48 research outputs found
A secular increase in continental crust nitrogen during the Precambrian
Recent work indicates the presence of substantial geologic nitrogen
reservoirs in the mantle and continental crust. Importantly, this geologic
nitrogen has exchanged between the atmosphere and the solid Earth over time.
Changes in atmospheric nitrogen (i.e. atmospheric mass) have direct effects on
climate and biological productivity. It is difficult to constrain, however, the
evolution of the major nitrogen reservoirs through time. Here we show a secular
increase in continental crust nitrogen through Earth history recorded in
glacial tills (2.9 Ga to modern), which act as a proxy for average upper
continental crust composition. Archean and earliest Palaeoproterozoic tills
contain 66 100 ppm nitrogen, whereas Neoproterozoic and Phanerozoic tills
contain 290 165 ppm nitrogen, whilst the isotopic composition has
remained constant at ~4\permil. Nitrogen has accumulated in the continental
crust through time, likely sequestered from the atmosphere via biological
fixation. Our findings support dynamic, non-steady state behaviour of nitrogen
through time, and are consistent with net transfer of atmospheric N to geologic
reservoirs over time.Comment: 14 pages, 2 figures, 2 tables, supplemental informatio
Effects of Ozone Levels on Climate Through Earth History
Molecular oxygen in our atmosphere has increased from less than a part per million in the Archean Eon, to a fraction of a percent in the Proterozoic, and finally to modern levels during the Phanerozoic. While oxygen itself has only minor radiative and climatic effects, the accompanying ozone has important consequences for Earth climate. Using the Community Earth System Model (CESM), a 3-D general circulation model, we test the effects of various levels of ozone on Earth's climate. When CO2 is held constant, the global mean surface temperature decreases with decreasing ozone, with a maximum drop of ~3.5 K at near total ozone removal. By supplementing our GCM results with 1-D radiative flux calculations, we are able to test which changes to the atmosphere are responsible for this temperature change. We find that the surface temperature change is caused mostly by the stratosphere being much colder when ozone is absent; this makes it drier, substantially weakening the greenhouse effect. We also examine the effect of the structure of the upper troposphere and lower stratosphere on the formation of clouds, and on the global circulation. At low ozone, both high and low clouds become more abundant, due to changes in the tropospheric stability. These generate opposing short-wave and long-wave radiative forcings that are nearly equal. The Hadley circulation and tropospheric jet streams are strengthened, while the stratospheric polar jets are weakened, the latter being a direct consequence of the change in stratospheric temperatures. This work identifies the major climatic impacts of ozone, an important piece of the evolution of Earth's atmosphere.</p
Low Simulated Radiation Limit for Runaway Greenhouse Climates
Terrestrial planet atmospheres must be in long-term radiation balance, with solar radiation absorbed matched by thermal radiation emitted. For hot moist atmospheres, however, there is an upper limit on the thermal emission which is decoupled from the surface temperature. If net absorbed solar radiation exceeds this limit the planet will heat uncontrollably, the so-called \runaway greenhouse". Here we show that a runaway greenhouse induced steam atmosphere may be a stable state for a planet with the same amount of incident solar radiation as Earth has today, contrary to previous results. We have calculated the clear-sky radiation limits at line-by-line spectral resolution for the first time. The thermal radiation limit is lower than previously reported (282 W/sq m rather than 310W/sq m) and much more solar radiation would be absorbed (294W/sq m rather than 222W/sq m). Avoiding a runaway greenhouse under the present solar constant requires that the atmosphere is subsaturated with water, and that cloud albedo forcing exceeds cloud greenhouse forcing. Greenhouse warming could in theory trigger a runaway greenhouse but palaeoclimate comparisons suggest that foreseeable increases in greenhouse gases will be insufficient to do this
Faint young Sun paradox remains
The Sun was fainter when the Earth was young, but the climate was generally
at least as warm as today; this is known as the `faint young Sun paradox'.
