Thomas M. Melvin

A 3,500-year tree-ring record of annual precipitation on the northeastern Tibetan Plateau

Significance This paper describes the production and climatic interpretation of a tree-ring width chronology that is currently the longest, absolutely dated series produced for the northeastern Tibetan Plateau and one of the longest in the world. The method of chronology construction enables comparison of variations in precipitation totals over long timescales as well as shorter periods. Precipitation in this region during the last 50 years has been historically high—likely higher than for any equivalent length period in at least 3,500 years, even when considering the chronology and interpretational uncertainty. Notable dry periods occurred in the 4th century BCE and in the second half of the 15th century CE. An annually resolved and absolutely dated ring-width chronology spanning 4,500 y has been constructed using subfossil, archaeological, and living-tree juniper samples from the northeastern Tibetan Plateau. The chronology represents changing mean annual precipitation and is most reliable after 1500 B.C. Reconstructed precipitation for this period displays a trend toward more moist conditions: the last 10-, 25-, and 50-y periods all appear to be the wettest in at least three and a half millennia. Notable historical dry periods occurred in the 4th century BCE and in the second half of the 15th century CE. The driest individual year reconstructed (since 1500 B.C.) is 1048 B.C., whereas the wettest is 2010. Precipitation variability in this region appears not to be associated with inferred changes in Asian monsoon intensity during recent millennia. The chronology displays a statistical association with the multidecadal and longer-term variability of reconstructed mean Northern Hemisphere temperatures over the last two millennia. This suggests that any further large-scale warming might be associated with even greater moisture supply in this region.

A Closer Look at Regional Curve Standardization of Tree-Ring Records: Justification of the Need, a Warning of Some Pitfalls, and Suggested Improvements in Its Application

Some background describing the rationale and early development of regional curve standardization (RCS) is provided. It is shown how, in the application of RCS, low-frequency variance is preserved in the mean values of individual series of tree indices, while medium-frequency variance is also preserved in the slopes. Various problems in the use of the RCS approach are highlighted. The first problem arises because RCS detrending removes the average slope (derived from the data for all trees) from each individual tree measurement series. This operation results in a pervasive ‘trend-in-signal’ bias, which occurs when the underlying growth-forcing signal has variance on timescales that approach or exceed the length of the chronology. Even in a long chronology (i.e., including subfossil data), this effect will bias the start and end of the RCS chronology. Two particular problems associated with the use of RCS on contemporaneously growing trees, which might represent a typical (i.e., modern) sample, are also discussed. The first is the biasing of the RCS curve by the residual climate signal in age-aligned samples and the undesirable subsequent removal of this signal variance in RCS application. The second is the ‘differing-contemporaneous-growth-rate’ bias that effectively imparts a spurious trend over the span of a modern chronology. The first of these two can be mitigated by the application of ‘signal-free’ RCS. The second problem is more insidious and can only be overcome by the use of multiple sub-RCS curves, with a concomitant potential loss of some longer-timescale climate variance. Examples of potential biasing problems in the application of RCS are illustrated by reference to several published studies. Further implications and suggested directions for necessary further development of the RCS concept are discussed.

Large‐scale variations in the vegetation growing season and annual cycle of atmospheric CO2 at high northern latitudes from 1950 to 2011

We combine satellite and ground observations during 1950-2011 to study the long-term links between multiple climate (air temperature and cryospheric dynamics) and vegetation (greenness and atmospheric CO(2) concentrations) indicators of the growing season of northern ecosystems (>45°N) and their connection with the carbon cycle. During the last three decades, the thermal potential growing season has lengthened by about 10.5 days (P < 0.01, 1982-2011), which is unprecedented in the context of the past 60 years. The overall lengthening has been stronger and more significant in Eurasia (12.6 days, P < 0.01) than North America (6.2 days, P > 0.05). The photosynthetic growing season has closely tracked the pace of warming and extension of the potential growing season in spring, but not in autumn when factors such as light and moisture limitation may constrain photosynthesis. The autumnal extension of the photosynthetic growing season since 1982 appears to be about half that of the thermal potential growing season, yielding a smaller lengthening of the photosynthetic growing season (6.7 days at the circumpolar scale, P < 0.01). Nevertheless, when integrated over the growing season, photosynthetic activity has closely followed the interannual variations and warming trend in cumulative growing season temperatures. This lengthening and intensification of the photosynthetic growing season, manifested principally over Eurasia rather than North America, is associated with a long-term increase (22.2% since 1972, P < 0.01) in the amplitude of the CO(2) annual cycle at northern latitudes. The springtime extension of the photosynthetic and potential growing seasons has apparently stimulated earlier and stronger net CO(2) uptake by northern ecosystems, while the autumnal extension is associated with an earlier net release of CO(2) to the atmosphere. These contrasting responses may be critical in determining the impact of continued warming on northern terrestrial ecosystems and the carbon cycle.

