[Gozra]

According to everything I can get my hands on and reading about Global warming:
The most likely cause is the Sun heating us up. The amount of green house gas that humans contribute in miniscule.
Carbon Dioxide is less that 3 percent of all green house gases and our contribution to that 3 percent is really small way less than 10 percent as I recall.
As far as the vulcanism in the artic I have not heard that one BUT The Greenlandic Ice cap has begun melting in the last 100 years and the trend is accelerating.
This in turn is causing the salinity of the water to become lower. Which in turn is causing less of the gulf stream to reach England and Europe lowering the temperature of the water up there which in turn can cause winters to be colder and longer. Since 1965 the amount of warm water moving north has been reduced by 30%
Some Scientist postulate that this could be a trigger into another ice Age.
The Space sun shade is a neat idea and it can be implemented fairly cheaply but I really think that our efforts should be concentrated in three areas.
1 Clean up our planet and research technologies that help recycle and cleanup our planet
2 Cheap energy
3 Get off this planet and have self-sufficient colonies other than this earth.
4 Promote a society that we are all in this together and we all own earth and everything we do affects everyone else.

If we don't then there will never be a human space empire