Abstract
About 2.5 billion years ago, microbes learned to harness plentiful solar energy to reduce CO
2
with H
2
O, extracting energy and producing O
2
as waste. O
2
production from this metabolic ...process was so vigorous that it saturated its photochemical sinks, permitting it to reach “runaway” conditions and rapidly accumulate in the atmosphere despite its reactivity. Here we argue that O
2
may not be unique: diverse gases produced by life may experience a “runaway” effect similar to O
2
. This runaway occurs because the ability of an atmosphere to photochemically cleanse itself of trace gases is generally finite. If produced at rates exceeding this finite limit, even reactive gases can rapidly accumulate to high concentrations and become potentially detectable. Planets orbiting smaller, cooler stars, such as the M dwarfs that are the prime targets for the James Webb Space Telescope (JWST), are especially favorable for runaway, due to their lower UV emission compared to higher-mass stars. As an illustrative case study, we show that on a habitable exoplanet with an H
2
–N
2
atmosphere and net surface production of NH
3
orbiting an M dwarf (the “Cold Haber World” scenario), the reactive biogenic gas NH
3
can enter runaway, whereupon an increase in the surface production flux of one order of magnitude can increase NH
3
concentrations by three orders of magnitude and render it detectable by JWST in just two transits. Our work on this and other gases suggests that diverse signs of life on exoplanets may be readily detectable at biochemically plausible production rates.
Abstract
The spectroscopic characterization of terrestrial exoplanets over a wide spectral range from the near- to the mid-infrared will be made possible for the first time with the JWST. One ...challenge is that it is not known a priori whether such planets possess optically thick atmospheres or even any atmospheres altogether. However, this challenge also presents an opportunity, the potential to detect the surface of an extrasolar world. This study explores the feasibility of characterizing with the JWST the atmosphere and surface of LHS 3844b, the highest signal-to-noise rocky thermal emission target among planets that are cool enough to have nonmolten surfaces. We model the planetary emission, including the spectral signal of both the atmosphere and surface, and we explore all scenarios that are consistent with the existing Spitzer 4.5
μ
m measurement of LHS 3844b from Kreidberg et al. In summary, we find a range of plausible surfaces and atmospheres that are within 3
σ
of the observationless reflective metal-rich, iron-oxidized, and basaltic compositions are allowed, and atmospheres are restricted to a maximum thickness of 1 bar, if near-infrared absorbers at ≳100 ppm are included. We further make predictions on the observability of surfaces and atmospheres and find that a small number (∼3) of eclipse observations should suffice to differentiate between surface and atmospheric features. We also perform a Bayesian retrieval analysis on simulated JWST data and find that the surface signal may make it harder to precisely constrain the abundance of atmospheric species and may falsely induce a weak H
2
O detection.
Previous studies have shown that increases in poleward ocean heat transport (OHT) do not strongly affect tropical SST. The goal of this paper is to explain this observation. To do so, the authors ...force two atmospheric global climate models (GCMs) in aquaplanet configuration with a variety of prescribed OHTs. It is found that increased OHT weakens the Hadley circulation, which decreases equatorial cloud cover and shortwave reflection, as well as reduces surface winds and evaporation, which both limit changes in tropical SST. The authors also modify one of the GCMs by alternatively setting the radiative effect of clouds to zero and disabling wind-driven evaporation changes to show that the cloud feedback is more important than the wind–evaporation feedback for maintaining constant equatorial SST as OHT changes. This work highlights the fact that OHT can reduce the meridional SST gradient without affecting tropical SST and could therefore serve as an additional degree of freedom for explaining past warm climates.
Full text
Available for:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A key legacy of the recently launched the Transiting Exoplanet Survey Satellite (TESS) mission will be to provide the astronomical community with many of the best transiting exoplanet targets for ...atmospheric characterization. However, time is of the essence to take full advantage of this opportunity. The James Webb Space Telescope (JWST), although delayed, will still complete its nominal five year mission on a timeline that motivates rapid identification, confirmation, and mass measurement of the top atmospheric characterization targets from TESS. Beyond JWST, future dedicated missions for atmospheric studies such as the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) require the discovery and confirmation of several hundred additional sub-Jovian size planets (Rp < 10 R⊕) orbiting bright stars, beyond those known today, to ensure a successful statistical census of exoplanet atmospheres. Ground-based extremely large telescopes (ELTs) will also contribute to surveying the atmospheres of the transiting planets discovered by TESS. Here we present a set of two straightforward analytic metrics, quantifying the expected signal-to-noise in transmission and thermal emission spectroscopy for a given planet, that will allow the top atmospheric characterization targets to be readily identified among the TESS planet candidates. Targets that meet our proposed threshold values for these metrics would be encouraged for rapid follow-up and confirmation via radial velocity mass measurements. Based on the catalog of simulated TESS detections by Sullivan et al., we determine appropriate cutoff values of the metrics, such that the TESS mission will ultimately yield a sample of ∼300 high-quality atmospheric characterization targets across a range of planet size bins, extending down to Earth-size, potentially habitable worlds.
