Why is the Amazon rainforest important? Water, carbon and life

The Amazon rainforest is important for three reasons that reach far beyond its borders. Its trees pump moisture into the air that falls as rain across South America; its wood and soil store 150–200 billion tonnes of carbon that would otherwise warm the planet; and it is home to roughly one in ten known species. That is why scientists watch closely how much of it is cleared, and whether it could pass a point of no return.

In short

• The forest makes much of its own rain, and that moisture also waters farms and cities far to the south and west.
• It holds carbon equal to 15–20 years of the world's carbon dioxide emissions, so losing it would speed up climate change.
• It is not "the lungs of the planet": its net contribution to the oxygen we breathe is close to zero.

How big is the Amazon rainforest?

The rainforest covers about 5.5 to 6 million square kilometers, most of a river basin of about 7 million square kilometers. Brazil holds roughly 60% of it, and the rest spreads over Peru, Colombia, Bolivia, Venezuela, Ecuador, Guyana, Suriname and French Guiana. A survey of forest plots published in 2013 estimated that it contains about 390 billion individual trees belonging to about 16,000 species, according to the Wikipedia summary of the research.

How does the Amazon make rain?

Trees draw water from the soil through their roots and release it as vapor from their leaves, a process called transpiration. Over the Amazon, that vapor rises, cools and falls again as rain, often several times on its way west across the continent. Isotope studies by the Brazilian scientist Eneas Salati in the 1970s showed that the forest generates about half of its own rainfall this way, as Thomas Lovejoy and Carlos Nobre recount in a 2018 editorial in Science Advances. The streams of moisture moving through the air are often called "flying rivers."

The rain doesn't stay in the forest. Winds carry the moisture toward the Andes and then south, where it helps water farmland and fill reservoirs in southern Brazil, Paraguay and Argentina. A 2024 review in Nature by Bernardo Flores and colleagues found that up to one-third of the yearly rainfall in the Amazonian parts of Bolivia, Peru, Colombia and Ecuador depends on water passed on by the forest in Brazil. Cutting down trees in one country can reduce the rain in another.

How much carbon does the Amazon store, and why does it matter?

Plants take carbon dioxide from the air and build it into wood, leaves and roots through photosynthesis. A forest as old and dense as the Amazon has piled up an enormous store. The Flores review puts it at 150–200 billion tonnes of carbon, which it calls equal to 15–20 years of global carbon dioxide emissions.

Worked example: what a tenth of the forest's carbon would mean

1. Carbon is counted on its own, but it reaches the air as carbon dioxide. Each tonne of carbon becomes about 3.67 tonnes of carbon dioxide, because an oxygen pair adds 32 units of mass to carbon's 12.
2. So 150–200 billion tonnes of carbon is about 550–730 billion tonnes of carbon dioxide.
3. Dividing by 15–20 years, as the review does, implies yearly global emissions of roughly 37 billion tonnes, which matches recent estimates.
4. If one-tenth of the store burned or rotted away, about 55–73 billion tonnes of carbon dioxide would be released: one and a half to two years of the whole world's emissions, added on top of everything else.

That is why the Amazon matters for climate change everywhere. Carbon dioxide strengthens the greenhouse effect wherever it is released, and a standing forest keeps that carbon out of the air. For decades the intact forest also took up more carbon each year than it lost, and studies have found that drought, fire and clearing are weakening that uptake.

Is the Amazon the lungs of the planet?

No, and the popular claim that it makes 20% of the world's oxygen is wrong. The forest's plants do release a great deal of oxygen through photosynthesis, but the same forest uses almost all of it again: trees respire, and microbes and animals consume oxygen as they break down dead leaves and wood. The ecologist Yadvinder Malhi of the University of Oxford told National Geographic that the net oxygen effect of the Amazon, as of any biome, is around zero. The air holds so much oxygen, about 21% of it, that even losing the whole forest would barely change what anyone breathes. The real reasons to care are rain, carbon and life.

Why is the Amazon so rich in species?

The Amazon is warm and wet all year, and it has stayed forested for tens of millions of years, giving species a long, stable time to diverge, as described in the note on how evolution works. Its rivers split populations of birds and monkeys, so different species live on opposite banks. About one in ten of the world's known species lives there, and many more have never been described. Each species lost is a set of genes, chemical compounds and ecological relationships that cannot be rebuilt.

Could the Amazon reach a tipping point?

A tipping point is a threshold past which a change feeds itself and becomes hard to reverse. For the Amazon, the fear is this loop: fewer trees means less transpiration, so less rain and longer dry seasons, which kill more trees and let fire spread, until parts of the forest turn into a degraded, savanna-like landscape that holds far less carbon.

How much forest has already gone depends on what is counted. The Flores review says the Amazon biome has lost 13% of its original forest area to clearing, or 15% counting young regrowth as lost. Other assessments, which count the wider region and degraded forest, put the figure nearer 20% cleared plus 6% badly degraded, as Carbon Brief reported in 2024.

The two best-known estimates of where the danger lies differ in what they expect and what they recommend.

	Lovejoy and Nobre, 2018	Flores and colleagues, 2024
Threshold named	A flip to non-forest across the east, south and center at 20–25% deforestation, below an earlier model's 40%	A critical threshold near 20% deforestation in one model; 1,000 mm of yearly rain and an eight-month dry season as other limits
Safe limit proposed	Bring deforestation back below 20% by replanting, as a margin of safety	Keep total deforestation under 10% of the original forest and warming under 1.5 °C
How change would come	A large, linked shift across much of the basin	Patchy local and regional transitions; 10–47% of the forest exposed to compounding stress by 2050

Still argued

Scientists agree that drought, warming, fire and clearing are stressing the forest. They disagree about where, how fast and how sharply it could change. The 2024 authors stress that the forest's complexity adds real uncertainty, and that a single basin-wide collapse is one possible path among several. The thresholds come from models and field studies with wide error bars, so no one can name the year or the percentage at which the forest would tip.

What happens in the Amazon reaches everyone

The Amazon is one of the clearest places to see how parts of the Earth system connect: rock and soil, described in the note on what the Earth is made of, feed the forest; the forest feeds the rain; and the rain and the stored carbon tie it to the climate of the whole planet. What keeps it standing, by every assessment above, is less clearing and less warming.

Check yourself

• Without looking back: explain how cutting trees in Brazil could reduce rainfall in Argentina or Bolivia.
• Why does the Amazon release a great deal of oxygen and yet add almost nothing to the air's supply?
• Using the worked example, roughly how many years of global emissions would one-tenth of the Amazon's carbon equal, and how did you get there?

Sources

• Critical transitions in the Amazon forest system — Flores and colleagues, Nature, 2024
• Amazon Tipping Point — Lovejoy and Nobre, Science Advances, 2018
• Why the Amazon doesn't really produce 20% of the world's oxygen — National Geographic, 2019
• Unprecedented stress in up to half of the Amazon may lead to tipping point by 2050 — Carbon Brief, 2024
• Amazon rainforest — Wikipedia