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The Cryosphere

Observed snow depth trends in the European Alps: 1971 to 2019

Here, we present an Alpine-wide analysis of snow depth from six Alpine countries – Austria, France, Germany, Italy, Slovenia, and Switzerland – including altogether more than 2000 stations of which more than 800 were used for the trend assessment. Linear trends of monthly mean snow depth between1971 and 2019 showed decreases in snow depth for most stations from November to May.

Earth Surface Dynamics

The enigma of relict large sorted stone stripes in the tropical Ethiopian Highlands

Large forms of sorted patterned ground belong to the most prominent geomorphic features of periglacial and permafrost environments of the mid-latitudes and polar regions, but they were hitherto unknown in the tropics. Here, we report on relict large sorted stone stripes (up to 1000 m long, 15 m wide, and 2 m deep) on the ca. 4000 m high central Sanetti Plateau of the tropical Bale Mountains in the southern Ethiopian Highlands.

Geochronology

geoChronR – an R package to model, analyze, and visualize age-uncertain data

Chronological uncertainty is a hallmark of the paleoenvironmental sciences and geosciences. While many tools have been made available to researchers to quantify age uncertainties suitable for various settings and assumptions, disparate tools and output formats often discourage integrative approaches. Here, we describe geoChronR, an open-source R package to facilitate these tasks.

Atmospheric Chemistry and Physics

Sensitivities to biological aerosol particle properties and ageing processes: potential implications for aerosol–cloud interactions and optical properties

Primary biological aerosol particles (PBAPs), such as bacteria, viruses, fungi, and pollen, represent a small fraction of the total aerosol burden. Given that PBAPs are highly efficient atmospheric ice nuclei (IN) at T > -10 ∘C, we suggest that small changes in their sizes or surface properties due to chemical, physical, or biological processing might translate into large impacts on ice initiation in clouds.

Atmospheric Measurement Techniques

High-resolution optical constants of crystalline ammonium nitrate forinfrared remote sensing of the Asian Tropopause Aerosol Layer

In this work, we have retrieved the first low-temperature data set of optical constants for crystalline AN in the 800–6000 cm-1 wave number range with a spectral resolution of 0.5 cm-1. We show that our data accurately fit aircraft-borne infrared measurements of ammonium nitrate particles in the upper tropospheric aerosol layer, which is formed during the Asian summer monsoon period.

Natural Hazards and Earth System Sciences

A statistical–parametric model of tropical cyclones for hazard assessment

Natural Hazards and Earth System Sciences
DOI 10.5194/nhess-21-893-2021
30 March 2021

We present the formulation of an open-source, statistical–parametric model of tropical cyclones (TCs) for use in hazard and risk assessment applications. We demonstrate the capability of the model to replicate observed behaviour of TCs, including coastal landfall rates which are of significant importance for risk assessments.

Weather and Climate Dynamics

A global climatological perspective on the importance of Rossby wavebreaking and intense moisture transport for extreme precipitation events

Extreme precipitation events (EPEs) frequently cause flooding with dramatic socioeconomic impacts in many parts of the world.Previous studies considered two synoptic-scale processes, Rossby wave breaking and intense moisture transport, typically in isolation, and their linkage to such EPEs in several regions. This study presents for the first time a global and systematic climatological analysis of these two synoptic-scale processes, in tandem and in isolation, for the occurrence of EPEs.

Annales Geophysicae

Characteristics of fragmented aurora-like emissions (FAEs) observed on Svalbard

This study analyses the observations of a new type of small-scale aurora-like feature, which is further referred to as fragmented aurora-like emission(s) (FAEs). One possible explanation for this is Farley–Buneman instabilities of strong local currents. In the present study, we provide an overview of the observations and discuss their characteristics and potential generation mechanisms.

Atmospheric Chemistry and Physics

Model physics and chemistry causing intermodel disagreement within the VolMIP-Tambora Interactive Stratospheric Aerosol ensemble

As part of the Model Intercomparison Project on the climatic response to Volcanic forcing (VolMIP), several climate modeling centers performed a coordinated pre-study experiment with interactive stratospheric aerosol models simulating the volcanic aerosol cloud from an eruption resembling the 1815 Mt. Tambora eruption (VolMIP-Tambora ISA ensemble).