Rosing et al. [1] claim that the paradox can be resolved by making the early
Earth's clouds and surface less reflective. We show that, even with the
strongest plausible assumptions, reducing cloud and surface albedos falls short
by a factor of two of resolving the paradox. A temperate Archean climate cannot
be reconciled with the low level of CO2 suggested by Rosing et al. [1]; a
stronger greenhouse effect is needed.Comment: 3 pages, no figures. In press in Nature. v2 corrects typo in author
list in original submissio
A 1D microphysical cloud model for Earth, and Earth-like exoplanets. Liquid water and water ice clouds in the convective troposphere
One significant difference between the atmospheres of stars and exoplanets is
the presence of condensed particles (clouds or hazes) in the atmosphere of the
latter.
The main goal of this paper is to develop a self-consistent microphysical
cloud model for 1D atmospheric codes, which can reproduce some observed
properties of Earth, such as the average albedo, surface temperature, and
global energy budget. The cloud model is designed to be computationally
efficient, simple to implement, and applicable for a wide range of atmospheric
parameters for planets in the habitable zone.
We use a 1D, cloud-free, radiative-convective, and photochemical equilibrium
code originally developed by Kasting, Pavlov, Segura, and collaborators as
basis for our cloudy atmosphere model. The cloud model is based on models used
by the meteorology community for Earth's clouds. The free parameters of the
model are the relative humidity and number density of condensation nuclei, and
the precipitation efficiency. In a 1D model, the cloud coverage cannot be
self-consistently determined, thus we treat it as a free parameter.
We apply this model to Earth (aerosol number density 100 cm^-3, relative
humidity 77 %, liquid cloud fraction 40 %, and ice cloud fraction 25 %) and
find that a precipitation efficiency of 0.8 is needed to reproduce the albedo,
average surface temperature and global energy budget of Earth. We perform
simulations to determine how the albedo and the climate of a planet is
influenced by the free parameters of the cloud model. We find that the
planetary climate is most sensitive to changes in the liquid water cloud
fraction and precipitation efficiency.
The advantage of our cloud model is that the cloud height and the droplet
sizes are self-consistently calculated, both of which influence the climate and
albedo of exoplanets.Comment: To appear in Icaru
Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating
Traditionally stellar radiation has been the only heat source considered
capable of determining global climate on long timescales. Here we show that
terrestrial exoplanets orbiting low-mass stars may be tidally heated at high
enough levels to induce a runaway greenhouse for a long enough duration for all
the hydrogen to escape. Without hydrogen, the planet no longer has water and
cannot support life. We call these planets "Tidal Venuses," and the phenomenon
a "tidal greenhouse." Tidal effects also circularize the orbit, which decreases
tidal heating. Hence, some planets may form with large eccentricity, with its
accompanying large tidal heating, and lose their water, but eventually settle
into nearly circular orbits (i.e. with negligible tidal heating) in the
habitable zone (HZ). However, these planets are not habitable as past tidal
heating desiccated them, and hence should not be ranked highly for detailed
follow-up observations aimed at detecting biosignatures. Planets orbiting stars
with masses <0.3 solar masses may be in danger of desiccation via tidal
heating. We apply these concepts to Gl 667C c, a ~4.5 Earth-mass planet
orbiting a 0.3 solar mass star at 0.12 AU. We find that it probably did not
lose its water via tidal heating as orbital stability is unlikely for the high
eccentricities required for the tidal greenhouse. As the inner edge of the HZ
is defined by the onset of a runaway or moist greenhouse powered by radiation,
our results represent a fundamental revision to the HZ for non-circular orbits.
In the appendices we review a) the moist and runaway greenhouses, b) hydrogen
escape, c) stellar mass-radius and mass-luminosity relations, d) terrestrial
planet mass-radius relations, and e) linear tidal theories. [abridged]Comment: 59 pages, 11 figures, accepted to Astrobiology. New version includes
an appendix on the water loss timescal