropogenic land cover change (ALCC) developed during the last ten years, (2) to discuss issues related to pollen-based reconstruction of the past land-cover and introduce a new method, REVEALS (Regional Estimates of VEgetation Abundance from Large Sites), to infer long-term records of past land-cover from pollen data, (3) to present a new project (LANDCLIM: LAND cover – CLIMate interactions in NW Europe during the Holocene) currently underway, and show preliminary results of REVEALS reconstructions of the regional land-cover in the Czech Republic for five selected time windows of the Holocene, and (4) to discuss the implications and future directions in climate and vegetation/land-cover modeling, and in the assessment of the effects of human-induced changes in land-cover on the regional climate through altered feedbacks. The existing ALCC scenarios show large discrepancies between them, and few cover time periods older than AD 800. When these scenarios are used to assess the impact of human land-use on climate, contrasting results are obtained. It emphasizes the need for methods such as the REVEALS model-based land-cover reconstructions. They might help to fine-tune descriptions of past land-cover and lead to a better understanding of how long-term changes in ALCC might have influenced climate. The REVEALS model is demonstrated to provide better estimates of the regional vegetation/landcover changes than the traditional use of pollen percentages. This will achieve a robust assessment of land cover at regional- to continental-spatial scale throughout the Holocene. We present maps of REVEALS estimates for the percentage cover of 10 plant functional types (PFTs) at 200 BP and 6000 BP, and of the two open-land PFTs “grassland” and “agricultural land” at five time-windows from 6000 BP to recent time. The LANDCLIM results are expected to provide crucial data to reassess ALCC estimates for a better understanding of the land suface-atmosphere interactions. NordForsk LANDCLIM VR LANDCLIM MERGE

We integrate 130 globally distributed proxy time series to refine the understanding of climate variability during the Holocene. Cyclic anomalies and temporal trends in periodicity from the Lower to the Upper Holocene are extracted by combining Lomb-Scargle Fourier-transformed spectra with bootstrapping. Results were cross-checked by counting events in the time series. Main outcomes are: First, the propensity of the climate system to fluctuations is a region specific property. Many records of adjacent sites reveal a similar change in variability although they belong to different proxy types (e.g., δ18O, lithic composition). Secondly, at most sites, irreversible change occured in the Mid-Holocene. We suggest that altered ocean circulation together with slightly modified coupling intensity between regional climate subsystems around the 5.5 kyr BP event (termination of the African Humid Period) were responsible for the shift. Fluctuations especially intensified along a pan-American corridor. This may have led to an unequal crisis probability for early human civilizations in the Old and New World. Our study did not produce evidence for millennial scale cyclicity in some solar activity proxies for the Upper Holocene, nor for a privileged role of the prominent 250, 550, 900 and 1450 yr cycles. This lack of global periodicities corroborates the regional character of climate variability.

We investigate the extent to which commonly considered quantities, based on total column ozone observations and simulations, are applicable as measures of ozone loss in the polar vortices. Such quantities have been used frequently in ozone assessments by the World Meteorological Organization (WMO) and to assess the performance of chemistry-climate models. The most commonly considered quantity is monthly mean column ozone poleward of a latitude of 63° in spring. For the Arctic, these monthly means were found to be insensitive to the exact choice of the latitude threshold, unlike the Antarctic where greater sensitivity was found. Choosing a threshold based on the location of the transport barrier at the vortex boundary instead of geometric latitude led to a roughly similar year-to-year variability of the monthly means, but in particular years deviations of several tens of Dobson units occurred. Moreover, the minimum of daily total ozone minima poleward of a particular latitude, another popular measure, is debatable, insofar as it relies on one single measurement or model grid point. For Arctic conditions, this minimum value occurred often in air outside polar vortex, both in the observations and in a chemistry-climate model. As a result, we recommend that the minimum of daily minima no longer be used when comparing polar ozone loss in observations and models. As a possible alternative, we suggest considering the minimum of daily average total ozone poleward of a particular equivalent latitude (or in the vortex) in spring. This definition both obviates relying on one single data point and reduces the impact of year-to-year variability in the Arctic vortex breakup on ozone loss measures. However, compact relations of such simple measures with meteorological quantities that describe the potential for polar heterogeneous chlorine activation and thus ozone loss were not found. Therefore, we argue that where possible, more sophisticated measures of chemical polar ozone loss that include additional information to disentangle the impact of transport and chemistry on ozone, should be employed.

The tracer-tracer correlation technique (TRAC) has been widely employed to infer chemical ozone loss from observations. Yet, its applicability to chemistry-climate model (CCM) data is disputed. Here, we report the successful application of TRAC on the results of a CCM simulation. By comparing TRAC-calculated ozone loss to ozone loss derived with the passive ozone method in a chemistry transport model we differentiate effects of internal mixing and cross vortex boundary mixing on a TRAC reference correlation. As a test case, we consider results of a cold Arctic winter/spring episode from an E39/C experiment, where typical features, for example, sufficient polar stratospheric cloud formation potential, denitrification and dehydration, and intermittent and final stratospheric warming events, are simulated. We find that internal mixing does not impact the TRAC-derived reference correlation at all. Mixing across the vortex boundary would lead to an underestimation of ozone loss by ∼10% when calculated with TRAC. We provide arguments that TRAC is a consistent and conservative method to derive chemical ozone loss and can be used to extract its chemical signature also from CCM simulations. As a consequence, we will be able to provide a lower bound for chemical ozone loss for model simulations where a passive ozone tracer is not available.

In the recent WMO assessment of ozone depletion, the minimum ozone column is used to assess the evolution of the polar ozone layer simulated in several chemistry-climate models (CCMs). The ozone column may be strongly influenced by changes in transport and is therefore not well-suited to identify changes in chemistry. The quantification of chemical ozone depletion can be achieved with tracer-tracer correlations (TRAC). For forty Antarctic winters ( 1960 - 1999), we present the seasonal chemical depletion simulated with the ECHAM4. L39(DLR)/CHEM model. Analyzing methane - ozone correlations, we find a mean chemical ozone loss of 80 +/- 10 DU during the 1990s, with a maximum of 94 DU. Compared to ozone loss deduced from HALOE measurements the model underestimates chemical loss by 37%. The average multidecadal trend in loss from 1960 to 1999 is 17 +/- 3 DU per decade. The largest contribution to this trend comes from the 62 +/- 11 DU ozone loss increase between the 1970s and 1990s.