The current explosion in detection and characterization of thousands of extrasolar planets from the Kepler mission, the Hubble Space Telescope, and large ground-based telescopes opens a new era in ...searches for Earth-analog exoplanets with conditions suitable for sustaining life. As more Earth-sized exoplanets are detected in the near future, we will soon have an opportunity to identify habitale worlds. Which atmospheric biosignature gases from habitable planets can be detected with our current capabilities? The detection of the common biosignatures from nitrogen-oxygen rich terrestrial-type exoplanets including molecular oxygen (O2), ozone (O3), water vapor (H2O), carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) requires days of integration time with largest space telescopes, and thus are very challenging for current instruments. In this paper we propose to use the powerful emission from rotational-vibrational bands of nitric oxide, hydroxyl and molecular oxygen as signatures of nitrogen, oxygen, and water rich atmospheres of terrestrial type exoplanets "highlighted" by the magnetic activity from young G and K main-sequence stars. The signals from these fundamental chemical prerequisites of life we call atmospheric "beacons of life" create a unique opportunity to perform direct imaging observations of Earth-sized exoplanets with high signal-to-noise and low spectral resolution with the upcoming NASA missions.
The lifetime of nitrous oxide, the third‐most‐important human‐emitted greenhouse gas, is based to date primarily on model studies or scaling to other gases. This work calculates a semiempirical ...lifetime based on Microwave Limb Sounder satellite measurements of stratospheric profiles of nitrous oxide, ozone, and temperature; laboratory cross‐section data for ozone and molecular oxygen plus kinetics for O(1D); the observed solar spectrum; and a simple radiative transfer model. The result is 116 ± 9 years. The observed monthly‐to‐biennial variations in lifetime and tropical abundance are well matched by four independent chemistry‐transport models driven by reanalysis meteorological fields for the period of observation (2005–2010), but all these models overestimate the lifetime due to lower abundances in the critical loss region near 32 km in the tropics. These models plus a chemistry‐climate model agree on the nitrous oxide feedback factor on its own lifetime of 0.94 ± 0.01, giving N2O perturbations an effective residence time of 109 years. Combining this new empirical lifetime with model estimates of residence time and preindustrial lifetime (123 years) adjusts our best estimates of the human‐natural balance of emissions today and improves the accuracy of projected nitrous oxide increases over this century.
Key Points
Nitrous oxide lifetime is computed empirically from MLS satellite data
Empirical N2O lifetimes compared with models including interannual variability
Results improve values for present anthropogenic and preindustrial emissions
Large solar coronal mass ejections pose a threat in the near-Earth space. As a cause of extreme periods of space weather, they can damage satellite-based communications and create geomagnetically ...induced currents in power and energy grids. Further, the solar wind energetic particles can reduce the protecting layer of atmospheric ozone and pose a threat to life on Earth. The large coronal mass ejection (CME) of July 2012, although directed away from the Earth, is often highlighted as a prime example of a potentially devastating super storm. Here we show, based on proton fluxes recorded by the instruments aboard the STEREO-A satellite, that the atmospheric response to the July 2012 event would have been comparable to those of the largest solar proton events of the satellite era. Significant impact on total ozone outside polar regions would require a much larger event, similar to those recorded in historical proxy data sets. Such an extreme event would cause long-term ozone reduction all the way to the equator and increase the size, duration, and depth of the Antarctic ozone hole. The impact would be comparable to predicted drastic and sudden ozone reduction from major volcanic eruptions, regional nuclear conflicts, or long-term stratospheric geoengineering.
We present a coupled Monte Carlo and multistream model simulating primary ionization and secondary electron ionization, respectively, from energetic proton precipitation in the Earth's upper ...atmosphere. Good agreement is obtained with previous model results. It is found that while secondary electrons make a negligible contribution to ionization from low‐energy (≤10 keV) auroral proton precipitation, their importance increases with increasing incident proton energy, confirming earlier findings. It becomes significant or even comparable to primary ionization from protons and generated hydrogen atoms in charge‐changing collisions. Our calculations of the mean energy loss per ion pair production show a nearly monotonic increase with incident proton energy, ranging from about 22 eV to 33 eV when incident energy increases from 100 eV to 1 MeV. To facilitate a fast calculation in large‐scale computations, we develop a parameterization for total (primary plus secondary) ionization from monoenergetic proton precipitation. This is obtained by fitting to a large set of numerical results from the coupled model. The quick method applies to a wide energy range of 100 eV to 1 MeV for incident monoenergetic protons, and its validity has been extensively tested under a variety of background atmospheric conditions. Our new parameterization can be used to rapidly calculate the ionization altitude profile from precipitating protons with any spectral distributions without any significant compromise in accuracy. By considering branching ratios of ionized atmospheric species, the fast calculation method is thus useful for self‐consistently including proton impact effects in large community models.
