Global Climate Forcing Research Articles

Segregated Supply of Sustainable Aviation Fuel to reduce Contrail Energy Forcing – Demonstration and Potentials

Aviation contributes about 4 % to net anthropogenic climate forcing, with contrails being the largest individual contributor to radiative forcing from aviation. One option to mitigate contrail-related climate impacts is using kerosene containing fewer or no aromatic components and thus showing a higher hydrogen content compared to conventional kerosene (i.e., fossil fuel-based). Such “low contrail” kerosene can be provided as a blend of conventional (crude oil-based) and synthetic kerosene or from hydroprocessing conventional kerosene.Low contrail kerosene reduces contrail lifetime and optical thickness and thus the magnitude of contrail climate forcing. However, market shares of such kerosene are presently very low. Simultaneously, a small fraction (< 10 %) of all flights globally accounts for the majority (> 80 %) of global warming contrail climate forcing. Hence, the targeted use of low contrail kerosene on those flights appears promising. But, such an approach would require additional operational efforts, such as a duplication of supply lines and storage tanks.This study evaluates the feasibility and operational efforts of a segregated supply of a 35 m-% SAF-blend (14.34 m-% hydrogen content) to 84 winter time demonstration flights to reduce contrail climate forcing. Between 17th January 2023 and 2nd February 2023, low contrail kerosene was supplied to commercial A320 type aircraft flights on the route between Stockholm and Copenhagen in northern Europe. The operational feasibility and related efforts to target flights with the highest contrail energy forcing as well as a large-scale application are described. The evolution of contrails is tracked using data from the Meteosat Second Generation (MSG) satellite. The contrail energy forcing is calculated for the corresponding flight trajectories assuming another, well-validated engine model (CFM56-5B4 for the simulations instead of LEAP1A-26 for the demonstration flights) using the Contrail Cirrus Prediction (CoCiP) model with meteorological input fields from European Reanalysis data (ERA5).For the first time, the experiment demonstrates the operational feasibility for a segregated supply of low contrail kerosene to medium range aircraft at Stockholm airport. The segregated supply of low contrail kerosene can be realized for short to medium range flights, which can be fueled by a refueller truck. Targeting individual flights via single line hydrant fueling systems seems impractical as of now. Operational efforts to target single flights with highest contrail energy forcing are almost identical to the efforts in this demonstration experiment.Simulations estimate that the segregated supply of the medium blend kerosene (14.34 m-% hydrogen content) can reduce contrail energy forcing by about 11 % assuming the use of a “Rich-Quench-Lean” (RQL) engine (CFM56-5B4). The contrail climate benefit increases to >20 % for a 50 % blend ratio (14.7 m-% hydrogen content). Also, the location and evolution of the demonstration flights’ 28 contrails calculated with CoCiP was tracked with satellite data.The uncertainty of absolute contrail climate forcing estimates is mainly limited due to meteorological data input and also by lacking information on fuel composition in terms of cycloalkane, mono- and polycyclic aromatics content. Contrarily, the uncertainty of relative changes in contrail climate forcing is subject to low uncertainty, since it compares the use of different fuels for an identical fleet and identical weather conditions.

Miocene ice sheet dynamics and sediment deposition in the central Ross Sea, Antarctica

