ATMOSPHERE × CLIMATE

The Sky Becomes a Filter

~430 million years ago

Phanerozoic · Paleozoic · Silurian

Oxygen split by sunlight makes ozone, and ozone absorbs the ultraviolet that breaks DNA. The oxygen came from the first land plants, and by an unexpected route: for a given amount of carbon their tissue holds far less phosphorus than marine life does, so the same phosphorus washed off the rocks built twenty times more carbon, and carbon buried unrotted is what leaves oxygen behind. By 420 to 400 million years ago there was enough of it to leave charcoal in the rocks, which means fire.

Why it matters

Life had been ashore in patches for billions of years, but always under a sky that let the hard ultraviolet through. Rising oxygen changed that in two ways: the ozone it made is what let life stand up in the open, and the oxygen itself brought fire. Fire then held that oxygen steady: too much of it and the forests burn, which is the feedback that has regulated the air ever since.

Dating & uncertainty

Redox proxies show the deep oceans oxygenated between about 435 and 392 Ma, and charcoal in the rocks shows atmospheric oxygen passed 15 percent by 420 to 400 Ma. For a given oxygen level, chemistry-climate models find a thinner ozone column than earlier estimates allowed, so the shield came later and more gradually than the textbook account has it.

Sources

1. Lenton et al. (2016), PNAS · Earliest land plants created modern levels of atmospheric oxygen
2. Cooke et al. (2022), Royal Society Open Science · A revised lower estimate of ozone columns during Earth's oxygenated history
3. Tostevin & Mills (2020), Interface Focus · Reconciling proxy records and models of Earth's oxygenation during the Neoproterozoic and Palaeozoic

Earth history as one day

21:43:362 h 16 min before midnight

if Earth’s 4.54 billion years were compressed into a single day