Why does threshold and feedback effects make global warming so scary?

Answer 1

Tipping over a threshold, and the positive feedbacks in global warming are scary, yes.

When global warming reaches a "tipping point" then it is practically impossible for it to go back. During 2016 carbon dioxide levels in the atmosphere were over 400 ppm the whole year, a symbolic tipping point,

Scripps Institute of Oceanography: "the concentration of carbon dioxide (CO2) in the atmosphere has stayed above the 400 ppm throughout the entire year up to this point. That all ensures that 2016 will be the year that carbon dioxide officially passed the symbolic 400 ppm mark, never to return below it in our lifetimes."

Feedback effects are cyclical effects like the Albedo (Reflectivity) and Melting of Arctic Ice. Ice reflects a lot of the sun's heat, slowing global warming. When the ice melts it reveals the darker ocean water below. This water does not reflect heat, in fact, it absorbs it, so the more the ice melts, the warmer the ocean gets, and the warmer ocean helps to melt more ice. This is a positive feedback effect.

Another positive feedback effect is the melting of tundra and other frozen land. This melting releases vast amounts of methane, which is a powerful greenhouse gas. The released methane makes the atmosphere warmer. This melts more frozen land, releasing more methane.

Methane hydrates at the bottom of the ocean have a similar positive feedback.