Trees Make Clouds Brighter!

We already know that trees absorb huge amounts of carbon dioxide, and so are vital regulators of climate change. But that’s not all they do.

Here, UBoC’s Dr Cat Scott explains to Paul Hudson and his listeners about the role of tree gases, (which are responsible for that lovely smell you get in  forests – especially pine ones), in the formation of clouds.

The gases become ‘kind of sticky,’ and combine with other gases to form nanometre-scale particles which clump together to influence the amount of sunlight reaching the surface of the earth, and also the brightness and therefore reflectiveness of clouds. (Brighter clouds reflect more light/heat back into space, just like bright land surfaces, thanks to the albedo effect).

We can see the role of trees as carbon sequesters in the CO2 graph below.  Because there is more land (and there are therefore more trees) in the northern hemisphere, there is more carbon dioxide being absorbed from the atmosphere when it is summer in the north, and all the deciduous trees are active. So the graph presents a saw-toothed pattern caused by fluctuations in the global average between winter and summer.

Graph from Wikimedia Commons

But research also tells us that trees, being darker than many types of land cover, also tend to reflect away a bit less of the radiation coming from the sun than paler surfaces – in an opposite effect to the albedo of the clouds they help to create.

This means that dark trees do have some warming effect (as Paul says ‘what we need is white trees!’).

However, when we model the effects, tropical trees deliver much more benefit through carbon sequestration and cloud formation than is lost by the negative albedo, keeping them alive and standing has to be our first priority.

And because Cat’s research suggests that the impact of deforestation may be as much as 10% worse than we thought – (fewer trees also means fewer clouds) – we now know we need to redouble our efforts to protect existing forests, and reverse deforestation.

You can hear the full programme, where Cat and Paul also discuss more of the science and talk about the proposed Northern Forest and our Leeds4Trees project with Leeds City Council, including our i-Tree survey of the University of Leeds campus which provides important evidence for the role of mature trees in cities, here: