Picture a thermometer that is already nudging the red zone. Now imagine someone turning up the heat beneath it. That is roughly the situation Earth faces, according to a fresh warning from the United Nations: a new phase of El Niño — the powerful climate pattern that originates in the tropical Pacific Ocean — could begin within a matter of weeks. El Niño works by releasing vast amounts of stored heat from the ocean into the atmosphere, warming air temperatures across much of the globe.

In its strongest years, it has been linked to devastating droughts in East Africa and South Asia, catastrophic floods in South America, and coral bleaching events on Australia's Great Barrier Reef. Scientists describe it as the planet's most influential natural climate driver, and they are watching its signals with unusual urgency right now. What makes this moment different is the backdrop. Climate change, driven by human greenhouse gas emissions, has already pushed global average temperatures to record levels in recent years.

When El Niño layers its own warming on top of that human-made heat, the combined effect can be startling.