Because it is not visible to people's eyes as it is taking place very slowly that it impossible to observe. For example, change in weather do not take place in one year, the sea level do not rise in 2 days to 5 metres. It is proven as scientists gathered data over 10 years. However, in southern countries, it is visible as during the summer, the highest temperature shows that it increased by 1c every year.
It's hard to prove the cause of human but again there many evidence to show that human are part of the cause of this global warming.
