Extreme Global Warming Possible From Stratocumulus Cloud Feedback

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The week, a paper by Schneider et al. was published in Journal Nature Geoscience discussing the effect of global warming on stratocumulus clouds. It spells out a rather sobering tipping point. When carbon dioxide equivalent emissions reach near 1,300 parts per million (ppm)...equivalent to a warming over +4 C/+7 F globally...causes an extremely abrupt collapse of stratocumulus cloud cover in the tropics and subtropical latitudes. This leads to a catastrophic warming of +10 C/+18 F in the the tropics and +8 C/14 F globally. These stratocumulus clouds provide a cooling effect for the tropics and the planet by high albedo (reflecting visible light). Their loss, as modeled explicitly, will accelerate the planet to a "hothouse" state. 

What are Stratocumulus Clouds?

Stratocumulus clouds are low-level convective clouds (clouds produced and shaped by upward/downward vertical motions) which cover large areas in blanket-like "sheets" as their upward motions are "capped" by warmer air above them (known as a temperature inversion). They are "warm clouds" in infrared images from their lower proximity to the surface, but are bright and reflective on visible imagery. 

Photo by Carlye Calvin/UCAR 

Stratocumulus clouds are common over the moist, low-level atmosphere in ocean basins, especially over cold water currents not supportive of deeper convection (which would otherwise produce puffy widely spaced cumulus clouds or thunderstorms). 


Stratocumulus circled over areas offshore South America Thursday afternoon. 

Stratocumulus clouds cover 20% of the tropical oceans and reflect 30-60% of the shortwave radiation shined upon them, so they are a significant regulator of global climate.


The Stratocumulus Tipping Point 

Schneider et al (2019) modeled the stability of cumulus clouds at different concentrations of carbon dioxide. Global Climate Models (GCMs) have lower resolutions which are unable to explicitly model the behavior of clouds on finer scales. They instead attempt to account for their behavior using changes in the larger-scale temperature and moisture fields in a process known as parameterization. Parameterizations are commonly used in numerical models to account for effects which cannot be explicitly predicted, but which are necessary to decrease model errors. It is a common feature of weather forecast models as well. However, parameterizations are notoriously inaccurate and so the researchers instead explicitly modeled the effects of rising temperatures on a high-resolution tropical domain to examine the effects on cloud behavior and coverage. 

Schneider et al. found that at higher than 1,200 ppm of equivalent CO2 (so accounting for all greenhouse gases, including carbon dioxide, methane, and increasing evaporation of water vapor), longwave (heat) radiative cooling from the cloud tops of stratocumulus clouds begins to notably decrease as the near-surface atmosphere moistens from increasing evaporation from the ocean surface. This causes the clouds to form at lower altitudes, but also become thinner. The evaporative cooling (like when one sweats) of the cloud tops at lower altitudes results in less cooling as the longwave radiation reaching the cloud tops from above originates from a lower altitude with warmer temperatures as well. By 1,300 ppm, the ocean warming from below becomes sufficient to break up the stratocumulus decks, leading to the development of widely-spaced cumulus and deeper convection. With open, exposed sea surface, this causes dramatic, abrupt warming (on an order of years given current rising greenhouse gases concentrate rates) of +8 C in the tropics (and globally) and +10 C in the subtropical zones. This is in addition to the ~4 C of global warming which occurred up to that point. 


Stratocumulus tipping point and cloud feedback illustrated by Schneider et al (2019). 


Context To Abrupt Climate Change

The research deals with the effect of equivalent carbon dioxide on clouds and ultimately the sensitivity of warming on the atmospheric concentration. The high emissions scenarios of the Intergovernmental Panel on Climate Change indicate the modeled tipping point for stratocumulus stability may be reached as early as the 2090s. However, this these scenarios do not include other greenhouse gases besides carbon dioxide. Methane and nitrous oxide emissions from agriculture and (for methane) fossil fuel extraction/production is increasing exponentially, in addition to CO2. These gases are in much lower concentrations than carbon dioxide, but are increasing exponentially and are much more powerful greenhouse gases molecule to molecule. 


In addition, other feedbacks are beginning to come to life in the past decade which will likely precede the collapse of stratocumulus clouds. These including what some once thought to be slower processes...

1) Increasing methane emissions from subsea permafrost from the shallow seas within the coastal Arctic Ocean along Siberia (including the potential for abrupt releases); 

2) Increasing emissions of carbon from land permafrost and peatlands (which include the potential for abrupt warming and organic matter decomposition by high soil temperatures based on rate of land air temperature warming). 

3) Carbon emissions from more and more dying and burning trees in increasing forest fires globally. 

4) Also looming is the Arctic sea ice, which like stratocumulus clouds in the tropics/subtropics provide a large surface of high albedo, keeping the Arctic sea surface and regional air cool. But Arctic sea ice is melting rapidly, causing the Arctic to warm at 2-3 times the global rate of warming. Dr. Peter Wadhams of Cambridge University has indicated that an ice free Arctic would have the same warming effect globally as the warming which has occurred since the 1980s. 

5) Aerosols from industrial activity have been limiting the rise in global temperatures over the 20th into the early 21st century. Although this "aerosol masking", reflecting sunlight and limiting global warming was more prominent in the 1950s-60s, this effect still has a measurable effect. Significant reductions in aerosols either in a controlled (ending of coal burning in particular) or uncontrolled (economic collapse reducing industrial output) would mean a significant (~0.5-1 degree C) increase in global temperature. The climate is still warming even though current modeling of aerosols would suggest cooler conditions. 

Schneider et al. (2019), although there is a range of uncertainty as to where the tipping point for greenhouse warming, they made note that early Eocene conditions seems to approximate the intense warming noted in their modeling, with frost-free Arctic regions below 2,000 ppm carbon dioxide levels. 

So it appears that the stratocumulus tipping point may be one of many which is capable of bringing the planet to the "hothouse state" discussed by Steffen et al. (2018). And given the tipping points in the climate system...of immediate threat is the near-term (decade or less) collapse of Arctic sea ice in the warm season which can initiate/accelerate further elements...it appears...while the exact timing is unknown...that this hothouse swing is much closer than 6-8 decades from being crossed. 

----Meteorologist Nick Humphrey

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