Inter-hemispheric temperature variability over the past millennium

The Earth’s climate system is driven by a complex interplay of internal chaotic dynamics and natural and anthropogenic external forcing. Recent instrumental data have shown a remarkable degree of asynchronicity between Northern Hemisphere and Southern Hemisphere temperature fluctuations, thereby questioning the relative importance of internal versus external drivers of past as well as future climate variability1, 2, 3. However, large-scale temperature reconstructions for the past millennium have focused on the Northern Hemisphere4, 5, limiting empirical assessments of inter-hemispheric variability on multi-decadal to centennial timescales. Here, we introduce a new millennial ensemble reconstruction of annually resolved temperature variations for the Southern Hemisphere based on an unprecedented network of terrestrial and oceanic palaeoclimate proxy records. In conjunction with an independent Northern Hemisphere temperature reconstruction ensemble5, this record reveals an extended cold period (1594–1677) in both hemispheres but no globally coherent warm phase during the pre-industrial (1000–1850) era. The current (post-1974) warm phase is the only period of the past millennium where both hemispheres are likely to have experienced contemporaneous warm extremes. Our analysis of inter-hemispheric temperature variability in an ensemble of climate model simulations for the past millennium suggests that models tend to overemphasize Northern Hemisphere–Southern Hemisphere synchronicity by underestimating the role of internal ocean–atmosphere dynamics, particularly in the ocean-dominated Southern Hemisphere. Our results imply that climate system predictability on decadal to century timescales may be lower than expected based on assessments of external climate forcing and Northern Hemisphere temperature variations5, 6 alone.

Contrasts, Contacts, and Interconnections — Tel Kinrot as an Early Iron Age Key Site in the Northern Jordan Rift Valley at the Dawn of the 1st Millennium BCE

Ancient Kinneret (Tēl Kinrōt [Hebrew]; Tell el-ʿOrēme [Arabic]) is located on a steep limestone hill on the northwestern shores of the Sea of Galilee (2508.7529 [NIG]). The site, whose settlement history began sometime during the Pottery-Neolithic or the early Chalcolithic period, is emerging as one of the major sites for the study of urban life in the Southern Levant during the Early Iron Age (c. 1130–950 BCE). Its size, accessibility by major trade routes, and strategic location between different spheres of cultural and political influence make Tēl Kinrōt an ideal place for studying the interaction of various cultures on urban sites, as well as to approach questions of ethnicity and regionalism during one of the most debated periods in the history of the ancient Levant. The paper will briefly discuss the settlement history of the site during the Early Iron Age. However, the main focus will lie on the material culture of the late Iron Age IB city that rapidly evolved to a regional center during the transition from the 11th to the 10th century BCE. During this period, ancient Kinneret features a multitude of cultural influences that reach from Egypt via the Central Hill Country until the Northern parts of Syria and the Amuq region. While there are indisputably close ties with the ‘Aramaean’ realm, there are also strong indications that there were – at the same time – vivid socio-economic links with the West, i.e. the Southern and Northern Mediterranean coasts and their hinterland. It will be argued that the resulting ‘cultural blend’ is a typical characteristic of the material culture of the Northern Jordan Rift Valley in the advent of the emerging regional powers of the Iron Age II.

Links between atmospheric carbon dioxide, the land carbon reservoir and climate over the past millennium

The stability of terrestrial carbon reservoirs is thought to be closely linked to variations in climate 1, but the magnitude of carbon–climate feedbacks has proved dificult to constrain for both modern 2–4 and millennial 5–13 timescales. Reconstructions of atmospheric CO2 concentrations for the past thousand years have shown fluctuations on multidecadal to centennial timescales 5–7, but the causes of these fluctuations are unclear. Here we report high-resolution carbon isotope measurements of CO2 trapped within the ice of the West Antarctic Ice Sheet Divide ice core for the past 1,000 years. We use a deconvolution approach 14 to show that changes in terrestrial organic carbon stores best explain the observed multidecadal variations in the 13 C of CO2 and in CO2 concentrations from 755 to 1850 CE. If significant long-term carbon emissions came from pre-industrial anthropogenic land-use changes over this interval, the emissions must have been offset by a natural terrestrial sink for 13 C-depleted carbon, such as peatlands. We find that on multidecadal timescales, carbon cycle changes seem to vary with reconstructed regional climate changes. We conclude that climate variability could be an important control of fluctuations in land carbon storage on these timescales.