Atmospheric Measurement Techniques

Comparison of ozone measurement methods in biomass burning smoke: anevaluation under field and laboratory conditions

In recent years wildland fires in the United States have had significant impacts on local and regional air quality and negative human health outcomes. Although the primary health concerns from wildland fires come from fine particulate matter (PM2.5), large increases in ozone (O3 have been observed downwind of wildland fire plumes. The chemiluminescence FRM method is highly recommended for accurate measurements of O3 in wildland fire plume studies and at regulatory ambient monitoring sites frequently impacted by wildland firesmoke.

Climate of the Past

Cryptotephra from the Icelandic Veiðivötn 1477 CE eruption in a Greenland ice core: confirming the dating of volcanic events in the 1450s CE and assessing the eruption’s climatic impact

Volcanic eruptions are a key source of climatic variability, and reconstructing their past impact can improve our understanding of the operation of the climate system and increase the accuracy of future climate projections. While many chronological mismatches have been resolved, the precise timing and climatic impact of two major sulfate-emitting volcanic eruptions during the 1450s  CE, including the largest atmospheric sulfate-loading event in the last 700 years, have not been constrained. Here we explore this issue through a combination of tephrochronological evidence and high-resolution ice-core chemistry measurements from a Greenland ice core, the TUNU2013 record.

The Cryosphere

The case of a southern European glacier which survived Roman and medieval warm periods but is disappearing under recent warming

We present here the first chronological study of a glacier located in the Central Pyrenees (NE Spain), Monte Perdido Glacier (MPG),carried out by different radiochronological techniques and a comparison with geochemical proxies from neighbouring palaeoclimate records. The chronological model evidences that the glacier persisted during the Roman period and the Medieval Climate Anomaly. The apparent absence of ice in the past∼ 600 years suggests that any ice accumulated during the Little Ice Age has since ablated.

The Cryosphere

Diverging responses of high-latitude CO2 and CH4 emissions in idealized climate change scenarios

The present study investigates the response of the high-latitude carbon cycle to changes in atmospheric greenhouse gas (GHG) concentrations in idealized climate change scenarios. We find that at a global mean temperature of roughly 1.75 K (±0.5 K) above pre-industrial levels the high-latitude ecosystem turns from a CO2 sink into a source of atmospheric carbon, with the net fluxes into the atmosphere increasing substantially with rising atmospheric GHG concentrations. This is very different from scenario simulations with the standard version of the MPI-ESM, in which the region continues to take up atmospheric CO2 throughout the entire 21st century.

Earth System Dynamics

A dynamical systems characterization of atmospheric jet regimes

Atmospheric jet streams are typically separated into primarily “eddy-driven” (or polar-front) jets and primarily “thermally driven” (or subtropical) jets. Here, we link the current understanding of dynamical jet maintenance mechanisms, mostly arising from conceptual or idealized models, to the phenomena observed in reanalysis data.

Climate of the Past

Lower oceanic δ13C during the last interglacial period compared to the Holocene

The last time in Earth’s history when high latitudes were warmer than during pre-industrial times was the last interglacial period (LIG, 129–116 ka  BP). Since the LIG is the most recent and best documented interglacial, it can provide insights into climate processes in a warmer world. However, some key features of the LIG are not well constrained, notably the oceanic circulation and the global carbon cycle. Here, we use a new database of LIG benthic δ13C to investigate these two aspects.

The Cryosphere

Mapping avalanches with satellites – evaluation of performance andcompleteness

The spatial distribution and size of avalanches are essential parameters for avalanche warning, avalanche documentation, mitigation measure design and hazard zonation. Despite its importance, this information is incomplete today and only available for limited areas and limited time periods.

Atmospheric Chemistry and Physics

A-Train estimates of the sensitivity of the cloud-to-rainwater ratio to cloud size, relative humidity, and aerosols

Precipitation efficiency has been found to play an important role in constraining the sensitivity of the climate through its role in controlling cloud cover, yet its controls are not fully understood. Here we use CloudSat observations to identify individual contiguous shallow cumulus cloud objects and compute the ratio of cloud water path to rainwater (WRR) path as a proxy for warm-rain efficiency.

