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Mt. Bromo on the island of Java, Indonesia. A current collision between a continental plate and this arc of volcanic islands exposes rocks that absorb carbon dioxide from the atmosphere, which tends to cool the Earth. In an earlier era, such collisions in the tropics led to global ice ages. (Photo courtesy of Sara Marlowe via Wikimedia Commons)
Scientists at the University of California think they know why the Earth's generally warm and mild climate over the past billions of years has sometimes been interrupted by cold snaps that envelop the ice poles and sometimes make the planet a snowball.
According to them, the main trigger is the formation of mountains in the tropics, while landmasses collide with arcs of volcanic islands, such as the chain of Aleutian Islands in Alaska.
The climate of the Earth depends to a large extent on the amount of carbon dioxide in the atmosphere, which traps heat and warms the planet. While the burning of fossil fuels since the industrial revolution has led the CO2 unprecedented levels since 3 million years, CO2 Levels have been even higher in Earth's past, coinciding with warm periods without any major ice cover.
In fact, the Earth's default climate seems to be warm and mild. Periods without glaciers have been dominated for three quarters of the last billion years.
However, half a dozen glaciations cooled the Earth during this period, two of them serious enough to turn the planet into a snowball Earth, covered with a layer of ice covering a large part of the surface. What is the cause of these frozen interludes?
In a study published in this week's edition of the newspaper Science, the team concludes that when volcanic arcs collide with continents in the tropics – an inevitable consequence of the planet's ever-changing tectonic plates – they cause global cooling, creating a glacial climate with vast ice caps .
Plate tectonics has displaced continents around 500 million years, inevitably causing collisions. When collisions with volcanic island arcs occurred in the tropics (green stripe), they exposed rocks called ophiolites (orange), which sucked carbon from the atmosphere and resulted in the formation of ice (blue) and global cooling. (Animation provided by Nicholas Swanson-Hysell)
Such a collision currently occurs when parts of the Indonesian archipelago are pushed up into the mountains of Australia's northern margin. The result is that there are mountains containing rocks called ophiolites that have a great ability to remove carbon from the atmosphere. During geological periods, there is a balance between CO2 emitted by volcanoes and CO2 consumed by chemical reactions with rocks. Rocks rich in calcium and magnesium, such as ophiolites, are the most effective for CO2. When these elements are released from the rocks, they combine with the CO2 and head for the ocean, where they form limestone, blocking the CO2 in rock, where he has been staying for millions of years.
"The Earth has a long-standing carbon sequestration program," said Nicholas Swanson-Hysell, assistant professor of Earth and Planetary Science, who designed the study with Francis Macdonald, a professor in the Department of Earth Sciences. Land of the University of Santa Barbara. "We know that these processes keep the Earth's climate in balance, but determining the causes of changes between non-glacial and glacial climates over time scales of millions of years is a long-standing puzzle. "
Unfortunately for the Earth's future, the geological processes that consume CO2 are slow and unable to fight massive CO2 emissions from the burning of oil, coal and natural gas. According to Swanson-Hysell, the Earth's natural carbon sequestration program will restore balance over the millennia, but it will take a long time for modern civilization to succeed so well in the colder climate of the Earth.
The Appalachians came with a major freeze
In 2017, Swanson-Hysell and Macdonald proposed that an important glacier, 445 million years ago, had been triggered by a collision similar to that occurring today in Indonesia. This collision occurred during the first phase of the construction of the Appalachian Mountain, when the present eastern United States was located in the tropics. In the hot, humid tropics, weathering reactions that ultimately sequester carbon are even more effective, which has reduced CO2 in the atmosphere and a colder planet for millions of years. The work of unified communications researchers was based on a similar proposal by Macdonald and Oliver Jagoutz of MIT that such processes were important for cooling over the past 90 million years.
During most of the past billion years, the Earth has had little or no ice (white regions). However, the ice has spread at least seven times since the poles to the south, including today (blue), and has twice covered almost the entire planet. (Courtesy of Nicholas Swanson-Hysell)
The new study reinforces the link between these tropical collisions and the global climate. It was conducted by Swanson-Hysell, Macdonald and Jagoutz, along with Yuem Park, a graduate student from UC Berkeley, and Lorraine Lisiecki from UC Santa Barbara.
In current research, the Berkeley / Santa Barbara / MIT team has used state-of-the-art models of Earth's paleogeography to reconstruct the position of such mountain-building events over the last fifty years. They found that the three major ice ages of this period had been preceded by volcanic-continental collisions in the tropics and that no collision outside the tropics had triggered an ice age.
"Although we thought this process was important, the relationship between such environments in the tropics and the ice climate was clearer than expected," Swanson-Hysell said.
Some 445 million years ago, the eastern United States (far left) sat in the tropics, where a collision of islands and continents created the Appalachian Mountains. This collision resulted in an ice age that lasted for millions of years. (Courtesy of Nicholas Swanson-Hysell image)
The team theory also explains why the ice periods are ending. As these collisions stop and fewer rocks are exposed, or the rocks leave the tropical rain belt, carbon sequestration becomes less effective, the CO2 levels increase as volcanic degassing continues and the Earth warms up again in a non-glacial climate.
Swanson-Hysell will present the group's findings on Friday, April 12 at the annual meeting of the European Geoscience Union in Vienna, Austria.
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