Karin Holmgren

Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data

A number of reconstructions of millennial-scale climate variability have been carried out in order to understand patterns of natural climate variability, on decade to century timescales, and the role of anthropogenic forcing. These reconstructions have mainly used tree-ring data and other data sets of annual to decadal resolution. Lake and ocean sediments have a lower time resolution, but provide climate information at multicentennial timescales that may not be captured by tree-ring data. Here we reconstruct Northern Hemisphere temperatures for the past 2,000 years by combining low-resolution proxies with tree-ring data, using a wavelet transform technique to achieve timescale-dependent processing of the data. Our reconstruction shows larger multicentennial variability than most previous multi-proxy reconstructions, but agrees well with temperatures reconstructed from borehole measurements and with temperatures obtained with a general circulation model. According to our reconstruction, high temperatures—similar to those observed in the twentieth century before 1990—occurred around ad 1000 to 1100, and minimum temperatures that are about 0.7 K below the average of 1961–90 occurred around ad 1600. This large natural variability in the past suggests an important role of natural multicentennial variability that is likely to continue.

A 3000-year high-resolution stalagmitebased record of palaeoclimate for northeastern South Africa

High-resolution stable isotope variations and growth structure analyses of the last three millennia of a 6600-year stalagmite record at Cold Air Cave, Makapansgat Valley, South Africa, are presented. Growth layers, which are measurable over the last 250 years, are shown to be annual. The correlation between the width of growth layers and precipitation is strongly positive. Changes inδ18O andδ13C are positively correlated and inversely correlated to changes in the colour of the growth layers in the stalagmite. Variations in colour are directly correlated with mean annual temperature. Dark colouration is the product of increased temperature and mobilization of organic matter from the soil, and is associated with wetter summers and enhanced growth of C4 grasses. Darker colouring and enrichedδ18O andδ13C reflect a warmer, wetter environment, whereas lighter colouring and depleted isotopic values are indicative of cooler, drier conditions. The dominant episode in the 3000-year record is the cool, dry 500-year manifestation of the‘Little Ice Age’, from ad 1300 to about 1800, with the lowest temperatures at around ad 1700. The four centuries from ad 900 to 1300, experiencing above-average warming and high variability, may be the regional expression of the medieval warming. Other cool, dry spells prevailed from around ad 800 to 900 and from about ad 440 to 520. The most prolonged warm, wet period occurred from ad 40 to 400. Some extreme events are shown to correspond well with similar events determined from the Greenland GISP2 ice-core record and elsewhere. Distinct periodicities occur within the record at around 120, 200–300, 500–600 and at about 800 years BP.

Rapid climate shifts in the southern African interior throughout the Mid to Late Holocene

A detailed climate proxy record based on δ18O, δ13O, and grey index of a well-dated stalagmite from Cold Air Cave in the Makapansgat Valley of north-eastern South Africa suggests that regional precipitation, temperatures and vegetation oscillated markedly and rapidly over the last ∼6500 years on centennial and multi-decadal scales. The mid-Holocene prior to 5200 years ago was humid and warm. A fundamental transition occurred 3200 years ago, leading to drier and cooler conditions that culminated at 1750 AD. Comparisons with ice core records suggest synchronous changes implicating rapid global teleconnections.

European summer temperatures since Roman times

The spatial context is criticalwhen assessing present-day climate anomalies, attributing them to potential forcings and making statements regarding their frequency and severity in a long-term perspective. Recent international initiatives have expanded the number of high-quality proxy-records and developed new statistical reconstruction methods. These advances allow more rigorous regional past temperature reconstructions and, in turn, the possibility of evaluating climate models on policy-relevant, spatiotemporal scales. Here we provide a new proxy-based, annually-resolved, spatial reconstruction of the European summer (June-August) temperature fields back to 755 CE based on Bayesian hierarchical modelling (BHM), together with estimates of the European mean temperature variation since 138 BCE based on BHM and composite-plus-scaling (CPS). Our reconstructions compare well with independent instrumental and proxy-based temperature estimates, but suggest a larger amplitude in summer temperature variability than previously reported. Both CPS and BHM reconstructions indicate that the mean 20th century European summer temperature was not significantly different from some earlier centuries, including the 1st, 2nd, 8th and 10th centuries CE. The 1st century (in BHM also the 10th century) may even have been slightly warmer than the 20th century, but the difference is not statistically significant. Comparing each 50 yr period with the 1951-2000 period reveals a similar pattern. Recent summers, however, have been unusually warm in the context of the last two millennia and there are no 30 yr periods in either reconstruction that exceed the mean average European summer temperature of the last 3 decades (1986-2015 CE). A comparison with an ensemble of climate model simulations suggests that the reconstructed European summer temperature variability over the period 850-2000 CE reflects changes in both internal variability and external forcing on multi-decadal time-scales. For pan-European temperatures we find slightly better agreement between the reconstruction and the model simulations with high-end estimates for total solar irradiance. Temperature differences between the medieval period, the recent period and the Little Ice Age are larger in the reconstructions than the simulations. This may indicate inflated variability of the reconstructions, a lack of sensitivity and processes to changes in external forcing on the simulated European climate and/or an underestimation of internal variability on centennial and longer time scales.

Climate model benchmarking with glacial and mid-Holocene climates

Abstract Past climates provide a test of models’ ability to predict climate change. We present a comprehensive evaluation of state-of-the-art models against Last Glacial Maximum and mid-Holocene climates, using reconstructions of land and ocean climates and simulations from the Palaeoclimate Modelling and Coupled Modelling Intercomparison Projects. Newer models do not perform better than earlier versions despite higher resolution and complexity. Differences in climate sensitivity only weakly account for differences in model performance. In the glacial, models consistently underestimate land cooling (especially in winter) and overestimate ocean surface cooling (especially in the tropics). In the mid-Holocene, models generally underestimate the precipitation increase in the northern monsoon regions, and overestimate summer warming in central Eurasia. Models generally capture large-scale gradients of climate change but have more limited ability to reproduce spatial patterns. Despite these common biases, some models perform better than others.

