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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 CO_2­ 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.

Sources & Further reading

Schneider, T., et al. (2019). "Possible climate transitions from breakup of stratocumulus decks under greenhouse warming." Nature Geoscience 12(3): 163-167.