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The Atacama Desert did not become hyperarid everywhere at one clearly established moment. Evidence from its hyperarid core points to aridification as early as the Eocene, roughly 56–34 million years ago, while other records show later changes in particular basins and adjacent Andean regions. The best-supported picture is of a long, uneven climate history shaped by atmospheric circulation, cold Pacific waters, topography, and changing access to moisture—with some variability even during prolonged aridity.
There is no single onset date for the whole Atacama
“When did the Atacama become a desert?” sounds like a question with one date for an answer. The geological record does not provide one. The Atacama includes a hyperarid core, a coastal zone, the Precordillera, Andean foothills, and highlands. These areas occupy different positions relative to Pacific moisture, the Andes, and sources of moisture farther east. Their climate histories and the evidence available for reconstructing them are not interchangeable.
It also matters what “became a desert” means. Initial aridification—the beginning of a trend toward drier conditions—is not necessarily the same as extreme aridity, hyperaridity, or the spread of hyperarid conditions across a broader region. A recent interpretation of evidence from the core places aridification in the Eocene. Earlier accounts emphasized later transitions, including the Oligocene–early Miocene. Those interpretations need not describe the same place or threshold.
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1Fix the driver behind crashes, sound loss and screen glitches2Clear out junk files and repair common Windows errors3Scan for outdated or missing drivers - takes under a minuteAccordingly, the Eocene finding revises the proposed history of the hyperarid core; it does not establish that every part of the modern Atacama was already hyperarid then. Later regional records document additional changes, and some preserve substantial gaps. The available evidence supports a history with different timings across the region rather than one continuous, desert-wide climate curve.
A timeline of the evidence
| Interval | What the evidence indicates | How to interpret it |
|---|---|---|
| Eocene, about 56–34 million years ago | A 2026 Nature Communications study reports cosmogenic-nuclide exposure-age evidence consistent with aridification in the hyperarid core. | This is evidence for an early change in the core, not proof that the entire modern desert had reached hyperaridity. |
| Oligocene–early Miocene, about 34–16 million years ago | Earlier research placed important stages of extreme aridity in this broad interval, with proposed links to Andean uplift and development of the Humboldt Current. | These proposed transitions may concern different thresholds or locations from the newer Eocene interpretation. |
| Miocene, about 23–5 million years ago | Regional stratigraphic and paleosol evidence records later transitions and intensification. A study of the Quillagua and Calama basins examines a 10-million-year interval. | The basin record includes two major hiatuses, so it is not continuous evidence of climate through the whole interval. |
| Last 3.8 million years | A 2024 sedimentary-record study reports climate variability and an increase in aridity at about 2.3 million years ago in alluvial-fan deposits. | Both figures describe that study’s record and deposits; neither supplies a single onset date for the entire Atacama. |
| Last 215,000 years | A 2019 study of the hyperarid core finds small but meaningful precipitation changes against a long-term hyperarid background. | Long-lasting aridity did not mean that precipitation was identical in every part of this interval. |
The dates in this timeline refer to different kinds of evidence. A surface exposure age, a sedimentary sequence, and a paleosol do not measure the same thing or necessarily identify the same climate threshold. Reading them as successive points on one uniform regional graph would give them more comparability than they have.
The scale of each interval also matters. Eocene and Miocene are geological epochs spanning millions of years, while the 215,000-year record captures a much shorter window within the Quaternary. The 3.8-million-year archive is longer, but its reported change at about 2.3 million years ago is tied specifically to alluvial-fan deposits. These studies complement one another because they illuminate different windows and settings—not because they form a single uninterrupted sequence.
Why the Atacama is so dry
The region’s aridity reflects several interacting barriers to rainfall. Subtropical atmospheric subsidence suppresses rain. Along the coast, cold upwelling associated with the Humboldt, or Peru–Chile, Current contributes to a temperature inversion that limits precipitation from Pacific moisture. Inland, the Andes and Altiplano restrict access to moisture arriving from the Amazon and Atlantic side. The region’s continental position is another part of the dry setting.
These controls act on different moisture pathways. The cold-current-related inversion helps explain why Pacific moisture does not readily produce rainfall across the coastal desert. The Andes and Altiplano affect moisture arriving from the east. Atmospheric subsidence further favors dry conditions. The resulting climate is not adequately explained by one mountain-building event or by a single ocean current alone.
The timing of these controls and their relative importance remain uncertain. A USGS synthesis associates long-term isolation from Amazonian moisture with uplift and/or the formation of the Altiplano. The 2026 study, by contrast, argues that aridification in the hyperarid core may have begun before major Andean uplift and modern Humboldt Current development. These accounts can coexist if early aridification in the core is distinguished from later intensification and the development of hyperaridity across a wider area.
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This distinction avoids a common oversimplification: the Andes did not simply “create” the Atacama at one moment. Topography helps restrict moisture from the east, but the core’s early aridification evidence suggests that the full regional pattern cannot be reduced to the later growth of a single barrier. Nor does the early-core interpretation make the ocean and atmosphere irrelevant; it shows why timing and geography must be considered when weighing the relative roles of each mechanism.
What each kind of geological evidence can—and cannot—show
Cosmogenic nuclides and exposed surfaces
The 2026 study uses cosmogenic-nuclide exposure ages from surface clasts to support an interpretation of very long surface preservation and Eocene aridification in the core. This evidence helps constrain how long surfaces have remained exposed and how active the landscape has been. It is not a direct measurement of rainfall, and it does not provide a continuous annual precipitation record. Its significance for climate comes through interpretation of landscape preservation, not from measuring past rain directly.
