Climatic and ecological responses to Bennu-type asteroid collisions

There is an estimated chance of 0.037% that asteroid Bennu will collide with Earth in 2182 CE. The potential collision of such medium-sized asteroids can inject massive amounts of dust into the atmosphere, with unknown consequences for terrestrial and marine ecosystems. Here, we use the coupled high-top Community Earth System Model Version 2 with interactive chemistry to investigate how medium-sized asteroid strikes would affect climate, vegetation, and marine productivity. Our simulations, which inject up to 400 million tons of dust into the stratosphere, show marked disruptions in climate, atmospheric chemistry, and global photosynthesis. Global mean temperatures are projected to drop by 4°C, and global precipitation decreases by 15% in our simulations. The largest relative reductions in global terrestrial and marine net primary productivity reach 36 and 25%, respectively. Depending on the iron amount of the asteroid and the subsequent marine dust deposition, large diatom blooms occur in iron-limited regions such as the Southern Ocean and the eastern equatorial Pacific.

Metals from spacecraft reentry in stratospheric aerosol particles

Large increases in the number of low earth orbit satellites are projected in the coming decades [L. Schulz, K.-H. Glassmeier, Adv. Space Res. 67, 1002-1025 (2021)] with perhaps 50,000 additional satellites in orbit by 2030 [GAO, Large constellations of satellites: Mitigating environmental and other effects (2022)]. When spent rocket bodies and defunct satellites reenter the atmosphere, they produce metal vapors that condense into aerosol particles that descend into the stratosphere. So far, models of spacecraft reentry have focused on understanding the hazard presented by objects that survive to the surface rather than on the fate of the metals that vaporize. Here, we show that metals that vaporized during spacecraft reentries can be clearly measured in stratospheric sulfuric acid particles. Over 20 elements from reentry were detected and were present in ratios consistent with alloys used in spacecraft. The mass of lithium, aluminum, copper, and lead from the reentry of spacecraft was found to exceed the cosmic dust influx of those metals. About 10% of stratospheric sulfuric acid particles larger than 120 nm in diameter contain aluminum and other elements from spacecraft reentry. Planned increases in the number of low earth orbit satellites within the next few decades could cause up to half of stratospheric sulfuric acid particles to contain metals from reentry. The influence of this level of metallic content on the properties of stratospheric aerosol is unknown.

Chlorine activation and enhanced ozone depletion induced by wildfire aerosol

Remarkable perturbations in the stratospheric abundances of chlorine species and ozone were observed over Southern Hemisphere mid-latitudes following the 2020 Australian wildfires^1,2. These changes in atmospheric chemical composition suggest that wildfire aerosols affect stratospheric chlorine and ozone depletion chemistry. Here we propose that wildfire aerosol containing a mixture of oxidized organics and sulfate^3-7 increases hydrochloric acid solubility^8-11 and associated heterogeneous reaction rates, activating reactive chlorine species and enhancing ozone loss rates at relatively warm stratospheric temperatures. We test our hypothesis by comparing atmospheric observations to model simulations that include the proposed mechanism. Modelled changes in 2020 hydrochloric acid, chlorine nitrate and hypochlorous acid abundances are in good agreement with observations^1,2. Our results indicate that wildfire aerosol chemistry, although not accounting for the record duration of the 2020 Antarctic ozone hole, does yield an increase in its area and a 3-5% depletion of southern mid-latitude total column ozone. These findings increase concern^2,12,13 that more frequent and intense wildfires could delay ozone recovery in a warming world.

