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Abstract:

White dwarf–brown dwarf short-period binaries (Porb ≲ 2 h) are some of the most extreme irradiated atmospheric environments known. These systems offer an opportunity to explore theoretical and modelling efforts of irradiated atmospheres different to typical hot Jupiter systems. We aim to investigate the three-dimensional (3D) atmospheric structural and dynamical properties of the brown dwarf WD 0137−349B. We use the 3D global circulation model (GCM) Exo-Flexible Modelling System (FMS) with a dual-band grey radiative transfer scheme to model the atmosphere of WD 0137−349B. The results of the GCM model are post-processed using the 3D Monte Carlo radiative transfer model CMCRT. Our results suggest inefficient day–night energy transport and a large day–night temperature contrast for WD 0137−349B. Multiple flow patterns are present, shifting energy asymmetrically eastward or westward depending on their zonal direction and latitude. Regions of overturning are produced on the western terminator. We are able to reproduce the start of the system near-infrared (IR) emission excess at ≳1.95 μm as observed by the Gemini Near-Infrared Spectrograph (GNIRS) instrument. Our model overpredicts the IR phase curve fluxes by factors of ≈1–3, but generally fits the shape of the phase curves well. Chemical kinetic modelling using VULCAN suggests a highly ionized region at high altitudes can form on the dayside of the brown dwarf. We present a first attempt at simulating the atmosphere of a short-period white dwarf–brown dwarf binary in a 3D setting. Further studies into the radiative and photochemical heating from the ultraviolet irradiation are required to more accurately capture the energy balance inside the brown dwarf atmosphere. Cloud formation may also play an important role in shaping the emission spectra of the brown dwarf.

Abstract:

The “liquid water habitable zone” (HZ) concept is predicated on the ability of the silicate weathering feedback to stabilize climate across a wide range of instellations. However, representations of silicate weathering used in current estimates of the effective outer edge of the HZ do not account for the thermodynamic limit on concentration of weathering products in runoff set by clay precipitation, nor for the energetic limit on precipitation set by planetary instellation. We find that when the thermodynamic limit is included in an idealized coupled climate/weathering model, steady-state planetary climate loses sensitivity to silicate dissolution kinetics, becoming sensitive to temperature primarily through the effect of temperature on runoff and to pCO2 through an effect on solute concentration mediated by pH. This increases sensitivity to land fraction, CO2 outgassing, and geological factors such as soil age and lithology, all of which are found to have a profound effect on the position of the effective outer edge of the HZ. The interplay between runoff sensitivity and the energetic limit on precipitation leads to novel warm states in the outer reaches of the HZ, owing to the decoupling of temperature and precipitation. We discuss strategies for detecting the signature of silicate weathering feedback through exoplanet observations in light of insights derived from the revised picture of weathering.

Abstract:

Supercontinent assembly and breakup can influence the rate and global extent to which insulated and relatively warm subcontinental mantle is mixed globally, potentially introducing lateral oceanic‐continental mantle temperature variations that regulate volcanic and weathering controls on Earth's long‐term carbon cycle for a few hundred million years. We propose that the relatively warm and unchanging climate of the Nuna supercontinental epoch (1.8–1.3 Ga) is characteristic of thorough mantle thermal mixing. By contrast, the extreme cooling‐warming climate variability of the Neoproterozoic Rodinia episode (1–0.63 Ga) and the more modest but similar climate change during the Mesozoic Pangea cycle (0.3–0.05 Ga) are characteristic features of the effects of subcontinental mantle thermal isolation with differing longevity. A tectonically modulated carbon cycle model coupled to a one‐dimensional energy balance climate model predicts the qualitative form of Mesozoic climate evolution expressed in tropical sea‐surface temperature and ice sheet proxy data. Applied to the Neoproterozoic, this supercontinental control can drive Earth into, as well as out of, a continuous or intermittently panglacial climate, consistent with aspects of proxy data for the Cryogenian‐Ediacaran period. The timing and magnitude of this cooling‐warming climate variability depends, however, on the detailed character of mantle thermal mixing, which is incompletely constrained. We show also that the predominant modes of chemical weathering and a tectonically paced abiotic methane production at mid‐ocean ridges can modulate the intensity of this climate change. For the Nuna epoch, the model predicts a relatively warm and ice‐free climate related to mantle dynamics potentially consistent with the intense anorogenic magmatism of this period.

Abstract:

The atmospheric lifetime and radiative impacts of different climate pollutants can both differ markedly, so metrics that equate emissions using a single scaling factor, such as the 100-year Global Warming Potential (GWP100), can be misleading. An alternative approach is to report emissions as 'warming-equivalents' that result in similar warming impacts without requiring a like-for-like weighting per emission. GWP*, an alternative application of GWPs where the CO2-equivalence of short-lived climate pollutant (SLCP) emissions is predominantly determined by changes in their emission rate, provides a straightforward means of generating warming-equivalent emissions. In this letter we illustrate the contrasting climate impacts resulting from emissions of methane, a short-lived greenhouse gas, and CO2, and compare GWP100 and GWP* CO2-equivalents for a number of simple emissions scenarios. We demonstrate that GWP* provides a useful indication of warming, while conventional application of GWP100 falls short in many scenarios and particularly when methane emissions are stable or declining, with important implications for how we consider 'zero emission' or 'climate neutral' targets for sectors emitting different compositions of gases. We then illustrate how GWP* can provide an improved means of assessing alternative mitigation strategies. GWP* allows warming-equivalent emissions to be calculated directly from CO2-equivalent emissions reported using GWP100, consistent with the "Paris Rulebook" agreed by the UNFCCC. It provides a direct link between emissions and anticipated warming impacts, supporting stocktakes of progress towards a long-term temperature goal and compatible with cumulative emissions budgets.

Abstract:

© 2019, © 2019 Bulletin of the Atomic Scientists. To halt global warming, the emission of carbon dioxide into the atmosphere by human activities such as fossil fuel burning, cement production, and deforestation needs to be brought all the way to zero. The longer it takes to do so, the hotter the world will get. Lack of progress towards decarbonization has created justifiable panic about the climate crisis. This has led to an intensified interest in technological climate interventions that involve increasing the reflection of sunlight to space by injecting substances into the stratosphere which lead to the formation of highly reflective particles. When first suggested, such albedo modification schemes were introduced as a “Plan B,” in case the world economy fails to decarbonize, and this scenario has dominated much of the public perception of albedo modification as a savior waiting in the wings to protect the world against massive climate change arising from a failure to decarbonize. But because of the mismatch between the millennial persistence time of carbon dioxide and the sub-decadal persistence of stratospheric particles, albedo modification can never safely play more than a very minor role in the portfolio of solutions. There is simply no substitute for decarbonization.