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The Sun drives Earth’s energy systems, influencing weather, ocean currents, and agricultural productivity. Understanding solar variability is critical, but direct observations are limited to 400 years of sunspot records. To extend this timeline, cosmic ray-produced radionuclides like 14 C in tree-rings provide invaluable insights. However, few records have the resolution or temporal span required to thoroughly investigate important short-term solar phenomena, such as the 11-year solar cycle, or 14 C production spikes most likely linked to solar energetic particle (SEP) events. Here we present a continuous, annually resolved atmospheric 14 C record from tree-rings spanning the first millennium BCE, confirming no new SEP’s and clearly defining the 11-year solar cycle, with a mean period of 10.5 years, and amplitude of approximately 0.4‰ in 14 C concentration. This dataset offers unprecedented detail on solar behavior over long timescales, providing insights for climatic research and solar hazard mitigation, while also offering enhanced radiocarbon calibration and dating accuracy. An annually resolved tree ring radiocarbon record reveals the 11-year solar cycle in the first millennium BCE, improving understanding of past solar activity and enhancing radiocarbon dating accuracy.

The Sun is magnetically active and often produces eruptive events on different energetic and temporal scales. Until recently, the upper limit of such events was unknown and believed to be roughly represented by direct instrumental observations. However, two types of extreme events were discovered recently: extreme solar energetic particle events on the multi-millennial time scale and super-flares on sun-like stars. Both discoveries imply that the Sun might rarely produce events, called extreme solar events (ESE), whose energy could be orders of magnitude greater than anything we have observed during recent decades. During the years following these discoveries, great progress has been achieved in collecting observational evidence, uncovering new events, making statistical analyses, and developing theoretical modelling. The ESE paradigm lives and is being developed. On the other hand, many outstanding questions still remain open and new ones emerge. Here we present an overview of the current state of the art and the forming paradigm of ESE from different points of view: solar physics, stellar–solar projections, cosmogenic-isotope data, modelling, historical data, as well as terrestrial, technological and societal effects of ESEs. Special focus is paid to open questions and further developments. This review is based on the joint work of the International Space Science Institute (ISSI) team #510 (2020–2022).

During solar storms, the Sun expels large amounts of energetic particles (SEP) that can react with the Earth’s atmospheric constituents and produce cosmogenic radionuclides such as 14 C, 10 Be and 36 Cl. Here we present 10 Be and 36 Cl data measured in ice cores from Greenland and Antarctica. The data consistently show one of the largest 10 Be and 36 Cl production peaks detected so far, most likely produced by an extreme SEP event that hit Earth 9125 years BP (before present, i.e., before 1950 CE), i.e., 7176 BCE. Using the 36 Cl/ 10 Be ratio, we demonstrate that this event was characterized by a very hard energy spectrum and was possibly up to two orders of magnitude larger than any SEP event during the instrumental period. Furthermore, we provide 10 Be-based evidence that, contrary to expectations, the SEP event occurred near a solar minimum. Cosmogenic radionuclides from ice cores and tree rings indicate that an extreme solar proton event has hit Earth about 9200 years ago. Contrary to expectations, the event occurred during a quiet phase of the Sun within the 11 year solar cycle.

Adsorption is one of the most successful physicochemical approaches for removing heavy metal contaminants from polluted water. The use of residual biomass for the production of adsorbents has attracted a lot of attention due to its cheap price and environmentally friendly approach. The transformation of Sargassum—an invasive brown macroalga—into activated carbon (AC) via phosphoric acid thermochemical activation was explored in an effort to increase the value of Sargassum seaweed biomass. Several techniques (nitrogen adsorption, pHPZC, Boehm titration, FTIR and XPS) were used to characterize the physicochemical properties of the activated carbons. The SAC600 3/1 was predominantly microporous and mesoporous (39.6% and 60.4%, respectively) and revealed a high specific surface area (1695 m2·g−1). To serve as a comparison element, a commercial reference activated carbon with a large specific surface area (1900 m2·g−1) was also investigated. The influence of several parameters on the adsorption capacity of AC was studied: solution pH, solution temperature, contact time and Cr(VI) concentration. The best adsorption capacities were found at very acid (pH 2) solution pH and at lower temperatures. The adsorption kinetics of SAC600 3/1 fitted well a pseudo-second-order type 1 model and the adsorption isotherm was better described by a Jovanovic-Freundlich isotherm model. Molecular dynamics (MD) simulations confirmed the experimental results and determined that hydroxyl and carboxylate groups are the most influential functional groups in the adsorption process of chromium anions. MD simulations also showed that the addition of MgCl2 to the activated carbon surface before adsorption experiments, slightly increases the adsorption of HCrO4− and CrO42− anions. Finally, this theoretical study was experimentally validated obtaining an increase of 5.6% in chromium uptake.