Geoscientific Model Development

Development of a MetUM (v 11.1) and NEMO (v 3.6) coupled operational forecastmodel for the Maritime Continent – Part 1: Evaluation of ocean forecasts

This article describes the development and ocean forecast evaluation of an atmosphere–ocean coupled prediction system for the Maritime Continent (MC) domain, which includes the eastern Indian and western Pacific oceans. Overall, the model forecast deviation of SST, SSH, and subsurface temperature and salinity fields relative to observation is within acceptable error limits of operational forecast models.

Climate of the Past

Long-term global ground heat flux and continental heat storage from geothermal data

Here, we provide new global estimates of changes in ground surface temperature, ground surface heat flux, and continental heat storage derived from geothermal data using an expanded database and new techniques.

Natural Hazards and Earth System Sciences

Radar-based assessment of hail frequency in Europe

Natural Hazards and Earth System Sciences
DOI 10.5194/nhess-21-683-2021
10 March 2021

In this study we present a unique 10  year climatology of severe convective storm tracks for a large European area covering Germany, France, Belgium and Luxembourg. For the period 2005–2014, a high-resolution hail potential composite of 1×1 km2 is produced from two-dimensional radar reflectivity and lightning data.

Climate of the Past

In situ cosmogenic 10Be–14C–26Al measurements from recently deglaciated bedrock as a new tool to decipher changes in Greenland Ice Sheet size

Sometime during the middle to late Holocene (8.2 ka to ∼ 1850–1900 CE), the Greenland Ice Sheet (GrIS) was smaller than its current configuration. Contemporary retreat of the GrIS from its historical maximum extent in southwestern Greenland is exposing a landscape that holds clues regarding the configuration and timing of past ice-sheet minima.

Biogeosciences

Reviews and syntheses: Impacts of plant-silica–herbivore interactions onterrestrial biogeochemical cycling

Researchers have known for decades that silicon plays a major role in biogeochemical and plant–soil processes in terrestrial systems. We review and synthesize 119 available studies directly investigating silicon and herbivory to summarize key trends and highlight research gaps and opportunities.

Ocean Science

Response of tidal flow regime and sediment transport in North Malé Atoll, Maldives, to coastal modification and sea level rise

Changes to coastlines and bathymetry alter tidal dynamics and associated sediment transport processes, impacting upon a number of threats facing coastal regions, including flood risk and erosion. Especially vulnerable are coral atolls such as those that make up the Maldives archipelago, which has undergone significant land reclamation in recent years and decades and is also particularly exposed to sea level rise.

Earth System Dynamics

Assessment of a full-field initialized decadal climate prediction system with the CMIP6 version of EC-Earth

In this paper, we present and evaluate the skill of an EC-Earth 3.3 decadal prediction system contributing to the Decadal Climate Prediction Project – Component A (DCPP-A). This prediction system is capable of skilfully simulating past global mean surface temperature variations at interannual and decadal forecast times as well as the local surface temperature in regions such as the tropical Atlantic, the Indian Ocean and most of the continental areas, although most of the skill comes from the representation of the external radiative forcings.

Hydrology and Earth System Sciences

Vapor plumes in a tropical wet forest: spotting the invisible evaporation

Forest evaporation exports a vast amount of water vapor from land ecosystems into the atmosphere. This work describes the formation process of vapor plumes in a tropical wet forest as evidence of evaporation processes happening during rain events.

Atmospheric Chemistry and Physics

Low-NO atmospheric oxidation pathways in a polluted megacity

The impact of emissions of volatile organic compounds (VOCs) to the atmosphere on the production of secondary pollutants, such as ozone and secondary organic aerosol (SOA), is mediated by the concentration of nitric oxide (NO). Polluted urban atmospheres are typically considered to be high-NO” environments, while remote regions such as rainforests, with minimal anthropogenic influences, are considered to be “low NO”. However, our observations from central Beijing show that this simplistic separation of regimes is flawed.

