EARTH’S EVOLVING CLIMATE
From hot to cold and back again
Just after the birth of the Solar System, in the Hadean Eon, the entire Earth was liquid, a hellscape of fire and lava. The planet was still being pummelled from all directions by meteor strikes. As Earth started to cool, a crust of solid rock began to appear on its surface, but at first it was very fragile. Repeated impacts and volcanic eruptions broke that crust into chunks called tectonic plates, which floated on the magma below.
During the Archaean Eon, 4 to 2.6 billion years ago, the plates started moving, but when they collided, the heat of the mantle would break them apart. They had to stop, cool down and recover before they could start moving again. By the time this eon came to an end, tectonic plates had become more stable, and they had started to move constantly. Across history the plates have continually collided and separated, forming and reforming different patterns of continents and oceans.
These movements have had dramatic effects on Earth’s climate. When continents break apart, the exposed rock absorbs carbon dioxide from the air, and global temperatures plummet. As volcanoes erupt, the greenhouse gases break free again and blow back into the atmosphere, trapping heat from the Sun. Events like these have triggered enormous freeze-thaw cycles in Earth’s past.
Earth’s position in space has influenced global climate too. After its collision with Theia, Earth’s axis fell into a tilt, creating the seasons. That tilt has always been unstable, and so has the shape of Earth’s orbit around the Sun. Our distance from the Sun changes on a 100,000-year cycle, tipping us in and out of ice ages. For the first part of Earth’s existence, its climate was entirely dominated by these quirks of space and geology, but when life evolved everything changed.
During the Proterozoic Eon, evolution invented photosynthesis. The atmosphere filled with oxygen and gained a protective shield called the ozone, making it possible for living organisms to start terraforming the land. Our impact on the climate has been monumental, and today humanity is one of the most powerful forces of climate change.
“As Earth started to cool, a crust of solid rock began to appear”