Peter Brannen writes:

Some 4 billion years after its creation, a small planet circling an unexceptional star in the outlying Orion-Cygnus spiral arm of the Milky Way enjoyed a brief and bustling season of complex life. The planet blushed with a breathable atmosphere, and for a few hundred million years it also hosted temperatures that somehow stayed within a surprisingly narrow window — one amenable to a biosphere that now teemed with energetic, multicellular creatures. For its entire prior history, it had hosted only microbes and muck; in this glorious season, the planet flourished with animal life.

For a time at least. For most of Earth’s history, it hadn’t, and soon after, cosmologically speaking, it wouldn’t, but let’s focus on these happier days. When we do, we find that the half-billion-year age of animal life — starting in earnest with the Cambrian explosion and on through today’s extreme moment of geologic carbon release and upheaval — has seen an extraordinary variety of climates. Animals have survived and mostly thrived throughout, as the Earth has expressed different moods, from viciously sultry, tropical worlds to parched ice-capped ones. These shifting climates intimately shaped the biosphere over this span (and vice versa), and in occasional terrifying pulses, threatened to end it.

“It gets pretty toasty,” said Jessica Tierney, a paleoclimatologist at the University of Arizona, about Earth’s steamiest days, which by her count include: a hot spell before the rise of terrestrial life some 400 million years ago; a near-fatal super-greenhouse when the supercontinent of Pangaea reigned 250 million years ago; the Cretaceous sauna haunted by dinosaurs; and the Eocene at the early age of mammals.

While paleoclimatologists routinely reconstruct this or that bit of Earth history — the dinosaur world that hosted polar rainforests, say, or the Pleistocene one that saw ice sheets overrun Kansas — a few groups are working through the entire story of Earth’s climate across the age of animals. Although they take different approaches to building their models of Earth’s ancient past, their simulations tell similar stories about the mind-bending variety of climates the planet has endured over hundreds of millions of years and their sensitivity to atmospheric levels of carbon dioxide. [Continue reading…]