An El Niño has been officially declared over the tropical Pacific Ocean by Australia's Bureau of Meteorology. This complex weather pattern, notably characterized by warmer surface temperatures in the equatorial Pacific, typically causes unusual weather and higher global temperatures on average.
El Niños are extremely complicated and difficult to predict, particularly from March to June. A declaration had already been made by the National Oceanic and Atmospheric Administration back in March, which predicted a weak El Niño which would likely dissipate soon. The Australian scientists, however, are calling for a "substantial" one that will likely persist through most of the year.
If the Australians are correct, then 2015 could shatter 2014's record for hottest year ever recorded. Almost all the heat trapped by global warming ends up in the ocean, and El Niño is one way some of it gets back into the atmosphere.