Medieval warm Period

Le connessioni tra quadri ambientali e fenomeni economici hanno visto di recente un crescente interesse. In un campo connotato da un così forte carattere interdisciplinare sono varie le difficoltà che si incontrano, non solo nello scambio di conoscenze, ma anche nella stessa impostazione delle ricerche. Ambiente, clima, biologia, ecosistemi sono terreni di studio su cui convergono varie discipline, che richiedono un fondamentale chiarimento delle diverse prospettive di conoscenza e dei rispettivi presupposti, senza dei quali si rimane inevitabilmente immersi in una specie di “babele” di linguaggi in cui non sempre è facile districarsi. Il testo intende portare un contributo nell'ambito della storia ambientale e climatica del Medioevo; e costituisce una rielaborazione della relazione tenuta e discussa in occasione del Convegno internazionale del Centro Italiano di Studi di Storia e Arte di Pistoia (2015) sul tema "La crescita economica dell'Occidente medievale. Un tema storico non ancora esaurito".

Values of δ13C are frequently reported with radiocarbon dates from organic materials. In C3 plants δ13C values have been linked to changes in water use efficiency as a response to arid conditions. By calculating 13C discrimination (Δ13C) from 13C isotopic composition (δ13C), archaeologists can gain potentially valuable inference into past climate conditions. Values of Δ13C reflect the process of discrimination against heavier 13C isotopes of carbon by comparing the δ13C of samples to that of the atmosphere, and can be calculated when records of atmospheric δ13CO2 are available. The present study examines a 1300 year history of radiocarbon-derived Δ13C from the Lower Alentejo of Portugal using charcoal recovered from excavations of a series of medieval habitation sites in the study area. To calculate Δ13C, the posterior means generated from Bayesian change-point analysis of δ13CO2 records were used. Archaeological data were then compared to contemporary ecological studies of Δ13C of the same taxa against instrumental records of climate. Values of Δ13C fell within mean ranges for the taxa through a period of population growth between the 7th and 10th centuries AD. During the height of the Medieval Warm Period in the 11th century AD Δ13C values frequently fell to low levels associated with arid conditions. At this time environmental degradation and erosion were documented. Values of Δ13C increased for a brief period in the early 12th century AD before the rural Lower Alentejo was largely abandoned for nearly two centuries. Another period of aridity occurred in the 16th and 17th centuries AD. Radiocarbon-derived Δ13C is a potentially useful paleoclimate proxy for archaeologists provided that results can be paired with observed Δ13C variation in studies that pair these data with instrumental climate records.

The loss of the Norse Western Settlement in Greenland around the mid-fourteenth century has long been taken as a prime example of the impact of changing climate on human populations. This study employs an interdisciplinary approach combining historical documents, detailed archaeological investigations, and a high-resolution proxy climate record from the Greenland Ice Sheet Project 2 (GISP2) to investigate possible causes

The level of Cliff Lake, a small, subalpine, moraine-dammed lake in California’s south central Sierra Nevada, was approximately 5 m lower than present for 50 years or more approximately 600 years ago, this determined by radiocarbon dating of wood recovered from a submerged tree stump found in the lake. This finding corresponds to commensurate data from throughout much of western North America, suggesting the duration and magnitude of terminal medieval megadrought was similar throughout the region. Ultimately this datum helps give credence to the perspective that though late Holocene climate in California was indeed variable, the effects of terminal Medieval megadrought was similar across both time and broad geographic expanse.