That makes the method especially useful for a question about long-term landscape stability, while setting limits on the climate detail it can provide. Exposure ages can inform an argument that surfaces experienced little disturbance over long spans; by themselves, they do not tell us how much rain fell in a particular year, or establish identical conditions at every nearby site. The 2026 Nature Communications paper therefore offers a new line of evidence for the core’s early aridification, rather than a rain-by-rain history of the entire desert.
Sediments, lakes, and basin stratigraphy
Sedimentary and lacustrine deposits can preserve evidence of wetter intervals and environmental change. But a gap in the sedimentary record may reflect erosion or a period when sediment was not deposited. The Quillagua and Calama basin study covers the last 10 million years and identifies two major hiatuses. Its findings therefore offer a discontinuous chronology, not an uninterrupted account of conditions throughout those 10 million years.
A hiatus is not evidence that climate remained constant during the missing interval. It means the archive does not preserve a usable sequence for that span, whether because material was removed or never accumulated. This is why the 2013 Earth-Science Reviews study’s 10-million-year scope should not be mistaken for 10 million years of continuous observations. Basin records can identify changes where deposits survive, but their gaps make broad extrapolations uncertain.
The 2024 study’s 3.8-million-year record adds a longer view of variation in the hyperarid setting. It reports increased aridity at about 2.3 million years ago in alluvial-fan deposits. That result is useful for understanding the record examined, but should stay attached to its location and deposit type rather than be generalized into a date for the entire desert.
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Paleosols, groundwater and wetland records, lake levels, and plant remains help reconstruct regional conditions, especially across the Altiplano and western Andean flank. Their geographic distribution is essential to interpretation: evidence from an adjacent highland or foothill is not automatically a direct record of the hyperarid core. A regional synthesis can compare these archives, but their differences are part of the story, not noise to be averaged away.
For example, a lake-level or wetland signal in the highlands can indicate a change in water availability at that archive’s location. It can help show how the wider central Andes responded to changing conditions, but it does not, on its own, establish the amount or timing of rainfall in the core. Likewise, plant remains and paleosols reflect the environments where they formed. Location is therefore not a technical footnote; it determines what a proxy can say about the Atacama.
Aridity persisted, but it was not climate stasis
Hyperaridity describes a long-term condition; it does not imply that rain never fell or that precipitation never changed. The 2019 Scientific Reports study documents small but meaningful precipitation fluctuations in the core over 215,000 years. A USGS synthesis also notes rare rainfall events in the Atacama itself and describes variable wet and dry periods east of the hyperarid zone.
This contrast matters because changes in the surrounding Andes or highlands need not map directly onto changes in the core. Places east of the desert have different access to moisture and can register wet and dry intervals differently. A rare rainfall event or a modest precipitation fluctuation is compatible with a persistently hyperarid background; neither, by itself, means the region ceased to be hyperarid.
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In other words, “persistently dry” and “unchanging” are not synonyms. Long-term averages and geological indicators can point to a hyperarid setting even as precipitation fluctuates over shorter intervals. The distinction is important when interpreting both rare events and small changes: they provide evidence of variability, not necessarily of a wholesale reversal in the desert’s climate.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to read claims about the desert’s climate history
When two accounts give different dates for the onset of aridity, first check whether they refer to the same location and definition. One may describe initial drying in the core; another may identify extreme aridity in a basin or a later transition in a broader region. Then consider what was measured: surface exposure, soil development, sediment, or an indicator of wetter conditions. Finally, check whether the record is continuous or contains gaps.
- Location: Is the evidence from the hyperarid core, coast, Precordillera, foothills, or Altiplano?
- Threshold: Does “onset” mean initial aridification, extreme aridity, or widespread hyperaridity?
- Proxy: Is the interpretation based on surface exposure, paleosols, alluvial fans, lake or basin deposits, or biological remains?
- Continuity: Does the archive preserve a continuous sequence, or are erosion and non-deposition known to leave hiatuses?
- Claim scope: Does the conclusion apply to the study site, or is it being extended beyond what that record establishes?
These checks help reconcile apparently competing timelines without forcing unlike records into a false consensus. The evidence for Eocene aridification is an important early-core interpretation. Later regional and basin records document other transitions, while younger records show variability within the arid setting. No single whole-desert onset age is established by these findings.
Frequently Asked Questions
Did the entire Atacama Desert become hyperarid in the Eocene?
No. A 2026 Nature Communications study reports evidence consistent with Eocene aridification in the hyperarid core. It does not establish that the coast, Precordillera, foothills, and highlands all became hyperarid at the same time.
Why do studies give different dates for the onset of Atacama aridity?
They may examine different locations, proxies, or thresholds. Initial aridification in the core is not necessarily the same event as extreme aridity in a basin or widespread hyperaridity across the region. Some sedimentary records also contain major gaps.
What caused the Atacama’s dry climate?
Several influences act together: subtropical atmospheric subsidence suppresses rain, cold coastal upwelling contributes to an inversion that limits Pacific precipitation, and the Andes and Altiplano restrict moisture from the east. The relative timing and importance of these factors remain unsettled.
Has the Atacama always been completely rain-free?
No. Hyperaridity is a long-term climate condition, not a claim that rain never falls. A 2019 study found small but meaningful precipitation fluctuations in the core over 215,000 years, and a USGS synthesis notes rare rainfall events in the Atacama.
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The Bottom Line
The Atacama’s climate history is not a single march toward dryness at one date. A 2026 study argues that the hyperarid core began aridifying in the Eocene, while basin, sediment, and regional archives record later transitions and variability in different places. The evidence supports long-lasting aridity, but also local differences, gaps in the record, and meaningful climate change within that dry setting.
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