Why are the concern over global cooling replaced with a concert over global warming?

Because global warming warms up the earth and even though it only warmed it up 4C, it is estimated to warm up more and more each year.

With all the pollution that the world puts into the atmosphere, it is slowly destroying the ozone layer. The ozone layer protects us from harmful rays that the sun gives off. If that ozone layer disappears, The earth will be much warmer, and you will probably get skin cancer. Human life will be drastically different if we couldn't go outside because of this.