It is a global issue because it affects the earth's entire climate.
And since the global climate is so sensitive, even minor changes can have enormous consequences (ie, floods, increased precipitation, heat-waves, more Category 5 hurricanes and typhoons, rising of sea level, droughts, etc.).
I believe the ability of the United Nations to determine which countries can survive politically and economically and which ones can not is the prime motivator of this issue.
This would mean that it is certainly a global issue. If the United States were not permitted to increase it's production and countries like China were, it would be a great boost for many third world countries and allows their standard of living to increase at the expense of the rest of the world.