Climate Forcing Research Articles

Biogeochemical responses to global warming during the Paleocene–Eocene Thermal Maximum in the eastern Tethys

The Paleocene–Eocene Thermal Maximum (PETM; ∼56 Ma) represents a rapid and sustained climatic perturbation that lasted for ∼170 kyr and coincided with the emplacement of the North Atlantic Igneous Province (NAIP). Global average temperature increased by 5 to 9 °C, which was likely triggered by a rapid emission of 13C-depleted CO2. Numerous sites from the deep oceans have been studied for the PETM, but subtropical shallow-water eastern Tethys remains poorly understood despite its importance in regulating heat and moisture transport. Prior studies on calcareous nannofossil biostratigraphy and stable isotopes of marine carbonates confirmed the occurrence of the PETM in the Tarim Basin of the eastern Tethys in northwestern China. Here we present new, high–resolution major, trace and rare earth element geochemical data, and clay mineral records at the Kuzigongsu section in the Tarim Basin to assess the biogeochemical responses of the eastern Tethys to climatic forcings across the PETM. Chemical weathering proxies (chemical index of alteration [CIA], chemical index of weathering [CIW], plagioclase index of alteration [PIA], and Rb/Sr) support an enhanced terrestrial input shortly after the PETM onset, possibly due to intensified chemical weathering. Furthermore, clay mineral assemblage is dominated by illite and illite/smectite during the PETM, implying increased physical weathering. The PETM is also characterized by an increase in nutrient-sensitive elements such as P, Ni, and Cu and higher abundance of nutrient–sensitive calcareous nannofossils, suggesting an intensification of marine primary productivity. Meanwhile, the Tarim Basin may have encountered episodic bottom water deoxygenation supported by an increase in the enrichment factor of redox–sensitive elements such as U and V. The integrated geochemical proxy records suggest that the eastern Tethys has encountered profound ecosystem stress imposed by elevated nutrient fluxes and ocean deoxygenation, which may have been amplified by simultaneously intensified chemical and physical weathering during the PETM.

Anomalous Meltwater From Ice Sheets and Ice Shelves Is a Historical Forcing

AbstractRecent mass loss from ice sheets and ice shelves is now persistent and prolonged enough that it impacts downstream oceanographic conditions. To demonstrate this, we use an ensemble of coupled GISS‐E2.1‐G simulations forced with historical estimates of anomalous freshwater, in addition to other climate forcings, from 1990 through 2019. There are detectable differences in zonal‐mean sea surface temperatures (SST) and sea ice in the Southern Ocean, and in regional sea level around Antarctica and in the western North Atlantic. These impacts mostly improve the model's representation of historical changes, including reversing the forced trends in Antarctic sea ice. The changes in SST may have implications for estimates of the SST pattern effect on climate sensitivity and for cloud feedbacks. We conclude that the changes are sufficiently large that model groups should strive to include more accurate estimates of these drivers in all‐forcing historical simulations in future coupled model intercomparisons.

Orbital CO2 reconstruction using boron isotopes during the late Pleistocene, an assessment of accuracy

Abstract. Boron isotopes in planktonic foraminifera are a widely used proxy to determine ancient surface seawater pH and by extension atmospheric CO2 concentration and climate forcing on geological timescales. Yet, to reconstruct absolute values for pH and CO2, we require a δ11Bforam-borate to pH calibration and independent determinations of ocean temperature, salinity, a second carbonate parameter, and the boron isotope composition of seawater. Although δ11B-derived records of atmospheric CO2 have been shown to perform well against ice-core-based CO2 reconstructions, these tests have been performed at only a few locations and with limited temporal resolution. Here we present two highly resolved CO2 records for the late Pleistocene from Ocean Drilling Program (ODP) Sites 999 and 871. Our δ11B-derived CO2 record shows a very good agreement with the ice core CO2 record with an average offset of 13±46 (2σ) and an RMSE of 26 ppm, with minor short-lived overestimations of CO2 (of up to ∼50 ppm) occurring during some glacial onsets. We explore potential drivers of this disagreement and conclude that partial dissolution of foraminifera has a minimal effect on the CO2 offset. We also observe that the general agreement between δ11B-derived and ice core CO2 is improved by optimising the δ11Bforam-borate calibration. Despite these minor issues, a strong linear relationship between relative change in climate forcing from CO2 (from ice core data) and pH change (from δ11B) exists over the late Pleistocene, confirming that pH change is a robust proxy of climate forcing over relatively short (<1 million year) intervals. Overall, these findings demonstrate that the boron isotope proxy is a reliable indicator of CO2 beyond the reach of the ice cores and can help improve determinations of climate sensitivity for ancient time intervals.

Resolution of the Tetrabothrius jagerskioeldi Cryptic Species Complex among Holarctic Alcidae (Charadriiformes): Cestodes among Fraterculinae—Exploring Marine Diversity, Host Range, and Dynamic Oceanography in the Greater North Pacific

In the biosphere, limits for diversity among species, communities, and biomes are revealed through intensive and extensive field-based inventory and assembly of voucher specimens and associated informatics examined in a phylogenetic, historical, ecological, and biogeographic arena. Archival resources for specimens and information contribute to a cumulative view of faunal structure and assembly under a comparative umbrella. Ultimately, species definitions, and inclusive partitions among populations and lineages, are fundamental in articulating hypotheses that examine interactions about evolution, the nature of organisms, and the condition of environments across space and time. We conclude our proposals establishing species limits for tapeworms of the cryptic complex historically accommodated in Tetrabothrius jagerskioeldi Nybelin, 1916 among Alcidae seabirds (Charadriiformes). Explorations of this facet of marine diversity summarize inventory data for species of Tetrabothrius Rudolphi, 1819 from field collections among 1,976 seabirds of 41 species representing 3 avian orders (Charadriiformes, Suliformes, Procellariiformes) examined across 58 oceanic/geographic localities from the greater North Pacific basin between 1949 and 2019, or over the past 70 years. Cestodes of the complex including T. jagerskioeldi sensu stricto, T. alcae Hoberg and Soudachanh, 2021, and T. sinistralis Hoberg and Soudachanh, 2021, along with 2 previously unrecognized taxa, are documented, occurring in 128 of 1976 seabirds examined (6%) and 17 of 41 marine avian species from 23 insular, coastal, and pelagic sites spanning the North Pacific. In completing an evaluation of this assemblage, we focus on those cestodes among 8 species of medium- to small-bodied alcids, subfamily Fraterculinae. Specimens designated as Tetrabothrius fraterculus n. sp. were observed among puffins (2 species of Fraterculus and Cerorhinca), whereas Tetrabothrius aithuia n. sp. is proposed for tapeworms in auklets (4 species of Aethia and Ptychoramphus) and a puffin (Cerorhinca); both cestodes are currently unknown among the Alcinae species of Cepphus, Uria, Brachyramphus, and Synthliboramphus based on the North Pacific inventory collections. These large-bodied cestodes, typical of the 5 species in the complex, are characterized among 46 currently valid species of Tetrabothrius in avian hosts, based on unique configurations of the genital atrium; male and female genital papillae; terminal genital ducts; numbers of testes; and the structure, position, and dimensions of the vaginal and atrial seminal receptacles. Ancillary characters contributing to differentiation include attributes of the scolex, structure, dimensions, and position of the vitelline gland and relative position of the dorsal and ventral osmoregulatory canals. A suite of complex attributes unequivocally separates 5 respective species that had historically been relegated to a single geographically widespread and morphologically variable taxon under T. jagerskioeldi sensu lato. Host range for respective cryptic species of the T. jagerskioeldi–complex reveal an intricate view of ecological isolation and connectivity across the greater North Pacific basin. Seabirds, marine mammals, and parasites are indicators of changing conditions over space and time. Oceanic regime shifts, prey cascades, and diversity for birds and parasites serve as proxies for revealing accelerating perturbation in marine foodwebs under climate forcing.