Old World megadroughts and pluvials during the Common Era

An atlas of megadroughts in Europe and in the Mediterranean Basin during the Common Era provides insights into climate variability. Climate model projections suggest widespread drying in the Mediterranean Basin and wetting in Fennoscandia in the coming decades largely as a consequence of greenhouse gas forcing of climate. To place these and other “Old World” climate projections into historical perspective based on more complete estimates of natural hydroclimatic variability, we have developed the “Old World Drought Atlas” (OWDA), a set of year-to-year maps of tree-ring reconstructed summer wetness and dryness over Europe and the Mediterranean Basin during the Common Era. The OWDA matches historical accounts of severe drought and wetness with a spatial completeness not previously available. In addition, megadroughts reconstructed over north-central Europe in the 11th and mid-15th centuries reinforce other evidence from North America and Asia that droughts were more severe, extensive, and prolonged over Northern Hemisphere land areas before the 20th century, with an inadequate understanding of their causes. The OWDA provides new data to determine the causes of Old World drought and wetness and attribute past climate variability to forced and/or internal variability.

Trends in recent temperature and radial tree growth spanning 2000 years across northwest Eurasia

This paper describes variability in trends of annual tree growth at several locations in the high latitudes of Eurasia, providing a wide regional comparison over a 2000-year period. The study focuses on the nature of local and widespread tree-growth responses to recent warming seen in instrumental observations, available in northern regions for periods ranging from decades to a century. Instrumental temperature data demonstrate differences in seasonal scale of Eurasian warming and the complexity and spatial diversity of tree-growing-season trends in recent decades. A set of long tree-ring chronologies provides empirical evidence of association between inter-annual tree growth and local, primarily summer, temperature variability at each location. These data show no evidence of a recent breakdown in this association as has been found at other high-latitude Northern Hemisphere locations. Using Kendalls concordance, we quantify the time-dependent relationship between growth trends of the long chronologies as a group. This provides strong evidence that the extent of recent widespread warming across northwest Eurasia, with respect to 100- to 200-year trends, is unprecedented in the last 2000 years. An equivalent analysis of simulated temperatures using the HadCM3 model fails to show a similar increase in concordance expected as a consequence of anthropogenic forcing.

New ice core evidence for a volcanic cause of the A.D. 536 dust veil

In New Zealand human cryptosporidiosis demonstrates spring and autumn peaks of incidence with the spring peak being three times greater in magnitude than the autumn peak. The imbalance between the two peaks is notable, and may be associated with the high livestock density in New Zealand. In the summer and autumn the cryptosporidiosis rate was positively associated with temperatures in the current and previous month, highlighting the importance of outdoor recreation to transmission. No associations between spring incidence and weather were found providing little support for the importance of drinking-water pathways. Imported travel cases do not appear to be an important factor in the aetiology of cryptosporidiosis in New Zealand.

Potential bias in ‘updating’ tree-ring chronologies using regional curve standardisation: Re-processing 1500 years of Torneträsk density and ring-width data:

We describe the analysis of existing and new maximum-latewood-density (MXD) and tree-ring width (TRW) data from the Torneträsk region of northern Sweden and the construction of 1500 year chronologies. Some previous work found that MXD and TRW chronologies from Torneträsk were inconsistent over the most recent 200 years, even though they both reflect predominantly summer temperature influences on tree growth. We show that this was partly a result of systematic bias in MXD data measurements and partly a result of inhomogeneous sample selection from living trees (modern sample bias). We use refinements of the simple Regional Curve Standardisation (RCS) method of chronology construction to identify and mitigate these biases. The new MXD and TRW chronologies now present a largely consistent picture of long-timescale changes in past summer temperature in this region over their full length, indicating similar levels of summer warmth in the medieval period (MWP, c. ce 900–1100) and the latter half of the 20th century. Future work involving the updating of MXD chronologies using differently sourced measurements may require similar analysis and appropriate adjustment to that described here to make the data suitable for the production of un-biased RCS chronologies. The use of ‘growth-rate’ based multiple RCS curves is recommended to identify and mitigate the problem of ‘modern sample bias’.

