When trees make rain: Could restoring forests help ease drought in Australia?
If you’ve ever walked in a rainforest or even a greenhouse, you’ll know that the air inside is heavy with moisture.
This phenomenon is caused by trees releasing water vapour through pores in their leaves called stomata.
We also know that many big forests, and rainforests in particular, tend to get more rain than surrounding areas — hence the name.
Although people have guessed that forests could help make rain, it’s always been a chicken-or-egg scenario: do forests make rain or do areas with high rainfall grow forests?
An expanding body of evidence supports the idea that forests, in the right conditions, not only make rain locally but also hundreds of kilometres away.
In Australia, we’ve cut down nearly 40 per cent of our forests in the past 200 years, leaving a fragmented landscape in their place.
In Queensland, more than one million hectares have been cleared since 2012, and New South Wales and the Northern Territory have also recently increased logging.
So if forests create rain, and we’ve chopped down almost half, have we affected the amount of rainfall we get?
And is there any evidence that returning more land to forest could bring more rain?
What’s the evidence that trees make rain?
In the southern Amazon, the tree canopy turns a light shade of green as trees put out fresh shoots two to three months before the onset of the official wet season.
Around the same time, researchers have noticed that the forest tends to build up low-lying cloud and rains increase.
Last year, researchers showed that the “greening” of the forest and increased atmospheric moisture were connected, and they used a hydrogen isotope to do it.
Isotopes are two or more forms of the same element that have different numbers of neutrons in their nuclei, and therefore have different atomic mass.
When water evaporates from the ocean, the heavier hydrogen isotope deuterium is usually left behind in favour of the lighter protium isotope.
Water vapour produced by trees, on the other hand, has a much higher deuterium concentration.
The researchers used NASA’s Aura satellite to analyse the level of deuterium present in the vapour above the Amazon during the early build up of rain.
They found the increased water vapour was almost certainly coming off the forest.
Later toward the wet season proper, the concentration of deuterium in the atmosphere above the forest reduced, as moisture was transported from the coast by increasing winds.
The researchers suspect that early rain from the forest triggers atmospheric circulation that begins pulling in moist coastal air, which eventuates in even more rain.
In other words, the moisture from the forest is kickstarting an early wet season.
If we’ve been chopping down forest in Australia, have we affected our rainfall?
Original article published by the ABC, 15 September 2018.