Obliquity forcing of Late Pliocene–Early Pleistocene aridification in the Qaidam Basin, NE Tibetan Plateau

Knowledge of the variations of aridification during past warm periods can help predict future aridification trends. However, the variation of aridity during the warm Pliocene epoch, a potential analog for the future climate, has not been sufficiently studied due to the lack of high-resolution records. Here, we present high-resolution (∼7 kyr) oxygen and carbon isotope records spanning the Late Pliocene–Early Pleistocene from the Qaidam Basin, northwestern China. We interpret enriched carbonate δ18O value as indicating stronger evaporation, based on the general correlation between the enrichment of carbonate δ18O and an increased sedimentary halite content. Our results reveal that the Qaidam Basin was already an extremely arid environment no later than ∼3.9 Ma, and that aridification was further intensified at ∼2.6 Ma. Interestingly, the Qaidam Basin experienced decreased aridity or climate wetting during the high insolation intervals within the middle Piacenzian warm period, in contrast with the intensified aridity in the semi-arid Chinese Loess Plateau. Strikingly, the Qaidam Basin experienced intensified aridification during high-obliquity intervals, with smaller global ice sheets, during 3.6–2.5 Ma. We attribute this obliquity signal of Asian aridification to the effects of insolation and possibly glacial forcing, via their impacts on regional evaporation and reduced monsoon precipitation. Using the Late Pliocene as an analogue, our findings suggest that future aridification variations in Central Asia during the next ∼100 kyr may be dominated by 40-kyr cycles, under natural climatic forcing.

Source apportionment of soot particles and aqueous-phase processing of black carbon coatings in an urban environment

Abstract. The impacts of soot particles on climate and human health depend on the concentration of black carbon (BC) as well as the thickness and composition of the coating material, i.e., organic and inorganic compounds internally mixed with BC. In this study, the size-resolved chemical composition of BC-containing aerosol was measured using a high-resolution soot particle aerosol mass spectrometer (SP-AMS) during wintertime in Fresno, California, a location influenced by abundant combustion emissions and frequent fog events. Concurrently, particle optical properties were measured to investigate the BC absorption enhancement. Positive matrix factorization (PMF) analysis was performed on the SP-AMS mass spectral measurements to explore the sources of soot particles and the atmospheric processes affecting the properties of BC coatings. The analysis revealed that residential woodburning and traffic are the dominant sources of soot particles. Alongside primary soot particles originating from biomass burning (BBOABC) and vehicles (HOABC) two distinct types of processed BC-containing aerosol were identified: fog-related oxidized organic aerosol (FOOABC) and winter-background OOABC (WOOABC). Both types of OOABC showed evidence of having undergone aqueous processing, albeit with differences. The concentration of FOOABC was substantially elevated during fog events, indicating the formation of aqueous secondary organic aerosol (aqSOA) within fog droplets. On the other hand, WOOABC was present at a relatively consistent concentration throughout the winter and was likely related to the formation of secondary organic aerosol (SOA) in both the gas phase and aerosol liquid water. By comparing the chemical properties and temporal variations in FOOABC and WOOABC, we gain insights into the key aging processes of BC aerosol. It was found that aqueous-phase reactions facilitated by fog droplets had a significant impact on the thickness and chemical composition of BC coatings, thereby affecting the light absorption and hygroscopic properties of soot particles. These findings underscore the important role of chemical reactions occurring within clouds and fogs and influencing the climate forcing of BC aerosol in the atmosphere.