Variability of the Aerosol Content in the Tropical Lower Stratosphere from 2013 to 2019: Evidence of Volcanic Eruption Impacts

This paper quantifies the tropical stratospheric aerosol content as impacted by volcanic events over the 2013–2019 period. We use global model simulations by the Whole Atmosphere Community Climate Model (WACCM) which is part of the Community Earth System Model version 1.0 (CESM1). WACCM is associated with the Community Aerosol and Radiation Model for Atmospheres (CARMA) sectional aerosol microphysics model which includes full sulphur chemical and microphysical cycles with no a priori assumption on particle size. Five main volcanic events (Kelud, Calbuco, Ambae, Raikoke and Ulawun) have been reported and are shown to have significantly influenced the stratospheric aerosol layer in the tropics, either through direct injection in this region or through transport from extra-tropical latitudes. Space-borne data as well as ground-based lidar and balloon-borne in situ observations are used to evaluate the model calculations in terms of aerosol content, vertical distribution, optical and microphysical properties, transport and residence time of the various volcanic plumes. Overall, zonal mean model results reproduce the occurrence and vertical extents of the plumes derived from satellite observations but shows some discrepancies for absolute values of extinction and of stratospheric aerosol optical depth (SAOD). Features of meridional transport of the plumes emitted from extra-tropical latitudes are captured by the model but simulated absolute values of SAOD differ from 6 to 200% among the various eruptions. Simulations tend to agree well with observed in situ vertical profiles for the Kelud and Calbuco plumes but this is likely to depend on the period for which comparison is done. Some explanations for the model–measurement discrepancies are discussed such as the inaccurate knowledge of the injection parameters and the presence of ash not accounted in the simulations.

Toward practical stratospheric aerosol albedo modification: Solar-powered lofting

Many climate intervention (CI) methods have been proposed to offset greenhouse gas-induced global warming, but the practicalities regarding implementation have not received sufficient attention. Stratospheric aerosol injection (SAI) involves introducing large amounts of CI material well within the stratosphere to enhance the aerosol loading, thereby increasing reflection of solar radiation. We explore a delivery method termed solar-powered lofting (SPL) that uses solar energy to loft CI material injected at lower altitudes accessible by conventional aircraft. Particles that absorb solar radiation are dispersed with the CI material and heat the surrounding air. The heated air rises, carrying the CI material to the stratosphere. Global model simulations show that black carbon aerosol (10 microgram per cubic meter) is sufficient to quickly loft CI material well into the stratosphere. SPL could make SAI viable at present, is also more energy efficient, and disperses CI material faster than direct stratospheric injection.

Marine wild-capture fisheries after nuclear war

Nuclear war, beyond its devastating direct impacts, is expected to cause global climatic perturbations through injections of soot into the upper atmosphere. Reduced temperature and sunlight could drive unprecedented reductions in agricultural production, endangering global food security. However, the effects of nuclear war on marine wild-capture fisheries, which significantly contribute to the global animal protein and micronutrient supply, remain unexplored. We simulate the climatic effects of six war scenarios on fish biomass and catch globally, using a state-of-the-art Earth system model and global process-based fisheries model. We also simulate how either rapidly increased fish demand (driven by food shortages) or decreased ability to fish (due to infrastructure disruptions), would affect global catches, and test the benefits of strong prewar fisheries management. We find a decade-long negative climatic impact that intensifies with soot emissions, with global biomass and catch falling by up to 18 ± 3% and 29 ± 7% after a US-Russia war under business-as-usual fishing-similar in magnitude to the end-of-century declines under unmitigated global warming. When war occurs in an overfished state, increasing demand increases short-term (1 to 2 y) catch by at most ∼30% followed by precipitous declines of up to ∼70%, thus offsetting only a minor fraction of agricultural losses. However, effective prewar management that rebuilds fish biomass could ensure a short-term catch buffer large enough to replace ∼43 ± 35% of today's global animal protein production. This buffering function in the event of a global food emergency adds to the many previously known economic and ecological benefits of effective and precautionary fisheries management.

Origins and Spatial Distribution of Non-Pure Sulfate Particles (NSPs) in the Stratosphere Detected by the Balloon-Borne Light Optical Aerosols Counter (LOAC)

