Question

Why was the concern over global cooling replaced with a concern over global warming?

Answer 1

The reason why the concern over global cooling was replaced with a concern over global warming was that steady surface temperature increases were noted between the 1970s and 90s. 

Answer 2

The correct answer is - b. Steady surface temperature increases were noted between the 1970's and 90's.

Once it has been noticed that the global temperature is actually rising instead of dropping, the scientists shifted their opinion and focused on a global warming instead of the global cooling. 
The temperature rise was noticed in most of the world with few exceptions, but the most concerning was the warming on the north and south pole because of the melting of the polar caps the sea levels will rise globally and this will contribute to lots of big problems apart from the global rise of the temperature (though on short term), and that lots of huge important cities may end up bellow water and that the world economy will collapse.