Trees can make it rain

February 18, 2023 7:41 AM

"Rainforests have this incredible capacity to generate rainfall. Transpiration is the process whereby trees draw moisture from the ground, via their roots, to the canopy and they emit moisture. They transpire through tiny holes in their leaves called stomata. By this transpiration process, the forests returns water from rainfall to the atmosphere, preventing it from simply running back down rivers to the sea. Forests also emit tiny organic particles into the air. Pollen, specks of vegetation and spores of fungi create a nucleus around which future raindrops can form. A process called cloud seeding. Some rainforests have been found to generate up to 75% of their own rainfall through these processes. In essence, old growth rainforests enhance and create their own climate..."

"Intact forests have an extraordinary capacity to absorb and transfer moisture, and that affects the health of ecosystems hundreds of kilometres away. Coastal forests hold water. Coastal forests contribute that moisture to the atmosphere and transfer that moisture further inland. That is a lifeline to forests that are further inland. And if there is an intact continuum of forests, that acts to transfer that moisture much further inland.

Where there are breaks in forests, where forest has been cleared and lost, the water cycle is cut and areas further inland or further afield get less lifegiving moisture. They get less rainfall" - Ecologist Mark Graham

posted by chariot pulled by cassowaries (15 comments total)

It is a great irony that the made-up bullshit of Rain Follows The Plow seems to have been not just wrong, but backwards, when it comes to forest clearance.
posted by notoriety public at 10:09 AM on February 18, 2023 [11 favorites]

It is a great irony that the made-up bullshit of Rain Follows The Plow seems to have been not just wrong, but backwards, when it comes to forest clearance

The more you learn about Australian ecology, the more you think "I wish the British settlers had listened to the Aboriginal people who had 80,000 years collective experience in successfully managing the Australian landscape, rather than assuming that The British Person Is Always Right."

British people assuming that The British Person Is Always Right also caused the failure of a Welsh socialist farming collective in South America - I think Patagonia - because the local people tried to tell the British people what seeds/crops/farming techniques would work for local soil/rainfall conditions, and the British people scoffed that they knew better because they were British/white, and as a result their farm failed miserably.
posted by chariot pulled by cassowaries at 10:17 AM on February 18, 2023 [4 favorites]

When we kill forests, we also kill ourselves. The "line must go up, stonks!" crowd will not rest until every bit of life is converted to money and waste.

Defend the forests
posted by anecdotal_grand_theory at 10:18 AM on February 18, 2023 [8 favorites]

One thing that really struck me about the fact that a coastal forest can affect rainfall hundreds of kilometres inland:

if someone is seeking planning permission to chop down a forest, the planning authorities shouldn't just consider how it will affect local conditions,

because chopping down the forest could also mean that hundreds of kilometres away, someone's wheat/canola/soy/linseed/sugarcane etc no longer gets enough rainfall.

Any major forest changes need to be considered at a *state* level, not a local council level!

And in addition to considering issues like habitat loss, erosion, etc the forest's effect on the water cycle needs to be taken into serious consideration.
posted by chariot pulled by cassowaries at 10:22 AM on February 18, 2023 [5 favorites]

An early example of terra-forming a landscape to bring rain was Joseph Hooker's proposal to the British Admiralty to import trees to Ascension Island. 20 years later there were thickets of trees comprising dozens of different species all propping each other up in a part-artificial, part-natural reduced-instruction-set ecosystem. The endemic species have been hard hit by the invasives and not just the goats. Of the ten native species of ferns known in 1850, three are now extinct. But before Hooker, the lack of water was so severe that Ascension was victualled like a ship at sea and was, indeed, named HMS Ascension. So these 19thC British people succeeded in their venture . . . by some definitions of success.
posted by BobTheScientist at 10:22 AM on February 18, 2023 [4 favorites]

The other thing that occurred to me: if forests create rainfall hundreds of kilometres further inland, to what extent did coastal and near-coastal tree-felling by European settlers in the USA and Canada contribute to the Dust Bowl on the American and Canadian prairies during the 1930s that devastated so many farms?
posted by chariot pulled by cassowaries at 10:26 AM on February 18, 2023 [5 favorites]

to what extent did coastal and near-coastal tree-felling by European settlers in the USA and Canada contribute to the Dust Bowl

Don't even need to invoke rain pattern arguments there, sadly. A much larger factor there was the overuse of plowing on the relatively poor soil. The native grasses grew very thick and intertwined root systems which held the soil in place. Sown crops do not do nearly as thorough a job of soil binding. A couple of dry years is all it took to turn that formerly well-anchored topsoil into dust, which then blew away.

Maybe forest destruction caused those dry years to happen harder and faster than they would have otherwise? But even without, the farming techniques used in the Great Plains then were horribly destructive to topsoil in any case, and they would still have bled out, only slower.
posted by notoriety public at 10:46 AM on February 18, 2023 [6 favorites]

Man, this is a plot point in the SF story I'm writing and I was getting a bit tangled. Thanks a ton for the link!
edit: seems like it's geoblocked
posted by dhruva at 11:44 AM on February 18, 2023

Ah, it's a link to ABC iView, which is indeed, geolocked.
posted by freethefeet at 12:21 PM on February 18, 2023 [1 favorite]

I suppose the hadley cell winds would help direct which way your forest-rain-pump sprays its rain. So the west coasts of mid latitudes and the east coasts of the tropics? so deforestation of the us midwest would lead to dryer east coast? Deforestation in the amazon by the coasts leads to drier interior?

IDK I Am not a treeteorologist
posted by anecdotal_grand_theory at 1:03 PM on February 18, 2023

One of these years I need to finish The Retreat of the Elephants: An Environmental History of China. It talks about the impacts on rainfall as the Han Chinese people expanded outward into indigenous areas over the course of many centuries and cut down forests in order to farm. Officialdom seems to have often viewed this positively, as in one account the book quotes from "the national gazetteer for the early nineteenth century":

The commandery of Lian used long ago to be described as a place of disease... on account of its deep valleys and dense woods... It was always overcast with rain...

At the present time, the forests are sparse... the light from the sky above shines down; the population is dense, and the secluded forests every day more opened up...

posted by clawsoon at 2:05 PM on February 18, 2023 [1 favorite]

If anybody needs help to work around the geoblock, shoot me a memail.
posted by flabdablet at 8:42 PM on February 18, 2023 [1 favorite]

coastal and near-coastal tree-felling

Not much contribution from the West, the further North you go because of the Rockies.

I'm super glad that there's proof, hoping that governments make use of it though.

I remember when I was in cub scouts 33 years ago (shit) that we got to visit a camp in the old growth in Southern British Columbia and it was always drizzling and wet and it was so interesting how the water shaped the environment and how the environment worked with the water.

Everything was so green and alive and breathing.

Actual temperate rainforest. Yeah, it was uncomfy sometimes, but adequate gear and warm dry places to retreat to made it tolerable.

Not sure how long that seasonal weather lasts now. I recall that it was for most of the year.
posted by porpoise at 9:30 PM on February 18, 2023 [1 favorite]

There's indigenous knowledge about this in the U.S., too. Native people talk about how the trees bring the rain, or as a friend says, "The tree people call the cloud people."
posted by lapis at 9:42 AM on February 19, 2023 [4 favorites]

It'll be catastrophic for South American and Asia if the Amazon and Russian forests go, respectively.

Related: biotic pump (thread), The Amazon is going dry. In one parched corner, a desperate wait for water is only just beginning (thread).
posted by jeffburdges at 4:14 PM on February 19, 2023