$CO_2$ and methane. $CO_2$ levels are increasing
due to burning of fossil fuel leading to global warming.
Effects of global warming
(i) Deleterious changes in the environment and resulting in odd climatic changes (e.g. El Nino effect).
(ii) Increased melting of polar ice caps as well as of other places like the Himalayan snow caps.
(iii) Rise in sea level that can submerge many coastal areas.