Drill cores from the Antarctic continental shelf are essential for directly constraining changes in past Antarctic Ice Sheet extent. Here, we provide a sedimentary facies analysis of drill cores from International Ocean Discovery Program (IODP) Site U1521 in the Ross Sea, which reveals a unique, detailed snapshot of Antarctic Ice Sheet evolution between ca. 18 Ma and 13 Ma. We identify distinct depositional packages, each of which contains facies successions that are reflective of past baseline shifts in the presence or absence of marine-terminating ice sheets on the outermost Ross Sea continental shelf. The oldest depositional package (&amp;gt;18 Ma) contains massive diamictites stacked through aggradation and deposited in a deep, actively subsiding basin that restricted marine ice sheet expansion on the outer continental shelf. A slowdown in tectonic subsidence after 17.8 Ma led to the deposition of progradational massive diamictites with thin mudstone beds/laminae, as several large marine-based ice sheet advances expanded onto the mid- to outer continental shelf between 17.8 Ma and 17.4 Ma. Between 17.2 Ma and 15.95 Ma, packages of interbedded diamictite and diatom-rich mudstone were deposited during a phase of highly variable Antarctic Ice Sheet extent and volume. This included periods of Antarctic Ice Sheet advance near the outer shelf during the early Miocene Climate Optimum (MCO)—despite this being a well-known period of peak global warmth between ca. 17.0 Ma and 14.6 Ma. Conversely, there were periods of peak warmth within the MCO during which diatom-rich mudstones with little to no ice-rafted debris were deposited, which indicates that the Antarctic Ice Sheet was greatly reduced in extent and had retreated to a smaller terrestrial-terminating ice sheet, most notably between 16.3 Ma and 15.95 Ma. Post-14.2 Ma, diamictites and diatomites contain unambiguous evidence of subglacial shearing in the core and provide the first direct, well-dated evidence of highly erosive marine ice sheets on the outermost continental shelf during the onset of the Middle Miocene Climate Transition (MMCT; 14.2−13.6 Ma). Although global climate forcings and feedbacks influenced Antarctic Ice Sheet advances and retreats during the MCO and MMCT, we propose that this response was nonlinear and heavily influenced by regional feedbacks related to the shoaling of the continental shelf due to reduced subsidence, sediment infilling, and local sea-level changes that directly influenced oceanic influences on melting at the Antarctic Ice Sheet margin. Although intervals of diatom-rich muds and diatomite indicating open-marine interglacial conditions still occurred during (and following) the MMCT, repeated advances of marine-based ice sheets since that time have resulted in widespread erosion and overdeepening in the inner Ross Sea, which has greatly enhanced sensitivity to marine ice sheet instability since 14.2 Ma.

Palaeoenvironmental Analysis of the Southern Part of the Danube–Tisza Interfluve (Hungary): The Northern Loess Wall of Katymár and the Hay Meadows and Loess Banks of Hajós

This study presents a comprehensive palaeoenvironmental reconstruction of the southern part of the Danube–Tisza interfluve in the Carpathian Basin from the Late Pleistocene to Early Holocene, addressing the region’s response to global climate forcings and local environmental factors based on multi-proxy analyses of two key protected areas: the Katymár brickyard and the hay meadows and loess banks of Hajós. By integrating radiocarbon-dated malacological, macrobotanical, pollen, phytolith, geochemical, and sedimentological analyses, it was possible to provide a picture of past climate–ecosystem interactions. The Katymár North sequence provides an extended chronology from Marine Isotope Stage 3 (MIS3, ca. 36,000 cal BP) through the Last Glacial Maximum (LGM) and into the Early Holocene, while the Hajós sequence offers high-resolution data for the LGM–Holocene transition. By the late Ice Age, humidity and surface moisture superseded temperature as primary palaeoecological limiting factors, promoting ecotone-like forest–steppe environments during cooling periods.

Expansion of C4 plants in the tropical Leizhou Peninsula during the Last Glacial Maximum: Modulating effect of regional sea-level change

