The past 11 years have been the warmest on record. But Brown researchers found that this warming may be occurring to an even greater degree than previously expected in the tropics, a region home to around 40% of the world’s population.
The study, published in the Proceedings of the National Academy of Sciences and authored by Lina Pérez-Angel, a postdoctoral researcher in earth, environmental and planetary sciences, relied on analysis of a sediment core — a drilled segment of earth that contains layers of organic material — from the Bogotá basin in Colombia.
The sediment core is among the longest continuous records from this period in the tropics, according to Pérez-Angel. The sample holds preserved bacterial lipids, which can be used to assess temperatures from the time they became fossilized.
Researchers grounded their analysis in the Pliocene Epoch — which extended from 5.3 to 2.6 million years ago — because during that epoch, Earth warmed under atmospheric CO2 levels similar to today, Pérez-Angel added.
This research is one of the “first applications of this (methodology) to go back many millions of years on a tropical continent,” according to study author James Russell, professor of earth, environmental and planetary sciences and senior associate dean of the faculty.
The fossilized bacterial lipids are able to provide a record of historical temperatures because bacteria cells increase or decrease the lipids’ abundance to regulate the cell’s internal temperature by altering cell membrane fluidity.
“In humans, we wear clothes to regulate our temperature: more layers when it’s winter, less layers when it’s summer,” Pérez-Angel said. “So bacteria works in a similar way.”
The fossilized lipids were found relatively close to the surface of the basin, so they’re unaffected by the geothermal gradient, Pérez-Angel said. The geothermal gradient refers to how increasing depth comes along with increasing temperatures, which can degrade molecules and make them harder to read for analysis.
The analysis found that land temperatures increased nearly twice as much as sea surface temperatures in the Pliocene period. Although land temperatures have been known to increase more than sea temperatures, this difference is more than previously hypothesized.
While this concept of terrestrial land amplification — the theory that land temperatures change more intensely than water — has been widely accepted, the findings of this research point to “additional warming that is not completely explained by this theory,” Pérez-Angel said
A factor that could contribute to this discrepancy is the “distribution of sea surface temperatures” that occurred in the Pliocene, Pérez-Angel explained.
While the Pliocene Epoch has several similarities to today’s climate conditions, “today greenhouse gas concentrations may already be higher than they were during the interval (Pérez-Angel) studied,” Russell said, suggesting a need to conduct further research on even warmer epochs. Increased greenhouse gas concentrations have been shown to contribute to global warming by trapping heat from the sun, and “the tropics are already the warmest place on Earth,” he added.
Daniel Ibarra, an assistant professor of earth, environmental and planetary sciences and of engineering who was not involved with the study, wrote in an email to The Herald that while the study’s results were “somewhat expected,” they have “not been shown this convincingly before.”
“Studies like these demonstrate that we should focus on producing records from underrepresented places,” Ibarra wrote.
These higher rates of warming could lead to environmental consequences like droughts and exacerbated heat, Pérez-Angel said.
“In many parts of the tropics that are already extremely warm, a couple more degrees matters a lot,” Russell said.
Nishita Malhan is a senior staff writer covering science and research.
