The next sentence in the page is "It is also the only known mass extinction of plants". I didn't include it because it says citation needed, though. Every scenario in which life is endangered globally involves harsh feedback loops and is therefore extremely hypothetical and unstudied, but the possibilities do exist. Climate change simulation is a best case of a best case scenario- how could you possibly account realistically for the impact of losing biodiversity on that scale?

The short term problem is probably ocean acidification, although 3+ degrees will directly kill a ton of food production and plant species. Ocean acidification is usually thought of in terms of corals and shellfish but it can also lead to a collapse of the ocean food chain when nearly all zooplankton die. Oceanic plants can survive acidic conditions and will thrive without predators or competition, but that leaves out the fact that the ocean will now be full of dead, rotting things. There are an incredible number of ways that scenario can become a nightmare. Poison by toxins, infection caused by too-rapid growth, explosion of predators as they feed on detritus, etc. A healthy population is required to survive a huge climate change, and there are a million ways for that to go wrong.

The major catastrophe would be 5+ degree warming, which has the potential to collapse current ocean circulation patterns including the nino cycle. That would cause major devastation to biodiversity, deoxygenation of the ocean, and kill most life by land area and ocean volume. Then the isolated patches of life would have to deal with extreme weather, air quality, acidity changes, water and temperature changes. The survivors would have to stabilize the biosphere and live for millions of years with an unprecedented lack of biodiversity, until they could evolve. Species die spontaneously all the time. Tasmanian devils developed contagious cancer.

The Sahara used to be a jungle. The wind changed a little bit, and converted everything to sand. As the climate changes, areas will be destroyed- even if they stay green, biodiversity will be massively reduced and those places will no longer be sustainable long term. Even if rain starts falling in the Sahara, it will be decades or centuries before it grows.

In a normal extinction event growth is moderated over tens or thousands of years, in a smooth transition to other species. We have a century until we reach the same changes.

Say you have a field with grass, moss and cows. Geological warming kills the grass over a hundred years, and the moss slowly grows in it's place, and all is good. If anthropogenic warming kills all the grass overnight, The cows eat all the moss except for a little hidden under a rock, which is killed when a cow dies on top of it and blocks the sun.

>It seems to me that preventing collapse of human civilization is the thing to focus on. It's what most people will care about, right? And it's far more likely than total global ecosystem collapse. I mean, we may see the first signs within a few years. The "end of all life on this planet" is something that humanity will never get near, because it'll be long gone before it happens.

Wow, that may actually change my feelings on reading this sentiment. It's still a little horrifying that we have to wait until humans are affected, but I do understand how someone could look around and say "Yep, still looks alive to me!". If focusing on the threat to humanity engenders the threat as a whole, I'm all for it.

Of course, the breakneck speed of extinction is still horrifying. Each day means hundreds of species that I will never see, and I'm already pissed about missing giant sloths. We only killed those 10,000 years ago.