Mathematical analysis of a time-delayed model for cocoa yield

Abstract Cocoa is an important crop that is predominantly grown in the western part of Africa. However, there have been fluctuations and declining trends in production and several factors have been identified to be responsible for this. A significant factor is the effect of climate variation, which could result in a low farm-level yield. Therefore, to understand the contribution of climate variability on the farm-level yield, we construct and analyse a time-delayed model to capture the effect of rainfall on cocoa production. This work uses a system of differential equations to model the crop transition from the flowering stage to pod formation, pod ripening and then to harvesting. We introduce a periodic forcing function into the model of flowering to account for the impact of seasonal rainfall variations. This leads to a novel nonlinear parametrically forced ODE for the flowering with periodically varying coefficients, which is coupled to a time-delayed model for the ripened pod formation and then harvesting. We perform an analysis of all parts of the system proving that it has a periodic solution when (parametrically) forced periodically, and we then conduct an asymptotic analysis on this periodic solution to show how its rich behaviour depends on the parameters of the climatic forcing in the model.

Investigation of spatial and temporal variability in lower tropospheric ozone from RAL Space UV–Vis satellite products

Abstract. Ozone is a potent air pollutant in the lower troposphere and an important short-lived climate forcer (SLCF) in the upper troposphere. Studies using satellite data to investigate spatiotemporal variability of troposphere ozone (TO3) have predominantly focussed on the tropospheric column metric. This is the first study to investigate long-term spatiotemporal variability in lower tropospheric column ozone (LTCO3, surface–450 hPa sub-column) by merging multiple European Space Agency–Climate Change Initiative (ESA-CCI) products produced by the Rutherford Appleton Laboratory (RAL) Space. We find that in the LTCO3, the degree of freedom of signal (DOFS) from these products varies with latitude range and season and is up to 0.8, indicating that the retrievals contain useful information on lower TO3. The spatial and seasonal variation of the RAL Space products are in good agreement with each other, but there are systematic offsets of up to 3.0–5.0 DU between them. Comparison with ozonesondes shows that the Global Ozone Monitoring Experiment (GOME-1, 1996–2003), the SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY, 2003–2010) and the Ozone Monitoring Instrument (OMI, 2005–2017) have stable LTCO3 records over their respective periods, which can be merged together. However, GOME-2 (2008–2018) shows substantial drift in its bias with respect to ozonesondes. We have therefore constructed a robust merged data set of LTCO3 from GOME-1, SCIAMACHY and OMI between 1996 and 2017. Comparing the LTCO3 differences between the 1996–2000 and 2013–2017 5-year averages, we find sizeable positive increases (3.0–5.0 DU) in the tropics/sub-tropics, while in the northern mid-latitudes, we find small-scale differences in LTCO3. Therefore, we conclude that there has been a substantial increase in tropical/sub-tropical LTCO3 during the satellite era, which is consistent with tropospheric column ozone (TCO3) records from overlapping time periods (e.g. 2005–2016).

The 1996 Mid-Atlantic Winter Flood: Exploring Climate Risk through a Storyline Approach

Abstract This article explores the application of thermodynamic perturbations to a historical midlatitude, wintertime, rain-on-snow flood event to evaluate how similar events may evolve under different climate forcings. In particular, we generate a hindcast of the 1996 Mid-Atlantic flood using an ensemble of 14-km variable-resolution simulations completed with the U.S. Department of Energy’s global Energy Exascale Earth System Model (E3SM). We show that the event is skillfully reproduced over the Susquehanna River Basin (SRB) by E3SM when benchmarked against in situ observational data and high-resolution reanalyses. In addition, we perform five counterfactual experiments to simulate the flood under preindustrial conditions and four different levels of warming as projected by the Community Earth System Model Large Ensemble. We find a nonlinear response in simulated surface runoff and streamflow as a function of atmospheric warming. This is attributed to changing contributions of liquid water input from a shallower initial snowpack (decreased snowmelt), increased surface temperatures and rainfall rates, and increased soil water storage. Flooding associated with this event peaks from around +1 to +2 K of global average surface warming and decreases with additional warming beyond this. There are noticeable timing shifts in peak runoff and streamflow associated with changes in the flashiness of the event. This work highlights the utility of using storyline approaches for communicating climate risk and demonstrates the potential nonlinearities associated with hydrologic extremes in areas that experience ephemeral snowpack, such as the SRB.

