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Possible Climate Transitions Resulting from Stratocumulus Deck Breakup
Under Greenhouse Warming
Over oceans in lower latitudes stratocumulus clouds are present,
especially in the subtropics. They cover large parts of the surface of
the Earth which are cooled by the shading from sunlight. Their dynamical
scales are, however, too small to be resolvable in global climate
models, with the result that their response to greenhouse warming has
remained uncertain. This instability, in addition to warming by CO2
levels, triggers a warming of the surface by about 8 K globally and 10 K
in the subtropics. When the stratocumulus have broken up, they reform
only once the concentrations of CO2 have dropped
significantly below the level at which the instability first occurred.
Climate transitions that have arisen from this instability may have
contributed significantly to hothouse conditions and changes in climate
in the geological past. Schneider et
al. suggest such transitions
to a much warmer climate may also occur in the future if CO2
levels continue to rise.
The stratocumulus clouds that form above turbulent boundary layers over
large swathes of subtropical oceans are unusual among the clouds of the
Earth: the turbulence that sustains them is driven by longwave radiative
cooling of the tops of the cloud, instead of heating at the surface.
This results from cloud droplets absorbing and emitting longwave
radiation so efficiently that the clouds are essentially opaque to
longwave radiation. In contrast to this, the free troposphere above them
is dry and has few clouds, which makes it relatively Transparent to
longwave radiation. The longwave flux downwelling just above the tops of
the clouds, therefore, originates in cloud layers that are higher and
cooler and is 50-90 W m-2 weaker than the longwave flux that
is emitted by the cloud tops (Wood, 2012). Convection which is driven by
the resulting longwave cooling of the cloud tops, which penetrates the
boundary layer and supplies the clouds with moisture from the surface.
Turbulent entrainment of dry and warm free-tropospheric air across the
trade inversion, the interface that is usually sharp, between the
boundary layer and the free tropospheric air above, balances the
convective moistening and radiative cooling of the cloud layer. When the
longwave cooling of the cloud decks becomes too weak to propel parcels
of air to the surface, or when the turbulent entrainment of dry and warm
free tropospheric air across the inversion becomes too strong, the
stratocumulus decks breakup.
Key processes in boundary layers that are topped by stratocumulus, such
as entrainment across the inversion, occur at scales of tens of metres
or smaller (Stevens et al., 2003; Stevens et al., 2005; Stevens et al.,
2007; Mellado, 2016. This is much too small to be resolvable in global
climate models (GCMs), which currently have horizontal resolutions of
10s of kilometres (Schneider et al., 2017). Therefore, GCMs resort to
parameterisations, which relate the occurrence of stratocumulus to
resolved large-scale variables, such as temperature and humidity. The
parameterisations are, however, notoriously inaccurate: GCMs severely
underestimate the prevalence of stratocumulus decks (Nam et al., 2012;
Lin et al., 2014), and confidence is low in the simulations of their
climate change response (Boucher et al., 2013). As stratocumulus clouds
cover 20% of the tropical oceans (Eastman, Warren & Hahn, 2011) and
critically affect the energy balance of the Earth, reflecting 30-60% of
the shortwave radiation that is incident on them back into space (Wood,
2012), problems simulating their response to climate change percolates
into the global climate response. E.g., uncertainties in the response of
stratocumulus and other clouds lead to large uncertainties in the
predictions of global temperatures and impacts of climate (Vial,
Dufresne & Bony, 2013; Webb, Lambert & Gregory, 2013; Seneviratne, Donat
& Pitman, 2016).
Implications for climates in the past and the future
When current Global Circulation Models are run with CO2
levels up to 9,000 ppm do not appear to exhibit the instability of the
stratocumulus (Flato et al., 2013; Huber & Caballero, 2011; Caballero &
Huber, 2013), which Schneider et al. suggest is probably the result of
interactions of turbulence, cloud processes, and radiation are
inadequately parameterised in them (Bretherton, 2015). Inadequacies in
parameterisations of clouds may account for the difficulties of GCMs in
simulations of past warm climates (Kopp et al., 2017). Schneider et al.
suggest that inadequacies in cloud parameterisations may account for the
difficulties GCMs have simulating warm past climates (Kopp et al.,
2017). E.g., during the Early Eocene, about 50 Ma, the Arctic was
frost-free. Current GCMs, however, only reproduce an Arctic that is
frost-free at atmospheric levels of CO2 above 4,000 ppm
(Huber & Caballero, 2011; Cramwinckel et al., 2018), which is much
higher than the levels below 2,000 ppm reconstructed for the Eocene
(Anagnostou et al., 2016). The explanation of how such hothouse climates
can exist without such implausibly high atmospheric levels of CO2
may be the instability of stratocumulus clouds. The instability of the
stratocumulus may also have had a role in past transitions of climate,
such as the Eocene-Oligocene transition to a cool climate 34 Ma (Liu et
al., 2009).
The results of the Schneider et
al. suggest that stratocumulus decks may break up if levels of CO2
continue rising. Under high-emission scenarios of the present
(Meinshausen, 2011), equivalent CO2 concentrations of about
1,300 ppm, the lowest level at which the instability of the
stratocumulus deck was present in the simulations of Schneider et
al. can be reached within a
century. It remains uncertain, however, at which level of CO2
the stratocumulus instability occurs as they needed to parameterise
instead of resolving the large-scale dynamics that interact with the
cloud cover. According to Schneider et
al. it is imperative to
improve the parameterisations of clouds and turbulence in climate
models, in order to be able to quantify more precisely at which level of
CO2 the stratocumulus instability occurs, how it interacts
with large-scale dynamics and what its global effects are.
Schneider, T., et al. (2019). "Possible climate transitions from breakup
of stratocumulus decks under greenhouse warming." Nature Geoscience
12(3): 163-167.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |