Why has an international treaty on carbon emissions been hard to reach?

21 youth have sued the U.S. government that the current
US approach to climate change is unconstitutional as it
is going to impact their futures,
and their right to life, liberty and the persuit of happiness.
This is a landmark case, and has reached the supreme court. 

In class, you heard background on historic and current greenhouse gas emissions.
That slide show is shared here.
Then you played the role of a group of nations with shared interests and decided which proposal your group supported the most.  The groups and the proposals can be found here, see pages 61-72 (Lesson 7) in this PDF.