CHANGING GRADIENTS OF CLIMATE CHANGE IN SOUTHERN AFRICA DURING THE PAST MILLENNIUM: IMPLICATIONS FOR POPULATION MOVEMENTS

Climates of equatorial East Africa and subtropical Southern Africa have varied inversely over long periods of time. The high-resolution δ 18O stalagmite record from Cold Air Cave in the Makapansgat valley in South Africa and a similar resolution lake-level record for Lake Naivasha in Kenya have been in anti-phase for much of the last thousand years. A similar relationship is evident in the twentieth century meteorological record. The changes in rainfall in the two regions on multi-decadal to centennial scales have influenced both settlement patterns and livelihoods of Iron Age agriculturalists. The resulting latitudinal gradient of change may have been a significant factor in promoting southward migration of Sotho-Tswana speaking people from equatorial East Africa during the first few centuries of the last millennium and earlier. This would have occurred at times when environments in the north were deteriorating and those to the south were ameliorating.

Trace element variation in speleothem aragonite: potential for palaeoenvironmental reconstruction

Trace element variation in annually laminated stalagmites has the potential to record long-term, high-resolution palaeoclimatic data provided both (1) the relationship between trace element content in the carbonate and the climate, and (2) the mechanism of stalagmite growth, are understood. Here we report a secondary ion mass spectrometry (SIMS) study of an aragonitic speleothem (T7) from Cold Air Cave, Southern Africa, which achieves small (μm) spatial resolution and ppm precision. T7 has a series of fine visible laminae believed to be annual to subannual. This article establishes (1) the usefulness of SIMS in analyzing speleothem aragonite at the spatial resolution and precision relevant to climatic studies, (2) the distribution of trace elements in aragonite on micron scales and their relationship to models for speleothem growth and (3) the potential of aragonite speleothems to encode climatic data at subannual resolution. Complex reproducible oscillations in Sr/Ca and Ba/Ca are observed parallel to the growth direction of the speleothem. However, in addition to this variability, we observe trace element oscillations perpendicular to the growth direction and infer that aragonite crystallites in the speleothem are laterally, as well as vertically zoned. Stepscans parallel to the growth direction demonstrate complex zoning patterns from which simple climatic data are difficult to extract due to small-scale lateral heterogeneity in the aragonite. However, time-series analysis of the whole data set reveals regular cycles in Sr/Ca and Ba/Ca at a scale that matches the magnitude of the visible laminae. On this basis, it appears that within the complex geochemical variability, there resides a regular environmental control, possibly variation in water throughput, hence rainfall.

How well do reanalyses represent the southern African precipitation

Monthly-mean precipitation observations over southern Africa are used to evaluate the performance of eight global reanalyses: ERA-40, ERA-interim, JRA-25, MERRA, CFSR, NCEP-R1, NCEP-R2 and 20CRv2. All eight reanalyses reproduce the regionally averaged seasonal cycle fairly well; a few spatial mismatches with the observations are found in the climate mean for the rainy season. Principal component analyses show a dipole in the leading modes of all reanalyses, however with crucial differences in its spatial position. Possible reasons for the differences between the reanalyses are discussed on the basis of the ERA-interim and 20CRv2 results. A comparison between the moisture transports shows that ERA-interim manifests a very strong moisture convergence over the eastern equatorial Atlantic, resulting in the strong precipitation here. This excessive convergence may be due to the water–vapor assimilation and convection parameterization. Over the Indian Ocean, the ITCZ is shifted northward in ERA-interim compared to its position in 20CRv2. This discrepancy is most likely attributable to the meridional SST gradients in the Indian Ocean which are significantly larger in the ERA-interim than those in the 20CRv2, and the resulting atmospheric response prevents a southward shift of the ITCZ. Overall, the consistent description of the dynamical circulation of the atmosphere and the hydrological cycle appears as a crucial benchmark for reanalysis data. Based on our evaluation, the preferential reanalysis for investigating the climate variability over southern Africa is 20CRv2 that furthermore spans the longest time period, hence permitting the most precise investigations of interannual to decadal variability.

Corroborated rainfall records from aragonitic stalagmites

Abstract Sr and Ba levels of two proximal aragonite speleothems from Cold Air Cave, South Africa, are examined at μm resolution. A compositional map derived from parallel secondary ion mass spectrometry linescans indicates heterogeneity perpendicular to the stalagmite growth axis, precluding the use of single linescans as a climatic proxy technique. A ∼40-yr averaged trace element record for the cave produced from multiple parallel linescans on both stalagmites is compared with regional and local climatic (rainfall, temperature) data for 1955–1996. This period includes two extensive drought episodes. There is poor correlation between trace elements and annual mean temperature. Droughts correspond to minima in trace element ratios and peaks correspond to annual maxima in rainfall. The onset and termination of droughts are notable inflexions. However, in detail annual rainfall magnitude does not correlate directly with trace element ratios, indicating that kinetic factors, notably variations in speleothem growth rate, play a key role in trace element uptake by aragonite. Averaged long data sets constrained by chronological control have potential in the study of sub-decadal precipitation patterns.

On the Phonetics of Infant Vocalization

This is a progress report on Normal Jollerutveckling (3), a project initiated with a view towards obtaining a detailed and clinically useful description of the vocalizations of normal Swedish infants. The present study focuses on certain aspects of the results, namely some findings on auditory and acoustic phonetic analyses.