The Sun sporadically produces eruptive events leading to intense fluxes of solar energetic particles (SEPs) that dramatically disrupt the near-Earth radiation environment. Such events have been directly studied for the last decades but little is known about the occurrence and magnitude of rare, extreme SEP events. Presently, a few events that produced measurable signals in cosmogenic radionuclides such as 14 C, 10 Be and 36 Cl have been found. Analyzing annual 14 C concentrations in tree-rings from Switzerland, Germany, Ireland, Russia, and the USA we discovered two spikes in atmospheric 14 C occurring in 7176 and 5259 BCE. The ~2% increases of atmospheric 14 C recorded for both events exceed all previously known 14 C peaks but after correction for the geomagnetic field, they are comparable to the largest event of this type discovered so far at 775 CE. These strong events serve as accurate time markers for the synchronization with floating tree-ring and ice core records and provide critical information on the previous occurrence of extreme solar events which may threaten modern infrastructure. Two extreme solar energetic particle events have been found by carbon isotopes measured in ancient tree rings in 7176 and 5259 BCE. The recorded ~2% increases of atmospheric 14 C for both events exceeds in amplitude of all previously observed events.

The Carrington event of 1859 has been the strongest solar flare in the observational history. It plays a crucial role in shedding light on the frequency and impacts of the past and future Solar Energetic Particle (SEP) events on human societies. We address the impact of the Carrington event by measuring tree‐ring 14C with multiple replications from high‐latitude locations around the event and by comparing them with mid‐latitude measurements. A transient offset in 14C following the event is observed with high statistical significance. Our state‐of‐the‐art 14C production and transport model does not reproduce the observational finding, suggesting features beyond present understanding. Particularly, our observation would require partially fast transport of 14C between the stratosphere and troposphere at high latitudes. The observation is consistent with the previous findings with the SEP events of 774 and 993 CE for which faster integration of 14C into tree rings is observed at high latitudes. Plain Language Summary Strong Earth‐directed solar eruptions can cause 14C concentration spikes in the atmosphere. Large enough events may leave a signal in the annually grown tree‐rings as they capture the isotopic carbon fingerprint through photosynthesis. Such rapid 14C increases have been detected, for instance, starting in years 774 and 993 CE. However, no increase has been observed following the Carrington event of 1859, despite it being the largest solar eruption of the modern era. Notably, all prior 14C measurements covering the Carrington event come from mid‐latitude trees. To achieve a broader geographical coverage, we have measured the event from several high‐latitude locations. After comparing the high‐ and mid‐latitude measurements, we have found a statistically significant difference lasting for several years post‐Carrington. To better understand the difference, we have adopted a 14C production and atmospheric transport model capable of simulating regional differences. Despite the improved model, we found it unable to reproduce the observational results, which suggests features beyond current understanding. Ultimately, the observation emphasizes the role of subtle 14C differences in tree‐ring 14C studies, potentially opening new ways to study past solar phenomena and atmospheric dynamics. Key Points A transient offset in 14C from high‐latitude Finnish Lapland tree rings was observed between years 1861 and 1863 The Carrington event and the stratosphere‐troposphere dynamics are discussed as potential explanations for the disparity The discovery underscores the importance of transient offsets, high‐latitude tree rings in spotting anomalous solar/geomagnetic phenomena

A Correction to this paper has been published: https://doi.org/10.1038/s41467-021-21647-w

Many studies have investigated the role of solar variability in Holocene climate. Beyond sunspot observations, solar activity can be reconstructed from 14C in tree rings. Due to the lack of sub-decadal resolution of 14C records, these studies focused on long-term processes. In this study, we use an annually-resolved 14C record to examine solar variability (e.g. 11-year Schwabe solar cycle) and its connection to European seasonal climate inferred from tree-ring records during the entire past millennium with spectral and wavelet techniques. The 11-year Schwabe solar cycle shows a significant impact in European moisture- and temperature-sensitive tree-ring records. Complex 'top-down'/'bottom-up' effects in the strato-tropoatmospheric system are assumed to affect European spring and summer climate with a temporal-shift as evident from observed changes in phase behavior. Significant evidence is also found for the ∼60- and ∼90-year band during the first half of the past millennium.

The Sun provides the principal energy input into the Earth system and solar variability represents a significant external climate forcing. Although observations of solar activity (sunspots) cover only the last about 400 years, radionuclides produced by cosmic rays and stored in tree rings or ice cores serve as proxies for solar activity extending back thousands of years. However, the presence of weather-induced noise or low temporal resolution of long, precisely dated records hampers cosmogenic nuclide-based studies of short-term solar variability such as the 11-yr Schwabe cycle. Here we present a continuous, annually resolved atmospheric 14 C concentration (fractionation-corrected ratio of 14 CO 2 to CO 2 ) record reconstructed from absolutely dated tree rings covering nearly all of the last millennium ( ad 969–1933). The high-resolution and precision 14 C record reveals the presence of the Schwabe cycle over the entire time range. The record confirms the ad 993 solar energetic particle event and reveals two new candidates ( ad 1052 and ad 1279), indicating that strong solar events that might be harmful to modern electronic systems probably occur more frequently than previously thought. In addition to showing decadal-scale solar variability over the last millennium, the high-temporal-resolution record of atmospheric radiocarbon also provides a useful benchmark for making radiocarbon dating more accurate over this interval. 11-year solar cycles consistently occurred throughout the last thousand years, according to a synthesis of annually resolved tree ring radiocarbon records from central Europe.