Due to the lack of relatively long-term, high-resolution terrestrial records in tropical southern China, there is limited published research on terrestrial vegetation changes and their responses to regional and/or global climate forcings since the last glacial period. In this study, a 170-cm-long peat core (covering the interval from ~44.1 to 9.3 cal kyr BP) recovered from the Xialu peatland in Leizhou Peninsula, South China, was analyzed for organic carbon isotope (δ13Corg), along with total organic carbon, total nitrogen, and bulk dry density, to investigate past vegetation and hydroclimatic changes. Our results showed that C4 plants dominated the study region during Marine Isotope Stage (MIS) 2 (29–14 cal kyr BP), indicating generally cooler and drier conditions during MIS 2 relative to late MIS 3 (~ 44.1–29 cal kyr BP) and early MIS 1 (14–9.3 cal kyr BP). In particular, the driest conditions occurred during the Last Glacial Maximum (~ 25–19 cal kyr BP) when sea level was at its lowest. In addition, several millennial-scale climatic events associated with the expansion of C4 plants were clearly identified. Our record is sensitive to a variety of glacial-interglacial forcings, including regional processes and global forcing, among which the inundation history of Beibu Gulf due to sea-level change during the late Quaternary, which has been neglected in previous studies, may have played an important role in modulating paleo-hydroclimatic changes in tropical southern China.

Predicting tail risks and the evolution of temperatures

This paper explores a range of simple models to study the relationship between global temperature anomalies and climate forcings. In particular, we consider quantile regression models with potentially time-varying parameters (TVP), implemented by Bayesian methods. In its most general specification, this approach is flexible in that it models distinct regions of distribution of global temperature anomalies, while also allowing us to investigate changes in the relationship between (natural and anthropogenic) climate forcings and temperatures. Our results indicate that there is indeed considerable variation over time in the relationship between temperatures and its drivers, and that these effects may be heterogeneous across different quantiles. We then perform a long-range forecasting exercise for temperatures, which suggests that incorporating TVP or explicitly modelling quantile levels or the combination of both features can improve prediction for different parts of the temperature distribution. In addition, we produce forecasts for 2030 considering the intermediate RCP 4.5 scenario: given that no single specification dominates, we account for model uncertainty by considering forecast averaging across all specifications. Our approach allows us to make statements about the probability of temperature levels — for instance, we find that a scenario of +1.8 °C will occur with a non-negligible probability under RCP 4.5.

Global climate forcing on late Miocene establishment of the Pampean aeolian system in South America

Wind-blown dust from southern South America links the terrestrial, marine, atmospheric, and biological components of Earth’s climate system. The Pampas of central Argentina (~33°–39° S) contain a Miocene to Holocene aeolian record that spans an important interval of global cooling. Upper Miocene sediment provenance based on n = 3299 detrital-zircon U-Pb ages is consistent with the provenance of Pleistocene–Holocene deposits, indicating the Pampas are the site of a long-lived fluvial-aeolian system that has been operating since the late Miocene. Here, we show the establishment of aeolian sedimentation in the Pampas coincided with late Miocene cooling. These findings, combined with those from the Chinese Loess Plateau (~33°–39° N) underscore: (1) the role of fluvial transport in the development and maintenance of temporally persistent mid-latitude loess provinces; and (2) a global-climate forcing mechanism behind the establishment of large mid-latitude loess provinces during the late Miocene.

Revised historical Northern Hemisphere black carbon emissions based on inverse modeling of ice core records

Black carbon emitted by incomplete combustion of fossil fuels and biomass has a net warming effect in the atmosphere and reduces the albedo when deposited on ice and snow; accurate knowledge of past emissions is essential to quantify and model associated global climate forcing. Although bottom-up inventories provide historical Black Carbon emission estimates that are widely used in Earth System Models, they are poorly constrained by observations prior to the late 20th century. Here we use an objective inversion technique based on detailed atmospheric transport and deposition modeling to reconstruct 1850 to 2000 emissions from thirteen Northern Hemisphere ice-core records. We find substantial discrepancies between reconstructed Black Carbon emissions and existing bottom-up inventories which do not fully capture the complex spatial-temporal emission patterns. Our findings imply changes to existing historical Black Carbon radiative forcing estimates are necessary, with potential implications for observation-constrained climate sensitivity.

