The weather phenomenon El Nino has arrived, the US National Oceanic and Atmospheric Administration (NOAA) said Thursday, and scientists expect it will intensify into the end of the year, potentially to historic strength. El Nino is a natural climate phenomenon that warms surface temperatures in the central and eastern equatorial Pacific Ocean, bringing worldwide changes in winds, rainfall patterns and erratic weather. And scientists fear it will exacerbate the heat of a planet already warming from burning fossil fuels, while amping up weather extremes.