Biogeosciences

A limited effect of sub-tropical typhoons on phytoplankton dynamics

Typhoons are assumed to stimulate primary ocean production through the upward mixing of nutrients into the ocean surface. This assumption is based largely on observations of increased surface chlorophyll concentrations following the passage of typhoons. This surface chlorophyll enhancement, occasionally detected by satellites, is often undetected due to intense cloud coverage.

Geoscience Communication

Volcanoes in video games: the portrayal of volcanoes incommercial off-the-shelf (COTS) video games and their learning potential

Volcanoes are a very common staple in mainstream video games. Particularly within the action–adventure genres, entire missions or even full storylines can require players to traverse an active volcano. Therefore, players can spend a lot of time immersed within the detailed graphics and unknowingly learn about volcanic traits while playing.

Atmospheric Chemistry and Physics

Mass accommodation and gas–particle partitioning in secondary organicaerosols: dependence on diffusivity, volatility, particle-phase reactions,and penetration depth

Mass accommodation is an essential process for gas–particle partitioning of organic compounds in secondary organic aerosols (SOA). The mass accommodation coefficient is commonly described as the probability of a gas molecule colliding with the surface to enter the particle phase. It is often applied, however, without specifying if and how deep a molecule has to penetrate beneath the surface to be regarded as being incorporated into the condensed phase (adsorption vs. absorption).

Earth System Dynamics

A new view of heat wave dynamics and predictability over the easternMediterranean

Skillful forecasts of extreme weather events have a major socioeconomic relevance. Here, we compare two complementary approaches to diagnose the predictability of extreme weather: recent developments in dynamical systems theory and numerical ensemble weather forecasts.

Atmospheric Chemistry and Physics

An overview of the ORACLES (ObseRvations of Aerosols above CLouds and theirintEractionS) project: aerosol–cloud–radiation interactions in the southeastAtlantic basin

Southern Africa produces almost a third of the Earth’s biomass burning (BB) aerosol particles, yet the fate of these particles and their influence on regional and global climate is poorly understood. ORACLES (ObseRvations of Aerosols above CLouds and their intEractionS) is a 5-year NASA EVS-2 (Earth Venture Suborbital-2) investigation with three intensive observation periods designed to study key atmospheric processes that determine the climate impacts of these aerosols.

Earth System Dynamics

Synchronized spatial shifts of Hadley and Walker circulations

Here, by examining the spatiotemporal relationship between Hadley and Walker cells in observations and climate model experiments, we demonstrate that the seasonally evolving warm-pool sea surface temperature (SST) anomalies in the decay phase of an El Niño event generate a meridionally asymmetric Walker circulation response, which couples the zonal and meridional atmospheric overturning circulations.

Natural Hazards and Earth System Sciences

A revision of the Combined Drought Indicator (CDI)used in the European Drought Observatory (EDO)

Natural Hazards and Earth System Sciences
DOI 10.5194/nhess-21-481-2021
19 February 2021

Building on almost 10 years of expertise and operational application of the Combined Drought Indicator (CDI), which is implemented within the European Commission’s European Drought Observatory (EDO) for the purposes of early warning and monitoring of agricultural droughts in Europe, this paper proposes a revised version of the index.

Ocean Science

The transient sensitivity of sea level rise

Ocean Science
DOI 10.5194/os-17-181-2021
18 February 2021

Here, we introduce a similar metric for sea level response. We find that future projections estimated on climate model responses fall below extrapolation based on recent observational records.This comparison suggests that the likely upper level of sea level projections in recent IPCC reports would be too low.

Geoscientific Model Development

Coordinating an operational data distribution network for CMIP6 data

The distribution of data contributed to the Coupled Model Intercomparison Project Phase 6 (CMIP6) is via the Earth System Grid Federation (ESGF). The ESGF is a network of internationally distributed sites that together work as a federated data archive.

Climate of the Past

Greenland climate simulations show high Eemian surface meltwhich could explain reduced total air content in ice cores

This study presents simulations of Greenland surface melt for the Eemian interglacial period (∼130,000 to 115, 000 years ago) derived from regional climate simulations with a coupled surface energy balance model.