Modes of Pangean lake level cyclicity driven by astronomical climate pacing modulated by continental position and pCO2

Orbital cyclicity is a fundamental pacemaker of Earth's climate system. The Newark-Hartford Basin (NHB) lake sediment record of eastern North America contains compelling geologic expressions of this cyclicity, reflecting variations of climatic conditions in tropical Pangea during the Late Triassic and earliest Jurassic (~233 to 199 Ma). Climate modeling enables a deeper mechanistic understanding of Earth system modulation during this unique greenhouse and supercontinent period. We link major features of the NHB record to the combined climatic effects of orbital forcing, paleogeographic changes, and atmospheric pCO[Formula: see text] variations. An ensemble of transient, orbitally driven climate simulations is assessed for nine time slices, three atmospheric pCO[Formula: see text] values, and two paleogeographic reconstructions. Climatic transitions from tropical humid to more seasonal and ultimately semiarid are associated with tectonic drift of the NHB from [Formula: see text] to [Formula: see text]. The modeled orbital modulation of the precipitation-evaporation balance is most pronounced during the 220 to 200 Ma interval, whereas it is limited by weak seasonality and increasing aridity before and after this interval. Lower pCO[Formula: see text] at around 205 Ma contributes to drier climates and could have led to the observed damping of sediment cyclicity. Eccentricity-modulated precession dominates the orbitally driven climate response in the NHB region. High obliquity further amplifies summer precipitation through the seasonal shifts in the tropical rainfall belt. Regions with other proxy records are also assessed, providing guidance toward an integrated picture of global astronomical climate forcing in the Late Triassic and ultimately of other periods in Earth history.

Downwind control of oceanic air by land: the land wake and its sensitivity to CO2

Oceans are well-known to be directly altered by global climate forcings such as greenhouse gas changes, but how oceans are indirectly influenced by land and its response to such forcings remains less explored. Here, we assess the present-day and projected future state of a little-explored feature of the climate system—a ‘land wake’ in relative humidity downwind of the east coast of North America, consisting of low-humidity continental air extending roughly 1000 km over the Atlantic ocean. The wake exists throughout the year, but is supported by high continental temperatures in summer and low continental moisture in winter. The wake is well represented in an ensemble of global climate models (GCMs), qualitatively matching reanalysis data. Under increasing atmospheric CO2, the land wake intensifies in GCM simulations through two pathways: the radiative effects of CO2 on surface temperatures, and the biogeochemical effect of CO2 on terrestrial vegetation. Vegetation responses to increased CO2 alter the summer wake from Florida to Newfoundland, and both the radiative and biogeochemical effects of CO2 drive reductions in coastal cloud cover. These changes illustrate the potential of rapidly changing terrestrial climate to influence coastal regions and the ocean environment downwind of continents through both light conditions and the energy balance of the surface ocean.

Glacial Warming in the Eastern Pacific Warm Pool

The Eastern Pacific Warm Pool (EPWP) modulates global climate through its connection with tropical Pacific circulation, but sparse paleoceanographic data from this region limits our understanding of its role in past climate variability. We present a 144 kyr alkenone-sea surface temperature (SST) reconstruction from core NH22P, located in the northern EPWP, that shows local warming occurred during periods of global cooling. Climate model simulations of the Last Glacial Maximum indicate that both ice sheet and greenhouse gas forcing slowed wind speeds over the EPWP, which attenuated glacial cooling of local SST via the wind-evaporation-SST feedback. Spectral analysis further suggests precessional pacing of the warming spikes. Vernal equinox insolation could explain this pacing as direct shortwave heating during boreal spring would have contributed to the early seasonal intensification of the EPWP. This work provides crucial constraints on tropical Pacific glacial climate variability and highlights the unique response of the EPWP to global climate forcings.