A 9111 year long conifer tree-ring chronology for the European Alps: a base for environmental and climatic investigations

An ultra-long tree-ring width chronology (9111 years long, 7109 BC to AD 2002) has been established based on the analysis and dating of 1432 subfossil/dry dead wood samples and cores from 335 living trees. The material was collected from treeline or near-treeline sites (c. 2000 to 2400 m a.s.l.) mainly in the Eastern Alps. The availability of preserved samples through time at high altitudinal sites is influenced by Alpine forest history and is partly climatically controlled, as shown by comparisons of the sample depth record of the Eastern Alpine Conifer Chronology (EACC) with the Holocene glacier record. The similarity of variations over time between the sample depth of the chronology and the mid-Holocene GISP2 10Be record suggest a relationship between sample depth and solar activity. The Eastern Alpine Conifer Chronology has already been used as a dating base in environmental studies, eg, on glacier fluctuations, as well as in archaeological studies.

Tree-ring growth variability in the Austrian Alps: the influence of site, altitude, tree species and climate

We present an extensive new network of ring-width chronologies, comprising data from 100 sites within the Austrian Eastern Alps, made up of multiple tree species. Principal components analysis and cluster analysis were used to identify five separate high-frequency tree-growth signals from these data. Tree-growth variability at these sites is explained by site altitude and species differences that moderate the effects of the annually varying climatic forcing on tree growth. Recently developed, two-century long gridded climate data sets for the Greater Alpine Region were used to assess the relationship between climate and tree growth. Tree growth at low altitudes is controlled mainly by spring-summer moisture availability. At high altitudes precipitation is no longer a limiting factor and growth is mostly determined by summer temperature. In the intermediate altitudinal range, we did not find any direct relationships with specific climatic variables. High-altitude chronologies suitable for reconstructing past temperatures and low-altitude chronologies suitable for reconstructing past precipitation were identified.

A tree-ring reconstruction of East Anglian (UK) hydroclimate variability over the last millennium

We present an annually resolved reconstruction of spring-summer precipitation variability in East Anglia, UK (52–53°N, 0–2°E) for the period AD 900–2009. A continuous regional network of 723 living (AD 1590–2009) and historical (AD 781–1790) oak (Quercus sp.) ring-width series has been constructed and shown to display significant sensitivity to precipitation variability during the March-July season. The existence of a coherent common growth signal is demonstrated in oaks growing across East Anglia, containing evidence of near-decadal aperiodic variability in precipitation throughout the last millennium. Positive correlations are established between oak growth and precipitation variability across a large region of northwest Europe, although climate-growth relationships appear time transgressive with correlations significantly weakening during the early twentieth century. Examination of the relationship between oak growth, precipitation, and the North Atlantic Oscillation (NAO), reveals no evidence that the NAO plays any significant role in the control of precipitation or tree growth in this region. Using Regional Curve Standardisation to preserve evidence of low-frequency growth variability in the East Anglian oak chronology, we produce a millennial length reconstruction that is capable of explaining 32–35% of annual-to-decadal regional-scale precipitation variance during 1901–2009. The full length reconstruction indicates statistically significant anomalous dry conditions during AD 900–1100 and circa-1800. An apparent prolonged wetter phase is estimated for the twelfth and thirteen centuries, whilst precipitation fluctuates between wetter and drier phases at near centennial timescales throughout the fourteenth to seventeenth centuries. Above average precipitation reconstructed for the twenty-first century is comparable with that reproduced for the 1600s. The main estimated wet and dry phases reconstructed here appear largely coherent with an independent tree-ring reconstruction for southern-central England.