While water and sulfuric acid droplets are the main component of stratospheric aerosols, measurements performed for about 30 years have shown that non-sulfate particles (NSPs) are also present. Such particles, released from the Earth mainly through volcanic eruptions, pollution or biomass burning, or coming from space, present a wide variety of compositions, sizes, and shapes. To better understand the origin of NSPs, we have performed measurements with the Light Optical Aerosol Counter (LOAC) during 151 flights under weather balloons in the 2013–2019 period reaching altitudes up to 35 km. Coupled with previous counting measurements conducted over the 2004–2011 period, the LOAC measurements indicate the presence of stratospheric layers of enhanced concentrations associated with NSPs, with a bimodal vertical repartition ranging between 17 and 30 km altitude. Such enhancements are not correlated with permanent meteor shower events. They may be linked to dynamical and photophoretic effects lifting and sustaining particles coming from the Earth. Besides, large particles, up to several tens of μm, were detected and present decreasing concentrations with increasing altitudes. All these particles can originate from Earth but also from meteoroid disintegrations and from the interplanetary dust cloud and comets.

Persisting volcanic ash particles impact stratospheric SO2 lifetime and aerosol optical properties

Volcanic ash is often neglected in climate simulations because ash particles are assumed to have a short atmospheric lifetime, and to not participate in sulfur chemistry. After the Mt. Kelut eruption in 2014, stratospheric ash-rich aerosols were observed for months. Here we show that the persistence of super-micron ash is consistent with a density near 0.5 g cm-3, close to pumice. Ash-rich particles dominate the volcanic cloud optical properties for the first 60 days. We also find that the initial SO2 lifetime is determined by SO2 uptake on ash, rather than by reaction with OH as commonly assumed. About 43% more volcanic sulfur is removed from the stratosphere in 2 months with the SO2 heterogeneous chemistry on ash particles than without. This research suggests the need for re-evaluation of factors controlling SO2 lifetime in climate model simulations, and of the impact of volcanic ash on stratospheric chemistry and radiation.

Rapidly expanding nuclear arsenals in Pakistan and India portend regional and global catastrophe

Pakistan and India may have 400 to 500 nuclear weapons by 2025 with yields from tested 12- to 45-kt values to a few hundred kilotons. If India uses 100 strategic weapons to attack urban centers and Pakistan uses 150, fatalities could reach 50 to 125 million people, and nuclear-ignited fires could release 16 to 36 Tg of black carbon in smoke, depending on yield. The smoke will rise into the upper troposphere, be self-lofted into the stratosphere, and spread globally within weeks. Surface sunlight will decline by 20 to 35%, cooling the global surface by 2° to 5°C and reducing precipitation by 15 to 30%, with larger regional impacts. Recovery takes more than 10 years. Net primary productivity declines 15 to 30% on land and 5 to 15% in oceans threatening mass starvation and additional worldwide collateral fatalities.

Improvement of stratospheric aerosol extinction retrieval from OMPS/LP using a new aerosol model

The Ozone Mapping and Profiler Suite Limb Profiler (OMPS/LP) has been flying on the Suomi NPP satellite since October 2011. It is designed to produce ozone and aerosol vertical profiles at ~2 km vertical resolution over the entire sunlit globe. Aerosol extinction profiles are computed with Mie theory using radiances measured at 675 nm. The operational Version 1.0 (V1.0) aerosol extinction retrieval algorithm assumes a bimodal lognormal aerosol size distribution (ASD) whose parameters were derived by combining an in situ measurement of aerosol microphysics with the SAGE II aerosol extinction climatology. Internal analysis indicates that this bimodal lognormal ASD does not sufficiently explain the spectral dependence of LP measured radiances. In this paper we describe the derivation of an improved aerosol size distribution, designated Version 1.5 (V1.5), for the LP retrieval algorithm. The new ASD uses a gamma function distribution that is derived from Community Aerosol and Radiation Model for Atmospheres (CARMA) calculated results. A cumulative distribution fit derived from the gamma function ASD gives better agreement with CARMA results at small particle radii than bimodal or unimodal functions. The new ASD also explains the spectral dependence of LP measured radiances better than the V1.0 ASD. We find that the impact of our choice of ASD on the retrieved extinctions varies strongly with the underlying reflectivity of the scene. Initial comparisons with co-located extinction profiles retrieved at 676 nm from the SAGE III/ISS instrument show a significant improvement in agreement for the LP V1.5 retrievals. Zonal mean extinction profiles agree to within 10% between 19-29 km, and regression fits of collocated samples show improved correlation and reduced scatter compared to the V1.0 product. This improved agreement will motivate development of more sophisticated ASDs from CARMA results that incorporate latitude, altitude, and seasonal variations in aerosol properties.