Aerosol absorption using in situ filter-based photometers and ground-based sun photometry in the Po Valley urban atmosphere

Abstract. Light-absorbing aerosols (LAAs) are short-lived climate forcers with a significant impact on Earth's radiative balance. LAAs include dust aerosols, black carbon (BC) and organic light-absorbing carbonaceous aerosol (collectively termed brown carbon, BrC), which have also been proven to be highly toxic. In this study, aerosol absorption at five wavelengths (ranging from ultraviolet to infrared) was monitored continuously using filter-based photometers during two winter seasons in 2020 and 2021 in the city of Modena (southern central Po Valley, northern Italy), at two regulatory air quality monitoring sites, along with other pollutants (coarse particulate matter, PM10; fine particulate matter, PM2.5; O3; NO; NO2; and C6H6) and the vehicular traffic rate. The aerosol optical depth (AOD) and other column aerosol optical properties were concurrently monitored at four wavelengths by an AErosol RObotic NETwork (AERONET) sun photometer under urban background conditions within Modena. In situ absorption levels were apportioned to both sources (fossil fuel and biomass burning) and species (BC and BrC), while columnar absorption was apportioned to BC, BrC and mineral dust. The combined analysis of the atmospheric aerosol and gas measurements and of the meteorological conditions (in situ and from the ERA5 reanalysis) identified the location of potential urban sources of BC and BrC, most likely related to traffic and biomass burning. In situ data show different diurnal/weekly patterns for BrC from biomass burning and BC from traffic, with minor differences between the background and the urban traffic conditions. AERONET version 3 absorption aerosol optical depth (AAOD) retrievals at four wavelengths allowed the estimation of the absorptive direct radiative effect due to LAAs over the same period under the reasonable assumption that the AOD signal is concentrated within the mixing layer. AERONET retrievals showed a modest correlation of columnar absorption with planetary boundary layer (PBL)-scaled in situ observations, although the correlation improved significantly during a desert dust transport event that affected both in situ aerosol and columnar absorption, particularly in the blue spectrum range. A low correlation occurred between the contribution of BrC to aerosol absorption for the in situ and the columnar observations, with the BrC contribution being generally larger for in situ observations. Finally, evidence of a highly layered atmosphere during the study period, featuring significant spatial mixing and modest vertical mixing, was shown by ERA5-based atmospheric temperature profiles and by the large correlation of concurrent AERONET AOD retrievals in Modena and in Ispra (on the northwestern side of the Po Valley, ca. 225 km from Modena).

Effect of Engine Design Parameters on the Climate Impact of Aircraft: A Case Study Based on Short-Medium Range Mission

The climate impact of aviation is considerably different from that of other transport modes. The turbofan engine’s efficiency can be increased by increasing the Operating Pressure Ratio (OPR), bypass ratio (BPR) and Turbine Inlet Temperature (TIT), thereby reducing CO2 and H2O emissions. However, this may have an adverse effect on the secondary emissions, such as NOx, soot, etc. Taking a holistic view in evaluating the climate impact of engine development trends considering all the climate forcers is imperative for design trends in the future. This research investigates the impact of some key engine design parameters on climate. The emission changes due to design variations in the CFM56-5B are estimated using in-house engine performance and emission prediction tools. Accordingly, the changes in the species’ Average Temperature Response for 100 years (ATR100) are analyzed using a climate assessment tool, AirClim. The results show that the overall climate impact increases by 40% when increasing OPR from 25 to 40. Meanwhile, the Twin Annular Premixed Swirler (TAPS-II) combustor reduces the total ATR100 drastically, in the range of 52–58%, due to lean combustion.

