CNN — 

Scientists have watched in astonishment as ocean temperatures have steadily risen over the past several years – even as the cooling La Niña phenomenon had a firm grip on the Pacific. The oceans have been record-warm for the past four years, scientists reported in January. Then in mid-March, climatologists noted that global sea surface temperature climbed to a new high.

The incredible trend worries experts about what could lie ahead, especially as forecasts predict El Niño is on its way starting this summer – and along with it, impacts like extreme heat, dangerous tropical cyclones and a significant threat to fragile coral reefs.