‘We humans have altered clouds — these are not the same now as in the pre-industrial era’

Daniel Cziczo is Professor of Earth, Atmospheric and Planetary Sciences at Purdue University. Speaking to Srijana Mitra Das at Times Evoke, he discusses clouds, changes in them — and why those impact us:
■What is the core of your research?
My group and I work on understanding and measuring particles in the atmosphere called aerosols — we’re researching the science and composition of these. They have many implications, including for human health which is impacted by breathing in particles, and our climate. Small particles influence cloud formation — clouds reflect sunlight back into space, offsetting some of Earth’s heating, but certain particles trap some heat which we’re studying. Another project we are working on is spacecraft re-entering Earth’s atmosphere, mixing with particles from meteorites — this can also create particles which impact our atmospheric chemistry and climate.
■We think of clouds as transitory phenomena — what is their role in climate systems?Most folk interested in climate change know about greenhouse gases like carbon dioxide or methane in the atmosphere — these cause global warming by trapping some outgoing heat, also called outgoing radiation. The physics of this has been relatively well-understood from the 1800s — clouds are much more poorly understood but they are also important players in our climate. Changes in clouds cause flooding or droughts. Such changes are brought about by particles composing clouds. Consider dark-coloured particles like black carbon which come from cars or engines — these trap heat trying to escape Earth. Particles are also the seeds on which droplets and ice crystals form to make clouds. Those clouds usually scatter some radiation back to space, cooling a portion of global warming. Now, as we change our emissions of greenhouse gases, we are changing these particles — estimates find we could have doubled the particulate content of our atmos phere. This means we humans are changing clouds and climate — clouds in the modern world are different from what they were in pre-industrial times. Understanding this is extremely important to get our climate projections right.

■Are there different kinds of clouds?
There are diverse kinds, like low-lying stratus clouds or a cloud-deck, cumulus clouds which are those puffy formations you can see on an otherwise clear day and cirrus clouds, high-altitude wispy clouds. Clouds are different in their form and properties — lower-level clouds tend to be warmer, with more liquid water, while cirrus clouds are very high and extremely cold, composed completely of ice crystals. They are unique in the climate system because this cloud type actually traps more heat than it reflects back to space — so, cirrus clouds warm the climate. Hence, how these clouds are set up, where they are and how we are impacting them influences our climate.

■Which particles compose clouds naturally versus ones coming from human activities?
Many particles in the atmosphere can’t be delineated as coming directly from anthropogenic or natural sources. For instance, there are sulphate particles — these can come from natural sources like volcanos but also from industry. Certain particles are almost entirely humanmade, including black carbon. One intriguing kind is mineral dust which comes off dry roads or during a dust storm. Mineral dust is natural, rising off the surface of the planet — but land use changes by humans, including cutting down trees, expanding agriculture or overgrazing animals, creates more of it. So, it’s a natural particle but it’s been greatly increased by human activities. This is one of the more effective cloud nucleating agents, especially for ice clouds — so, changes in its quantity have a direct impact on our climate.

■What does ‘cloud forcing’ mean?
Particles on their own, like black carbon which absorbs heat or an organic particle which can scatter some sunlight back to space, influence climate — this is ‘direct forcing’. ‘Cloud forcing’ is the same thing except there is the intermediate step of a particle making a droplet or an ice crystal which is part of a cloud — then, that cloud either traps heat or reflects sunlight back to space. This ‘cloud forcing’ changes our environment.

■Which other anthropogenic activities are impacting the clouds now?
We humans are changing the whole world around us — some activities are less obvious but make more particles in the environment. Deforestation normally couples with intentional burning of forests which creates a lot of particles. We are also studying human-caused effects coming from the top of the atmosphere now via the re-entry of satellites. The space industry has looked at deorbiting spacecraft almost as a way of getting rid of junk, so it doesn’t obstruct future launches. However, those satellites don’t come down as intact pieces — they burn up in the upper atmosphere. That material re-condenses and forms small particles which then fall through our atmosphere. We are studying their impacts — we must be cognisant of these, given the changes seen already in our atmosphere and the number of satellite launches likely to increase greatly.


■Can you tell us about your work on the North American monsoon and the clouds there?
Most people are aware of the Asian monsoon which is so big and impactful on India. The North American monsoon is also seasonal and while much smaller in scale, it is very important for the water balance of North America. We’ve been researching the impacts of climate change here — warmer climates will mean greater convection and more intense storms. One aspect we are studying is how these storms can become powerful enough to not just remain in our part of the atmosphere, which is the lower part called the troposphere, but penetrate the next level up, which is the stratosphere. Such overshooting is becoming more frequent with climate change — the effects of the water, particles and gases these storms can push into the stratosphere are poorly-known.

■How do you physically study clouds?
First, we use aircraft to actually fly into clouds — we’ve developed different inlets for the aircraft which allow us to bring in small particles or the cloud, meaning ice crystals or droplets. We measure these in an instrument which tells us their composition. The second method is making clouds in our laboratory — we have chambers which let us set temperature and relative humidity very accurately. We can then put particles in those chambers and see what clouds form.

When you fly into a cloud, it’s already existing — you don’t really know at what temperature and relative humidity it formed. You have to do this in a lab — so, we need both the field and laboratory to understand this world of clouds.