A new method for measuring airborne elemental carbon using PUF disk passive samplers

Carbonaceous aerosol species, such as elemental carbon (EC), are important Short-Lived Climate Forcers (SLCFs), contributing to climate and health effects of air pollution. The quantification of carbonaceous aerosols has been conventionally carried out using active air sampling followed by various analytical techniques, such as thermal/thermal-optical analysis. Active sampling requires specific equipment and infrastructure with electricity and therefore may not be the best choice for studying carbonaceous aerosols at remote locations. Passive sampling on the other hand provides a simple and cost-effective alternative to study time-weighted temporal and spatial trends. For the first time in this study, we have developed a method to examine the viability of measuring EC using polyurethane foam passive air samplers (PUF-PAS) coupled with a thermal analysis, i.e., EnCan-Total-900 (ECT9). The method was found reproducible with coefficients of variation as low as 3% for EC measured in ambient passive samples. The method had relatively low background with EC levels in blanks being as low as 0.1% of those in deployed samples, allowing quantification within a wide range of concentrations. The results indicate a homogenous distribution of particles within the PUF-PAS substrate. EC concentrations measured with the passive method were not significantly different from those obtained from active samples at the study sites (p > 0.01). This proof of concept of the PUF-PAS method provides an opportunity to cost-effectively expand measurements of elemental carbon at the global scale, and could be further extended to include other carbonaceous aerosol species in the future. This helps address regional data gaps for improving uncertainties of SLCF impacts on global climate forcing and to inform policy decisions.

Identifying Global‐Scale Patterns of Vegetation Change During the Last Deglaciation From Paleoclimate Networks

AbstractDuring the last deglaciation (∼19–11 ka before present), the global mean temperature increased by 3–8 K. The concurrent hydroclimate and land cover changes are not well constrained. Here, we use a pollen database to quantify global‐scale vegetation changes during this transitional period at orbital (∼104 years) and millennial timescales (∼103 years). We focus on the proportion of tree and shrub pollen, the arboreal pollen (AP) fraction. Temporal similarities over long distances are identified by a paleoclimate network approach. At the orbital scale, we find coherent AP variations in the low and mid‐latitudes which we attribute to the global climate forcing. While AP fractions predominantly increased through the deglaciation, we identify regions where AP fractions decreased. For millennial timescales, we do not observe spatially coherent similarity structures. We compare our results with networks computed from three deglacial climate simulations with the CCSM3, HadCM3, and LOVECLIM models. Networks based on simulated precipitation patterns reproduce the characteristics of the AP network. Sensitivity experiments with statistical emulators indicate that, indeed, precipitation variations explain the diagnosed patterns of vegetation change better than temperature and CO2 variations. Our findings support previous interpretations of deglacial forest evolution in the mid‐latitudes being the result of atmospheric circulation changes. The network analysis identifies differences in the vegetation‐climate‐CO2 relationship simulated by CCSM3 and HadCM3. We conclude that network analyses are a promising tool to benchmark transient climate simulations with dynamical vegetation changes. This may result in stronger constraints of future hydroclimate and land cover changes.

Transferability of an individual- and trait-based forest dynamics model: A test case across the tropics

Individual-based forest models (IBMs) are useful to investigate the effect of environment on forest structure and dynamics, but they are often restricted to site-specific applications. To build confidence for spatially distributed simulations, model transferability, i.e. the ability of the same model to provide reliable predictions at contrasting sites, has to be thoroughly tested. We tested the transferability of a spatially explicit forest IBM, TROLL, with a trait-based species parameterization and global gridded climate forcing, by applying it to two sites with sharply contrasting climate and floristic compositions across the tropics, one in South America and one in Southeast Asia. We identified which parameters are most influential for model calibration and assessed the model sensitivity to climatic conditions for a given calibration. TROLL produced realistic predictions of forest structure and dynamics at both sites and this necessitates the recalibration of only three parameters, namely photosynthesis efficiency, crown allometry and mortality rate. All three relate to key processes that constrain model transferability and warrant further model development and data acquisition, with mortality being a particular priority of improvement for the current generation of vegetation models. Varying the climatic conditions at both sites demonstrate similar, and expected, model responses: GPP increased with temperature and irradiance, while stem density and aboveground biomass declined as temperature increased. The climate dependence of productivity and biomass was mediated by plant respiration, carbon allocation and mortality, which has implications both on model development and on forecasting of future carbon dynamics. Our detailed examination of forest IBM transferability unveils key processes that need to improve in genericity before reliable large-scale implementations can be envisioned.

