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An ancient warming event may have lasted longer than we thought

Fifty-six million years ago, during the Paleocene-Eocene Thermal Maximum (PETM), global temperatures rose by more than 5°C over 100,000 or more years. Between 3,000 and 20,000 petagrams of carbon were released into the atmosphere during this time, severely disrupting ecosystems and ocean life globally and creating a prolonged hothouse state.

Modern anthropogenic global warming is also expected to upend Earth's carbon cycle for thousands of years. Between 1850 and 2019, approximately 2,390 petagrams of carbon dioxide (CO₂) were released into the atmosphere, and the release of another 5,000 petagrams in the coming centuries is possible with continued fossil fuel consumption. However, estimates of how long the disruption will last range widely, from about 3,000 to 165,000 years.

Understanding how long the carbon cycle was disrupted during the PETM could offer researchers insights into how severe and how long-lasting disruptions stemming from anthropogenic climate change may be. Previous research used carbon isotope records to estimate that the PETM lasted 120,000–230,000 years. Victor Piedrahita and colleagues now suggest that the warming event lasted almost 269,000 years.

Evidence of the PETM is indicated in the geological record by a substantial drop in stable carbon isotope ratios. This drop is split into three phases, each representing a different part of the carbon cycle's disruption and recovery. Previous estimates of when the isotopic drop ended have varied widely because of noise in the data on which they're based.

In the new research, in Geophysical Research Letters, scientists studied six sedimentary records whose ages have been reliably estimated in previous work: one terrestrial record from Wyoming's Bighorn Basin and five marine sedimentary records from various locations. Rather than using only raw data, as in previous studies, they used a probabilistic-based detection limit to account for analytical and chronological uncertainties and constrain the timeframe of the PETM.

The recovery period in particular, this new study suggests, took much longer than previous estimates indicated—more than 145,000 years. The extended recovery time during the PETM likely means that future climate change scenarios will influence the carbon cycle for longer than most carbon cycle models predict, according to the researchers.