Heatwaves and summer droughts across Europe are likely to intensify under anthropogenic global warming thereby affecting ecological and societal systems. To place modern trends and extremes in the context of past natural variability, annually resolved and absolutely dated climate reconstructions are needed. Here, we present a network of 153 yew (Taxus baccata L.) tree-ring width (TRW) series from 22 sites in southern England that cover the past 310 years. Significant positive correlations were found between TRW chronologies and both April–July precipitation totals (r > 0.7) and July drought indices (r > 0.59) back to 1901 CE (p < 0.05). We used a suite of residual and standard TRW chronologies to reconstruct interannual to multi-decadal spring–summer precipitation and mid-summer drought variability over western Europe, respectively. Our yew hydroclimate reconstructions capture the majority of reported summer droughts and pluvials back to 1710 CE. Clusters of severe drought spells occurred in the second half of the 18th and mid-twentieth century. Our study suggests that the frequency and intensity of recent hydroclimate extremes over western Europe are likely still within the range of past natural variability.

As blue intensity (BI) methods are increasingly employed to generate temperature-sensitive tree-ring records around the globe, the influence of intra-site variation in elevation on climate-growth relationships for BI parameters remains largely unresolved. Here, we develop six latewood blue intensity (LWBI) chronologies along an elevational gradient for two montane conifer species, Abies concolor var. concolor (Gordon & Glend.) Lindl. Ex Hilderb and Picea engelmannii Parry ex Engelm., growing in the arid southwestern United States. In this first documented study to examine the climate response of LWBI from A. concolor, we find positive, significant (p < 0.05) correlations between the LWBI chronology from the highest elevation plot and spring–summer temperatures (April–August, r > 0.46). Moreover, the positive temperature response of A. concolor is generally stronger and more temporally stable than for P. engelmannii across varying seasonal windows. In comparing the differences in climate response across species and elevation, we document distinct clinal relationships between the temperature response of LWBI for A. concolor, where both the strength and temporal stability of the positive temperature signal increases with elevation. Meanwhile, the mid-elevation P. engelmannii demonstrate the highest climate sensitivity. As such, our findings contribute to a more comprehensive understanding of how elevation influences the type and strength of the climatic information embedded within the LWBI parameter from arid, montane conifers growing near their historical range margins.

Background Chir pine (Pinus roxburghii Sarg.) forests are distributed in the dry valleys of Bhutan Himalaya. In the past, these forests have been heavily influenced by human activities such as grazing, burning, resin tapping, and collection of non-timber forest products. Bhutan’s Forest Act of 1969, which shifted forest management from local community control to centralized governmental control, greatly restricted these activities. To understand the implications of the Forest Act on the chir pine forests, we used tree-rings and fire scars to reconstruct the fire history of a chir pine forest in eastern Bhutan. This provided an opportunity to characterize the fire regime before and after the Forest Act of 1969 was implemented and assess the scale and magnitude of changes that have occurred.

Results We developed a 120-year chir pine fire chronology from nine sites within a single forested landscape. Between 1900 and ~ 1970, fires were small and patchy. When fires occurred, they were limited to one to two sites within the larger study area. After 1970, there was a distinct shift in fire activity, with fires in 1985, 1989, 1996, 2000, and 2013 burning > 90% of sample plots. Fire activity was positively associated with La Niña conditions (wetter, cooler) in the preceding year. This is likely the result of increased accumulation and connectivity of fuels on the forest floor in wetter years.

Conclusions Prior to 1970, the fire regime in the studied chir pine landscape in eastern Bhutan was dominated by patchy, low-intensity fires indicating that the fire regime was fuel limited. After 1970, fires became larger and more frequent. This shift was associated with the enactment of the Bhutan Forest Act in 1969, which regulated grazing and implemented a policy of strict fire exclusion in government-reserved forests. This likely led to a large buildup of fuels, particularly after La Niña years. Historical patterns of grazing and low-intensity fires prior to the Forest Act kept fuel loads low and disconnected. The cessation of most human activities in these forests after 1969 resulted in an increase in fuel loads and connectivity within the landscape. This has greatly reshaped fire regimes in the chir pine forests of eastern Bhutan over the past half century.

Antecedentes Los bosques de pino chir, o pino indio de hoja larga (Pinus roxburghii Sarg.) están distribuidos en los valles secos del Himalaya en Bután. En el pasado, estos bosques fueron fuertemente influenciados por actividades humanas como el pastoreo, fuegos, colección de resinas, y recolección de productos no forestales. La ley forestal de Bután de 1969, que cambió el manejo forestal desde el control comunitario local a uno ejercido por las autoridades gubernamentales centrales, restringió fuertemente esas actividades. Para entender las implicancias de esta ley forestal sobre los bosques de pino chir, usamos los anillos de crecimiento y cicatrices de fuego para reconstruir la historia de fuego en un bosque de pino chir en el este de Bután. Esto proveyó de una oportunidad para caracterizar el régimen de fuegos antes y después que la aplicación de esta ley forestal fuera implementada en 1969, y determinar así la escala y magnitud de los cambios ocurridos.

