January 2025 has been confirmed as the warmest on record globally, driven by human-caused climate change, despite emerging La Niña conditions in the tropical Pacific.

La Niña is known for its temporary cooling effect on global temperatures, so in this article we explore what it is and why it matters.

There is a large natural climate phenomenon called El Niño-Southern Oscillation (ENSO). The cooler phase of ENSO is known as La Niña. La Niña is a climate pattern defined by an unusual cooling of surface and sub-surface waters in the Pacific Ocean, and events can last between 1–3 years.

The conditions for declaring La Niña differ between different weather and climate agencies, but during an event sea temperatures in the equatorial Pacific can often fall 3-5 °C below average. 

El Niño events typically have the opposite effect to La Niña. El Niño is the name given to the warm phase of ENSO. When there is neither a La Niña or El Niño event ongoing, it is referred to as ENSO-neutral.

A strong La Niña or El Niño can bring big changes in temperatures, wind patterns and rainfall patterns around the world. 

La Niña changes temperatures in the tropics, in turn affecting atmospheric circulation, which then influences global climate and weather patterns worldwide. La Niña is mainly known for its temporary cooling effect on global temperatures. 

Between 2020 and 2022, the world experienced an unusually long period of La Niña conditions, which helped keep a lid on global temperature rise due to human-driven climate change. Whereas in 2023 – 2024 a string of monthly global temperature records were broken, boosted by El Niño.

During La Niña events, trade winds in the Pacific are even stronger than usual, pushing more warm water toward Asia. Trade winds in the Pacific are winds that blow from east to west across the tropics, caused by the Earth’s rotation and high pressure areas. Due to the shift of warm water towards Asia, upwelling increases, bringing cold, nutrient-rich water to the surface off the west coast of the Americas. 

These cold waters in the Pacific push the jet stream northward. This tends to lead to drought in southern USA and heavy rains and flooding in the Pacific Northwest and Canada.  La Niña can also lead to a more severe hurricane season. 

When more cold water comes to the surface off the west coast of South America during La Niña events, more nutrients rise from the bottom of the ocean. That means there is more food available for marine species like squid and salmon, in turn increasing stocks for South American fishing communities.

It’s clear that La Niña has knock on effects for people, infrastructure, food, and energy systems around the world.