How convinced are you that human activities are a significant cause of changes to the Earth's climate and long-term weather patterns?

It all depends how you define "significant". The Earth has wild weather swings between heat periods and ice ages and not only does it's weather patterns change but even it's land masses move around. By comparison man made change is insignificant. If you mean significant for the wellbeing of humans and the weather they have been used to in the last few centuries then yes the evidence of man made weather change is well documented but also overrated when contrasted to the natural weather evolution. There are other things to focus on like toxicity and deforestation, weather is probably the least of our polution related problems.