The history of molecular oxygen (O 2 ) in Earth’s atmosphere is still debated; however, geological evidence supports at least two major episodes where O 2 increased by an order of magnitude or more: the Great Oxidation Event (GOE) and the Neoproterozoic Oxidation Event. O 2 concentrations have likely fluctuated (between 10 −3 and 1.5 times the present atmospheric level) since the GOE ∼2.4 Gyr ago, resulting in a time-varying ozone (O 3 ) layer. Using a three-dimensional chemistry-climate model, we simulate changes in O 3 in Earth’s atmosphere since the GOE and consider the implications for surface habitability, and glaciation during the Mesoproterozoic. We find lower O 3 columns (reduced by up to 4.68 times for a given O 2 level) compared to previous work; hence, higher fluxes of biologically harmful UV radiation would have reached the surface. Reduced O 3 leads to enhanced tropospheric production of the hydroxyl radical (OH) which then substantially reduces the lifetime of methane (CH 4 ). We show that a CH 4 supported greenhouse effect during the Mesoproterozoic is highly unlikely. The reduced O 3 columns we simulate have important implications for astrobiological and terrestrial habitability, demonstrating the relevance of three-dimensional chemistry-climate simulations when assessing paleoclimates and the habitability of faraway worlds.

Recently, Cooke et al . (Cooke et al . 2022 R. Soc. Open Sci. 9 , 211165. ( doi:10.1098/rsos.211165 )) used a three-dimensional coupled chemistry-climate model (WACCM6) to calculate ozone column depths at varied atmospheric O 2 levels. They argued that previous one-dimensional (1-D) photochemical model studies, e.g. Segura et al . (Segura et al . 2003 Astrobiology 3 , 689–708. ( doi:10.1089/153110703322736024 )), may have overestimated the ozone column depth at low pO 2 , and hence also overestimated the lifetime of methane. We have compared new simulations from an updated version of the Segura et al . model with those from WACCM6, together with some results from a second three-dimensional model. The discrepancy in ozone column depths is probably due to multiple interacting parameters, including H 2 O in the upper troposphere, lower boundary conditions, vertical and meridional transport rates, and different chemical mechanisms, especially the treatment of O 2 photolysis in the Schumann–Runge (SR) bands (175–205 nm). The discrepancy in tropospheric OH concentrations and methane lifetime between WACCM6 and the 1-D model at low pO 2 is reduced when absorption from CO 2 and H 2 O in this wavelength region is included in WACCM6. Including scattering in the SR bands may further reduce this difference. Resolving these issues can be accomplished by developing an accurate parametrization for O 2 photolysis in the SR bands and then repeating these calculations in the various models.

The Great Oxidation Event was a period during which Earth’s atmospheric oxygen (O2) concentrations increased from ∼10−5 times its present atmospheric level (PAL) to near modern levels, marking the start of the Proterozoic geological eon 2.4 billion years ago. Using WACCM6, an Earth System Model, we simulate the atmosphere of Earth-analogue exoplanets with O2 mixing ratios between 0.1 per cent and 150 per cent PAL. Using these simulations, we calculate the reflection/emission spectra over multiple orbits using the Planetary Spectrum Generator. We highlight how observer angle, albedo, chemistry, and clouds affect the simulated observations. We show that inter-annual climate variations, as well short-term variations due to clouds, can be observed in our simulated atmospheres with a telescope concept such as LUVOIR or HabEx. Annual variability and seasonal variability can change the planet’s reflected flux (including the reflected flux of key spectral features such as O2 and H2O) by up to factors of 5 and 20, respectively, for the same planetary phase. This variability is best observed with a high-throughput coronagraph. For example, HabEx (4 m) with a starshade performs up to a factor of two times better than a LUVOIR B (6 m) style telescope. The variability and signal-to-noise ratio of some spectral features depends non-linearly on atmospheric O2 concentration. This is caused by temperature and chemical column depth variations, as well as generally increased liquid and ice cloud content for atmospheres with O2 concentrations of <1 per cent PAL.

The climate of a terrestrial exoplanet is controlled by the type of host star, the orbital configuration and the characteristics of the atmosphere and the surface. Many rocky exoplanets have higher eccentricities than those in the Solar System, and about 18% of planets with masses <10 M⊕ have e > 0.1. Underexplored are the implications of such high eccentricities on the atmosphere, climate, and potential habitability on such planets. We use WACCM6, a state-of-the-art fully-coupled Earth-system model, to simulate the climates of two Earth-like planets; one in a circular orbit (e = 0), and one in an eccentric orbit (e = 0.4) with the same mean insolation. We quantify the effects of eccentricity on the atmospheric water abundance and loss given the importance of liquid water for habitability. The asymmetric temperature response in the eccentric orbit results in a water vapour mixing ratio in the stratosphere (>20 ppmv) that is approximately five times greater than that for circular orbit (∼4 ppmv). This leads to at most ∼3 times increases in both the atmospheric hydrogen loss rate and the ocean loss rate compared with the circular case. Using the Planetary Spectrum Generator, we simulate the idealised transmission spectra for both cases. We find that the water absorption features are stronger at all wavelengths for the e = 0.4 spectrum than for the circular case. Hence, highly-eccentric Earth-like exoplanets may be prime targets for future transmission spectroscopy observations to confirm, or otherwise, the presence of atmospheric water vapour.