For the first time, climate scientists have compiled a continuous, high-fidelity record of changes in the Earth’s climate spanning 66 million years in the past. The record reveals four distinctive climate states, which the researchers dubbed the Hothouse, Warmhouse, Coolhouse, and Icehouse.
These major climatic states persisted for millions and sometimes tens of millions of years, and within each one the climate shows rhythmic variations corresponding to changes in the Earth’s orbit around the sun. But each climate state has a distinctive response to orbital variations, which result in relatively small changes in global temperatures compared to dramatic changes between different climate states.
The new findings, published on 10 September in Science, are the result of decades of work and a great international collaboration. The challenge was to determine past climatic variations on a time scale fine enough to see the variability attributable to orbital variations (in the eccentricity of the Earth’s orbit around the sun and in the precession and inclination of its rotation axis).
“We have known for a long time that glacial-interglacial cycles are stimulated by changes in Earth’s orbit, which alter the amount of solar energy reaching the Earth’s surface, and astronomers have calculated these orbital variations back in time,” explained the co-author. James Zachos, distinguished professor of earth and planetary sciences and Ida Benson Lynn Professor of Ocean Health at UC Santa Cruz.
“As we reconstructed past climates, we could see gross changes in the long term quite well. We also knew there had to be finer-scale rhythmic variability due to orbital variations, but for a long time it was considered impossible to recover that signal,” Zachos She said. “Now that we have been able to capture natural climate variability, we can see that the predicted anthropogenic warming will be much greater than that.”
For the past 3 million years, the Earth’s climate has been in a frozen state characterized by alternating glacial and interglacial periods. Modern humans evolved during this period, but greenhouse gas emissions and other human activities are now driving the planet towards the climatic states of Warmhouse and Hothouse that have not been seen since the Eocene epoch, which ended around 34 million. Years ago. During the early Eocene, there were no polar ice caps and global average temperatures were 9 to 14 degrees Celsius higher than today.
“The IPCC projections for 2300 in the business as usual scenario will potentially bring global temperature to a level the planet has not seen in 50 million years,” Zachos said.
Crucial to compiling the new climate record was obtaining high-quality sediment cores from deep ocean basins through the International Ocean Drilling Program (ODP, later Integrated Ocean Drilling Program, IODP, replaced in 2013 by the International Ocean Discovery Program ). Traces of past climates are recorded in the shells of microscopic plankton (called foraminifera) preserved in the sediments of the seabed. After analyzing the sediment cores, the researchers had to develop an “astrochronology” by matching the climatic variations recorded in the sediment layers with the variations in the Earth’s orbit (known as Milankovitch cycles).
“The community figured out how to extend this strategy to older time frames in the mid-1990s,” said Zachos, who led a study published in 2001 in Science which showed the climatic response to orbital changes over a period of 5 million years covering the transition from the Oligocene to the Miocene epoch, about 25 million years ago.
“This changed everything, because if we could do it, we knew we could go back to maybe 66 million years ago and put these transient events and major transitions in Earth’s climate into the context of orbital-scale variations,” he said. .
Zachos has collaborated for years with lead author Thomas Westerhold at the Center for Marine Environmental Sciences (MARUM) of the University of Bremen in Germany, which houses a large sediment core deposit. The Bremen lab together with Zachos’ group at UCSC generated much of the new data for the oldest part of the record.
Westerhold oversaw a critical step, combining overlapping segments of the climate record obtained from sediment cores from different parts of the world. “It’s a tedious process to assemble this long mega-ply of climate records, and we also wanted to replicate the records with separate sediment cores to verify the signals, so this was a big effort from the international community working together,” Zachos said.
Now that they have compiled a continuous and astronomically dated climate record of the past 66 million years, researchers can see that the climate response to orbital changes depends on factors such as greenhouse gas levels and the extent of polar ice caps.
“In an extreme ice-free greenhouse world, there will be no feedback involving ice sheets, and that changes the dynamics of the climate,” explained Zachos.
Most major climate transitions over the past 66 million years have been associated with changes in greenhouse gas levels. Zachos has done extensive research on the Paleocene-Eocene Thermal Maximum (PETM), for example, showing that this episode of rapid global warming, which brought the climate to a greenhouse state, was associated with a massive release of carbon into the atmosphere. . Similarly, in the late Eocene, when atmospheric carbon dioxide levels were decreasing, ice sheets began to form in Antarctica and the climate shifted to Coolhouse state.
“The climate can become unstable as it approaches one of these transitions and we see less predictable responses to orbital forcing, so that’s something we’d like to understand better,” Zachos said.
The new climate record provides a valuable framework for many research areas, he added. It is not only useful for testing climate models, but also for geophysicists who study different aspects of terrestrial dynamics and for paleontologists who study how changes in environments drive the evolution of species.
“It is a significant advance in Earth science and an important legacy of the international ocean drilling program,” said Zachos.
The study analyzes the impact of carbon dioxide on the earth’s climate 30 million years ago
T. Westerhold el al., “An astronomically dated record of the Earth’s climate and its predictability over the past 66 million years”, Science (2020). science.sciencemag.org/cgi/doi… 1126 / science.aba6853
High-fidelity recording of Earth’s climate history puts current changes into context (2020, September 10)
recovered on 10 September 2020
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