Full text
Available for:
BFBNIB, NMLJ, NUK, PNG, UL, UM, UPUK
Clouds and Snowball Earth deglaciation Abbot, Dorian S.; Voigt, Aiko; Branson, Mark ...
Geophysical research letters,
28 October 2012, Volume:
39, Issue:
20
Journal Article
Peer reviewed
Open access
Neoproterozoic, and possibly Paleoproterozoic, glaciations represent the most extreme climate events in post‐Hadean Earth, and may link closely with the evolution of the atmosphere and life. ...According to the Snowball Earth hypothesis, the entire ocean was covered with ice during these events for a few million years, during which time volcanic CO2 increased enough to cause deglaciation. Geochemical proxy data and model calculations suggest that the maximum CO2 was 0.01–0.1 by volume, but early climate modeling suggested that deglaciation was not possible at CO2 = 0.2. We use results from six different general circulation models (GCMs) to show that clouds could warm a Snowball enough to reduce the CO2required for deglaciation by a factor of 10–100. Although more work is required to rigorously validate cloud schemes in Snowball‐like conditions, our results suggest that Snowball deglaciation is consistent with observations.
Key Points
We run a suite of GCMs with Snowball Earth boundary conditions
We find that clouds can reduce the CO2 needed to deglaciate by 10‐100
Snowball deglaciation no longer seems a serious problem for the hypothesis
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Abstract CO 2 absorbs and emits radiation, which allows it to act both as radiative forcing and feedback. Recent work has shown CO 2 ’s feedback effect becomes dominant in hothouse climates, giving ...rise to a non‐monotonic climate sensitivity around 310 K. However, CO 2 ’s feedback effect in colder climates is less clear. We use a line‐by‐line model to explore the CO 2 ‐dependence of the longwave clear‐sky feedback and identify a dividing temperature. Above 290 K, feedback increases with CO 2 concentration; below 290 K, feedback decreases with CO 2 concentration. We explain this dependence in terms of spectral competition under CO 2 increases. In hot climates, CO 2 ’s moderate feedback replaces near‐zero feedback from the H 2 O bands; in cold climates, CO 2 ’s moderate feedback replaces the large feedback from the surface. Given that global mean temperature is currently close to 290 K, our results suggest that feedback CO 2 ‐dependence is weak at present but can be important in past and future climates.
Plain Language Summary CO 2 traps heat, causing warming. But CO 2 also emits heat to space, acting as radiative feedback. Recent work has shown CO 2 ’s feedback effect crucially helps to stabilize very hot climates, but how does it affect present‐day Earth? We show that in hot climates, more CO 2 increases Earth’s feedback, while in cold climates, more CO 2 decreases it. To understand why, we explain that the surface is an effective emitter, CO 2 is a moderate emitter, while H 2 O is a poor emitter. At high temperatures, adding CO 2 to the atmosphere thus replaces feedback that would have otherwise come from H 2 O, increasing the overall feedback; at low temperatures, adding CO 2 replaces feedback that would have otherwise come from the surface, decreasing the overall feedback. Currently, Earth’s global‐mean temperature falls between these two temperature regimes, where CO 2 ’s effect on feedback is nearly zero. Our results explain why CO 2 ’s impact on feedback is small now but can be significant in past or future climates.
Key Points An increase in CO 2 concentration strengthens Earth’s feedback in hot climates, ∂λ / ∂ CO 2 > 0, but weakens it in colder climates, ∂λ / ∂ CO 2 < 0 Whether feedback CO 2 ‐dependence, ∂λ / ∂ CO 2 , is positive or negative primarily depends on the extent of the H 2 O window Feedback CO 2 ‐dependence and forcing temperature‐dependence, ∂λ / ∂ CO 2 = − ∂F 2 x / ∂ T s , can be important for past or future climates
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Satellite measurements and radiative calculations show that Earth's outgoing longwave radiation (OLR) is an essentially linear function of surface temperature over a wide range of temperatures (≳60 ...K). Linearity implies that radiative forcing has the same impact in warmer as in colder climates and is thus of fundamental importance for understanding past and future climate change. Although the evidence for a nearly linear relation was first pointed out more than 50 y ago, it is still unclear why this relation is valid and when it breaks down. Here we present a simple semianalytical model that explains Earth's linear OLR as an emergent property of an atmosphere whose greenhouse effect is dominated by a condensable gas. Linearity arises from a competition between the surface's increasing thermal emission and the narrowing of spectral window regions with warming and breaks down at high temperatures once continuum absorption cuts off spectral windows. Our model provides a way of understanding the longwave contribution to Earth's climate sensitivity and suggests that extrasolar planets with other condensable greenhouse gases could have climate dynamics similar to Earth's.
