Last time I promised you a forest so large it makes a good part of its own weather — rain that falls, is lifted back into the air by the trees and falls again downwind — and the argument over whether the trees are merely recycling the rain or pulling it in from the sea.
Start with two rivers. The Amazon delivers a little over seventeen billion tons of water a day to the Atlantic. By one widely quoted estimate, the trees of the basin put about twenty billion tons a day into the air. The bigger river is the one overhead, and it runs the other way — inland, then south along the Andes, made of vapour the forest has just breathed out.
The rain has already fallen
Rain carries a record of where it has been. Water made with the heavy isotope of oxygen condenses a little more readily than ordinary water, so as sea air blows inland and rains out along the way, each shower should be lighter than the last. Across most continents it is. In the 1970s Enéas Salati and his colleagues collected rain from the Atlantic coast to the Peruvian border and found that across some three thousand kilometres the heavy water barely thinned. Something was putting it back. A tree does not sort isotopes: it draws water out of the soil whole and lets it out of its leaves unchanged, heavy and light together. The rain falling on the western Amazon was, in good part, rain that had already fallen on the eastern Amazon and been lifted up again.
How good a part depends on how you ask. Draw a box around the basin and ask what fraction of the rain inside it came from evaporation inside it, and most methods answer a quarter to a third, and one says forty per cent. The slogan — that the Amazon makes half its own rain, recycled five or six times on the way west — is a round-up of that. The models that follow each parcel of vapour put the water that has been up and down more than once at roughly a tenth of the continent’s rainfall, rising to a quarter in the western basin in the dry season, when the moisture walks from east to west through the trees.
The sealed jar of No. 3 recycles everything and exports nothing. The Amazon is open at both ends: it uses part of its rain a second time and ships the surplus out, by river to the sea and by air to the farmland far to the south. And No. 37 was the water’s climb up the tree. This is what it does after it leaves the leaf.
It sends for the season
South of the equator the Amazon has a dry season, and the wet season that ends it begins in the middle of October — two or three months before the monsoon winds and the belt of converging trades that are supposed to bring it. In 2017 a team reading a satellite that can tell heavy water from light in the air answered why. Late in the dry season, under clear skies, with deep roots still finding stored water, the forest transpires harder than at any other time of year — near Manaus, transpiration rises from under half of the water leaving the surface in the wet season to almost all of it in the driest weeks. That vapour carries the soil’s isotopic signature into the middle of the atmosphere, where it builds shallow clouds; the shallow clouds moisten and destabilise the air above the forest until the convection goes deep and the rains come. The forest does not wait for the season. It sends for it — and the season it sends for fills the deep soil its roots will draw on to send for the next.
The argument
Now the pulling. In 2007 two physicists in St Petersburg pointed out that over open land, rainfall dies away within a few hundred kilometres of the coast, while over the Amazon, equatorial Africa and the Yenisey basin of Siberia it hardly falls off across thousands — and proposed that a forest actively draws sea air inland: that the air rising over so much evaporating leaf sucks the moist ocean wind in after it. The biotic pump. Earth scientists in Amsterdam answered in the same journal that the force it rests on is not supported by basic physics; the authors answered back, and the exchange is still running. The observation is not much argued. The mechanism is. Either way, the forest is load-bearing.
A loop that starts itself can fail to start
Pasture transpires far less than forest through a dry season. Over the southern Amazon the wet season now begins almost a month later than it did in the 1970s, and in 2018 two of the scientists who have watched the basin longest put the flip — from forest to something like savanna across the east and south — at twenty to twenty-five per cent cleared. There is, they wrote, “no point in discovering the precise tipping point by tipping it.”
Rain that lifts itself; a season that calls itself in. What holds the Amazon up is the Amazon, and it has been enough for sixty-five million years.
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One loop I’m watching
Next: an infection that never ends — not because anyone stays sick, but because a city is large enough that there is always someone new to hand it to; and the size below which it burns itself out and has to be carried back in from outside. Next time.