Planet Earth - Cloud Forming Trees

12 June 2011

Interview with

Catherine Scott, University of Leeds' Institute of Climate and Atmospheric Science

Kat -   The benefits of planting trees, from improving air quality to providing a habitat for wildlife, are well known.  Now there's another, much loftier, effect.  Small particles released by tree leaves can also cause clouds to form.  Planet Earth podcast presenter Sue Nelson met Catherine Scott from the University of Leeds Institute of Climate and Atmospheric Science.

Catherine -   During the life of a tree it will take in a certain amount of carbon from the atmosphere in the form of carbon dioxide.  It does that as part of photosynthesis when it also releases oxygen back into the atmosphere.  What we're interested in looking at are other gasses that trees release into the atmosphere.  So if you're walking through a forest you can smell a kind of piney odour and that's because of these other compounds, volatile organic compounds. They're things like isoprene and monoterpenes.Cirrus Clouds

Sue - What in particular are you interested in about them?

Catherine -   These compounds are incredibly important because when they're released into the atmosphere they undergo reactions with a class of compounds called oxidants and that's things like ozone.  Following those reactions they're able to form tiny particles in the atmosphere via a number of different mechanisms that scientists are still trying to get a clear idea about, but we know that it happens.  So it's the impact on the climate of these particles that we're really interested in looking at.

Sue -   So what role do these particles play then?

Catherine - Well, we know that they have two main effects.  Firstly, while they're present in the atmosphere they can interact with incoming solar radiation, the energy from the sun essentially, and perturb its path so that it doesn't make it to the earth's surface and scatters it.  Additionally, and what we're most interested in looking at here, are the role that these particles play in brightening the clouds that are above the forests.  They do this because when they're in the atmosphere they grow and get to a certain size where they're able to form cloud droplets.  The more of these droplets that there are in a cloud, the whiter and brighter it becomes. That means that it will reflect away more of the incoming solar radiation.

Sue - That's amazing.  So if you've got an area where you've got a lot of trees in a forest and they're producing these volatile organic compounds which produce particles, you're likely to see the brighter whiter clouds above them.  Is it that they're sort of producing their own clouds then - brighter, whiter, fluffier clouds?

Cloud ForestCatherine -   Essentially, yes.  There's a number of other processes that govern the actual formation of the clouds but what we're interested in looking at is just how significant the impact of these particular particles are on the clouds, how much of that effect we can credit to the original compounds that are released by the trees essentially.

Sue -   What would a brighter whiter cloud then do for a forest?  Because normally nature has a way of, you know, what's in it for them sort of thing.  There's normally a beneficial effect for the forest perhaps, but maybe not necessarily on the climate, or is it beneficial to both the climate and the forest?

Catherine -   Well, we think that these particles are beneficial to the forest because of the way they scatter the radiation as it comes in. It's scattered into different directions which means that more of it is available for the leaves of the trees to use, and that's something that we think is really quite important.  As for the climate, the problem that we've got at the moment with climate change is that there's an inbalance between the amount of solar energy that's coming into the earth's system and the amount of energy that's allowed to escape from the earth's system through the atmosphere.  The more carbon dioxide and other greenhouse gases that we have in the atmosphere the less of this radiation is allowed to escape.  So the main way that we're trying to address this is by reducing greenhouse gases in the atmosphere, allowing more of the radiation to escape. Something else that we can do is try and reflect away more of the sun's radiation so that less of it gets in in the first place. That's another way we can address this energy inbalance.

Sue - Because a white cloud will reflect more solar radiation.

Catherine -   Yes, essentially.  So what we're trying to do is quantify this effect using computer simulations so that we can understand exactly the impact that forests are having on the earth's system at the moment.

Kat -   That was Catherine Scott from the University of Leeds chatting to Sue Nelson about the impact of forests on fluffy white clouds and the climate.  And you can find more Planet Earth online resources at

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