I suspect that man's activities, and particularly GHG emissions and deforestation, at the very least exacerbate global climate change. Just how much, what are the precise mechanisms for doing so, and how best to reduce that impact, will probably change over time as our study of earth sciences improves. But given the possible (note that I did not say "probable") stakes and timeline involved, it is prudent that we act now to reduce that impact based on the best assessments we can make at this time.
Lack of complete scientific certainty shouldn't be an excuse for inaction. Given how often meterologists make mistakes in weather forecasts (daily?), one cannot argue that meterology is an exact science. Yet when they tell you a Force 4 hurricane is headed directly for you, the prudent thing to do is to prepare for it, even if it ends out vering away at the last minute or hits with less wind velocity and precipitation than predicted. I think the same principle applies to global climate change. As to what steps mankind can take beyond adaptation strategies, I think at the very least we can act in areas that make sense in other ways (including economic) -- greatly increasing energy and fuel efficiency, switching as much as we can to renewable fuel sources that have lesser adverse environmental impacts, etc.
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