Composition, Mixing State and Water Affinity of Meteoric Smoke Analogue Nanoparticles Produced in a Non-Thermal Microwave Plasma Source

The article reports on the composition, mixing state and water affinity of iron silicate particles which were produced in a non-thermal low-pressure microwave plasma reactor. The particles are intended to be used as meteoric smoke particle analogues. We used the organometallic precursors ferrocene (Fe(C5H5)2) and tetraethyl orthosilicate (TEOS, Si(OC2H5)4) in various mixing ratios to produce nanoparticles with radii between 1 nm and 4 nm. The nanoparticles were deposited on sample grids and their stoichiometric composition was analyzed in an electron microscope using energy dispersive X-ray spectroscopy (EDS). We show that the pure silicon oxide and iron oxide particles consist of SiO2 and Fe2O3, respectively. For Fe:(Fe+Si) ratios between 0.2 and 0.8 our reactor produces (in contrast to other particle sources) mixed iron silicates with a stoichiometric composition according to FexSi(1−x)O3 (0≤x≤1). This indicates that the particles are formed by polymerization of FeO3 and SiO3 and that rearrangement to the more stable silicates ferrosilite (FeSiO3) and fayalite (Fe2SiO4) does not occur at these conditions. To investigate the internal mixing state of the particles, the H2O surface desorption energy of the particles was measured. We found that the nanoparticles are internally mixed and that differential coating resulting in a core-shell structure does not occur.

On transient climate change at the Cretaceous-Paleogene boundary due to atmospheric soot injections

Climate simulations that consider injection into the atmosphere of 15,000 Tg of soot, the amount estimated to be present at the Cretaceous-Paleogene boundary, produce what might have been one of the largest episodes of transient climate change in Earth history. The observed soot is believed to originate from global wildfires ignited after the impact of a 10-km-diameter asteroid on the Yucatán Peninsula 66 million y ago. Following injection into the atmosphere, the soot is heated by sunlight and lofted to great heights, resulting in a worldwide soot aerosol layer that lasts several years. As a result, little or no sunlight reaches the surface for over a year, such that photosynthesis is impossible and continents and oceans cool by as much as 28 °C and 11 °C, respectively. The absorption of light by the soot heats the upper atmosphere by hundreds of degrees. These high temperatures, together with a massive injection of water, which is a source of odd-hydrogen radicals, destroy the stratospheric ozone layer, such that Earth's surface receives high doses of UV radiation for about a year once the soot clears, five years after the impact. Temperatures remain above freezing in the oceans, coastal areas, and parts of the Tropics, but photosynthesis is severely inhibited for the first 1 y to 2 y, and freezing temperatures persist at middle latitudes for 3 y to 4 y. Refugia from these effects would have been very limited. The transient climate perturbation ends abruptly as the stratosphere cools and becomes supersaturated, causing rapid dehydration that removes all remaining soot via wet deposition.

The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth

Recognizing whether a planet can support life is a primary goal of future exoplanet spectral characterization missions, but past research on habitability assessment has largely ignored the vastly different conditions that have existed in our planet's long habitable history. This study presents simulations of a habitable yet dramatically different phase of Earth's history, when the atmosphere contained a Titan-like, organic-rich haze. Prior work has claimed a haze-rich Archean Earth (3.8-2.5 billion years ago) would be frozen due to the haze's cooling effects. However, no previous studies have self-consistently taken into account climate, photochemistry, and fractal hazes. Here, we demonstrate using coupled climate-photochemical-microphysical simulations that hazes can cool the planet's surface by about 20 K, but habitable conditions with liquid surface water could be maintained with a relatively thick haze layer (τ ∼ 5 at 200 nm) even with the fainter young Sun. We find that optically thicker hazes are self-limiting due to their self-shielding properties, preventing catastrophic cooling of the planet. Hazes may even enhance planetary habitability through UV shielding, reducing surface UV flux by about 97% compared to a haze-free planet and potentially allowing survival of land-based organisms 2.7-2.6 billion years ago. The broad UV absorption signature produced by this haze may be visible across interstellar distances, allowing characterization of similar hazy exoplanets. The haze in Archean Earth's atmosphere was strongly dependent on biologically produced methane, and we propose that hydrocarbon haze may be a novel type of spectral biosignature on planets with substantial levels of CO2. Hazy Archean Earth is the most alien world for which we have geochemical constraints on environmental conditions, providing a useful analogue for similar habitable, anoxic exoplanets. Key Words: Haze-Archean Earth-Exoplanets-Spectra-Biosignatures-Planetary habitability. Astrobiology 16, 873-899.

