"Nobody knows which day of the week a six mile-wide asteroid crashed into what would someday be the Yucatan Peninsula. What people do know is that day was around 65 million years ago, and that the days that came after were colder, darker, and filled with fewer and fewer dinosaurs.
The collision reconfigured Earth's life support systems by kicking up huge amounts of dust, vaporizing massive volumes of water, and triggering hundreds of earthquakes and volcanic eruptions. The strike—and ensuing mass extinction—marks one of the most well-known geologic divisions, between the Cretaceous twilight and the Paleogenic dawn. In terms of global impact, humans are like a scattershot version of that asteroid; they have changed the planet so much that many scientists believe modern society deserves its own geologic epoch—the Anthropocene. And while nobody knows which day humans became a force of nature, a pair of scientists believe they have an equation that can pinpoint the year.
This planet is about 4.5 billion years old. For at least three quarters of that time, it has supported life. "The Earth is generally in a state of balance, with feedback loops that keep things like the atmosphere and temperature at equilibrium for deep spans of time," says Owen Gaffney, a writer and co-author of the new study, published in the Anthropocene Review. During those times of equilibrium, lifeforms evolve slowly, extinction is rare, and biodiversity increases. Then along comes an asteroid strike or megavolcano eruption. Or the Earth tilts half a degree on its axis. Each cataclysm alters the atmosphere, temperature, ocean composition, and dozens of other processes that determine what is fit enough to survive.
Item: In 1700, human-used land covered about 5 percent of the Earth. In 2000, it was about 55 percent.
Item: Human greenhouse gas emissions are causing the ocean to acidify at nearly the rate it did prior to the Permian extinction—the planet's largest—about 300 million years ago…"
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