If you want a glimpse of our climate future, one of the best places to look is a time when Earth last had carbon dioxide levels similar to today’s.
That is exactly what a new study did using sediments from Colombia’s Bogotá Basin – and the result is a little unsettling.
The record suggests that parts of the tropical Andes may warm far more on land than scientists have typically expected as carbon dioxide levels continue to rise.
A past world like today
The team, led by a researcher at Brown University, reconstructed temperatures back into the Pliocene, the period from about 5.2 million to 2.5 million years ago.
The Pliocene matters because it is the most recent stretch of Earth’s history when atmospheric carbon dioxide levels were comparable to modern values, making it a useful real-world check on climate theory.
What they found was a large gap between a high-CO2 world and the cooler world that followed. Average temperatures in the Bogotá Basin were about 4.8°C (8.6°F) warmer during the Pliocene.
Temperatures dropped in the Pleistocene, a more recent epoch that ended roughly 12,000 years ago, when carbon dioxide levels were much lower.
That temperature jump was larger than the team expected – especially when compared with what ocean-based reconstructions suggest for the same general time frames.
Why the tropics heated faster
Much of what scientists know about ancient climate comes from the oceans, along with some work in high latitudes.
Tropical regions on land are harder to study, with fewer long records, complex geography, and fewer widely used climate archives.
“Most of what we know about past temperature comes from the oceans or terrestrial high latitudes, and there has been a lot of theoretical work in recent years on how low-latitude ocean temperatures relate to the land,” said lead author Lina Pérez-Ángel, a senior researcher at Brown.
“I think the big takeaway here is that we found significantly more warming on land in this region than you would expect from theory.”
In simple terms, theory often predicts that land heat in the tropics should rise by about 1.4°C (2.5°F) for every 1°C (1.8°F) of sea-surface warming in the tropics.
But the Bogotá Basin record points to a much stronger land response, close to twice what the tropical oceans show for the same broad transition.
Revisiting an old drill core
The backbone of the study is a 585-meter-long (1,919-foot) sediment core from central Colombia, drilled along the eastern Andes.
It’s basically a stacked timeline of ancient lake, river, and wetland deposits – the kind of material that quietly preserves chemical clues about the environment year after year, century after century.
The core itself isn’t new: it was drilled and first studied in the late 1980s. But Pérez-Ángel wanted to revisit it with better tools. She reanalyzed the sediments using more precise dating methods and newer temperature “proxies.”
To nail down the ages, the team used zircons found in thin layers of volcanic ash inside the core. Zircons are extremely durable crystals that lock in uranium, and because uranium decays at a predictable rate, it acts like a clock.
With that approach, the researchers showed that the core reaches back to about 3.7 million years ago – right into the high-CO2 middle Pliocene.
Bacteria record ancient heat
For temperature, the team relied on brGDGTs – tough lipid molecules made by bacteria. These fats change their structure depending on temperature. When preserved in sediments, they can be used to reconstruct a continuous temperature history.
That let the researchers follow rising and falling heat across the tropics over millions of years, not just isolated snapshots.
The timing of the temperature shifts lined up broadly with what scientists have inferred from tropical sea-surface records, but the land temperature swing was larger than expected.
The researchers do not claim to have a single, neat explanation yet, but they outline several possible contributors.
One idea is that warming behaves differently at altitude. High-elevation regions like the Andes may amplify temperature changes in ways that are not fully captured by existing frameworks.
However, the team argues that altitude effects alone probably cannot explain the entire mismatch.
Another possibility is that regional ocean patterns during the Pliocene – something like a long-lived, El Niño-style background state – could have pushed extra warmth into this part of the Andes.
There is also the broader possibility that scientists may be oversimplifying how ocean warming translates into land warming in complex tropical mountain environments.
Heat threats across the tropics
The big message here is not “we have solved the tropics.” Instead, the findings suggest that scientists may be underestimating how hot some tropical land regions can become, even when the oceans do not appear much warmer.
“The land is where the people are, and they experience climate change at a regional level,” Pérez-Ángel said.
“If we want to produce reconstructions that are useful to people, we should do more to understand climate mechanisms at a regional scale.”
That is especially relevant for the Bogotá Basin today. It is home to more than 11 million people.
If the Pliocene is any guide, the tropics could be facing stronger-than-expected heat as greenhouse gas levels continue to rise.
The study is published in the journal Proceedings of the National Academy of Sciences.
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