Climate models have falsely assumed a (strong) cloud brightening cooling feedback, researchers of Yale University (Ivy Tan & Trude Storelvmo) and the Lawrence Livermore National Laboratory (Mark Zelinka) write in Science. Refining cloud behaviour in a warming atmosphere leads to far higher calculation of climate sensitivity – and therefore expected 21st century warming.

Yes. Part 10 of our series to unveil the ‘Real’ Global Temperature Trend is a real shocker.

Lovely image don’t you think? It comes from a different study (Nature, 2012) but it deals with the same subject: mixed-phase clouds. If ever you reach a point where you say you understand climate science, just look at this diagramme until you are as confused again as you should be…

A summary first: climate sensitivity 5 degrees or more…

Clouds have a lower ice content – that means on average they contain less ice crystals and more undercooled (sub-zero) water droplets. That in turn means under climatic warming the presumed cloud albedo (cooling) feedback is smaller than previously thought, and cloud and water vapour heat absorbtion (warming) feedbacks are of relatively larger importance.

Remodelling this insight in 3D global climate models leads to a far higher best estimate of Equilibrium Climate Sensitivity (ECS), a Yale University-led research group concludes based on special NASA satellite observations of clouds and in situ measurements of ice crystals and water droplets.

Their bombshell study was published in Science a couple of days ago. Bombshell we say because it adds to a growing pile of scientific evidence supporting the thought that (the low end of) IPCC’s climate sensitivity range leads to underestimation of probable global warming under rising atmospheric CO2 (and therefore political emissions budgets are overly optimistic still).

The climate model work of the Yale researchers was already in the high end of that spectrum, with initial calculation of Equilibrium Climate Sensitivity of 4 degrees, which they have now – based on their attempt to better model the cloud feedback – raised to 5-5.3 degrees Celsius.

(Mind you – the latest IPCC report, dating from 2014, mentioned a climate sensitivity range between 1.5-4.5 degrees – so these new numbers are high!)

The new study. Where to start. Clouds in general first

Clouds are infinitely interesting. Just look at them. They are an intricate part of Earth’s climate system, both influencing and being influenced by other factors, like temperature, precipitation, air pressure distribution, humidity and evaporation – and even terrestrial vegetation, marine life and airborne bacteria – and stuff that is too complicated for any sane person, like ammonia clustering.

Clouds have both a warming and a cooling effect on the climate, because they both absorb infrared energy and reflect sunlight. Depending on cloud type (cumulus or stratus mainly) but also cloud altitude warming or cooling effects are dominant – all combined clouds are a net warming feedback [one that consists of various smaller feedbacks, both positive and negative – and yes, clouds are a feedback only, not a climate change initiator, of course].

Now CO2-induced climate change also has a large influence on clouds – cloud formation, cloud prevalence and cloud morphology. Due to CO2-induced climate change the general circulation intensifies and both net evaporation and precipitation increase – that is for the global average.

Geographically the picture is more complicated still: the tropics may see an increase in (warming) cumulus clouds whereas the subtropics may see a decrease in (cooling) stratus clouds. Indeed, another (double) feedback we’ve already paid attention to in our special series and one that is also linked to raised climate sensitivity calculations.

But before you understand clouds you need to understand something far more fundamental first:

Water. The most potent greenhouse gas – when it is a gas:

A large part of CO2’s warming effect is caused by the direct water vapour feedback: Extra CO2 → Warming → More evaporation → More water vapour → Amplified Warming.

Water seems easy to understand, but is in fact a very complex climate factor

Water boils at 100 degrees Celsius, we’ve all learned at school. But some evaporation clearly also takes place at far lower temperatures, we all conclude after leaving a glass of water for a day or two.

Something similar happens at very low temperatures. The temperature range Anders Celsius devised is arranged around a water freezing point of zero degrees Celsius (equalling 273 Kelvin and 32 degrees Fahrenheit)

But go to the Arctic Ocean and you can (these days, sadly) find open water with temperatures below 0 Celsius. The salt content of seawater – the ions to be more precise – act as a force against ice crystals, tearing them apart. Hence seawater (depending on exact salinity) freezes not at zero, but somewhere closer to -4 degrees Celsius.

When it comes to sublimation (the phase transition from gas to solid or solid to gas) and condensation (the phase transition from gas to liquid, so the opposite of evaporation) the story gets far more complex still. But that is where things get very relevant for understanding cloud physics – and therefore cloud-climate interactions. Water vapour for instance needs a condensation nucleus (an aerosol) to become a droplet – or an ice crystal. And depending on the number of nuclei (and no doubt also the type) present – water droplet and ice crystal size is influenced. This size in turn is an important factor for cloud colour – the smaller the whiter, and the more reflective a cloud becomes.

Icy clouds reflect less sunlight – tiny water droplets are whitest

The last bit deals with relative reflectivity (albedo) of droplets versus ice crystals. Intuition may differ there, but climate scientists say clouds consisting of droplets reflect more light than clouds consisting of ice crystals.

And there one of the presumed cloud feedbacks comes into play. In a warmer world the freezing point moves higher up in the troposphere – hence a larger portion of clouds will consist of droplets, rather than ice crystals. As this would make the clouds more reflective, this is -finally- a mechanism that would count as a negative feedback on climate warming:

Extra CO2 → Warming → Less icy clouds → More Reflection → Cooling

Well now that is about as far as we can go in explaining clouds – because that is where our understanding ends. But fortunately the people at Yale walk an extra mile – and have introduced something else: the ratio between ice crystal and (sub-zero) water droplets, so the amount of ice inside (high altitude, cold) clouds.

This ratio has been overestimated by climate models they say – partly deducing this from NASA satellite observations and flying through a couple of clouds themselves.

The reason for this overestimation is simple: the climate models assumed schoolbook physics: all water becomes ice at temperatures below 0 degrees Celsius – which does not represent the complexity of the real world atmosphere – one in which a large part of those large cumulus clouds you see rising many kilometres high into a deeply frozen atmosphere consist not of ice – but of tiny ‘supercooled water droplets’. Mixed-phase clouds, climatologists call these. ‘Real-world clouds’ the researchers at Yale add.

Those cold watery clouds will remain equally watery when they warm a bit – and that’s where climate science just lost an important cooling feedback…

© Rolf Schuttenhelm | www.bitsofscience.org