Data basis for these calculations is the oak tree-ring based reconstructed time series of May-June precipitation for the North Aegean and Northwestern Anatolia provided by Criggs et. al., 2007.
A part of statistical analysis is based on a binary time series (1 = presence of a drought in that year, 0 = absence); a „drought“ is defined for a reconstructed precipitation values of 1 standard deviation below the mean value for the period 1089-1600 CE.
Mean waiting times between years with events were calculated on the basis of an expectation test for a poisson process for simple columns of event times for the five phenomena.
Probabilities of transition between years with events and years without were calculated with the help of Markov chain analysis on the basis of the above-mentioned binary time series.
The entire project is work in progress and includes also similar analyses for other medieval polities during this period (until now Byzantium, China, Egypt, England and Hungary; cf. also J. Preiser-Kapeller, (Not so) Distant Mirrors: a complex macro-comparison of polities and political, economic and religious systems in the crisis of the 14th century. Working paper for the International Conference “The Angevin Dynasty (14th Century)” in Targoviste (Romania), October 21st-23rd 2011; online:  http://www.academia.edu/512025/_Not_so_Distant_Mirrors_a_complex_macro-comparison_of_polities_and_political_economic_and_religious_systems_in_the_crisis_of_the_14th_century – also with indication of data sources for these calculations, and http://www.academia.edu/3077069/A_statistical_analysis_and_a_first_simple_model_for_internal_instability_in_the_Byzantine_Empire_1200-1453_CE). Comments are always welcome.
All calculations on and graphs of the time series were done by the author with the help of the software programmes Microsoft-Excel* and PAST* (Version 2.17); any usage of material or data only with approval of the author!

This multi-proxy paleoenvironmental study from Lake Sanabria (NW Iberian Peninsula), based on pollen, diatom, and sedimentology, provides evidences of climatic oscillations attributed to the Late Roman and Medieval Warm Periods as well as the Little Ice Age (LIA). From 440 to 950 AD, the climate was characterized by mild temperatures and a Mediterranean rainfall regime, although climatic cold periods were recorded at ca. 530 and 700. Evidence from pollen indicators of land-use suggests that grazing and farming were widespread activities. This period corresponds to the end of the Roman Warm Period and the Medieval Warm Period. The onset of new climate conditions occurred between 950 and 1100 AD, as minimum values of organic matter, arboreal pollen, diatom concentration, total nitrogen (TN), and grain size indicate low temperatures and a more regular rainfall regime. This period corresponds to the LIA and ended at 1590 AD, when lake productivity tended to recover to previous values in spite of the occurrence of cool events. Total organic carbon, TN, and diatom content covary with the temperature index for the NW Iberia, suggesting that Lake Sanabria was mainly controlled by climate before the industrial period. Since 1920 AD, lake productivity has been mainly influenced by human activity.

We review the post-glacial climate variability along the East Antarctic coastline using terrestrial and shallow marine geological records and compare these reconstructions with data from elsewhere. Nearly all East Antarctic records show a near-synchronous Early Holocene climate optimum (11.5–9 ka BP), coinciding with the deglaciation of currently ice-free regions and the optimum recorded in Antarctic ice and marine sediment cores. Shallow marine and coastal terrestrial climate anomalies appear to be out of phase after the Early Holocene warm period, and show complex regional patterns, but an overall trend of cooling in the terrestrial records. A Mid to Late Holocene warm period is present in many East Antarctic lake and shallow coastal marine records. Although there are some differences in the regional timing of this warm period, it typically occurs somewhere between 4.7 and 1 ka BP, which overlaps with a similar optimum found in Antarctic Peninsula terrestrial records. The differences in the timing of these sometimes abrupt warm events in different records and regions points to a number of mechanisms that we have yet to identify. Nearly all records show a neoglacial cooling from 2 ka BP onwards. There is no evidence along the East Antarctic coastline for an equivalent to the Northern Hemisphere Medieval Warm Period and there is only weak circumstantial evidence in a few places for a cool event crudely equivalent in time to the Northern Hemisphere's Little Ice Age. There is a need for well-dated, high resolution climate records in coastal East Antarctica and particularly in Terre Adélie, Dronning Maud Land and Enderby Land to fully understand the regional climate anomalies, the disparity between marine and terrestrial records, and to determine the significance of the heterogeneous temperature trends being measured in the Antarctic today.

