La Niña is a part of El Niño southern oscillation, a climate fluctuation that slowly sloshes vast bodies of water and heat between different ocean basins and disrupts weather patterns around the world. El Niño was first identified and christened by Peruvian fishermen who noticed a dismal drop in their catch of sardines that coincided with much warmer than usual coastal waters.

El Niño is now well known to be part of a grander climate reorganization that also has a reverse cool phase, La Niña. As vast swathes of the eastern Pacific cool down during La Niña, this has knock-on effects for atmospheric weather patterns, shifting the most vigorous storms from the central Pacific to the west and disrupting the prevailing winds across the globe.

This atmospheric reaction also helps to amplify sea surface temperature changes. Typically, La Niña will lower the global temperature by a couple of tenths of a degree Celsius.

In 2024, the Pacific swung from moderate El Niño conditions to a weak La Niña. However, this time around, it’s apparently not enough to stop the world warming, even temporarily. So what’s different this time?

Scientists aren’t entirely surprised. Each El Niño and La Niña cycle is unique. Following a surprisingly lengthy “triple dip” La Niña starting in 2020, the El Niño that developed in 2023 was also unusual, struggling to stand out against globally warm seas. The switch to a weak La Niña has only slightly cooled a narrow band along the equatorial Pacific, while surrounding waters have remained unusually hot.

Recent research shows that human caused warming of the ocean is accelerating, meaning a year-on-year rise in temperature is itself getting bigger, and this is dominating, to an ever greater extent, over El Niño and other natural oscillations in the climate. This means that even during La Niña, when equatorial eastern Pacific waters are cooler than normal, the rest of the world’s oceans have remained remarkably warm.

There is also a sense of inevitability as greenhouse gas levels continue to grow, even despite the demise of El Niño. During El Niño years, the land tends to absorb less carbon from the atmosphere as large continental areas, such as parts of South America, temporarily dry out causing less plant growth and more carbon-emitting plant decay.

La Niña tends to have the opposite effect. In the strong La Niña of 2011, so much extra rain fell on the normally dry lands of Australia and parts of South America and Southeast Asia that sea levels dropped as the land held on to this excess moisture borrowed temporarily from the ocean. This meant more carbon was taken from the atmosphere to feed extra plant growth. But despite the switch to La Niña, the rate of rise in atmospheric carbon in 2024 and January 2025 remains above the already high levels of previous years.

To this we can also add the diminishing effects of particle pollution from industry, big ships and other sources of “aerosols”, which in some regions had added a reflective haze in the atmosphere, meaning the world absorbed less sunlight. Clean air policies introduced over time have made the world less smoggy, but they also seem to have caused clouds to reflect less sunlight back to space, adding to global heating.

As industrial activity continues to spew greenhouse gases into the air, while air cleansed of particle pollution causes more sunlight to reach the ground, this growing heating effect is beginning to drown out natural fluctuations, tipping the balance toward record warmth and worsening hot, dry and wet extremes.

But, just as one swallow doesn’t make a summer, a single month is not reflective of the overall trajectory of climate change. Changing weather patterns from week to week can rapidly shift temperatures, especially over large landmasses, which warm up and cool down more quickly than the oceans.

Large areas of Europe, Canada and Siberia experienced much less cold weather than is normal for January (by up to about 7 degrees). Parts of South America, Africa, Australia and Antarctica also experienced above average temperatures. Along with the balmy oceans, this all contributed to an unexpectedly warm start to 2025.

While this particular warm January isn’t necessarily cause for immediate alarm, it suggests natural cooling phases may become less effective at temporarily offsetting the impact of rising greenhouse gas levels on global temperatures.

And to limit the scale of the inevitable, ensuing climate change, there is a clear, urgent need to rapidly and massively cut greenhouse gas emissions and to properly account for the true cost of our lifestyles on societies and the ecosystems that underpin them.

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The writer is a professor of climate science at University of Reading. This article is republished under a Creative Commons license.