Climate of the Past

The Eocene–Oligocene transition: a review of marine and terrestrial proxy data, models and model–data comparisons

We find that CO2 forcing involving a large decrease in CO2 of ca. 40 % (∼325 ppm drop) provides the best fit to the available proxy evidence,with ice sheet and palaeogeographic changes playing a secondary role.

The Cryosphere

Macroscopic water vapor diffusion is not enhanced in snow

The Cryosphere
DOI 10.5194/tc-15-389-2021
12 February 2021

Here we show using theory and numerical simulations of idealized and measured snow microstructures that, although sublimation and deposition of water vapor onto snow crystal surfaces do enhance microscopic diffusion in the pore space, this effect is more than countered by the restriction of diffusion space due to ice.

Geoscientific Instrumentation, Methods and Data Systems

Experiments on magnetic interference for a portable airborne magnetometry system using a hybrid unmanned aerial vehicle (UAV)

Geoscientific Instrumentation, Methods and Data Systems
DOI 10.5194/gi-10-25-2021
11 February 2021

An experiment concerning the static magnetic interference of the UAV was conducted to assess the severity of the interference of a hybrid vertical take-off and landing (VTOL) UAV.

Climate of the Past

Last Glacial Maximum (LGM) climate forcing and ocean dynamical feedback and their implications for estimating climate sensitivity

Here we conduct a suite of Last Glacial Maximum (LGM) simulations using the Community Earth System Model version 1.2 (CESM1.2) to quantify the forcing and efficacy of land ice sheets (LISs) and greenhouse gases (GHGs) in order to estimate equilibrium climate sensitivity.

The Cryosphere

Review article: Earth’s ice imbalance

We combine satellite observations and numerical models to show that Earth lost 28 trillion tonnes of ice between 1994 and 2017.

Natural Hazards and Earth System Sciences

A methodology for attributing the role of climate change in extreme events: a global spectrally nudged storyline

Natural Hazards and Earth System Sciences
DOI 10.5194/nhess-21-171-2021
8 February 2021

Extreme weather events are generally associated with unusual dynamical conditions, yet the signal-to-noise ratio of the dynamical aspects of climate change that are relevant to extremes appears to be small, and the nature of the change can be highly uncertain.

Atmospheric Chemistry and Physics

Opinion: Cloud-phase climate feedback and the importance of ice-nucleatingparticles

Shallow clouds covering vast areas of the world’s middle- and high-latitude oceans play a key role in dampening the global temperaturerise associated with CO2. These clouds, which contain both ice andsupercooled water, respond to a warming world by transitioning to a statewith more liquid water and a greater albedo, resulting in a negative“cloud-phase” climate feedback component. Here we argue that the magnitudeof the negative cloud-phase feedback component depends on the amount andnature of the small fraction of aerosol particles that can nucleate icecrystals. We propose that a concerted research effort is required to reducesubstantial uncertainties related to the poorly understoodsources, concentration, seasonal cycles and nature of these ice-nucleatingparticles (INPs) and their rudimentary treatment in climate models. Thetopic is important because many climate models may have overestimated themagnitude of the cloud-phase feedback, and those with better representationof shallow oceanic clouds predict a substantially larger climate warming. Wemake the case that understanding the present-day INP population in shallowclouds in the cold sector of cyclone systems is particularly critical fordefining present-day cloud phase and therefore how the clouds respond towarming. We also need to develop a predictive capability for future INPemissions and sinks in a warmer world with less ice and snow and potentiallystronger INP sources.

