Question

why must we look for alternative sources of energy

Answer 1

 Alternative sources are important because we are ruining the planet's climate through carbon dioxide.We are emitting so much in our atmosphere.
 If we continue to use fossil fuels, we may increase the temperature of the planet in ways that will harm us and our entire ecosystem.We cannot keep using fossil fuels forever, as the population increases they will gradually run out.

Answer 2

because we cannot keep using fossil fuels forever.  They will eventually run out, even as the population of Earth grows.