(i) Increase in the level of greenhouse gases ($C0_2$, CFCs, etc.) in the atmosphere allow the heat waves to reach earth but prevent their escape and thus, the earth becomes warm.
(ii) Effects of increased temperature are:
(a) Leads to deleterious changes in environment, resulting into odd climatic changes such as El Nino effect.
(b) Melting of polar ice caps which will cause the rise in sea level and many coastal areas may also get submerged.