(CN) — Scientists have developed a high-resolution record of the past 66 million years’ natural climate changes and their relationship to changes in the planet’s orbit and wobble, all thanks to microfossils found in dirt dug from the oceans’ depths.
Their research also shows that if humanity fails to address the growing concentration of atmospheric greenhouse gases, Earth may experience global average temperatures not seen in at least 40 million years — as soon as the year 2300.
“We have at least 66 million years of climate change in detail, a high fidelity record of the record in detail, and we can put it into context with projections for future climate,” said James Zachos, paleoclimatologist at the University of California, Santa Cruz. “The amplitude of anthropogenic [human-caused] climate change, by 2300, will be larger than the natural variability associated with orbital forcing throughout the past 66 million years.”
The Shells Tell
Zachos is one of 24 co-authors who contributed to the research, published Thursday in Science and borne of decades of deep ocean drilling and international laboratories’ coordinated efforts to splice together a climate record from samples obtained from ocean sediments.
“The hard part was actually drilling in the right places of the seafloor to pull up sedimentary archives that were the right quality to be able to resolve these higher frequency variations in climate,” Zachos said. “There’s not a single place where you can go and recover the entire last 66 million years in a single core with all the high-frequency variability; you actually have to jump around from place to place.”
Zachos and his colleagues spent as many as seven years at a time preparing proposals and gathering funding to survey different ocean basins in the Atlantic and the Pacific, first visiting them to locate suitable sites to take sediment samples from, and later returning for monthslong voyages to collect the cores.
“It requires different expertise from the scientists involved: micropaleontologists, to geochemists, to climate scientists,” Zachos said. “It wasn’t easy to put together.”
The cores contained key signatures of past climates as recorded in the shells of benthic foraminifera, single-celled organisms that live on the seafloor sediment. The microfossils — and, crucially, the oxygen isotopes contained within them — were preserved in seafloor sediments.
“The variability of deep ocean temperature is very small,” Zachos said. “Most of the water filling the ocean comes from polar regions, where wintertime cooling lowers the temperature and increases its density, so it begins to sink, and that’s how the deep ocean fills with water, or at least how it circulates.”
Because the microscopic foraminifera produce shells made of calcium and immersed in this deep ocean water, they become records of ancient ocean temperatures. The scientists just had to measure the ratio of oxygen-18 isotopes to oxygen-16 in the shells.
“Ratio changes are the partitioning of O-18 to O-16 from seawater into the shell. It’s sensitive to temperature … because of the mass difference of the two isotopes,” Zachos said. “That allows us to get the temperature of [the seawater] that particular shell was precipitated in. That also then gives us a sense of the high-latitude temperatures. … As the poles get warmer going back through time, or are getting colder and colder going forward through time, that’s reflected in the temperatures of the deep ocean.”
Lead author Thomas Westerhold, an environmental scientist at the University of Bremen, oversaw the analysis and verification of the samples’ records. He spliced the researchers’ work into one continuous “climate reference curve,” dubbed CENOGRID.
A Brief History of Climate, as Told by Ocean Dirt
CENOGRID charts the Earth’s average global surface temperature through four distinct climate states over the last 66 million years — the geological era known as the Cenozoic Era, the era of “new life,” named for mammals and birds’ diversification and spread after the death of dinosaurs.
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