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Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →The Atacama did not become hyperarid everywhere at one moment. Evidence suggests aridification in its hyperarid core may reach back to the Eocene, while later changes unfolded at different times in the core, along the coast, and across the Andes. The history is one of uneven drying, long-lived aridity, and smaller climate fluctuations—not a single date or an unchanging desert.
There is no single date when the whole Atacama became hyperarid
“The Atacama” covers environments that do not share one climate record. The hyperarid core, coastal belt, Precordillera, Andean foothills, and Altiplano differ in elevation, exposure to ocean air, and access to moisture from the east. A date inferred from one basin or a surface in the core cannot automatically be applied to every part of the desert.
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It also matters what a study means by “onset.” Initial aridification is not necessarily the same as extreme aridity, and neither proves that hyperarid conditions had already spread across a broad region. Some evidence records a landscape that remained exposed and little disturbed; other evidence records sediment deposited during wetter episodes, or changes in soils and vegetation. Those observations answer related but distinct questions.
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A 2026 Nature Communications study makes the timeline especially notable: it reports cosmogenic-nuclide exposure-age evidence consistent with Eocene aridification of the hyperarid core, roughly 56–34 million years ago. This is a new interpretation for a specific part of the Atacama, not proof that the entire modern desert was already hyperarid in the Eocene. Earlier work often emphasized later transitions, and evidence from other places and proxies continues to describe additional stages.
A timeline of drying and later change
| Interval | What the evidence indicates | How to interpret it |
|---|---|---|
| Eocene, about 56–34 million years ago | A 2026 study reports exposure-age evidence consistent with aridification of the hyperarid core. | A proposed early stage for the core, not a whole-desert onset date. |
| Oligocene–early Miocene, about 34–16 million years ago | Earlier research placed important development of extreme aridity in this broad interval, with proposed links to Andean uplift and the Humboldt Current. | These interpretations concern different sites, proxies, and definitions of aridity from the newer Eocene result. |
| Miocene, about 23–5 million years ago | Regional stratigraphy and paleosols record later transitions and intensification. A study of the Quillagua and Calama basins examines the last 10 million years. | The basin record contains two major hiatuses; it is not a continuous climate curve. |
| Last 3.8 million years | A 2024 sedimentary study reports climate variability and increased aridity at about 2.3 million years ago in alluvial-fan deposits. | The age and change are findings from that record and deposit type, not a date for the whole Atacama. |
| Last 215,000 years | A 2019 study finds small but meaningful precipitation changes in the hyperarid core. | Long-term hyperaridity can coexist with measurable fluctuations. |
| Recent geological and historical timescales | Regional archives show contrasting wet and dry periods east of the hyperarid zone, while rare rainfall events also occur in the Atacama. | Conditions in surrounding highlands and foothills need not match those in the core. |
Eocene: evidence for an earlier start in the core
The Eocene interpretation revises the commonly emphasized chronology by proposing that aridification in the hyperarid core began before the later stages of Andean uplift and the development of the modern Humboldt Current system. Its basis is cosmogenic-nuclide exposure dating of surface clasts. These measurements help constrain how long surfaces have been exposed and how much landscape activity has occurred; they do not directly measure ancient rainfall year by year.
The result should therefore be read as evidence about long-term surface preservation and aridification at the study’s sites. It does not settle when every area became extremely dry, nor does it rule out later regional intensification. The apparent difference from older timelines can partly reflect the fact that “aridification” and “hyperaridity” may refer to different thresholds, and that the studies examine different places.
Oligocene through Miocene: later stages, not a replacement for the Eocene evidence
Earlier studies proposed major development of extreme aridity during the Oligocene–early Miocene, roughly 34–16 million years ago, and discussed the roles of Andean uplift and the cold Pacific current. Regional stratigraphic and paleosol evidence also points to later changes during the Miocene, about 23–5 million years ago. These findings can coexist with an earlier start to aridification in the core: a region may begin drying in one place and become more intensely or more widely hyperarid later.
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The Quillagua and Calama basin study illustrates the limits of assigning a precise regional history from sediment alone. It examines a 10-million-year interval but identifies two major periods when sediment was not preserved. A missing sequence may reflect non-deposition or erosion, rather than a continuous record of stable climate. The deposits that remain can document particular conditions, but the gaps constrain what can be said about timing and continuity.
The last few million years: variation within an arid setting
A 2024 study describes a sedimentary record spanning 3.8 million years. Within its alluvial-fan deposits, it reports an increase in aridity at about 2.3 million years ago. Both numbers belong to that study’s archive: 3.8 million years is the record’s duration, and 2.3 million years ago marks a change identified in those deposits. Neither is a universal start date for the Atacama.
