Geologists have spent decades debating the temperatures during the first few hundred million years of Earth's history, as well as the early ocean's chemistry. A new paper provides evidence for relative moderation, similar to the world today. The authors conclude that the same forces that kept the planet in balance as life first appeared are likely to apply to many other worlds, increasing the chances the universe abounds with life.
Our understanding of stellar evolution indicates the Sun was considerably fainter 4 billion years ago than today. This has inspired some scientists to think the Earth's origins were in a deep freeze. However, warming gasses were probably more abundant, leading to estimates of average temperatures above 70ºC (160ºF).
University of Washington graduate student Joshua Krissansen-Totton has included factors left out of such estimates, such as ocean acidity. In Proceedings of the National Academy of Sciences, Krissansen-Totton and co-authors write: “We find that the Archean [4-2.5 billion years ago] climate was likely temperate (0-50°C) due to the combined negative feedbacks of continental and seafloor weathering.”
While the Archean oceans were probably slightly acidic, and therefore unsuitable for modern life forms such as corals, the pH was never below 6.6, and by the end of the Archean, it had already reached 7, neutrality between acidic and alkaline ions.
"Our results show that Earth has had a moderate temperature through virtually all of its history, and that is attributable to weathering feedbacks – they do a good job at maintaining a habitable climate," Krissansen-Totten said in a statement.
The findings are consistent with other recent work indicating it's not just biological forces that tend to keep the Earth from running too far in either temperature direction, with carbon dioxide levels rising and falling within a range that keeps things in balance.
"Seafloor weathering was more important for regulating temperature of the early Earth because there was less continental landmass at that time, the Earth's interior was even hotter, and the seafloor crust was spreading faster, so that was providing more crust to be weathered," Krissansen-Totton said.
If the reasons for the Earth's mild state were unusual, or even unknown, this might be bad for the prospects of finding life elsewhere. However, the authors think the opposite is true. The same forces are likely to be at play for other planets within stars' habitable zones, at least around stars with masses similar to the Sun, rather than red dwarfs, where radiation outbursts probably strip away atmospheres and oceans.
Remarkably, the results didn't change much unless Krissansen-Totton varied certain assumptions, such as the rate of volcanic release of carbon dioxide, to highly improbable degrees, strengthening the likelihood things will be similar on other worlds.
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