Landscape dynamics and the Phanerozoic diversification of the biosphere

The long-term diversification of the biosphere responds to changes in the physical environment. Yet, over the continents, the nearly monotonic expansion of life started later in the early part of the Phanerozoic eon1 than the expansion in the marine realm, where instead the number of genera waxed and waned over time2. A comprehensive evaluation of the changes in the geodynamic and climatic forcing fails to provide a unified theory for the long-term pattern of evolution of life on Earth. Here we couple climate and plate tectonics models to numerically reconstruct the evolution of the Earth’s landscape over the entire Phanerozoic eon, which we then compare to palaeo-diversity datasets from marine animal and land plant genera. Our results indicate that biodiversity is strongly reliant on landscape dynamics, which at all times determine the carrying capacity of both the continental domain and the oceanic domain. In the oceans, diversity closely adjusted to the riverine sedimentary flux that provides nutrients for primary production. On land, plant expansion was hampered by poor edaphic conditions until widespread endorheic basins resurfaced continents with a sedimentary cover that facilitated the development of soil-dependent rooted flora, and the increasing variety of the landscape additionally promoted their development.

A North–South Dipole Response of the South Atlantic Convergence Zone During the Mid‐Holocene

AbstractThe South Atlantic Convergence Zone (SACZ) profoundly modulates precipitation from central to southeastern Brazil in the present‐day climate. However, the understanding of its long‐term behavior responding to various climate forcings remains limited. Here, we use an isotope‐enabled atmospheric general circulation model (ECHAM4.6) to examine the precipitation response of the SACZ during the mid‐Holocene about six thousand years ago. The model simulates a northward intensification of the SACZ in the mid‐Holocene, resulting in a dipole anomaly pattern relative to today's climate. The mid‐Holocene precipitation increased along the northern margin of the SACZ due to the strengthening of easterly winds across the tropical Atlantic, while an eastward deflection of the South American low‐level jet reduced moisture transport to southern Brazil, resulting in reduced precipitation along the southern margins of the SACZ. The north–south dipole response in precipitation is consistent with the mid‐Holocene hydroclimate change observed in proxy records from the region.

Quantifying the benefits of BIPV windows in urban environment under climate change: A comparison of three Chinese cities

Building-integrated photovoltaic (BIPV) windows are an effective way to increase distributed PV capacity in the urban environment. However, the effects of BIPV windows on an annual scale have not been rigorously studied. In this study, we compared their performances with different window-to-wall ratios, canyon aspect ratios, and orientations in three Chinese cities. Our results show that in Beijing, BIPV windows with the southward orientation have the highest potential for power generation of 44.7 kWh/(m2·yr), which is 1.5 times the potential in Shenzhen and Nanjing. Compared to clear windows, BIPV windows are advantageous when the window coverage is greater than 50 % in a shallow canyon. The highest electricity saving ratios are found in Beijing (29.7 %), followed by Shenzhen (26.3 %) and Nanjing (23.3 %). However, the electricity saving ratio decreases by over 10 % in deep canyons. Under future climate forcing, the saving potential of BIPV windows increases slightly in Nanjing and remain stable in Shenzhen and Beijing. The study also found that BIPV windows could increase canyon air temperatures by over 0.2 °C during winter days but slightly cool the canyon in summer. The findings guide BIPV application in the built environment and cast light on the construction of low-carbon neighbourhoods.

Globally limited but severe shallow-shelf euxinia during the end-Triassic extinction

One of the most severe extinctions of complex marine life in Earth’s history occurred at the end of the Triassic period (~201.4 million years ago). The marine extinction was initiated by large igneous province volcanism and has tentatively been linked to the spread of anoxic conditions. However, the global-scale pattern of anoxic conditions across the end-Triassic event is not well constrained. Here we use the sedimentary enrichment and isotopic composition of the redox-sensitive element molybdenum to reconstruct global–local marine redox conditions through the extinction interval. Peak δ98Mo values indicate that the global distribution of sulfidic marine conditions was similar to the modern ocean during the extinction interval. Meanwhile, Tethyan shelf sediments record pulsed, positive δ98Mo excursions indicative of locally oxygen-poor, sulfidic conditions. We suggest that pulses of severe marine de-oxygenation were restricted largely to marginal marine environments during the latest Triassic and played a substantial role in shallow-marine extinction phases at that time. Importantly, these results show that global marine biodiversity, and possibly ecosystem stability, were vulnerable to geographically localized anoxic conditions. Expanding present-day marine anoxia in response to anthropogenic marine nutrient supply and climate forcing may therefore have substantial consequences for global biodiversity and wider ecosystem stability.

