Collapse would be a significant negative feedback on emissions, although initially it might spur calls for a final fossil fuel binge to repair the economic damage done and "save lives".
This might be why some deep ecologists occasionally mention it as a potential advantage to Earth, if not human civilization. There was an article by Anders Levermann in the UK press discussing this and arguing that this feedback would effectively prevent the most extreme degrees of warming - 10C plus. To quote:
"It is the unanticipated impacts on fragile infrastructures and supply networks that constitute the largest threat of global warming. While climate change is often considered to be a problem for the global poor and for fragile ecosystems, the impact of extreme events on the global economic network will test the stability of America as much as that of Europe.
No one knows where the limits of our adaptive capacity are, but a path towards 10C of warming will likely challenge these limits. The wall we are speeding towards may be hidden in the fog, but not knowing where it is does not make it vanish. The warnings provided by weather impacts on our society are quite clear. We can either take them seriously and turn around or find out the hard way."
http://www.theguardian.com/environment/blog/2014/jan/31/climate-change-extreme-weather-earth