Science Matters

Forget two Amazon rainforest myths

Boyce Rensberger

(3/2024) The vast and forbidding Amazon rainforest holds not just exotic wildlife but two common myths. I’ll tell you both right now and then go into the details.

One, it is wrong to call the forest "the lungs of the Earth." It is the opposite.

Two, the Amazon has not been a tropical wilderness since time immemorial. What we have today is only about 400 years old.

First, the "lungs" bit. This line misunderstands what lungs do. Lungs take in oxygen and give off carbon dioxide. The Amazon does the opposite. All green plants take in carbon dioxide and give off oxygen. That’s during the daytime.

At night the process reverses as plants switch over to doing what animals do—consume oxygen and give off carbon dioxide. This is when plants do most of their growing. In the process the Amazon consumes almost all the oxygen it produces, leaving only 6 percent to 9 percent to go into the global atmosphere. So, the Amazon is not the lungs of anything and not even a significant contributor to the world’s oxygen.

The much bigger source of "new" atmospheric oxygen is algae in the oceans, which produce about three-quarters of the oxygen released to the atmosphere. Amazingly, there is way more plant biomass in the world’s oceans than in all land plants put together.

Let’s step back and examine that daily reversal in how plants handle oxygen and carbon dioxide. In sunlight all green plants take in carbon dioxide and water to make a sugar called glucose. Solar energy drives the chemical reactions. Glucose stores the energy in its bonds between atoms.

Plants then use sugar’s energy to carry out processes needed to grow. Also, some sugars are linked to make starch and, with still more linkage, cellulose. This process, called respiration, consumes oxygen and gives off carbon dioxide—just as we animals do.

Now for that second myth. The truth is that the Amazon forest has not been a "pristine" ecosystem since time immemorial. Studies in recent decades have found that centuries ago much of the region had a drier climate with a more open landscape, home to cities with ceremonial centers containing large pyramids. Around the cities were farms with canals for drainage or irrigation. In one small part of the forest two more such cities have recently been found, each linked to suburbs by raised causeways that run for miles. In the same region scientists found at least 79 smaller villages and hamlets.

To be clear, the scientists are not referring to the small tribal bands that inhabit parts of the Amazon today such as the Yanomami or the Xingu. Today’s rainforest tribes haven’t built large earthworks or ceremonial centers, though they are probably descended from the ancient peoples.

Legends of "lost cities" in the Amazon have been around for a long time. The earliest evidence that some may exist began to emerge in the 1970s. In the decades since, more and more clues have emerged.

"The civic-ceremonial architecture of these large settlement sites includes stepped platforms, on top of which lie U-shaped structures, rectangular platform mounds and conical pyramids," a German-British team of archaeologists wrote in their report in a recent issue of the international journal Nature. The tallest pyramids are more than 70 feet high. The central plaza in one city was a tenth of a mile across.

The archaeologists who found these two cities searched only a tiny fraction of the forest. They believe many more wait to be found.

The discovery of these earthworks, now covered with tall trees and many other kinds of tropical vegetation, was made possible by an aerial survey using instruments that can see through the jungle. The instruments use LiDAR (Light Detection and Ranging), which sends wavelengths of laser light that penetrate vegetation but bounce back from hard surfaces hundreds of thousands of times per second. The timing of the returned light pulses is a measure of the distance below the aircraft as it flies back and forth in a pattern called "mowing the lawn." A computer assembles the data points into an image. (The picture above is one example.)

Archaeologists will now try to visit the newly discovered sites. A few have already yielded pottery containing residues of corn, beans, manioc, and sweet potatoes.

LiDAR studies in the Ecuadorian part of the forest have found the ruins of cities that once were home to tens of thousands of people, beginning around 2,500 years ago.

After studying another part of the Amazon, an American and Brazilian team wrote in Nature: "Evidence of large, well-engineered public works (such as plazas, roads, moats, and bridges) in and between pre-Columbian settlements suggests a highly elaborate built environment, rivaling that of many contemporary complex societies of the Americas and elsewhere."

The researchers estimate that walled towns they found were each home to about 2,500 people with outlying settlements of 100 to 250. The people went to great lengths to build roads kept straight by cutting as much as 15 feet into a hillside when it would have been easier to go around the hill. The longest road found so far runs at least 15 miles.

Taken together, the evidence shows that much of the Amazon forest is far younger than once believed. Over thousands of square miles, there were not only small cities but networks of reservoirs, canals, farms, and roads across raised causeways. Evidence shows that the cultures that built the cities existed for nearly a thousand years, dying out by the year 1600. That was before Europeans reached the area.

What we learn from this research is not only that such ancient cultures existed, perhaps rivaling the well-known Maya or Inca societies. In just 400 years the local climate can turn wetter, causing the rainforest to expand and hide the cities. That’s a long time in human years, but of course a mere blip in nature’s lifespan. The damage being done to the Amazon rainforest today by modern agriculture and mining may not cause a permanent erasure of the ecosystem. If Brazil and the eight other countries that share pieces of the forest can get their act together, nature should be able to reclaim lands that are now barren.

More questionable is what condition human society will be in 400 years from now.

Boyce Rensberger retired to New Midway after more than 40 years as a science writer and editor, mostly for The New York Times and The Washington Post. Write him at boycerensberger@gmail.com.