Evaluation of river flood extent simulated with multiple global hydrological models and climate forcings

Global flood models (GFMs) are increasingly being used to estimate global-scale societal and economic risks of river flooding. Recent validation studies have highlighted substantial differences in performance between GFMs and between validation sites. However, it has not been systematically quantified to what extent the choice of the underlying climate forcing and global hydrological model (GHM) influence flood model performance. Here, we investigate this sensitivity by comparing simulated flood extent to satellite imagery of past flood events, for an ensemble of three climate reanalyses and 11 GHMs. We study eight historical flood events spread over four continents and various climate zones. For most regions, the simulated inundation extent is relatively insensitive to the choice of GHM. For some events, however, individual GHMs lead to much lower agreement with observations than the others, mostly resulting from an overestimation of inundated areas. Two of the climate forcings show very similar results, while with the third, differences between GHMs become more pronounced. We further show that when flood protection standards are accounted for, many models underestimate flood extent, pointing to deficiencies in their flood frequency distribution. Our study guides future applications of these models, and highlights regions and models where targeted improvements might yield the largest performance gains.

A New Messinian87Sr/86Sr Curve for the Ain El Beida Section (Morocco): An Atlantic Perspective of the Mediterranean Salinity Crisis

AbstractWe present a new87Sr/86Sr curve for the time interval between 6.5 and 5.5 Ma of the Ain El Beida section (AEB, Atlantic side of Morocco). The location and the precessional‐scale resolution of our curve allow to investigate the possible relationships between global paleoclimatic and palaeoceanographic trends and the high‐amplitude hydrological changes of the Mediterranean during the Messinian salinity crisis (MSC). The values, obtained from planktic foraminifers, plot in the upper portion of the global ocean curve and show long‐ (at eccentricity scale) and short‐term (precessional scale) oscillations; the latter are more evident during the maximum of eccentricity for both the 100 and 400 ka components, between 6.00 and 5.85 Ma. Negative87Sr/86Sr anomalies are observed at insolation minima in phase with δ18O maxima, marking the glacial stages from TG34 to TG26. It is unclear whether these oscillations originated from local (river runoff) or global (glacial/interglacial) climatic forcing. Conversely, the evaporites of the coeval Primary Lower Gypsum (5.97–5.62 Ma) accumulated in the Mediterranean at insolation minima show positive anomalies at glacial stages TG32, 30, 28, and 26. Such an opposite trend is possibly related to the greater sensitivity of the Mediterranean Sea to river runoff during insolation maxima compared to the global ocean, as recorded at the Moroccan Atlantic margin. These observations and the deviation toward lower values of the Mediterranean Sr isotope curve starting at around 6.5 Ma, support the hypothesis of a progressive restriction of the Atlantic connections since the early Messinian, leading to the MSC.

