First, even the sun's life is finite.  It won't be for eternity.

Second, you might have seen some pink stuff when your were rooting around in your attic.  It's called insulation, and it's function is to reduce heat loss.  Keeps you warmer in winter, and cooler in summer.  If you put more insulation in there, it does a better job.

Now imagine the upper atmosphere as your attic.  If you add more insulation, what happens?  Think really, really hard now.....  Yes, you got it!  It will hold more heat in!

Now, what the scientists are telling us is that the CO2 that we produce when we burn fossil fuels acts like......yes, yes!  Insulation!

(See, it really isn't all that hard)