Re: Sleep is a Good Thing(TM)
"catastrophic global climate change"
The question is "How Catastrophic?"
About 250 million years ago at the end of the Permian. era, Flood-basalt volcanism on a titanic scale, in what is now Siberia, injected enormous amounts of carbon dioxide into the air as it punched through coalfields in the area.
The result: Sudden global warming, which reached a point at which methane hydrate formations in the oceans became unstable and released vast amounts of methane into the atmosphere. Which warmed up the atmosphere even more. Which prompted more methane hydrate releases, in a vicious cycle.
It wasn't so much the rising temperatures that killed off most of the plants, but the increasing acidity of rain. For a sizable period of time (long enough to be observed but not long enough to be estimated), paleopalynologists find virtually no pollen -- only spores of fungi adapted to feeding on rotting vegetation -- and the ten million years after the crisis is the only known period in natural history since the arrival of plants on land that no coal formed anywhere on earth. Which is a very good marker of how far-reaching the death of land plants was.
All of those fungi and microorganisms feeding on the dead plants (and, soon after, the dead animals) consumed oxygen and excreted carbon dioxide -- making their own contribution to the vicious cycle, with no living plants to take up CO2 and damp the effect. Some palaeontologists believe that this cycle, once triggered, could have played out very, very rapidly -- on a scale of months.
Atmospheric oxygen levels dropped from ~20% to around 12-14%. The oceans became oxygen-depleted as well; what little fauna survived was notably adapted to low-oxygen conditions. With plant life nearly extinguished, food webs collapsed. Between the increased acidity of rainfall and the denudation of slopes, erosion increased to a furious pitch.
The result: the greatest mass extinction in Earth history -- no other even approaches it. 75-90% of species died out, and the biosphere was nearly handed back to the single-celled organisms. Biodiversity plummeted to the point at which a single species of land animal, a pig-sized herbivore, made up 80% of land animal biomass.
It didn't just happen once, although that was the biggest one. This time around the essential ingredients (rapidly rising CO2 levels and very large marine methane clathrate deposits - which are already bubbling out along the siberian continental margin - lookup "leptav sea methane emissions) are still there. I wouldn't bet on H. sapiens sapiens making it through the coming challenge, let alone its "civilisations".