Second-warmest May and El Niño: Is 2026 the next 2023?
The Copernicus Climate Change Service (C3S), one of six organizations whose temperature updates are used by the World Meteorological Organization, showed in its global temperature data for May, published this week, that May 2026 was the second-warmest May on record – the first time any month has ranked among the top three this year.
This, combined with the ongoing El Niño phenomenon, which is expected to intensify later this year, suggests that the global temperature anomaly this year may follow a curve more like 2023. Here are some charts that attempt to examine this possibility in detail.
Before we understand how temperatures in 2026 could follow a similar curve to 2023, it helps to understand how global temperatures typically deviate over the course of the year. This can be seen from the monthly deviation relative to the pre-industrial average over the past 10 years, as shown in the accompanying graph.
The average year in the past 10 years has followed a U-shaped curve. It started with high levels of warming relative to the pre-industrial average at the beginning of the year, which declined during the Northern Hemisphere summer season of June to August, and then returned to the same levels seen in January to March.
However, as expected, not all years have followed this trend in the past 10 years. For example, the 2021 and 2022 curves appear relatively flat throughout the year, while the warming in 2016, 2017 and 2020 appears to decrease as the year progresses. The opposite happened in 2023, when temperatures appeared to increase as the year progressed.
Why do these years show unusual trends in increasing temperatures during the year? One possible reason may be one of the biggest factors influencing annual temperature fluctuations: temperature trends in the central eastern tropical Pacific. This is tracked by the Ocean El Niño Index (ONI), which measures deviations in sea surface temperatures in a region compared to the previous average and is updated every five years. When the three-month average ONI is 0.5 or higher, El Niño conditions are said to exist, causing global temperatures to rise. When the three-month running average of the ONI is -0.5 or less, La Niña conditions are said to be present, which has a cooling effect on global temperatures. Neutral conditions are said to prevail when ONI values range between -0.5 and 0.5.
As the accompanying chart shows, in 2016, 2017 and 2020, when warming appeared to be decreasing throughout the year, Pacific Ocean conditions changed from El Niño to La Niña throughout the year. In 2021 and 2022, when the warming trend did not change much during the year, Pacific conditions also remained relatively stable. In 2023, when warming appeared to be increasing throughout the year, the Pacific Ocean shifted from neutral conditions to El Niño.
The trends described above are an important reason why warming trends in 2026 are likely to follow a similar pattern to 2023, with warming increasing as the year progresses. 2026 began with the continued influence of La Niña conditions, keeping the upward anomaly in the first part of the year relatively low. By May, El Niño appears to have already begun, at least according to the ONI (there are other indicators that track El Niño as well). This means that the two- to four-month lag in which Pacific sea surface temperatures begin to affect global temperatures will begin to take effect long before the end of the year, and global average temperatures may rise more toward the end of the year than they did at the beginning of the year.
Certainly, the trends described above must be interpreted with caution. The El Niño phenomenon is not the only factor affecting global temperatures. For example, since 2020, regulation of marine fuels has led to lower sulfur emissions along major shipping routes in the North Atlantic, allowing more sunlight to reach the Earth’s surface and contribute to global warming. An adverse effect of other factors is possible.
Another reason for caution is that El Niño and La Niña explain only a small portion of the rises or falls in temperatures. Most of the warming we are currently seeing in global temperatures comes from long-term climate change. This means that while 2026 may see increased intra-year warming, as occurred in 2023, the average warming for the year will likely be much higher than in 2023. Global temperatures averaged 1.28°C above the pre-industrial level through May 2023, and ended the year at 1.48°C. The average for 2026 through May is already 1.46°C.
