66 Million Years of Earth’s Climate History Unpublished – Current Changes in Context

Past and future trends in global average temperature over the last 67 million years. Oxygen isotope values ​​in deep-sea bentic foraminifera from sediment cores are a measure of global temperature and ice volume. Temperature 1961–1990 relative to global mean. Data from ice core records of the last 25,000 years describe the transition from the last glaciers to the current warm period, the Holocene. Historical data from 1850 to the present date show the rise after 1950, marking the beginning of the Anthropocene. Regarding the three representative concentration pathways (RCP) scenarios for global temperature for future projections, which is in accordance with benign deep-sea records, the climate state Miocene by 2100 is better than the climate optimum (~ 16 million years ago). Would be way beyond the limit. To munch continental ice sheets. If emissions remain stable after 2100 and not stable before 2250, then by 2300 the global climate enters the world of early eosin (~ 50 million years ago) with many of its global warming events and no large ice sheets at the poles. Can do. Credit: Westerold et al., Cenogrid

A consistent record of the past 66 million years reflects natural climate variability due to changes in the Earth’s orbit around the Sun compared to future warming caused by greenhouse gas emissions.

For the first time, climate scientists have compiled a continuous, high-fidelity record of variations in the Earth’s climate. The record reveals four distinct climatic states, which the researchers termed hothouses, warmhouses, coolhouses, and icehouses.

These major climatic states persisted for millions and sometimes millions of years, and within each, the climate shows rhythmic variations corresponding to changes in the Earth’s orbit around the Sun. But each climate state has a specific response to orbital variations, which bring relatively small changes in global temperature compared to dramatic changes between different climate states.

New findings published today (September 10, 2020) in the Journal Science, Are the result of decades of work and a large international collaboration. The challenge was to determine climate change precisely on a time scale to see the variability caused by orbital variations (the eccentricity of the Earth’s orbit around the Sun and the tilt and tilt of its rotational axis).

“We have long known that glacial-interglacial cycles are affected by changes in the Earth’s orbit, changes in the amount of solar energy reaching the Earth’s surface, and astronomers calculated these orbital variations back in time. “Zachos, Distinguished Professor of Earth and Planetary Sciences and Ida Benson Lin Professor of Ocean Health at UC Santa Cruz, told Koathor James.

“As we recreate previous seasons, we can see the long-term drastic changes well. We also knew that there should be rhythmic variability at finer scales due to orbital variations, but for a long time it was considered impossible to recover that signal, ”said Zachos. “Now that we have succeeded in capturing natural climate variability, we can see that the estimated anthropogenic warming will be much higher than this.”

For the past 3 million years, the Earth’s climate has been in an iceberg state, characterized by alternating glaciers and interstitials. Modern humans evolved during this time, but greenhouse gas emissions and other human activities are now driving the planet toward warmhouse and hothouse climate states that had not been seen since the Eocene era, which ended about 34 million years ago. happened. During the early Eocene, there were no polar ice caps, and the average global temperature was 9 to 14 degrees Celcius It is more than today.

In a ‘business-as-usual’ scenario, Zachos said, the IPCC estimate for 2300 would potentially bring down global temperatures the planet hasn’t seen in 50 million years.

Getting high-quality sediment cores from deep-sea basins through the International Ocean Drilling Program (ODP, later successful in 2013 by the Integrated Ocean Drilling Program, IODP, International Ocean Discovery Program) to compile new climate records Was. Signs of past climates are recorded in the shells of microscopic plankton (called foraminifera) preserved in seafloor sediments. After analyzing the sediment core, the researchers then had to develop “astrobiology” by matching the climatic variations recorded in the sedimentary layers with variations in the Earth’s orbit (known as the Milankovich cycle).

Zachos, who led a study published in the mid-1990s, said, “The old time gap in the mid-1990s found ways to expand this strategy.” Science The climatic response to orbital variations appeared for a period of 5 million years covering the transition from the Oligocene era to the Miocene, about 25 million years ago.

“He changed everything, because if we could do that, we knew that we could probably go all the way back 66 million years ago and that these transient events and major transitions in the Earth’s climate caused orbital-scale variation In terms of, “he said. .

Zachos has for years collaborated with lead author Thomas Westrold at the University of Bremen Center for Marine Environmental Sciences (MARUM) in Germany, which has a large reservoir of sediment cores. Bremen Lab, along with the group of Zachos at UCSC, generated a lot of new data for the older part of the record.

Westerhold observes an important step, piecing together overlapping segments of climate records derived from sediment cores from different parts of the world. Zachac said, “It’s a tedious process to collect this long megasplice of the climate record, and we also wanted to replicate the record with separate sediment cores to validate the signals, so it would work together Wale was a major effort of the international community, ”Zach said.

Now that they have compiled a consistent, astronomically dated climate record of the last 66 million years, researchers can see that the climate’s response to orbital variations depends on factors such as greenhouse gas levels and polar ice sheets .

“In an extreme greenhouse world with no snow, there will be no reaction involving ice sheets, and this changes the dynamics of the climate,” Zakos explained.

Most major climate changes in the last 66 million years have been associated with changes in greenhouse gas levels. Zachos has done extensive research on the Paleocene-Eocene Thermal Maxim (PETM), for example, showing that this episode of rapid global warming, which plunged the climate into the hothouse state, was associated with a massive release of carbon into the atmosphere . Likewise, in the late Euzean, as atmospheric carbon dioxide levels were falling, ice sheets began to form in Antarctica and the climate changed to a coolhouse state.

“When these are near one of these transitions, the climate can be unstable, and we see more deterministic responses to orbital compulsions, so this is something we would like to understand better,” Zachos said.

The new climate record provides a valuable framework for many fields of research. It is useful not only for testing climate models, but also by geoscientists to study various aspects of Earth’s dynamics and paleontologists on how changing environments drive the evolution of species.

“This is an important advance in earth science, and a major legacy of the International Ocean Drilling Program,” Zachos said.

Reference: 10 September 2020, Science.
DOI: Science

Coauthors Steven Bohati, now at the University of Southampton, and Kate Littler, now at the University of Exeter, both worked at UC Santa Cruz with the Zachos. The paper’s colleagues also include researchers from more than a dozen institutions around the world. The work was funded by the German Research Foundation (DFG), the Natural Environment Research Council (NERC), the European Union’s Horizon 2020 program, the National Science Foundation of China, the Netherlands Earth Systems Science Center and the US National Science Foundation.