Resultados Desarrollamos una cronología del pino chir de 120 años en nueve sitios dentro de un paisaje de bosque uniforme. Desde 1900 y hasta 1970, los incendios fueron pequeños y ocurrían en parches. Cuando estos ocurrían, estaban limitados a 1–2 sitios dentro de la gran área de estudios. Después de 1970, hubo un cambio distintivo en la actividad de los incendios, con ocurrencia en los años 1985, 1989, 1996, 2000, y 2013, quemando > 90% de las parcelas de muestreo. La actividad de los incendios fue asociada positivamente con condiciones de la Niña (más húmedas y frías) en el año precedente al evento de incendio. Este es probablemente el resultado de una acumulación incremental y mayor conectividad del combustible en el suelo forestal en años húmedos.

Conclusiones Antes del año 1970, el régimen de fuego en el paisaje de pino chir estudiado en el este de Bután, estaba dominado por fuegos de baja intensidad y que ocurrían en parches, indicando que ese régimen estaba limitado por la disponibilidad del combustible. Luego de 1970, los incendios comenzaron a ser más frecuentes y abarcaban mayores superficies. Este cambio estuvo asociado con la puesta en vigencia de la Ley Forestal de 1969, que regulaba el pastoreo e implementaba una política estricta de exclusión del fuego en reservas forestales gubernamentales. Esto llevó a un gran crecimiento y acumulación de combustibles, particularmente luego de años Niña. Los patrones históricos de pastoreo y baja intensidad de los incendios previos a la puesta en vigencia de la Ley forestal, mantenían la carga de combustibles a niveles bajos y desconectados entre sí dentro de ese paisaje. La implementación de esta ley reconfiguró dramáticamente los regímenes de fuego en los bosques de pino chir en el este de Bután en los últimos 50 años.

The jet stream is an important dynamic driver of climate variability in the Northern Hemisphere mid-latitudes1–3. Modern variability in the position of summer jet stream latitude in the North Atlantic–European sector (EU JSL) promotes dipole patterns in air pressure, temperature, precipitation and drought between northwestern and southeastern Europe. EU JSL variability and its impacts on regional climatic extremes and societal events are poorly understood, particularly before anthropogenic warming. Based on three temperature-sensitive European tree-ring records, we develop a reconstruction of interannual summer EU JSL variability over the period 1300–2004 ce (R2 = 38.5%) and compare it to independent historical documented climatic and societal records, such as grape harvest, grain prices, plagues and human mortality. Here we show contrasting summer climate extremes associated with EU JSL variability back to 1300 ce as well as biophysical, economic and human demographic impacts, including wildfires and epidemics. In light of projections for altered jet stream behaviour and intensified climate extremes, our findings underscore the importance of considering EU JSL variability when evaluating amplified future climate risk.

Existing global mean surface temperature reconstructions for the Holocene lack high-frequency variability that is essential for contextualising recent trends and extremes in the Earth's climate system. Here, we isolate and recombine archive-specific climate signals to generate a frequency-optimised record of interannual to multi-millennial temperature changes for the past 12 000 years. Average temperatures before ∼8000 years BP and after ∼4000 years BP were 0.26 (±2.84) °C and 0.07 (±2.11) °C cooler than the long-term mean (0–12 000 years BP), while the Holocene Climate Optimum ∼7000–4000 years BP was 0.40 (±1.86) °C warmer. Biased towards Northern Hemisphere summer temperatures, our multi-proxy record captures the spectral properties of transient Earth system model simulations for the same spatial and season domain. The new frequency-optimised trajectory emphasises the importance and complex interplay of natural climate forcing factors throughout the Holocene, with an approximation of the full range of past temperature changes providing novel insights for policymakers addressing the risks of recent anthropogenic warming.

‘Blue rings’ (BRs) are visual indicators of less lignified cell walls typically formed towards the end of a tree’s growing season. Though BRs have been associated with ephemeral surface cooling, often following large volcanic eruptions, the intensity of cold spells necessary to produce BRs, as well as the consistency of their formation within and between trees still remains uncertain. Here, we report an exceptionally high BR occurrence within and between Scots pine (Pinus sylvestris L.) trees at two sites in Estonia, including the first published whole-stem analysis for BRs. Daily meteorological measurements from a nearby station allowed us to investigate the role temperature has played in BR formation since the beginning of the twentieth century. The single year in which BRs were consistently formed within and amongst most trees was 1976. While the summer of 1976 is well known for an exceptional heatwave in Northwest Europe, mean September and October temperatures were remarkably low over Eastern Europe, and 3.8 °C below the 1961–1990 mean at our sites. Our findings contribute to a better eco-physiological interpretation of BRs, and further demonstrate their ability to reveal ephemeral cooling not captured by dendrochronological ring width and latewood density measurements. 

