Clouds are great. They come in a vast array of forms and shapes (a teddy bear riding a shark, an elephant, amorphous blobs etc.), but more importantly, they’re a key component of the Earth system. Clouds are vital in the water cycle for delivering precipitation, and they also affect the Earth’s energy balance, i.e. the amount of radiation that goes in vs. what comes out and gets reflected back to space.
I’ve already done a podcast episode about clouds here but I thought it might be good to go into a bit more detail.
Why clouds matter for climate
The Intergovernmental Panel on Climate Change (IPCC), the body which synthesises the enormous body of scientific literature on climate change, and distills it into one mega-doc every seven years or so, notes in its most recent report [pdf] that “clouds and aerosols continue to contribute the largest uncertainty to estimates and interpretations of the Earth’s changing energy budget” (Boucher et al., 2013: 573).
It is perhaps unsurprising – clouds are incredibly hard to measure (they are in the sky, after all) and they vary massively over time and space. Besides, they are complex beasts: their properties and effects on the atmosphere depend on the height of the cloud; its chemical composition; whether it is made of liquid, ice or a combination of the two; how large the particles are; what shape the particles are… I could go on.
How to make a cloud
The presence of clouds hinges considerably on the concentration of atmospheric aerosols. Aerosols aren’t just what’s in hairspray – in this context they refer to particles in the atmosphere like dust, sea salt or clay minerals that act as nuclei onto which water can condense to form the ice crystals or liquid water droplets that make up clouds. Essentially, they are the seed for a cloud particle to form.
Aerosols act as cloud condensation nuclei (CCN) for liquid droplets, or as ice nuclei (IN) for ice crystals in clouds. Liquid droplets form relatively easily when the humidity is high enough and the temperature is low enough. They form even more easily when there are aerosols. However, the formation of ice crystals is a lot harder. When there are very few aerosols present, the temperature has to get extremely low for water droplets to spontaneously freeze into solid form. This process is called homogeneous nucleation: below -39°C, all drops freeze.
When there are aerosols around, clouds can form more easily because there is already a seed to get the process rolling. Once the particle is ‘activated’, the particle will continue to grow. It’s like a snowball effect (almost literally): the more water vapour that sticks to the particle (either via condensation if it’s liquid, or by a long list of processes if it’s ice), the less likely it is to lose mass to the atmosphere. In fact, for liquid droplets, this is related to something called the Kelvin effect, which means that bigger drops that are less curved lose water molecules from the surfaces less quickly.
Ice cloud processes are more uncertain
Different aerosols act as CCN and IN. CCN are generally soluble, while IN are usually not. What’s more important for IN is their size and shape. We know a lot more about the formation of liquid droplets than ice crystals – it’s not exactly known what makes a good IN. One thing we do know, though, is that they are much, much less common than CCN: by five or six orders of magnitude in fact. The number of IN or CCN is important because that determines how big the particles can grow inside the cloud. When there are lots of them, the water present in the cloud gets spread across more particles, which means they’re smaller. However, bigger particles are more likely to get heavy enough to fall, and falling particles become precipitation.
We don’t know a great deal about ice formation for a few reasons. Firstly, because there are so many fewer IN than CCN, they’re less easy to study. Secondly, there are many more pathways to ice crystal formation than to liquid droplet formation, for which there is one. For instance, ice crystals can form by collision with other ice crystals, by the freezing of liquid onto an existing ice crystal, by being deposited directly onto an existing ice crystal, … the list goes on. Thirdly, the technology to measure ice isn’t quite as sophisticated as that for measuring liquid droplets, so there is more error in the data. You can hear Ukrike Lohmann going into much more detail about these points on Youtube.
Different cloud types have different effects on radiation
The ‘microphysics’, a term used to refer to the in-cloud properties like particle size, shape, or ice content, of a cloud affects the way that radiation travels through it. The size distribution of the different particles (the number of particles of different sizes) determines the optical and radiative properties of the cloud i.e. how clouds interact with incoming radiation from the sun and outgoing radiation reflected back off the Earth’s surface. Because the number of CCN or IN strongly affects how big the particles are, the concentration of aerosols to act as these particles is important.
Different cloud types have different properties. For instance, ice crystals of different shapes (called habits) reflect and scatter light in different ways, which means ice-containing clouds vary in their characteristics. Some types of clouds are more reflective and therefore have a net cooling effect, whereas others are less reflective and have a net warming effect. Understanding this balance is crucial for our understanding of climate and how it might change in future. Of course, this is hindered somewhat by what we don’t know, and it is exactly these gaps in our knowledge give rise to the huge uncertainties cited in the IPCC’s recent report.
Why clouds matter for Larsen C
We’ve seen that clouds are really important for determining the balance of energy going in vs. what goes out of the Earth’s surface. There are few types of surface where that balance is more important than over ice: melting ice sheets, ice shelves and glaciers contribute to global sea level rise and can even alter the ocean’s circulation. Ice-covered regions are one of the most sensitive to climate change and it is consequently vital to get a better handle on the processes that happen there.
Let’s take an ice shelf like Larsen C. I’ve just convinced you (hopefully) that clouds affect the energy balance of a surface. Crucially, the energy balance at the ice surface determines whether the ice is melting or not, so you can see why we should care about clouds.
Polar microphysics: a lot of question marks
As I’ve mentioned, there are lots of unknowns with clouds and their effect on the energy balance. The microphysics strongly affects the radiative properties of the cloud. Problematically though, we don’t know a huge amount about the microphysics of polar clouds, and our climate models struggle to model them too as a result. Importantly, the microphysical make-up of clouds can vary considerably from place to place, so what we know about clouds over the UK will not necessarily apply over Larsen C.
Unfortunately, we have very few direct measurements of clouds over Antarctica. That’s because it’s so difficult to collect data there – the main way in situ (in-place) measurements are taken is by flying through clouds with aircraft-mounted instruments. Larsen C is remote, and flying in Antarctic conditions is difficult and dangerous. Those measurements that we do have are limited to summertime, which means we know comparatively little about what happens in winter.
More field campaigns please!
I’m sure you can see what I’m getting at. This lack of data can be solved by – you guessed it – more data collection.
Luckily, there are more and more cloud flight campaigns in the works (I’m even going on one) and projects like the cloud feedback model intercomparison project (CFMIP) are working to improve the representation of clouds in models.
More data will help constrain the properties of clouds, determine how different types of cloud affect how much radiation reaches the surface, and will begin to bring that range of uncertainty down so that we can more accurately predict what is likely to happen in future.
Well, we’ve finally got to the end. Congratulations if you made it this far, and sorry for the long read! To refresh your memory, here are some of the key points:
- Clouds are very important for Earth’s energy balance, and therefore climate.
- Polar regions are very sensitive to climate change.
- Polar microphysics are poorly understood but determine what effects clouds have on the energy balance.
- We need more research!