The surface temperature changes for the last 4000 years in northern inland Iberia (an area particularly sensitive to climate change) are determined by a high resolution study of carbon stable isotope records of stalagmites from three caves (Kaite, Cueva del Cobre, and Cueva Mayor) separated several tens of kilometers away in N Spain. Despite the local conditions of each cave, the isotopic series show a good overall coherence, and resulted to be strongly sensitive to surface temperature changes. The record reflects alternating warmer and colder intervals, always within a temperature range of 1.6 °C. The timing and duration of the intervals were provided by 43 230Th- 234U (ICP-MS) ages. Main climatic recognized periods are: (1) 3950-3000 yr BP: warm period punctuated by cool events around ~ 3950, 3550 and 3250 yr BP; (2) 2850-2500 yr BP cold interval (Iron Age Cold Period); (3) 2500-1650 yr BP moderate warm period (Roman Warm Period), with maximum temperatures between 2150 and 1750 yr BP; (4) 1650-1350 yr BP cold interval (Dark Ages Cold Period), with a thermal minimum at ~ 1500 yr BP; (5) 1350-750 yr BP warm period (Medieval Warm Period) punctuated by two cooler events at ~ 1250 and ~ 850 yr BP; (6) 750-100 yr BP cold period (Little Ice Age) with extremes occurring at 600-500 yr BP, 350-300 yr BP, and 150-100 yr BP; and (7) the last 150 years, characterized by rapid but no linear warming (Modern Warming). Remarkably, the presented records allow direct comparison of recent warming with former warm intervals such as the Roman or the Medieval periods. That comparison reveals the 20th century as the time with highest surface temperatures of the last 4000 years for the studied area. Spectral analysis of the time series shows consistent climatic cycles of ~ 400, ~ 900 and ~ 1300 yr, comparable with those recognized in the North Atlantic marine record, the Greenland ice cores, and other terrestrial records for the middle-late Holocene, suggesting common climate forcing mechanisms related to changes in solar irradiance and North Atlantic circulation patterns.

The genesis and development of the submerged notch occurring along the Northern Adriatic limestone coasts, surveyed at depth ranging between −0.35 and −2.8m and amplitude consistent with the local tide is discussed using new and published data on its elevation and data on limestone lowering rates, collected using a micro-erosion meter and a traversing micro-erosion meter. Coastal limestone lowering rates

The hypoxic zone in the Baltic Sea has increased in area about four times since 1960 and widespread oxygen deficiency has severely reduced macro benthic communities below the halocline in the Baltic Proper and the Gulf of Finland, which in turn has affected food chain dynamics, fish habitats and fisheries in the entire Baltic Sea. The cause of increased hypoxia is believed to be enhanced eutrophication through increased anthropogenic input of nutrients, such as nitrogen and phosphorus. However, the spatial variability of hypoxia on long time-scales is poorly known: and so are the driving mechanisms. We review the occurrence of hypoxia in modern time (last c. 50 years), modern historical time (AD 1950–1800) and during the more distant past (the last c. 10 000 years) and explore the role of climate variability, environmental change and human impact. We present a compilation of proxy records of hypoxia (laminated sediments) based on long sediment cores from the Baltic Sea. The cumulated results show that the deeper depressions of the Baltic Sea have experienced intermittent hypoxia during most of the Holocene and that regular laminations started to form c. 8500–7800 cal. yr BP ago, in association with the formation of a permanent halocline at the transition between the Early Littorina Sea and the Littorina Sea s. str. Laminated sediments were deposited during three main periods (i.e. between c. 8000–4000, 2000–800 cal. yr BP and subsequent to AD 1800) which overlap the Holocene Thermal Maximum (c. 9000–5000 cal. yr BP), the Medieval Warm Period (c. AD 750–1200) and the modern historical period (AD 1800 to present) and coincide with intervals of high surface salinity (at least during the Littorina s. str.) and high total organic carbon content. This study implies that there may be a correlation between climate variability in the past and the state of the marine environment, where milder and dryer periods with less freshwater run-off correspond to increased salinities and higher accumulation of organic carbon resulting in amplified hypoxia and enlarged distribution of laminated sediments. We suggest that hydrology changes in the drainage area on long time-scales have, as well as the inflow of saltier North Sea waters, controlled the deep oxic conditions in the Baltic Sea and that such changes have followed the general Holocene climate development in Northwest Europe. Increased hypoxia during the Medieval Warm Period also correlates with large-scale changes in land use that occurred in much of the Baltic Sea watershed during the early-medieval expansion. We suggest that hypoxia during this period in the Baltic Sea was not only caused by climate, but increased human impact was most likely an additional trigger. Large areas of the Baltic Sea have experienced intermittent hypoxic from at least AD 1900 with laminated sediments present in the Gotland Basin in the Baltic Proper since then and up to present time. This period coincides with the industrial revolution in Northwestern Europe which started around AD 1850, when population grew, cutting of drainage ditches intensified, and agricultural and forest industry expanded extensively.