Jan. 15 (UPI) -- Approximately 55.5 million years ago, some combination of volcanism and continental shift significantly disrupted Earth's carbon cycles and sent global temperatures sky-rocketing.
Surprisingly, the planet's ancient oceans remained relatively stable, mostly unaffected by the episode of global warming.
When temperatures rise, oxygen levels in the ocean drop. The phenomenon has been confirmed by computer models, as well as modern observations and ancient paleoclimate data.
But according to a new study, published Friday in the journal Nature Communications, oxygen depletion, or anoxia, encompassed no more 2 percent of the global seafloor during the episode of warming known as the Paleocene Eocene Thermal Maximum, or PETM.
"The good news from our study is that the Earth system was resilient to seafloor deoxygenation 56 million years ago despite pronounced global warming," lead study author Matthew Clarkson said in a news release.
"However, there are reasons why things are different today. In particular, we think the Paleocene had higher atmospheric oxygen than today, which would have made anoxia less likely," said Clarkson, a climate scientist at ETH Zurich in Switzerland.
Of course, there's also the issue of humans. The PETM was triggered by natural changes in Earth's carbon cycle -- humans are mostly responsible for modern global warming.
In addition to emitting greenhouse gases, accelerating rises in the global temperature, human activities have been leaching large amounts of nutrients into the planet's oceans via fertilizer runoff and pollution, accelerating oxygen losses.
Scientists were able to trace ancient oxygen levels by analyzing ratios of different uranium isotopes in ancient sediment layers dated to the 200,000-year-long warming period.
The analysis suggests there's an upper limit to ocean oxygen depletion. Using computer models based on the sediment data, scientists pegged the limit at ten times baseline levels of seafloor anoxia.
Such an increase would extend anoxia conditions across 2 percent of the ocean floor, still enough to have significant adverse impacts on marine ecosystems.
"This study shows how the resilience of the Earth's climate system has changed over time," said study co-author Tim Lenton.
The order of mammals we belong to -- the primates -- originated in the PETM. Unfortunately, as we primates have been evolving for the last 56 million years, it looks like the oceans have been getting less resilient," said Lenton, a professor at the University of Exeter and director of the Global Systems Institute.
"Although the oceans were more resilient than we thought at this time in the past, nothing should distract us from the urgent need to reduce emissions and tackle the climate crisis today," Lenton said.