WASHINGTON — January 2025 marked another unprecedented monthly heat record, according to the European climate service Copernicus, despite experiencing unseasonably cool temperatures across the United States and the influence of a cooling La Nina. Additionally, predictions suggest that 2025 may not be as warm as previous years.
This remarkable January heat coincides with insights from a recent study involving prominent climate scientist and former NASA chief James Hansen, which posits that global warming is progressing at an accelerated pace. This assertion has sparked debate within the scientific community, with some researchers supporting it while others remain skeptical.
Copernicus reported that January 2025 was 0.09 degrees Celsius (0.16 degrees Fahrenheit) warmer than January 2024, which had previously held the title of the hottest January. Compared to pre-industrial levels, the temperature difference was a significant 1.75 C (3.15 F). This occurrence marked the 18th time in the last 19 months that the global average temperature reached or exceeded the internationally recognized threshold of 1.5 C (2.7 F) above pre-industrial averages, although scientists require sustained temperatures above this limit for a period of 20 years to consider it officially crossed.
The records kept by Copernicus date back to 1940, while additional data from U.S. and British sources extend to 1850. Studies leveraging natural indicators, such as tree rings, suggest that the current climatic period is the warmest experienced in approximately 120,000 years, since the inception of human civilization.
The main contributor to these record-high temperatures is the accumulation of greenhouse gases, primarily from the combustion of fossil fuels like coal, oil, and natural gas. However, Samantha Burgess, the strategic lead for climate at Copernicus, pointed out that natural influences on temperature shifts have behaved unexpectedly.
Typically, variations in the central Pacific Ocean’s temperature significantly impact global climate. When the ocean is especially warm, it leads to El Nino conditions, causing a spike in global temperatures. Last year was marked by a strong El Nino that concluded in June, resulting in record warmth, surpassing projections.
Conversely, La Nina conditions cool the atmosphere and tend to mitigate the effects of global warming, making extreme temperatures less likely. Recently, a La Nina phase commenced in January following its development over several months. Climate experts had initially forecasted that 2025 would be cooler compared to the record heat of 2023 and 2024, attributing much of this prediction to the presence of La Nina.
Burgess noted that even with cooler Pacific conditions, record temperatures are still being observed, largely due to unusually warm waters across other global oceans. She explained that normally, after a significant El Nino, global temperatures would decrease rapidly, but the anticipated drop has not materialized.
For Americans, the news of a sweltering January appears paradoxical, given the colder weather experienced domestically. However, Burgess emphasized that the U.S. accounts for a small area of the Earth’s surface; significant portions worldwide are experiencing much warmer-than-average temperatures.
Remarkably, January recorded unusually warm conditions in the Arctic, with parts of the Canadian Arctic experiencing temperatures that were 30 C (54 F) higher than average, leading to localized sea ice melt. Copernicus indicated that sea ice levels in the Arctic during this month were at a record low for January. In the U.S., the National Snow and Ice Data Center categorized it as the second-lowest level, trailing only behind the year 2018.
As February has commenced cooler than the same month last year, Hansen, the renowned climate scientist now at Columbia University, cautioned not to dismiss 2025 in the race for the warmest year record. His study, published in the journal Environment: Science and Policy for Sustainable Development, highlighted that recent warming trends have occurred at approximately double the rate observed in the preceding 40 years.
Hansen expressed confidence that this elevated warming rate will persist for several more years, predicting a close contest between the years 2024 and 2025 for the title of the hottest year. He mentioned that the temperature rise has been evident since 2020, even when accounting for the variations linked to El Nino events and general climate change. Recent regulations on shipping have also curtailed sulfur pollution, which typically helps reflect sunlight away from the Earth, thereby reducing warming impacts—a trend that is set to continue.
Jonathan Overpeck, Dean of the Environment School at the University of Michigan and not a participant in Hansen’s study, remarked on the unsettling persistence of record warmth from 2023, through 2024, and into early 2025, asserting the accelerating effects of climate change.
Conversely, some researchers like Gabe Vecchi from Princeton and Michael Mann of the University of Pennsylvania express skepticism towards Hansen’s claims of acceleration, arguing that there is insufficient data to conclude that current trends do not stem from random variability. Mann contended that the observed temperature increases are still consistent with predictions made by current climate models.
