CLIMATE × LIFE

The Fern That Cooled the Arctic

~49 to 48 million years ago

Phanerozoic · Cenozoic · Paleogene · Eocene

For about a million years the Arctic Ocean was a freshwater lake covered by a floating fern that doubles its mass in a few days, among the fastest-growing plants alive. It grew, died and sank into a basin with no oxygen in it, where nothing could rot it. Grown today under high carbon dioxide, the same fern grows faster still, which would have made the burial self-reinforcing.

Why it matters

It sits exactly where the greenhouse world begins sliding towards the icehouse we live in. How much carbon dioxide the burial removed is a calculation rather than a measurement, and the estimates run from 55 to 470 parts per million, which is the difference between a footnote and a cause.

Dating & uncertainty

The layers of fern spores in Arctic sea-floor cores are unambiguous; how much carbon dioxide their burial removed is a calculation, and the estimates span 55 to 470 parts per million.

Sources

1. Speelman et al. (2009), Geobiology · The Eocene Arctic Azolla bloom: environmental conditions, productivity and carbon drawdown
2. Barke et al. (2012), Palaeogeography, Palaeoclimatology, Palaeoecology · Coeval Eocene blooms of the freshwater fern Azolla in and around Arctic and Nordic seas
3. Hutchinson et al. (2021), Climate of the Past · The Eocene-Oligocene transition: a review of proxy data, models and comparisons

Earth history as one day

23:44:2715 min 33 s before midnight

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