Changes in the Total Solar Irradiance and climatic effects

The correlation between the averaged reconstructed March temperature record for Kyoto, Japan, and the reconstructed Total Solar Irradiance (TSI) over 660 years from 1230 to 1890 gives evidence with 98% probability that the Little Ice Age with four cold periods is forced by variations of TSI. If the correlation is restricted to the period 1650–1890, with two cold periods in the 17th and 19th century and for which two independent reconstructed March temperature records are available, the probability of solar forcing increases to 99.99%. As solar irradiance variations have a global effect there has to be a global climatic solar forcing impact. However, by how much global temperature were lower during these minima and with what amplitude TSI was varying is not accurately known. The two quantities, global temperature and TSI, are linked by the energy equilibrium equation for the Earth system. The derivation of this equation with respect to a variation of the solar irradiance has two terms: A direct forcing term, which can be derived analytically and quantified accurately from the Stefan-Boltzmann law, and a second term, describing indirect influences on the surface temperature. If a small TSI variation should force a large temperature variation, then it has to be the second indirect term that strongly amplifies the effect of the direct forcing. The current knowledge is summarized by three statements:During the minima periods in the 13th, 15/16th, 17th, and 19th centuries the terrestrial climate was colder by 0.5–1.5 °C;Indirect Top-down and Bottom-up mechanisms do not amplify direct forcing by a large amount, i.e. indirect solar forcing is of the same magnitude (or smaller) as direct solar forcing;The radiative output of the Sun cannot be lower by more than 2 Wm−2 below the measured present-day TSI value during solar cycle minimum.These three statements contradict each other and it is concluded that at least one is not correct. Which one is a wrong statement is presently not known conclusively. It is argued that it is the third statement and it is speculated that over centennial time scales the Sun might vary its radiance significantly more than observed so far during the last 40 years of space TSI measurements. To produce Maunder minimum type cold climate excursions, a TSI decrease of the order of 10 Wm−2 is advocated.

Examining the Climate Effects of a Regional Nuclear Weapons Exchange Using a Multiscale Atmospheric Modeling Approach

AbstractRecent studies examine the potential for large urban fires ignited in a hypothetical nuclear exchange of one hundred 15 kt weapons between India and Pakistan to alter the climate (e.g., Mills et al., 2014, https://doi.org/10.1002/2013EF000205, and Reisner et al., 2018, https://doi.org/10.1002/2017JD027331). In this study, the global climate forcing and response is predicted by combining two atmospheric models, which together span the micro‐scale to global scale processes involved. Individual fire plumes are modeled using the Weather Research and Forecasting (WRF) model, and the climate response is predicted by injecting the WRF‐simulated black carbon (BC) emissions into the Energy Exascale Earth System Model (E3SM) atmosphere model Version 1 (EAMv1). Consistent with previous studies, the radiative forcing depends on smoke quantity and injection height, examined here as functions of fuel loading and atmospheric conditions. If the fuel burned is 1 g cm−2, BC is quickly removed from the troposphere, causing no global mean climate forcing. If the fuel burned is 16 g cm−2 and 100 such fires occurred simultaneously with characteristics similar to historical large urban firestorms, BC reaches the stratosphere, reducing solar radiation and causing cooling at the Earth's surface. Uncertainties in smoke composition and aerosol representation cause large uncertainties in the magnitude of the radiative forcing and cooling. The approximately 4 yr duration of the radiative forcing is shorter than the 8 to 15 yr that has previously been simulated. Uncertainties point to the need for further development of potential nuclear exchange scenarios, quantification of fuel loading, and improved understanding of fire propagation and aerosol modeling.

Biogenic particles formed in the Himalaya as an important source of free tropospheric aerosols

Aerosols of biogenic and anthropogenic origin affect the total radiative forcing of global climate. Poor knowledge of the pre-industrial aerosol concentration and composition, in particular of particles formed directly in the atmosphere from gaseous precursors, constitutes a large uncertainty in the anthropogenic radiative forcing. Investigations of new particle formation at pre-industrial-like conditions can contribute to the reduction of this uncertainty. Here we present observations taken at the remote Nepal Climate Observatory Pyramid station at 5,079 m above sea level, a few kilometres from the summit of Everest. We show that up-valley winds funnel gaseous aerosol precursors to higher altitudes. During this transport, these are oxidized into compounds of very low volatility, which rapidly form a large number of aerosol particles. These are then transported into the free troposphere, which suggests that the whole Himalayan region may act as an ‘aerosol factory’ and contribute substantially to the free tropospheric aerosol population. Aerosol production in this region occurs mainly via organic precursors of biogenic origin with little evidence of the involvement of anthropogenic pollutants. This process is therefore likely to be essentially unchanged since the pre-industrial period, and may have been one of the major sources that contributes to the upper tropospheric aerosol population during that time. Newly formed biogenic particles in the Himalaya increase free-tropospheric background aerosol concentration by a factor of up to two.