On a shorter geological timescale, a 2019 study examines precipitation variation in the hyperarid core over the past 215,000 years. Its central implication is that an exceptionally dry long-term background does not mean precipitation was always identical or absent. Small changes can still matter to local water availability and leave a detectable signal in an appropriate archive.
Why the Atacama is so dry
The desert’s aridity reflects several interacting barriers to moisture rather than a single cause. Subtropical atmospheric subsidence suppresses rainfall. Along the Pacific coast, cold-water upwelling associated with the Humboldt, or Peru–Chile, Current helps maintain a temperature inversion: stable air limits the vertical development of rain-producing clouds, even when marine air is present.
Moisture from the east faces a different obstacle. The Andes and Altiplano restrict access to moisture carried from the Amazon and Atlantic side, while the Atacama’s continental position contributes to its dryness. The regional synthesis by the USGS describes long-term isolation from Amazonian moisture and relates it to uplift and/or the formation of the Altiplano. The exact timing and relative contribution of each control remain unsettled.
It is therefore too simple to say that Andean uplift alone created the Atacama. The newer Eocene interpretation suggests that core aridification may predate major uplift and modern Humboldt Current development; it does not make topography or ocean circulation irrelevant to later conditions. An early drying trend in the core and subsequent intensification associated with evolving circulation and topography are compatible possibilities. The evidence supports a changing combination of controls, not a single switch.
What the different climate archives can—and cannot—show
Ancient rainfall is not measured directly by most geological evidence. Researchers infer climate from traces left by landscape processes, water, soils, or organisms. Each archive has its own geographic footprint, time resolution, and gaps, so the strongest reconstruction comes from comparing records without treating them as interchangeable.
| Archive or proxy | What it can indicate | Main limitation |
|---|---|---|
| Cosmogenic nuclides in surface clasts | Exposure age and long-term surface preservation, which can help assess landscape stability and aridification. | Not a direct or continuous record of annual precipitation; evidence applies to the sampled surfaces. |
| Lake, basin, and other sedimentary deposits | Conditions under which sediment accumulated, including wetter episodes and changes in erosion or deposition. | Erosion and non-deposition produce hiatuses; a preserved sequence may be discontinuous. |
| Alluvial fans | Changes recorded in fan sediments, including the aridity increase reported at about 2.3 million years ago in one 2024 record. | A finding from a specific deposit cannot be generalized to all environments or the entire desert. |
| Paleosols and regional environmental archives | Past soil-forming conditions and evidence from groundwater, wetlands, lake levels, or plant remains, especially across the Altiplano and western Andean flank. | Archives outside the hyperarid core describe their own locations, not necessarily conditions in the core. |
Temporal resolution differs too. A long exposure age can suggest how long a surface persisted without major disturbance but may not reveal short-lived wet periods. Sediment layers can capture particular episodes in more detail, yet they may be separated by long gaps. A lake-level or plant record from the Altiplano can help explain regional moisture shifts without serving as a direct gauge for the core. These are complementary clues, not competing versions of one continuous rainfall chart.
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The USGS synthesis emphasizes that the hyperarid zone is not the only relevant part of the central Andes. Areas east of the core experienced variable wet and dry periods, while rare rainfall events also occurred within the Atacama. The region can therefore show a sharp contrast: the core remains extremely dry over long spans, while nearby highlands or foothills respond differently to changing access to moisture.
This contrast explains why a regional archive may record a substantial wet interval without implying that the core became generally wet. Conversely, a long-lasting arid signal from the core does not mean all adjoining environments were equally dry. When comparing records, location is as important as age: a coastal observation, an Altiplano lake, and a surface in the core occupy different climatic settings.
How to read claims about the desert’s age
When a study or popular account gives a date for the Atacama’s “formation” or the “onset of hyperaridity,” check what that date actually describes. The evidence assembled here does not establish one whole-desert onset age. Instead, it records several places, thresholds, and intervals.
- Check the location. Is the evidence from the core, coast, Precordillera, foothills, or Altiplano?
- Check the climate claim. Does “onset” mean initial aridification, extreme aridity, or widespread hyperaridity?
- Check the proxy. Exposure ages, paleosols, fans, lake sediments, and plant remains do not measure the same thing.
- Check continuity. A study covering millions of years may still contain major hiatuses or preserve only selected episodes.
- Check the scale of the conclusion. A result from one basin or surface is not automatically a climate history for the whole desert.
Applied to the current timeline, this approach places the 2026 Eocene result as important evidence for early aridification in the hyperarid core, while leaving room for later intensification and distinct regional histories. It also makes sense of the older Oligocene–Miocene proposals without forcing either set of evidence to represent more than it measures.
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