La Nina, a cyclical cooling of the equatorial Pacific Ocean, had arrived by the second week of December. A La Nina is expected to cool down global temperatures. Yet, not only was December 2024 ranked the second warmest December month in recorded history, but warming relative to the pre-industrial average was more than 1.5°C in the month in most datasets. Why is the La Nina not cooling down global temperature? One reason for this is that there is a lag from the onset of Nina to a cooling in global temperatures. The second reason is that the La Nina cannot affect the long-term warming that anthropogenic emissions are causing. Here is how.
Snow covers a rock as drifting broken sea ice, floats on the surface, off the extreme western point of Replot Island, in Kvarken archipelago, near Bjorkoby, in the northern part of the Baltic Sea on January 15, 2025. Due to a mild winter and temperatures above zero degrees centigrade, the open sea has not frozen. (Photo by Olivier MORIN / AFP) (AFP)
The onset of La Nina (or its opposite phenomena, the El Nino) is usually tracked using sea surface temperature (SST) deviations in the Nino3.4 region (5N-5S, 170W-120W). When SSTs cooldown by at least 0.5°C consistently compared to a 30-year average updated every five years, a La Nina is declared. With hindsight (this trend has held beyond the second week of December) and forecasts, one can say that La Nina arrived by the second week of December.
Despite the arrival of La Nina, five out of six global temperature datasets showed that December last year was warmer than the pre-industrial average by more than the pre-industrial average. It was also the second warmest December month, just behind December 2023. In fact, daily ERA5 data from the Copernicus Climate Change Service (C3S) shows that global temperatures have not cooled down so far in January either.
Each of the first 13 days of the month (the period for which this data is available) was at least 1.64°C warmer than the pre-industrial average. Even allowing for the fact that ERA5 usually shows a higher level of warming than other datasets, this is far too warm (2025 was the first year when the annual average crossed 1.5°C warming in multiple datasets) for comfort. For example, each day from January 3 to January 11 was the warmest this year, and other days were the second warmest this year. If these trends continue, January 2025 will at least end up as the second warmest January on record, a fact that most datasets are likely to agree on.
Chart 1 (Abhishek Jha)
Why is La Nina not cooling down global temperatures? As explained above, there is a lag between the onset of La Nina (or El Nino) and its impact on global temperatures. How long is this lag? Around three months if previous trends hold. To find this expected duration, HT removed the long-term trend from global temperatures (where factors other than cyclical ones, such as El Nino and La Nina) and SSTs in the Nino 3.4 region and calculated the correlation between them. This shows that, for the 1950-2024 period, the correlation is strongest at a 3-month lag, although the strength of the correlation at a 2-month or 4-month lag is not drastically different. Nonetheless, that the correlation is stronger at a lag suggests that we cannot expect air temperatures (or what is commonly known simply as temperature) to cool down at the same time as the equatorial Pacific.
To be sure, even in a few months, the cooling is unlikely to be anything more than cyclical. In other words, we might expect temperatures cooler than what they were when an El Nino was underway (such as last year), but not drastically cooler. This can be seen in the super-imposed chart of SST deviations in the Nino3.4 region and global temperatures. In the long-term, global temperatures during La Nina are now warmer than what they were during La Nina events in the past.
The trends described above mean that the impact of La Nina and El Nino events is relatively insignificant compared to the long-term warming that greenhouse gas emissions have caused. Therefore, the relief from La Nina is likely to be only relative to the past year of El Nino. In fact, since the La Nina is expected to be weak and short-lived, its influence may be even more insignificant than in recent years. Therefore, it is not surprising that the UK Met Department has forecast that 2025 will likely be one of the three warmest years on record.
Abhishek Jha, HT’s senior data journalist, analyses one big weather trend in the context of the ongoing climate crisis every week, using weather data from ground and satellite observations spanning decades.
Abhishek Jha is Assistant Editor-Data at Hindustan Times. He uses statistical programming to generate newsworthy insights from large datasets. He is part of the team that produces Number Theory, a daily data story feature of the paper’s print edition. Since March 2024, he has been writing Weather Bee, a weekly column for the Hindustan Times website. He is a chemical engineer by training, who specialises in stories related to weather, climate, and the environment.
Jha has been at HT since 2018, where he offers data-driven perspective and analysis on politics, environment, weather, climate, economy and society. His work includes data coverage of elections in India and abroad, including the 2019 and 2024 Lok Sabha elections; the disasters and extreme weather resulting from changing climate, such as floods, droughts, heat waves, cold waves, and dwindling snow cap in the Himalayas; the factors that drive poor air quality in northern India; the changing patterns of land use; the Covid-19 pandemic and its impact on labour market conditions; the changing pattern of consumer spending seen in the new consumer spending surveys; and social norms seen in the surveys such as the National Family Health Survey.Read More
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