El Niño is coming. Above-average sea surface temperatures have developed off the west coast of South America and seem poised to grow into a full-fledged El Niño event, in which unusually warm water temperatures spread across the equatorial East Pacific. Models indicate a 75 percent chance of El Niño this fall, which could bring devastating droughts to Australia or heavy rains to the southern United States.

The debate over climate change, however, brings additional significance to this round of El Niño, which will probably increase global temperatures, perhaps to the highest levels ever. It could even inaugurate a new era of more rapid warming, offering vindication to maligned climate models and re-energizing climate activists who have struggled to break through in a polarized political environment.

For a decade, climate scientists have battled a public-relations challenge: Even though atmospheric temperatures are higher than at any time in the past 4,000 years, surface temperature increases seem to have slowed down since 1998. The planet has gotten warmer over the last decade, but climate change skeptics have used this so-called hiatus or pause in warming to take aim at the accuracy of the climate models, which appeared to predict more significant warming than has so far happened.