Atmospheric Measurement Techniques

ModIs Dust AeroSol (MIDAS): a global fine-resolution dust optical depth data set

Monitoring and describing the spatiotemporal variability in dust aerosols is crucial for understanding their multiple effects, related feedbacks, and impacts within the Earth system. This study describes the development of the ModIs Dust AeroSol (MIDAS) data set. MIDAS provides columnar daily dust optical depth (DOD) at 550 nm at a global scale and fine spatial resolution (0.1∘ × 0.1∘) over a 15-year period (2003–2017). This new data set combines quality filtered satellite aerosol optical depth (AOD) retrievals from MODIS-Aqua at swath level (Collection 6.1; Level 2), along with DOD-to-AOD ratios provided by the Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA-2) reanalysis to derive DOD on the MODIS native grid. The uncertainties of the MODIS AOD and MERRA-2 dust fraction, with respect to the AEronet RObotic NETwork (AERONET) and LIdar climatology of vertical Aerosol Structure for space-based lidar simulation (LIVAS), respectively, are taken into account for the estimation of the total DOD uncertainty. MERRA-2 dust fractions are in very good agreement with those of LIVAS across the dust belt in thetropical Atlantic Ocean and the Arabian Sea; the agreement degrades in North America and the Southern Hemisphere, where dust sources are smaller. MIDAS, MERRA-2, and LIVAS DODs strongly agree when it comes to annual and seasonal spatial patterns, with colocated global DOD averages of 0.033, 0.031, and 0.029, respectively; however, deviations in dust loading are evident and regionally dependent. Overall, MIDAS is well correlated with AERONET-derived DODs (R=0.89) and only shows a small positive bias (0.004 or 2.7 %). Among the major dust areas of the planet, the highest R values (>0.9) are found at sites of North Africa, the Middle East, and Asia. MIDAS expands, complements, and upgrades the existing observational capabilities of dust aerosols, and it is suitable for dust climatological studies, model evaluation, and data assimilation.

Atmospheric Chemistry and Physics

On the ice-nucleating potential of warm hydrometeors in mixed-phase clouds

The question as to whether or not the presence of warm hydrometeors in clouds mayplay a significant role in the nucleation of new ice particles has been debatedfor several decades. While the early works of and indicated that it might be irrelevant, the more recentstudy of suggested otherwise. In this work, weattempt to quantify the ice-nucleating potential using high-fidelity flowsimulation techniques around a single hydrometeor and use favorable considerationsto upscale the effects to a collective of ice particles in clouds. While we find that ice nucleationmay be significantly enhanced in the vicinity of a warm hydrometeorand that the affected volume of air is much larger than previously estimated, it isunlikely that this effect alone causes the rapidenhancement of ice nucleation observed in some types of clouds, mainly due to the lowinitial volumetric ice concentration. Furthermore, it is demonstrated that the excess nucleationrate does not primarily depend on the rate at which cloud volume is sampled by the meteors’wakes but is rather limited by the exposure time of ice-nucleating particles to the wake,which is estimated to be of the order of few microseconds. It is suggested to further investigatethis phenomenon by tracking the trajectories of ice-nucleating particles in order to obtaina parametrization which can be implemented into existing cloud models to investigate second-order effectssuch as ice enhancement after the onset of glaciation.

Atmospheric Chemistry and Physics

Stratospheric aerosol layer perturbation caused by the 2019 Raikoke and Ulawun eruptions and their radiative forcing

In June 2019 a stratospheric eruption occurred at Raikoke (48∘ N,153∘ E). Satellite observations show the injection of ash and SO2 into the lower stratosphere and an early entrainment of the plume into a cyclone. Following the Raikoke eruption, stratospheric aerosol optical depth (sAOD) values increased in the whole Northern Hemisphere and tropics and remained enhanced for more than 1 year, with peak values at 0.040 (short-wavelength, high northern latitudes) to 0.025 (short-wavelength, Northern Hemisphere average). Discrepancies between observations and global model simulations indicate that ash may have influenced the extent and evolution of the sAOD. Top of the atmosphere radiative forcings are estimated at values between -0.3 and -0.4Wm-2 (clear-sky) and of -0.1 to -0.2Wm-2 (all-sky), comparable to what was estimated for the Sarychev eruption in 2009. Almost simultaneously two significantly smaller stratospheric eruptions occurred at Ulawun (5∘ S, 151∘ E) in June and August. Aerosol enhancements from the Ulawun eruptions mainly had an impact on the tropics and Southern Hemisphere. The Ulawun plume circled the Earth within 1 month in the tropics. Peak shorter-wavelength sAOD values at 0.01 are found in the tropics following the Ulawun eruptions and a radiative forcing not exceeding -0.15 (clear-sky) and -0.05 (all-sky). Compared to the Canadian fires (2017), Ambae eruption (2018), Ulawun (2019) and the Australian fires (2019/2020), the highest sAOD and radiative forcing values are found for the Raikoke eruption.

