After two consecutive years of record-breaking droughts, the river levels in the Amazon Basin are slowly starting to recover. According to the Geological Survey of Brazil, the Madeira River, home to the most diverse fish life in the region, left the “drought” classification as its levels surpassed the 13-foot mark in late November. In both Madeira and Negro rivers, navigability is back to normal after months of suspension.

However, the drought is far from over, and its impacts are still felt across the region. In 2024, 69% of municipalities in the Amazon recorded more intense drought levels than in 2023. In the Madeira River, the levels were so low that the Geological Survey had to install a new ruler to measure it.

In Amazonas state alone, 850,000 people were affected and Indigenous communities still struggle with food insecurity and lack of access to drinking water. In the state of Pará, mass mortality of fish caused by the drought increases the vulnerability of riverine communities.

Although river levels are beginning to rise, the rains from the Amazonian wet season are less than expected, and the drought may further intensify into December.

While notably extreme, the droughts experienced by the Amazon River Basin in the past two years are not unusual. For the past 20 years, the nine countries that make up the Amazon Basin have seen drastic reductions in their total water surface area, with nine out of the past 10 years among the driest ever registered. They were also made more likely by climate change.

These chronic droughts in the Amazon cannot be attributed solely to less rain, as shown by a new study. Part of the explanation for this issue can be found at the opposite margin of the continent, in a region that is quite different from the tropical rainforest: the Andean glaciers.

A new study conducted by a team led by Boston College researchers in Colombia, Peru and Bolivia has found that tropical glaciers have retreated to the lowest levels in at least 11,700 years, when the Holocene, Earth’s current geological age, began. Faster than previously thought, the image of white caps covering the Andes is becoming a distant memory.

Home to most of the world’s tropical glaciers, the Andes offer a clearer picture of the global impacts of climate change. “The size of glaciers is directly related to the climate they exist in, making them essentially thermometers,” Andrew Gorin, the study’s lead investigator and now a University of California, Berkeley, Ph.D. candidate, told Mongabay.

With actual thermometers and rain gauges, tools used nowadays to measure climate variables, dating to a mere few centuries ago, Gorin explained that the experiment resorted to rock records and chemistry. “There is a rare class of chemicals, isotopes beryllium-10 and carbon-14, that accumulate in rocks when they are exposed to cosmic radiation, making it possible to measure how long it has been able to see the sky.”

“Kind of like how a sunburn can tell you how long someone was out in the sun,” in the words of Boston College associate professor of Earth and environmental sciences Jeremy Shakun.

Originally, the study aimed to re-create the history of advancing and retreating tropical glaciers throughout the Holocene. “But once we got to analyze the samples in the lab, there was nothing there to be measured. That was not what we expected,” Gorin said.

While the study doesn’t pinpoint exactly when that happened, the absence of the rare isotopes indicates that it was only in recent years that these parts of the Andes started seeing the sun again, after at least 11,700 years.

“At least” being the key expression. While the methods used only allow for measures up to that point in time, Gorin said it is reasonable to assume that tropical glaciers haven’t been this small in more than 125,000 years. “You’d have to go back to a time before the Holocene to find glaciers this small. But before that, there was an ice age. So it would have to be before that.”

“Essentially, our findings show that the glaciers in question are now smaller than they have been at any point of the Holocene, when all human civilizations were built,” Gorin explained.

For some scientists, this is a benchmark that indicates Earth is leaving the Holocene and entering a new epoch: the Anthropocene, characterized by the impact of human activity on Earth’s geology and ecosystems.

“We have crossed an epoch. Alarmingly so,” Gorin said.

From the Andes to the Amazon

The consequences of this crossing, however, are not restricted to tropical glaciers. On the other end of the South American mountain range, the Amazon Basin is strongly affected by ice loss in the glaciers.

“Everything that happens in the Andes ends up affecting the Amazon Rainforest and vice versa,” Efrain Turpo, a specialist at the Bien Común Institute and MapBiomas Peru, told Mongabay.

From the mountains toward the forest, creeks and rivers that arise from the melting of the Andes glaciers give birth to the subsidiaries that will in turn come together to make up the world’s largest river, the Amazon.

As the ice caps disappear, these creeks and rivers become more irregular, leading to longer and more intense droughts in the Amazon. Scientists estimate that the ice loss in the Andes could reduce the water flow to the Amazon rivers by up to 20%, with devastating consequences to the forest and the climate.

“As the birthplace of the Amazon rivers, the Andes have a vital importance for the rainforest,” Rogério Ribeiro Marinho, a professor of physical geography at the Federal University of Amazonas, told Mongabay. “While they correspond to less than 10% of the basin area and contribute with a very small volume of water, the Andes are responsible for over 90% of the sediments transported across the basin.”

The main water source for the Amazon Basin is ocean evaporation, but Marinho and Turpo explained that the Andes play a pivotal role in this cycle. The humidity that comes from the Atlantic, once incorporated into the basin and the forest, finds a barrier in the mountains, where it is redirected toward the forest as rain. “That promotes a recycling of the precipitation in the region and a resulting series of interconnected hydrological and morphological processes,” Marinho said.

The Amazon-Andes relationship is mutualistic and crosses geological eras. It was the presence and characteristics of the South American mountain range that allowed for the tropical rainforest to thrive as it is today.

“In Earth’s geological history, the Andes are of vital importance for the Amazon, given that it was its formation which made it possible for the continent to be home to the world’s largest watershed,” Marinho said.

