I do believe it's happening but I'm sceptical about whether it's all man-made rather than the natural cycle that happens over billions of years. After all, climate change was recorded long before we had cars and planes.
If it is all down to us I think the biggest culprit is probably population growth as millions more people breathe out carbon dioxide.
I recycle everything I can and buy locally produced food where possible but it does seem a bit like the earthquake and dustpan analogy.