Why is the sun releasing more radiation,which causes global warming now than before?

Global warming is not due to increased energy output of the sun. It is well known that the power output of the sun fluctuates somewhat over long time scales - it's generally believed that reduced output has been responsible for the periodic ice ages that the earth experiences. There is also some regular fluctuation that occurs over an 11-year cycle corresponding with sun spot activity. However, if that fluctuation was responsible for global warming we would see global temperatures rising and falling on an 11-year cycle, but we don't. So in summary global warming is not due to changing power output of the sun.

In a few billion years, the sun will have used up it's hydrogen and become a red giant. You will then have real "global warming". Life will be long gone as the inner planets, Mercury and Venus, will have vaporized and Earth will be uninhabitable. Stick around!!!