PH dependence of brown-carbon optical properties in cloud water

Abstract. Light-absorbing organic species present in aerosols, collectively called brown carbon (BrC), have important but highly uncertain effects on climate. Clouds likely represent a significant medium for secondary BrC production and for bleaching reactions, though the relative importance of the formation and loss processes in clouds is unknown at present. The acidity (or pH) of atmospheric particles and clouds affects the optical properties of BrC and bleaching rates. Given the wide variability of pH in the atmosphere (pH in particles and clouds ranges from −1 to 8), the optical properties of BrC and its bleaching behavior are expected to vary significantly, and the link between pH and BrC is yet another uncertainty in attempts to constrain its climate forcing effects. In this work, we characterize the pH dependence of BrC optical properties – including light absorption at 365 nm (Abs365), the mass absorption coefficient (MAC365), and the absorption Ångström exponent (AAE) – in bulk cloud water sampled from the summit of Whiteface Mountain, NY. In all samples (n=17), Abs365 and MAC365 increased linearly with increasing pH, highlighting the importance of reporting pH in studies of BrC in aqueous media. There was strong variability in the sensitivity of Abs365 to pH, with normalized slopes that ranged from 5.1 % to 17.2 % per pH unit. The normalized slope decreased strongly with increasing cloud water [K+], suggesting that the non-biomass-burning BrC has optical properties that are more sensitive to pH than BrC associated with biomass burning. AAE also showed a distinct pH dependence as it was relatively flat between pH 1.5–5 and then decreased significantly above pH 5. The cloud water composition was used to inform thermodynamic predictions of aerosol pH upwind and/or downwind of Whiteface Mountain and the subsequent changes in BrC optical properties. Overall, these results show that, in addition to secondary BrC production, photobleaching, and the altitudinal distribution, the climate forcing of BrC is quite strongly affected by its pH-dependent absorption.

The Holocene silicon biogeochemistry of Yellowstone Lake, USA

Silicon (Si) is an essential macronutrient for diatoms, an important component of lacustrine primary productivity that represents a link between the carbon and silicon cycles. Reconstructions of lake silicon cycling thus provide an underexploited window onto lake and catchment biogeochemistry. Silicon isotope geochemistry has potential to provide these reconstructions, given the competing source and process controls can be deconvolved. The silica-rich volcanic and hydrothermal systems in Yellowstone National Park are a great source of dissolved silicon into Yellowstone Lake, a system with high silicon, and thus carbon, export rates and the formation of diatom–rich sediment. Yellowstone Lake sediments should be an archive of past silicon biogeochemistry, although the effect of sublacustrine hydrothermal activity or hydrothermal explosion events is unclear.Here, we analysed lake water, tributaries, and hydrothermal vent fluids from Yellowstone Lake for their dissolved Si concentrations, isotope composition (δ30Si) and Ge/Si ratios to evaluate the sources of variability in the lake's Si cycle. Bulk elemental composition and biogenic SiO2 (bSiO2) content, together with δ30Si and Ge/Si ratios from a single diatom species, Stephanodiscus yellowstonensis, were analysed in two sediment cores spanning the last 9880 cal. yr BP. We investigate these datasets to identify long term Holocene changes in hydrothermal activity and effects of large and short-term events i.e., hydrothermal and a volcanic eruption.Combinations of bSiO2, δ30Si and Ge/Si with XRF and lithology data revealed that Yellowstone Lake has a resilient biogeochemical system: hydrothermal explosions are visible in the lithology but have no identifiable impact on bSiO2 accumulation or on the δ30Si signature. Both cores show similarities that suggest a stable and homogeneous dSi source across the entire lake. A narrow range of δ30Si and Ge/Si values suggests that the productive layer of the lake was well mixed and biogeochemically stable, with consistently high hydrothermal inputs of Si throughout the Holocene to buffer against the disturbance events. Variation in bSiO2 concentration through time is weakly correlated with an increase towards younger sediment in the δ30Si fossil diatom record in both cores. This increase mirrors that seen in ocean records, and follows changes known in summer insolation, summer temperatures and lake water-column mixing since the deglaciation. This suggests that climate forcing, and soil formation ultimately govern the silicon isotope record, which we suggest is via a combination of changes in weathering stoichiometry, diatom production, and relative proportion of dSi sources.

