It doesn't have to end all life on earth, it just has to end ours. Moss and bacteria will survive a lot more than we will.

Plus, to be rational, you have to evaluate not only probability but also potential losses, and multiply the two. A messed-up climate causing crop failures and population reverting to, say, middle ages counts would cost, say, 100 years' worth of 95% of current world GDP. Human extinction, from a human point of view, has infinite cost.