Evaluations of tropospheric aerosol properties simulated by the community earth system model with a sectional aerosol microphysics scheme

A sectional aerosol model (CARMA) has been developed and coupled with the Community Earth System Model (CESM1). Aerosol microphysics, radiative properties, and interactions with clouds are simulated in the size-resolving model. The model described here uses 20 particle size bins for each aerosol component including freshly nucleated sulfate particles, as well as mixed particles containing sulfate, primary organics, black carbon, dust, and sea salt. The model also includes five types of bulk secondary organic aerosols with four volatility bins. The overall cost of CESM1-CARMA is approximately ∼2.6 times as much computer time as the standard three-mode aerosol model in CESM1 (CESM1-MAM3) and twice as much computer time as the seven-mode aerosol model in CESM1 (CESM1-MAM7) using similar gas phase chemistry codes. Aerosol spatial-temporal distributions are simulated and compared with a large set of observations from satellites, ground-based measurements, and airborne field campaigns. Simulated annual average aerosol optical depths are lower than MODIS/MISR satellite observations and AERONET observations by ∼32%. This difference is within the uncertainty of the satellite observations. CESM1/CARMA reproduces sulfate aerosol mass within 8%, organic aerosol mass within 20%, and black carbon aerosol mass within 50% compared with a multiyear average of the IMPROVE/EPA data over United States, but differences vary considerably at individual locations. Other data sets show similar levels of comparison with model simulations. The model suggests that in addition to sulfate, organic aerosols also significantly contribute to aerosol mass in the tropical UTLS, which is consistent with limited data.

Composition and physical properties of the Asian Tropopause Aerosol Layer and the North American Tropospheric Aerosol Layer

Recent studies revealed layers of enhanced aerosol scattering in the upper troposphere and lower stratosphere over Asia (Asian Tropopause Aerosol Layer (ATAL)) and North America (North American Tropospheric Aerosol Layer (NATAL)). We use a sectional aerosol model (Community Aerosol and Radiation Model for Atmospheres (CARMA)) coupled with the Community Earth System Model version 1 (CESM1) to explore the composition and optical properties of these aerosol layers. The observed aerosol extinction enhancement is reproduced by CESM1/CARMA. Both model and observations indicate a strong gradient of the sulfur-to-carbon ratio from Europe to the Asia on constant pressure surfaces. We found that the ATAL is mostly composed of sulfates, surface-emitted organics, and secondary organics; the NATAL is mostly composed of sulfates and secondary organics. The model also suggests that emission increases in Asia between 2000 and 2010 led to an increase of aerosol optical depth of the ATAL by 0.002 on average which is consistent with observations.

KEY POINTS

The Asian Tropopause Aerosol Layer is composed of sulfate, primary organics, and secondary organics The North American Tropospheric Aerosol Layer is mostly composed of sulfate and secondary organics Aerosol Optical Depth of Asian Tropopause Aerosol Layer increases by 0.002 from 2000 to 2010.