Biogeosciences

Fire and vegetation dynamics in northwest Siberia during the last 60 years based on high-resolution remote sensing

The rapidly warming Arctic undergoes transitions that can influence global carbon balance. One of the key processes is the shift towards vegetation types with higher biomass underlining a stronger carbon sink. The shift is predicted by bioclimatic models based on abiotic climatic factors, but it is not always confirmed with observations. Recent studies highlight the role of disturbances in the shift. Here we use high-resolution remote sensing to study the process of transition from tundra to forest and its connection to wildfires in the 20 000 km2 area in northwest Siberia. Overall, 40 % of the study area was burned during a 60-year period. Three-quarters of the burned areas were dry tundra. About 10 % of the study area experienced two–three fires with an interval of 15–60 years suggesting a shorter fire return interval than that reported earlier for the northern areas of central Siberia (130–350 years). Based on our results, the shift in vegetation (within the 60-year period) occurred in 40 %–85 % of the burned territories. All fire-affected territories were flat; therefore no effect of topography was detected. Oppositely, in the undisturbed areas, a transition of vegetation was observed only in 6 %–15 % of the territories, characterized by steeper topographic slopes. Our results suggest a strong role of disturbances in the tree advance in northwest Siberia.

Biogeosciences

Millennial-age glycerol dialkyl glycerol tetraethers (GDGTs) in forested mineral soils: 14C-based evidence for stabilization of microbial necromass

Understanding controls on the persistence of soil organic matter (SOM) is essential to constrain its role in the carbon cycle and inform climate–carbon cycle model predictions. Emerging concepts regarding the formation and turnover of SOM imply that it is mainly comprised of mineral-stabilized microbial products and residues; however, direct evidence in support of this concept remains limited. Here, we introduce and test a method for the isolation of isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGTs) – diagnostic membrane lipids of archaea and bacteria, respectively – for subsequent natural abundance radiocarbon analysis. The method is applied to depth profiles from two Swiss pre-Alpine forested soils. We find that the Δ14C values of these microbial markers markedly decrease with increasing soil depth, indicating turnover times of millennia in mineral subsoils. The contrasting metabolisms of the GDGT-producing microorganisms indicates it is unlikely that the low Δ14C values of these membrane lipids reflect heterotrophic acquisition of 14C-depleted carbon. We therefore attribute the 14C-depleted signatures of GDGTs to their physical protection through association with mineral surfaces. These findings thus provide strong evidence for the presence of stabilized microbial necromass in forested mineral soils.

Biogeosciences

Increased carbon capture by a silicate-treated forested watershed affected by acid deposition

Meeting internationally agreed-upon climate targets requirescarbon dioxide removal (CDR) strategies coupled with an urgent phase-down offossil fuel emissions. However, the efficacy and wider impacts of CDR arepoorly understood. Enhanced rock weathering (ERW) is a land-based CDRstrategy requiring large-scale field trials. Here we show that a low 3.44 t ha-1 wollastonite treatment in an 11.8 ha acid-rain-impacted forested watershed in New Hampshire, USA, led to cumulative carbon capture by carbonic acid weathering of 0.025–0.13 t CO2 ha-1 over 15 years. Despite a 0.8–2.4 t CO2 ha-1 logistical carbon penalty from mining,grinding, transportation, and spreading, by 2015 weathering together withincreased forest productivity led to net CDR of 8.5–11.5 t CO2 ha-1. Our results demonstrate that ERW may be an effective, scalableCDR strategy for acid-impacted forests but at large scales requiressustainable sources of silicate rock dust.

Hydrology and Earth System Sciences

Intercomparison of freshwater fluxes over ocean and investigations into water budget closure

The net exchange of water between the surface and atmosphere is mainly determined by the freshwater flux: the difference between evaporation ( E ) and precipitation ( P ), or E−P . Although there is consensus among modelers that with a warming climate E−P will increase, evidence from satellite data is still not conclusive, mainly due to sensor calibration issues. We here investigate the degree of correspondence among six recent
satellite-based climate data records and ERA5 reanalysis E−P data.