The Andean uplift shaped the evolutionary history and the biodiversity patterns of both plants and animals in the Amazon and created the most biodiverse region on the planet. The unique combination of a long and narrow mountain range right beside a humid tropical rainforest allowed species to find refuge during climate oscillation cycles, in the lower plains for cooling periods like ice ages and up the mountains during the heat.

The Andes-Amazon Transition Belt, as the region is known, is a highly connected area where animals feed, reproduce and migrate. It is also a sacred place for the peoples whose cultural traditions have been flourishing for centuries, as well as a key economic pole for communities whose livelihoods depend on the glaciers and the forest.

However, human activity has been rapidly altering its structure. Deforestation in the Amazon has serious impacts on these processes. “If there’s a decline in the forest, the levels of precipitation will also suffer. A loss in water recycling in the Amazon also means less rain in the Andes,” Turpo explained.

Rising global temperatures are the obvious main reason behind melting glaciers, the scientists noted. But wildfires and industrial activities in the region also bring another element to the plight: black carbon. Produced by the incomplete combustion of fossil fuels or organic materials, black carbon is an ash-like particle that gets carried up by the smoke and ends up being transported by atmospheric currents, where it will be incorporated in the water cycle and precipitate with rain.

“When black carbon falls and accumulates over the glaciers, it reduces their albedo, the reflective property, turning them into a darker surface that will in turn heat faster, leading to faster melting,” Turpo said. “That means everything we do in the Amazon region has direct effects in accelerating the retreat of the Andean glaciers.”

“In order to understand the relationship between the loss of ice in the Andes and the droughts in the Amazon, we need to look at the system as a whole. It is not a simple local causal relation,” Marinho explained.

Crossing a geological epoch

The Holocene marks the beginning of an anomalously stable period in Earth’s climate history. Before that, the world experienced an ice age that lasted from roughly 125,000 to 18,000 years ago, followed by nearly 6,000 years of deglaciation, in a heterogeneous transition that meant the ice age ended in different places of the planet at different times.

“It was a moment when major ice sheets had just finished retreating and Earth’s climate system stabilized in terms of major climate variables,” Gorin explained.

Humans were mostly nomads until then. They didn’t live in one place nor did they domesticate animals. Holocene’s climate allowed them to start farming and build the global civilizations known today.

Yet, contrary to what someone living through 2024, the hottest year on record, would imagine of a world that had just exited an ice age, the glaciers experienced by early Holocene humans were smaller than those seen by early industrial societies. Until the 1800s, glaciers just kept growing. And, if it weren’t for human action, they would probably still be.

But human activity reverted this cooling trend. Accumulation of greenhouse gases in the atmosphere over the past two centuries and the rise in global temperatures catapulted the planet into a spiral of extreme climate changes — among which, the disappearing Andes tropical glaciers.

Between 1990 and 2020, glaciers receded 42% across different countries. Peru alone, home to 92% of all areas covered by glaciers in the tropics, saw a decrease in size of about 30% between 2000 and 2016. Meanwhile, Venezuela is on the verge of becoming the first country in modern history to lose all its glaciers. Earlier this year, the country’s only remaining glacier was reclassified as an ice field.

“We have built our civilizations in places that were good for farming, where land was arable, where access to water was easy. Now, the climate has changed to an extent unseen in the time when we developed all of our infrastructure. We have built that in, and for, the Holocene,” Gorin said.

The future is already here

Glaciers are the largest freshwater reservoirs on the planet, collecting water in the form of snow during the winter and releasing it as it melts during the summer. Roughly 10% of the world’s population depends on them as their main water source. In the Andes region, communities are already suffering from record-breaking droughts and water shortages.

Meltwater represents roughly 5% of the water supply in Quito, Ecuador; 61% in La Paz, Bolivia; and 67% in Huaraz, Peru. During droughts, this contribution escalates to 15%, 85% and 91%, respectively.

“We can no longer rely on the periodicity of the rain, so the glaciers become even more important for our survival,” Saul Luciano, a small-scale farmer and mountain guide from Huaraz, told Mongabay.

Luciano, who described seeing firsthand as the glaciers retreat in front of his eyes, said he fears they will soon not be able to provide enough water. “There are zones where farming communities are already experiencing water scarcity. For those who raise livestock, pasture is also scant, as the water that kept valleys humid and fertile no longer reaches lower parts of the mountain.”

Turpo explained that cutting emissions could retard the loss but, most importantly, local governments will have to start investing in adaptation and in responding to the needs of communities that are already suffering with the consequences of ice retreat.

Yet, he said he believes there is no turning back. “It is evident that the glaciers will disappear.”

Banner image: Receding glacier in the Sinakara Valley in the southern highlands Cusco Region of Peru. Image by Amanda Magnani. 

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Citations:

Gorin, A. L., Shakun, J. D., Jones, A. G., Kennedy, T. M., Marcott, S. A., Goehring, B. M., … Caffee, M. W. (2024). Recent tropical andean glacier retreat is unprecedented in the Holocene. Science, 385(6708), 517-521. doi:10.1126/science.adg7546

Hoorn, C., Palazzesi, L., & Silvestro, D. (2022). Editorial preface to special issue: Exploring the impact of andean uplift and climate on life evolution and landscape modification: From Amazonia to Patagonia. Global and Planetary Change, 211, 103759. doi:10.1016/j.gloplacha.2022.103759

Sierra, J., Espinoza, J. C., Junquas, C., Polcher, J., Saavedra, M., Molina Carpio, J. A., … Ticona, L. (2021). Deforestation impacts on Amazon-Andes hydroclimatic connectivity. doi:10.5194/egusphere-egu21-9251

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