Full text
Available for:
BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Abstract
Climate models and observations robustly agree that Earth’s clear-sky longwave feedback has a value of about −2 W m
−2
K
−1
, suggesting that this feedback can be estimated from first ...principles. In this study, we derive an analytic model for Earth’s clear-sky longwave feedback. Our approach uses a novel spectral decomposition that splits the feedback into four components: a surface Planck feedback and three atmospheric feedbacks from CO
2
, H
2
O, and the H
2
O continuum. We obtain analytic expressions for each of these terms, and the model can also be framed in terms of Simpson’s law and deviations therefrom. We validate the model by comparing it against line-by-line radiative transfer calculations across a wide range of climates. Additionally, the model qualitatively matches the spatial feedback maps of a comprehensive climate model. For present-day Earth, our analysis shows that the clear-sky longwave feedback is dominated by the surface in the global mean and in the dry subtropics; meanwhile, atmospheric feedbacks from CO
2
and H
2
O become important in the inner tropics. Together, these results show that a spectral view of Earth’s clear-sky longwave feedback elucidates not only its global-mean magnitude, but also its spatial pattern and its state dependence across past and future climates.
Significance Statement
The climate feedback determines how much our planet warms due to changes in radiative forcing. For more than 50 years scientists have been predicting this feedback using complex numerical models. Except for cloud effects the numerical models largely agree, lending confidence to global warming predictions, but nobody has yet derived the feedback from simpler considerations. We show that Earth’s clear-sky longwave feedback can be estimated using only pen and paper. Our results confirm that numerical climate models get the right number for the right reasons, and allow us to explain regional and state variations of Earth’s climate feedback. These variations are difficult to understand solely from numerical models but are crucial for past and future climates.
Full text
Available for:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
We spectrally resolve the conventional clear‐sky temperature and water vapor feedbacks in an idealized single‐column framework, and show that the well‐known partial compensation of these feedbacks is ...actually due to an almost perfect cancellation of the spectral feedbacks at wavenumbers where H2O is optically thick. This cancellation is a natural consequence of “Simpson's Law”, which says that H2O emission temperatures do not change with surface warming if relative humidity (RH) is fixed. We provide an explicit formulation and validation of Simpson's Law, and furthermore show that this spectral cancellation of feedbacks is naturally incorporated in the alternative RH‐based framework proposed by Held and Shell (2012, https://doi.org/10.1175/jcli-d-11-00721.1) and Ingram (2012, https://doi.org/10.1029/2011jd017221, 2013b, https://doi.org/10.1007/s00382-012-1294-3), thus bolstering the case for switching from conventional to RH‐based feedbacks. We also find a negligible RH‐based clear‐sky lapse rate feedback, suggesting that the impact of changing lapse rates depends crucially on whether relative or specific humidity is held fixed.
Plain Language Summary
Feedback analyses aim to isolate processes in the climate system which may amplify or diminish its response to an external forcing. In calculating feedbacks due to the warming of the atmosphere, however, a choice must be made as to whether the absolute or relative humidity (RH) is to be held fixed as the impact of warming is assessed. Here we examine these impacts frequency‐by‐frequency in the infrared spectrum, and find that fixing the RH leads to a much simpler picture than fixing absolute humidity.
Key Points
Conventional feedbacks exhibit strong spectral cancellation
This cancellation follows from “Simpson's Law” for water vapor thermal emission
Relative humidity‐based feedbacks naturally incorporate this cancellation, and more naturally manifest Simpson's Law
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Atmospheric circulation in a Snowball Earth is critical for determining cloud behavior, heat export from the tropics, regions of bare ice, and sea glacier flow. These processes strongly affect ...Snowball Earth deglaciation and the ability of oases to support photosynthetic marine life throughout a Snowball Earth. Here we establish robust aspects of the Snowball Earth atmospheric circulation by running six general circulation models with consistent Snowball Earth boundary conditions. The models produce qualitatively similar patterns of atmospheric circulation and precipitation minus evaporation. The strength of the Snowball Hadley circulation is roughly double modern at low CO2 and greatly increases as CO2 is increased. We force a 1‒D axisymmetric sea glacier model with general circulation model (GCM) output and show that, neglecting zonal asymmetry, sea glaciers would limit ice thickness variations to O(10%). Global mean ice thickness in the 1‒D sea glacier model is well‒approximated by a 0‒D ice thickness model with global mean surface temperature as the upper boundary condition. We then show that a thin‒ice Snowball solution is possible in the axysymmetric sea glacier model when forced by output from all the GCMs if we use ice optical properties that favor the thin‒ice solution. Finally, we examine Snowball oases for life using analytical models forced by the GCM output and find that conditions become more favorable for oases as the Snowball warms, so that the most critical time for the survival of life would be near the beginning of a Snowball Earth episode.
Key Points
Snowball Hadley cell is stronger than modern and increases with CO2 in six GCMs
GCMs produce a similar Snowball P-E pattern with net ablation in the tropics
Sea glacier thickness variations and oases are more favorable as Snowball ages
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
PDF
