Scientists at the University of California believe they know why the generally warm and temperate climate of the Earth during the past billion years has been interrupted by occasional cold spells that wrap the poles in ice and sometimes turn the planet in a snowball.
The key trigger, they say, is the formation of mountains in the tropics when continental mbades collide with arcs of volcanic islands, such as the Aleutian Islands chain in Alaska.
Earth's climate is, to a large extent, driven by 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 driven the COtwo Levels at heights not seen in 3 million years, CO.two levels have been even higher in the Earth's past, coinciding with warm periods when there were no large ice sheets.
In fact, the Earth's predetermined climate seems to be warm and soft. The periods without glaciers dominated during three quarters of the last billion years.
However, half a dozen ice ages cooled the Earth during that time, two of them severe enough to turn the planet into a Snowball Land with ice covering much of the surface. What caused these frigid interludes?
In a study that appears in this week's edition of the magazine Science, the team concludes that when the volcanic arcs collide with the continents in the tropics, an inevitable consequence of the tectonic plates in constant movement of the planet, cause a global cooling, which results in a glacial climate with extensive layers of ice.
Such a collision is now taking place as parts of the Indonesian archipelago are pushed into the mountains on the northern margin of Australia. The result is that there are mountains that contain rocks known as ophiolites that have a great capacity to remove carbon from the atmosphere. Throughout the geological periods, there is an act of equilibrium between the COtwo emitted by volcanoes and COtwo Consumed through chemical reactions with rocks. Rocks with abundant calcium and magnesium, such as ophiolites, are the most efficient to consume COtwo. When these elements are released from the rocks, they are combined with the CO.two and they go towards the ocean, where they form limestone, blocking COtwo In rock, where it remains for millions of years.
"The Earth has a long-term carbon sequestration program," said Nicholas Swanson-Hysell of UC Berkeley, badistant professor of earth sciences and planetariums who designed the study with Francis Macdonald, a professor in the Department of Earth Sciences at UC Santa Barbara. "We know that these processes keep Earth's climate balanced, but determining what causes changes between non-glacial and glacial climates over a million-year time scales is a long-standing puzzle."
Unfortunately for the future of the Earth, the geological processes that consume COtwo They are slow and unable to deal with the mbadive COtwo Emissions resulting from the burning of oil, coal and natural gas. Over the millennia, the Earth's natural carbon sequestration program will restore balance, Swanson-Hysell said, but this will be a long wait for modern civilization, which has been so successful in the current climate and cooler climate. Land.
The Appalachians arrived with a big freeze
In 2017, Swanson-Hysell and Macdonald proposed that a large ice age of 445 million years ago was triggered by a collision similar to that which occurs today in Indonesia. That collision took place during the first phase of the Appalachian mountain construction, when the current U.S. Eastern was in the tropics. In the hot and humid tropics, the weathering reactions that finally sequester carbon are even more efficient, resulting in less COtwo In the atmosphere and on a colder planet for millions of years. The work of UC researchers was based on a similar proposal by Macdonald and Oliver Jagoutz of MIT that these processes were important for cooling during the last 90 million years.
The new study strengthens the link between such tropical collisions and global climate and was conducted by Swanson-Hysell, Macdonald and Jagoutz, along with Yuem Park, a graduate student at UC Berkeley, and Lorraine Lisiecki, UC Santa Barbara.
In current research, the Berkeley / Santa Barbara / MIT team used cutting-edge models of Earth's paleogeography to reconstruct the status of such mountain-building events over the past 500 million years. They discovered that the three main ice ages during this time had been preceded by arc-continent volcanic collisions in the tropics, and that no collision outside the tropics caused an ice age.
"While we thought this process was important, the relationship between such environments in the tropics and the glacial climate was clearer than we expected," said Swanson-Hysell.
The team theory also explains why ice ages come to an end. As such collisions stop and less rock is exposed, or when rocks leave the tropical rain belt, carbon sequestration becomes less efficient, COtwo levels increase as volcanic degbading continues and the Earth once again warms up in a non-glacial climate.
Swanson-Hysell will present the group's findings on Friday, April 12 at the annual meeting of the European Union of Geosciences in Vienna, Austria.