Climate change

http://www.npr.org/templates/story/story.php?storyId=99888903

http://news.slashdot.org/article.pl?sid=09%2F01%2F27%2F0224215

This assumes we just stop outputting CO2 at some point and are passive. But humans are anything but passive, this is what got us into this mess in the first place.
So the article doesn't take into things like sequestration and other active attempts to reverse the trend.
I am confident in 50 year we will be able to completely re-terraform earth back to normal.
We will be able to deliberately control the amount of atmospheric gases, solar radiation reaching earth, and chemistry of the oceans and become the masters of our climate.

Why do I think this?

GA, AI, super computers.

And progress in Material science, renewable energy, Self replicating robotics, genetics and artificially accelerated evolution, Computational chemistry and simulation of quantum molecular dynamics.
The machines (beyond computers) under human guidance will do the research needed to reverse things.
So it's will be just a matter of energy required needed to do this.
I believe it will take more power to put the CO2 back then when we released it from burning fossil fuels.

Essentially give back that energy that we used for the past 100 years + interest.

As much as I disagree with Kurzwell on many things, some parts of his Singularity theory are dead on and will be able to reverse this trend.
So getting fusion working, space solar or some other massive power source going is critical to do this.
The key here is to use the amount of available resources as best as possible to devise future solutions before we get wiped out as a species.

I have faith that technology will save us.
But it may get a lot worse before it gets better.