Humans Could Reverse Over 40 Million Years of Climate Cooling by 2300

The climate reference curve known as CENOGRID shows Earth has experienced four distinct climate states over the last 66 million years. The detailed climatic changes of the past can be studied like a colorful barcode and provide context for ongoing anthropogenic change and how exceptional it is. Earth’s climate has gradually cooled for the last 50 million years, but unmitigated human impact on the environment reverses this cooling trend. (Credit: Thomas Westerhold)

(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.

“Over the last 66 million years or so, for most of that time, the Earth has been warmer than it has been in the last couple thousand years. That’s something we can see a lot more detail now, and that gives us a nice perspective on what’s happening in terms of anthropogenic warming and where we’re headed over the next several centuries,” Zachos said.

From 66 million years ago until 34 million years ago, the Earth drifted from “warmhouse” temperatures — more than 41 degrees Fahrenheit warmer than today — up to a period of extreme “hothouse” temperatures between 56 and 47 million years ago, when the global mean temperature was over 50 degrees hotter than today.

The hotter periods correlate with high atmospheric carbon concentrations and minimal or no polar ice volume, while the cooler periods saw low greenhouse gas emissions and greater ice extents.

“There’s probably no ice on Antarctica, and certainly no ice on the Northern Hemisphere,” Zachos said of the hothouse period. “That’s an extreme greenhouse state.”

For most of the past 34 million years, global average surface temperatures lowered to temperatures more familiar to us.

“The last couple million years, we’ve been locked in this glacial state of big glacials and little glacials. Right now we’re in a little glacial state,” Zachos said. “During glacials, of course, large parts of the Northern Hemisphere continents get covered in ice. The frequency of that is roughly 1,000-year cycles that have been occurring for several billion years.”

From Sea to Sky

These natural variations in global climate track not only with polar ice cap extent and ancient outpours of greenhouse gases into the atmosphere, but also with astronomical events. 

Earth revolves around the sun in neither a perfect circle nor a perfect oval. Its path around our star is eccentric, changing shape in subtle ways that affect the climate, because Jupiter and Mars exert their own gravitational pulls on Earth as they too round the sun.

Additionally, the Earth’s tilt angle oscillates between about 22 and 24.5 degrees relative to the orbital plane; this changing axial tilt accounts in part for the varying severity of the seasons, but also explains portions of the ancient climate record.

“If the tilt is high, the summers tend to be warmer in both the Northern Hemisphere and Southern Hemisphere’s high latitudes. It’s hard for ice to accumulate; snow that fell the previous winter will melt if you have hot summers. If the tilt angle is lower, you have cooler summers,” Zachos said.

These orbital variations came and went throughout the Earth’s history, and with this new finely detailed record of ancient climate changes, the scientists can see clearly that the Earth’s climate responded to these cycles dynamically.

“Greenhouse gas levels, if they’re really high, it won’t matter — you just won’t accumulate ice year after year, no matter what the tilt is. As you lower the CO2 levels and get to some threshold, then in Antarctica, during a cool summer when you have low tilt, all of a sudden the snow from the previous winter doesn’t completely melt,” Zachos said. “Now you start to accumulate snow and ice.”

The authors’ new climate curve is so fine-grained, Zachos says, that astronomers can use it to test their calculations of past orbital variations.

In a way, this new view into the climate’s ancient history puts both the future and the past into clearer perspective.

“The view into the past is also a glimpse into the future. We can learn something about the staggeringly rapid anthropogenic changes of our present century from the slow natural climate fluctuations occurring over millions of years,” co-author Norbert Marwan, complexity scientist at the Postdam Institute for Climate Impact Research, said in a statement.

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