Stratospheric sulfur and its implications for radiative forcing simulated by the chemistry climate model EMAC

Multiyear simulations with the atmospheric chemistry general circulation model EMAC with a microphysical modal aerosol module at high vertical resolution demonstrate that the sulfur gases COS and SO₂, the latter from low-latitude and midlatitude volcanic eruptions, predominantly control the formation of stratospheric aerosol. Marine dimethyl sulfide (DMS) and other SO₂ sources, including strong anthropogenic emissions in China, are found to play a minor role except in the lowermost stratosphere. Estimates of volcanic SO₂ emissions are based on satellite observations using Total Ozone Mapping Spectrometer and Ozone Monitoring Instrument for total injected mass and Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat or Stratospheric Aerosol and Gases Experiment for the spatial distribution. The 10 year SO₂ and COS data set of MIPAS is also used for model evaluation. The calculated radiative forcing of stratospheric background aerosol including sulfate from COS and small contributions by DMS oxidation, and organic aerosol from biomass burning, is about 0.07W/m². For stratospheric sulfate aerosol from medium and small volcanic eruptions between 2005 and 2011 a global radiative forcing up to 0.2W/m² is calculated, moderating climate warming, while for the major Pinatubo eruption the simulated forcing reaches 5W/m², leading to temporary climate cooling. The Pinatubo simulation demonstrates the importance of radiative feedback on dynamics, e.g., enhanced tropical upwelling, for large volcanic eruptions.

On the detection of mesospheric meteoric smoke particles embedded in noctilucent cloud particles with rocket-borne dust probes

Mesospheric nanoparticles in the forms of water ice particles and meteoric smoke particles (MSPs) exist in the middle atmosphere where they often play a decisive role in cloud formation and in chemical processes. Direct in situ observations of mesospheric nanoparticles have been made possible by rocket probes developed during the last two decades. Although progress has been made in mapping properties such as electric charge, sizes, and interaction with the plasma and neutral gas, more observations are needed on the size distribution, chemical content, and structure of the MSP to determine their role in cloud formation and chemistry in the mesosphere and stratosphere. We here present the result of a detailed analysis of the performance of a new dust probe MUltiple Dust Detector (MUDD) [O. Havnes et al., J. Atmos Soll.-Terr. Phys. 118, 190 (2014); O. Havenes et al., ibid. (in press)], which should give information of the size distribution of MSP by fragmenting impacting ice particles and releasing a fraction of the MSP which most probably are embedded in them [O. Havnes and L. I. Naesheim, Ann. Geophys. 25, 623 (2007); M. E. Hervig et al., J. Atmos. Sol.-Terr. Phys. 84-85, 1 (2012)]. We first determine the electric field structure and neutral gas condition in the interior of the probe and from this compute, the dynamics and current contribution of the charged fragments to the currents measured as the probe scans the fragment energy. For the single MUDD probe flown in July 2011 on the PHOCUS payload, we find that the fragment currents at the three retarding potentials for MUDD of 0, 10, and 20 V correspond to fragment sizes of ≳0.6 nm, >1.5 nm, and >1.8 nm if the fragments have a negative unit charge. We also discuss the optimum choice of retarding potentials in future flights of MUDD probes. By launching 2 to 3 mechanically identical MUDD probes but with different retarding potentials, we will obtain a much more detailed and reliable fragment (MSP) size distribution.

Bite-outs and other depletions of mesospheric electrons

The ionised mesosphere is less understood than other parts of the ionosphere because of the challenges of making appropriate measurements in this complex region. We use rocket borne in situ measurements of absolute electron density by the Faraday rotation technique and accompanying DC-probe measurements to study the effect of particles on the D-region charge balance. Several examples of electron bite-outs, their actual depth as well as simultaneous observations of positive ions are presented. For a better understanding of the various dependencies we use the ratio β/α_i (attachment rate over ion-ion recombination coefficient), derived from the electron and ion density profiles by applying a simplified ion-chemical scheme, and correlate this term with solar zenith angle and moon brightness. The probable causes are different for day and night; recent in situ measurements support existing hypotheses for daytime cases, but also reveal behaviour at night hitherto not reported in the literature. Within the large range of β/α_i values obtained from the analysis of 28 high latitude night flights one finds that the intensity of scattered sunlight after sunset, and even moonlight, apparently can photodetach electrons from meteoric smoke particles (MSP) and molecular anions. The large range of values itself can best be explained by the variability of the MSPs and by occasionally occurring atomic oxygen impacting on the negative ion chemistry in the night-time mesosphere under disturbed conditions.