Key Points
A coupled Monte Carlo and multi‐stream model for studying proton precipitation
A fast method for calculating ionization altitude profiles from proton impact
New parameterization is accurate and applies to 100 eV to 1 MeV incident protons
The Whole Atmosphere Community Climate Model (WACCM3) has been used to study the long‐term (more than a few months) effects of solar proton events (SPEs). Extremely large solar proton events occurred ...in 1972, 1989, 2000, 2001, and 2003 and caused some longer‐lasting atmospheric changes. The highly energetic solar protons produced odd hydrogen (HOx) and odd nitrogen (NOy), which then led to ozone variations. Some statistically significant long‐term effects on mesospheric ozone were caused by the HOx increases due to a very active time period for SPEs (years 2000–2004), even though the HOx increases were short‐lived (days). The long‐term stratospheric ozone effects were caused by the NOy enhancements. Very large NOy enhancements lasted for months in the middle and lower stratosphere after a few of the largest SPEs. SPE‐caused NOy increases computed with WACCM3 were statistically significant at the 95% level throughout much of the polar stratosphere and mesosphere in the recent solar maximum 5‐year period (2000–2004). WACCM3‐computed SPE‐caused polar stratospheric ozone decreases of >10% continued for up to 5 months past the largest events; however, statistically significant ozone decreases were computed for only a relatively small fraction of this time in relatively limited altitudes in the lower mesosphere and upper stratosphere. Annually averaged model output showed statistically significant (to 95%) stratospheric ozone loss in the polar Northern Hemisphere for years 2000–2002. The computed annually averaged temperature and total ozone change in these years were not statistically significant.
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► A monophyletic New Zealand clade, most closely related to Australian Diplodactylidae. ► Hoplodactylus is paraphyletic and composed of broad- and narrow-toed clades. ► Recognition of ...16 new species (36 total) and 5 new or resurrected genera (seven total). ► New Zealand radiation split after opening of Tasman Sea, refuting Gondwanan vicariance. ► Age of crown group suggests New Zealand was not completely inundated during Oligocene.
We used a multi-gene approach to assess the phylogenetic relationships of New Zealand diplodactylid geckos to their Australian and New Caledonian relatives and to one another. Data from nuclear (RAG-1, PDC) and mitochondrial (ND2, 16S) genes from >180 specimens representing all 19 recognized New Zealand taxa and all but two of 20 putatively new species suggested by previous studies were analyzed using Maximum Parsimony, Maximum Likelihood and Bayesian inference. All analyses retrieved a monophyletic New Zealand clade, most closely related to the Australian Diplodactylidae exclusive of Pseudothecadactylus. Hoplodactylus is paraphyletic and composed of two morphological groups: a broad-toed clade, consisting of the island-restricted, largest extant species, Hoplodactylus duvaucelii, and the species-rich, wide-ranging Hoplodactylus maculatus clade; and a narrow-toed clade, comprising five monophyletic subgroups: Naultinus, the Hoplodactylus pacificus and Hoplodactylus granulatus clades, and the distinctive species Hoplodactylus rakiurae and Hoplodactylus stephensi. Each of these lineages is here recognized at the generic level. Our data support recognition of 16 new species (36 total), and five new or resurrected genera (seven total). The New Zealand diplodactylid radiation split from its Australian relatives 40.2mya (95% highest posterior density estimate 28.9–53.5), after the opening of the Tasman Sea. Their distribution cannot, therefore, be regarded as derived as a result of Gondwanana vicariance. The age of the New Zealand crown group, 24.4mya (95% highest posterior density estimate 15.5–33.8), encompasses the period of the ‘Oligocene drowning’ of New Zealand and is consistent with the hypothesis that New Zealand was not completely inundated during this period. Major lineages within New Zealand geckos diverged chiefly during the mid- to late Miocene, probably in association with a suite of geological and climatological factors that have characterized the region’s complex history.
We present a new parameterization of the altitude profile of the ionization rate in the Earth's atmosphere due to precipitating energetic electrons. Precipitating electrons are assumed to have a ...Maxwellian energy distribution and an isotropic pitch angle distribution above the atmosphere. In this study, two electron transport models (whose validity has been verified by observations) are employed to calculate the ionization rate, to which we have fit our new parameterization. To derive a new parameterization, we follow a similar scheme to that of Roble and Ridley (1987) but take into account further functional dependence on the incident electron energy. As a result, the new method presented in this paper provides a highly improved prediction for electron impact in a significantly extended energy range from 100 eV to 1 MeV, spanning 4 orders of magnitude. Note that we have neglected the contribution of bremsstrahlung X rays generated by energetic electrons, which are mostly important below 50 km altitude. The comparison of parameterization results with model calculations shows that the errors generally fall well within ±5% in both the altitude‐integrated total ionization rate and the peak value. The altitude profile as a whole is also accurately predicted, with errors in the altitudes of the peak and e‐folding ionization rates significantly less than 5 km. The proposed new parameterization method with high accuracy is thus ready to be implemented into global models to assess the electron impact on the ionosphere and the atmosphere.