14C and 10Be dated Late Holocene fluctuations of Patagonian glaciers in Torres del Paine (Chile, 51°S) and connections to Antarctic climate change

Southern Hemisphere Holocene glacier chronologies are important for unraveling past climate change, mid-to-high latitude teleconnections, and regional to global climate forcing. At present, a significant number of glacier chronologies for Patagonia are based on 14C dating, which may afford only maximum- or minimum-limiting dates. Here, we combine geomorphology and stratigraphy with radiocarbon (14C) and beryllium-10 (10Be) surface exposure-age dating at three outlet glaciers, Zapata, Tyndall, and Pingo. These glaciers drain the southernmost tip of the Southern Patagonian Ice Field at Torres del Paine National Park, Chile (51°S). After an expansion that we date at 3200 yr. B.P., the Torres del Paine glaciers expanded to their last major Late Holocene maxima at 600 and 340 yr. B.P., with the final dated readvance after 190 yr. B.P. We use these data, together with other Patagonian glacier records, to define the early, mid and final glacial stages of the last millennium. These cold events were separated by warm conditions that allowed the Nothofagus forest to colonize deglaciated land. The presence of near-concurrent glacier fluctuations in Patagonia and Antarctic Peninsula indicates widespread cooling punctuated the Late Holocene, including the last millennium, across much of the extratropical Southern Hemisphere. We link this cooling to cold oceanographic-atmospheric conditions forced by a northern shift or intensification of the Southern Westerly Winds. Such scenario increased northward advection of cold Antarctic circumpolar water along western Patagonia and favored decreased upwelling of warm circumpolar deep water together with expanded sea ice around the Antarctic Peninsula.

Trophism, climate and paleoweathering conditions across the Eocene-Oligocene transition in the Massignano section (northern Apennines, Italy)

The Eocene-Oligocene transition represents the latest greenhouse-icehouse shift faced by Earth, a major global climate change associated with carbon cycle perturbation. In this paper, we investigate the Massignano stratigraphic section (northern Apennines, Italy), GSSP of the Eocene-Oligocene boundary, by X-ray diffraction analysis of clay minerals and carbon and oxygen stable isotopes to explore the link among climate, paleoweathering and runoff, and carbon cycle in the Neotethys across this major climatic transition. We examine the interplay between global climate forcing and orogen evolution controlling the Massignano hemipelagic sedimentation. The late Eocene clay mineral assemblages indicate that the Neotethys was sensitive to global eustatic changes as well as changes in composition and rates of weathered sediments accumulating into the basin. The carbon isotope record matches with the global signal, showing productivity pulses linked to cooling phases and sea-level drops before 34.8 Ma. Subsequently, a constant negative trend is recorded, which is consistent with the global carbon isotope curve, and attests for a decrease of primary productivity linked to decreasing atmospheric CO2. This negative trend ends at the Eocene-Oligocene boundary, when the onset of the global carbon isotope positive excursion is recorded also within the Massignano hemipelagic succession. On the contrary, the clay mineral assemblage is quite constant across the Eocene-Oligocene boundary reflecting the complex interplay among fluvial discharge, sea level changes and orogen dynamics which clouded the global climate shift. In this context, the enhanced fluvial discharge likely contributed to sustain high trophic conditions in the Adriatic waters and, in turn, the Oligocene positive carbon isotope shift. These results highlight how clay minerals proved to be a useful proxy to identify the interplay between global and regional controlling factors on hemipelagic sedimentation and, their integration with the carbon isotope record, provides insights into carbon cycle dynamics.