Land surface dynamics and meteorological forcings modulate land surface temperature characteristics

This study examines the effect of land cover, vegetation health, climatic forcings, elevation heat loads, and terrain characteristics (LVCET) on land surface temperature (LST) distribution over West Africa (WA). We employ fourteen machine-learning models, which preserve nonlinear relationships, to downscale LST and other predictands while preserving the geographical variability of WA. Our results showed that the random forest model performs best in downscaling predictands. This is important for the sub-region since it has limited access to mainframes to power multiplex machine-learning algorithms. In contrast to the northern regions, the southern regions consistently exhibit healthy vegetation. Also, areas with unhealthy vegetation coincide with hot LST clusters. The positive Normalized Difference Vegetation Index (NDVI) trends in the Sahel underscore rainfall recovery and subsequent Sahelian greening. The southwesterly winds cause the upwelling of cold waters, lowering LST in southern WA and highlighting the cooling influence of water bodies on LST. Identifying regions with elevated LST is paramount for prioritizing greening initiatives, and our study underscores the importance of considering LVCET factors in urban planning. Topographic slope-facing angles, heat loads, and diurnal anisotropic heat all contribute to variations in LST, emphasizing the need for a holistic approach when designing resilient and sustainable landscapes.

Arctic Tropospheric Ozone Trends

AbstractObserved trends in tropospheric ozone, an important air pollutant and short‐lived climate forcer (SLCF), are estimated using available surface and ozonesonde profile data for 1993–2019, using a coherent methodology, and compared to modeled trends (1995–2015) from the Arctic Monitoring Assessment Program SLCF 2021 assessment. Increases in observed surface ozone at Arctic coastal sites, notably during winter, and concurrent decreasing trends in surface carbon monoxide, are generally captured by multi‐model median trends. Wintertime increases are also estimated in the free troposphere at most Arctic sites, with decreases during spring months. Winter trends tend to be overestimated by the multi‐model medians. Springtime surface ozone increases in northern coastal Alaska are not simulated while negative springtime trends in northern Scandinavia are not always reproduced. Possible reasons for observed changes and model performance are discussed including decreasing precursor emissions, changing ozone dry deposition, and variability in large‐scale meteorology.

Free-tropospheric aerosols contribute to large aerosol optical depth at a remote, inland location with insignificant anthropogenic emissions

In-situ measurements of atmospheric aerosols from remote environments marked with negligible human activities are required to reduce the uncertainties associated with natural aerosols in pre-industrial climate forcings (particularly the aerosol-cloud interactions). In this study, we present long-term continuous aerosol measurements, carried out using ground-based and satellite observations, both close to the surface and in the vertical column, over a remote, rural, inland location in southern peninsular India. The near-surface aerosol concentrations were low, in general conformity to the typical pattern expected for a remote location, with the temporal changes being primarily governed by the background meteorology. However, the columnar aerosol optical depth (AOD) was surprisingly high and resembled that of a typical semi-urban continental location, with an yearly mean AOD of 0.47 ± 0.06. Spaceborne lidar observations in conjunction with ground-based radiometer measurements revealed that around 60% of this AOD was contributed by free-tropospheric aerosol layers during summer and monsoon seasons. Airmass back-trajectory analysis along with particle depolarisation ratio profiles indicate that these aerosol layers are mostly associated with long-range transported dust aerosols. Our findings indicate that benign surface concentration of aerosols at a location need not necessarily be sufficient to consider it as an optimal choice for studying background aerosols.