A chrysophyte-based quantitative reconstruction of winter severity from varved lake sediments in NE Poland during the past millennium and its relationship to natural climate variability

Chrysophyte cysts are recognized as powerful proxies of cold-season temperatures. In this paper we use the relationship between chrysophyte assemblages and the number of days below 4 °C (DB4 °C) in the epilimnion of a lake in northern Poland to develop a transfer function and to reconstruct winter severity in Poland for the last millennium. DB4 °C is a climate variable related to the length of the winter. Multivariate ordination techniques were used to study the distribution of chrysophytes from sediment traps of 37 low-land lakes distributed along a variety of environmental and climatic gradients in northern Poland. Of all the environmental variables measured, stepwise variable selection and individual Redundancy analyses (RDA) identified DB4 °C as the most important variable for chrysophytes, explaining a portion of variance independent of variables related to water chemistry (conductivity, chlorides, K, sulfates), which were also important. A quantitative transfer function was created to estimate DB4 °C from sedimentary assemblages using partial least square regression (PLS). The two-component model (PLS-2) had a coefficient of determination of View the MathML sourceRcross2 = 0.58, with root mean squared error of prediction (RMSEP, based on leave-one-out) of 3.41 days. The resulting transfer function was applied to an annually-varved sediment core from Lake Żabińskie, providing a new sub-decadal quantitative reconstruction of DB4 °C with high chronological accuracy for the period AD 1000–2010. During Medieval Times (AD 1180–1440) winters were generally shorter (warmer) except for a decade with very long and severe winters around AD 1260–1270 (following the AD 1258 volcanic eruption). The 16th and 17th centuries and the beginning of the 19th century experienced very long severe winters. Comparison with other European cold-season reconstructions and atmospheric indices for this region indicates that large parts of the winter variability (reconstructed DB4 °C) is due to the interplay between the oscillations of the zonal flow controlled by the North Atlantic Oscillation (NAO) and the influence of continental anticyclonic systems (Siberian High, East Atlantic/Western Russia pattern). Differences with other European records are attributed to geographic climatological differences between Poland and Western Europe (Low Countries, Alps). Striking correspondence between the combined volcanic and solar forcing and the DB4 °C reconstruction prior to the 20th century suggests that winter climate in Poland responds mostly to natural forced variability (volcanic and solar) and the influence of unforced variability is low.

Antarctic temperature changes during the last millennium: evaluation of simulations and reconstructions

Temperature changes in Antarctica over the last millennium are investigated using proxy records, a set of simulations driven by natural and anthropogenic forcings and one simulation with data assimilation. Over Antarctica, a long term cooling trend in annual mean is simulated during the period 1000–1850. The main contributor to this cooling trend is the volcanic forcing, astronomical forcing playing a dominant role at seasonal timescale. Since 1850, all the models produce an Antarctic warming in response to the increase in greenhouse gas concentrations. We present a composite of Antarctic temperature, calculated by averaging seven temperature records derived from isotope measurements in ice cores. This simple approach is supported by the coherency displayed between model results at these data grid points and Antarctic mean temperature. The composite shows a weak multi-centennial cooling trend during the pre-industrial period and a warming after 1850 that is broadly consistent with model results. In both data and simulations, large regional variations are superimposed on this common signal, at decadal to centennial timescales. The model results appear spatially more consistent than ice core records. We conclude that more records are needed to resolve the complex spatial distribution of Antarctic temperature variations during the last millennium.

Variability of daily winter wind speed distribution over Northern Europe during the past millennium in regional and global climate simulations

We analyse the variability of the probability distribution of daily wind speed in wintertime over Northern and Central Europe in a series of global and regional climate simulations covering the last centuries, and in reanalysis products covering approximately the last 60 years. The focus of the study lies on identifying the link of the variations in the wind speed distribution to the regional near-surface temperature, to the meridional temperature gradient and to the North Atlantic Oscillation. Our main result is that the link between the daily wind distribution and the regional climate drivers is strongly model dependent. The global models tend to behave similarly, although they show some discrepancies. The two regional models also tend to behave similarly to each other, but surprisingly the results derived from each regional model strongly deviates from the results derived from its driving global model. In addition, considering multi-centennial timescales, we find in two global simulations a long-term tendency for the probability distribution of daily wind speed to widen through the last centuries. The cause for this widening is likely the effect of the deforestation prescribed in these simulations. We conclude that no clear systematic relationship between the mean temperature, the temperature gradient and/or the North Atlantic Oscillation, with the daily wind speed statistics can be inferred from these simulations. The understand- ing of past and future changes in the distribution of wind speeds, and thus of wind speed extremes, will require a detailed analysis of the representation of the interaction between large-scale and small-scale dynamics.