The largest explosive volcanic eruption of the Common Era in terms of estimated sulphur yield to the stratosphere was identified in glaciochemical records 40 years ago, and dates to the mid-thirteenth century. Despite eventual attribution to the Samalas (Rinjani) volcano in Indonesia, the eruption date remains uncertain, and the climate response only partially understood. Seeking a more global perspective on summer surface temperature and hydroclimate change following the eruption, we present an analysis of 249 tree-ring chronologies spanning the thirteenth century and representing all continents except Antarctica. Of the 170 predominantly temperature sensitive high-frequency chronologies, the earliest hints of boreal summer cooling are the growth depressions found at sites in the western US and Canada in 1257 CE. If this response is a result of Samalas, it would be consistent with an eruption window of circa May-July 1257 CE. More widespread summer cooling across the mid-latitudes of North America and Eurasia is pronounced in 1258, while records from Scandinavia and Siberia reveal peak cooling in 1259. In contrast to the marked post-Samalas temperature response at high-elevation sites in the Northern Hemisphere, no strong hydroclimatic anomalies emerge from the 79 precipitation-sensitive chronologies. Although our findings remain spatially biased towards the western US and central Europe, and growth-climate response patterns are not always dominated by a single meteorological factor, this study offers a global proxy framework for the evaluation of paleoclimate model simulations.

Linked to major volcanic eruptions around 536 and 540 CE, the onset of the Late Antique Little Ice Age has been described as the coldest period of the past two millennia. The exact timing and spatial extent of this exceptional cold phase are, however, still under debate because of the limited resolution and geographical distribution of the available proxy archives. Here, we use 106 wood anatomical thin sections from 23 forest sites and 20 tree species in both hemispheres to search for cell-level fingerprints of ephemeral summer cooling between 530 and 550 CE. After cross-dating and double-staining, we identified 89 Blue Rings (lack of cell wall lignification), nine Frost Rings (cell deformation and collapse), and 93 Light Rings (reduced cell wall thickening) in the Northern Hemisphere. Our network reveals evidence for the strongest temperature depression between mid-July and early-August 536 CE across North America and Eurasia, whereas more localised cold spells occurred in the summers of 532, 540–43, and 548 CE. The lack of anatomical signatures in the austral trees suggests limited incursion of stratospheric volcanic aerosol into the Southern Hemisphere extra-tropics, that any forcing was mitigated by atmosphere-ocean dynamical responses and/or concentrated outside the growing season, or a combination of factors. Our findings demonstrate the advantage of wood anatomical investigations over traditional dendrochronological measurements, provide a benchmark for Earth system models, support cross-disciplinary studies into the entanglements of climate and history, and question the relevance of global climate averages.

Global temperatures are rising at an unprecedented rate, but environmental responses are often difficult to recognize and quantify. Long-term observations of plant phenology, the annually recurring sequence of plant developmental stages, can provide sensitive measures of climate change and important information for ecosystem services. Here, we present 419 354 recordings of the first flowering date from 406 plant species in the UK between 1753 and 2019 CE. Community-wide first flowering advanced by almost one month on average when comparing all observations before and after 1986 (p < 0.0001). The mean first flowering time is 6 days earlier in southern than northern sites, 5 days earlier under urban than rural settings, and 1 day earlier at lower than higher elevations. Compared to trees and shrubs, the largest lifeform-specific phenological shift of 32 days is found in herbs, which are generally characterized by fast turnover rates and potentially high levels of genetic adaptation. Correlated with January-April maximum temperatures at -0.81 from 1952-2019 (p < 0.0001), the observed trends (5.4 days per decade) and extremes (66 days between the earliest and latest annual mean) in the UK's first flowering dataset can affect the functioning and productivity of ecosystems and agriculture.

A steep decline in the quality and quantity of available climate proxy records before medieval times challenges any comparison of reconstructed temperature and hydroclimate trends and extremes between the first and second half of the Common Era. Understanding of the physical causes, ecological responses and societal consequences of past climatic changes, however, demands highly-resolved, spatially-explicit, seasonally-defined and absolutely-dated archives over the entire period in question. Continuous efforts to improve existing proxy records and reconstruction methods and to develop new ones, as well as clear communication of all uncertainties (within and beyond academia) must be central tasks for the paleoclimate community.