'El Nino' effect is closely associated with:

Global warming

The correct answer is Option (2) → Global warming

Increase in the level of greenhouse gases has led to considerable heating of Earth leading to global warming. During the past century, the temperature of Earth has increased by 0.6^oC, most of it during the last three decades. Scientists believe that this rise in temperature is leading to deleterious changes in the environment and resulting in odd climatic changes (e.g. El Nino effect) , thus leading to increased melting of polar ice caps as well as of other places like the Himalayan snow caps.