Nitrate ion spikes in polar ice cores are contentiously used to estimate the intensity, frequency, and probability of historical solar proton events, quantities that are needed to prepare for ...potentially society‐crippling space weather events. We use the Whole Atmosphere Community Climate Model to calculate how large an event would have to be to produce enough odd nitrogen throughout the atmosphere to be discernible as nitrate peaks at the Earth's surface. These hypothetically large events are compared with probability of occurrence estimates derived from measured events, sunspot records, and cosmogenic radionuclides archives. We conclude that the fluence and spectrum of solar proton events necessary to produce odd nitrogen enhancements equivalent to the spikes of nitrate ions in Greenland ice cores are unlikely to have occurred throughout the Holocene, confirming that nitrate ions in ice cores are not suitable proxies for historical individual solar proton events.
Key Points
SPE‐enhanced atmospheric NOy does not account for NO3− spikes in ice cores
NO3− in ice cores is a poor proxy for historical SPEs
A key stratospheric loss process for ozone depleting substances (ODSs) and greenhouse gases (GHGs) is reaction with the O(1D) atom. In this study, rate coefficients, k, for the O(1D) atom reaction ...were measured for the following key halocarbons: chlorofluorocarbons (CFCs) CFCl3 (CFC-11), CF2Cl2 (CFC-12), CFCl2CF2Cl (CFC-113), CF2ClCF2Cl (CFC-114), CF3CF2Cl (CFC-115); hydrochlorofluorocarbons (HCFCs) CHF2Cl (HCFC-22), CH3CClF2 (HCFC-142b); and hydrofluorocarbons (HFCs) CHF3 (HFC-23), CHF2CF3 (HFC-125), CH3CF3 (HFC-143a), and CF3CHFCF3 (HFC-227ea). Total rate coefficients, k T, corresponding to the loss of the O(1D) atom, were measured over the temperature range 217–373 K using a competitive reactive technique. k T values for the CFC and HCFC reactions were >1 × 10–10 cm3 molecule–1 s–1, except for CFC-115, and the rate coefficients for the HFCs were in the range (0.095–0.72) × 10–10 cm3 molecule–1 s–1. Rate coefficients for the CFC-12, CFC-114, CFC-115, HFC-23, HFC-125, HFC-143a, and HFC-227ea reactions were observed to have a weak negative temperature dependence, E/R ≈ −25 K. Reactive rate coefficients, k R, corresponding to the loss of the halocarbon, were measured for CFC-11, CFC-115, HCFC-22, HCFC-142b, HFC-23, HFC-125, HFC-143a, and HFC-227ea using a relative rate technique. The reactive branching ratio obtained was dependent on the composition of the halocarbon and the trend in O(1D) reactivity with the extent of hydrogen and chlorine substitution is discussed. The present results are critically compared with previously reported kinetic data and the discrepancies are discussed. 2D atmospheric model calculations were used to evaluate the local and global annually averaged atmospheric lifetimes of the halocarbons and the contribution of O(1D) chemistry to their atmospheric loss. The O(1D) reaction was found to be a major global loss process for CFC-114 and CFC-115 and a secondary global loss process for the other molecules included in this study.
Gamma-ray bursts (GRBs) are likely to have made a number of significant impacts on the Earth during the last billion years. The gamma radiation from a burst within a few kiloparsecs would quickly ...deplete much of the Earth's protective ozone layer, allowing an increase in solar UVB radiation reaching the surface. This radiation is harmful to life, damaging DNA and causing sunburn. In addition, NO sub(2) produced in the atmosphere would cause a decrease in visible sunlight reaching the surface and could cause global cooling. Nitric acid rain could stress portions of the biosphere, but the increased nitrate deposition could be helpful to land plants. We have used a two-dimensional atmospheric model to investigate the effects on the Earth's atmosphere of GRBs delivering a range of fluences, at various latitudes, at the equinoxes and solstices, and at different times of day. We have estimated DNA damage levels caused by increased solar UVB radiation, reduction in solar visible light due to NO sub(2) opacity, and deposition of nitrates through rainout of HNO sub(3). For the "typical" nearest burst in the last billion years, we find globally averaged ozone depletion up to 38%. Localized depletion reaches as much as 74%. Significant global depletion (at least 10%) persists up to about 7 yr after the burst. Our results depend strongly on time of year and latitude over which the burst occurs. The impact scales with the total fluence of the GRB at the Earth but is insensitive to the time of day of the burst and its duration (1-1000 s). We find DNA damage of up to 16 times the normal annual global average, well above lethal levels for simple life forms such as phytoplankton. The greatest damage occurs at mid- to low latitudes. We find reductions in visible sunlight of a few percent, primarily in the polar regions. Nitrate deposition similar to or slightly greater than that currently caused by lightning is also observed, lasting several years. We discuss how these results support the hypothesis that the Late Ordovician mass extinction may have been initiated by a GRB.
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