Keeping our Eyes on the First Milestone

In his most presentation to the UN in September 2007, John Holdren's slides had a stunning number:

If worldwide CO2 emissions peak in 2015 – that’s seven years from now – we have a 50 percent chance of avoiding catastrophic effects of climate change.

50 percent chance
of avoiding catastrophic effects

Those aren’t odds I like to play with given the stakes. What is catastrophic?

50-75% species loss this century
5 meter sea rise this century
10-20% reductions in corn, wheat, and rice yields this century

I imagine there are many many other things that could be added to this list.