Weather Bee: Is it 2023 again for global temperatures?

THe nicolaus copernicus mood commute serve (C3S), 1 of the six organisations whose temperature updates are used by the World Meteorological Organisation, 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, which is forecast to intensify later this year, suggests that the deviation in global temperatures this year might follow a 2023-like curve. Here are some charts that try to examine this possibility in detail.Before understanding how 2026 temperatures might follow a 2023-like curve, it is helpful to understand how global temperatures usually deviate over the course of a year. This can be seen in the monthly deviation relative to the pre-industrial average for the past ten years, shown in the accompanying chart.The average year in the past ten years 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 decreased during the Northern Hemisphere summer season of June-August, and returned to the same levels seen in January-March. However, as expected, not all years have followed this trend in the past 10 years. For example, the curves for 2021 and 2022 look relatively flat throughout the year, while warming in 2016, 2017, and 2020 seemed to decrease as the year progressed. The opposite happened in 2023, when warming appeared to increase as the year progressed. What is the reason behind these years showing unusual trends in warming through the year? One possible reason could be one of the biggest factors affecting interannual variability in temperature: temperature trends in the central-eastern equatorial Pacific Ocean. This is tracked through the Oceanic Niño Index (ONI), which measures deviations in sea surface temperatures in the region relative to a past average updated every five years. When the three-month running mean of the ONI is 0.5 or above, El Niño conditions are said to be in place, which has a warming effect on global temperatures. When the three-month running mean of the ONI is -0.5 or lower, La Niña conditions are said to be in place, which has a cooling effect on global temperatures. Neutral conditions are said to prevail when ONI values are between -0.5 and 0.5.As the accompanying chart shows, in 2016, 2017, and 2020, when warming seemed to decrease through the year, Pacific conditions changed from El Niño to La Niña over the course of the year. In 2021 and 2022, when the global warming trend did not change much through the year, Pacific conditions also remained relatively stable. In 2023, when warming appeared to increase through the year, the Pacific shifted from neutral conditions to El Niño. The trends described above are an important reason why warming trends in 2026 might follow a 2023-like pattern, with warming increasing as the year progresses. 2026 started with the lingering impact of La Niña conditions, keeping the upward deviation in the early part of the year relatively low. By May, El Niño appears to have already set in, at least according to the ONI (there are other indices for tracking El Niño as well). This means that the 2-4 month lag with which sea surface temperatures in the Pacific begin affecting global temperatures will come into play well before the end of the year, and global average temperatures could see a larger spike toward year-end than was seen at the beginning of the year.To be sure, the trends described above must be interpreted with caution. El Niño is not the only factor affecting global temperatures. For example, since 2020, a marine fuel regulation has led to lower sulphur emissions along major shipping routes in the North Atlantic Ocean, allowing more sunlight to reach the Earth's surface and contributing to warming. An opposite effect from other factors is just as possible.Another reason for caution is that El Niño and La Niña explain only a small part of the spikes or dips in temperatures. Most of the warming currently seen in global temperatures comes from long-term climate change. This means that while 2026 may see warming increase through the year, as happened in 2023, it is quite possible that the average warming for the year will be much higher than in 2023. Global temperatures averaged 1.28°C above the pre-industrial level up to May 2023 and ended the year at 1.48°C. The average for 2026 up to May is already 1.46°C.