995. Is Global Warming Accelerating?

The Copernicus Climate Change Service, the European Union's meteorological information agency, announced that global temperatures reached a record high in March 2024, marking the 10th consecutive month of record-high monthly temperatures.

The average temperature in March 2024 was 14.14℃, 0.73℃ higher than the average temperature in March from 1991 to 2020. It broke the record set in 2016, which was the warmest March to date, by 0.10℃, and marked the 10th consecutive month of record monthly temperature highs. It was 1.68℃ higher than during the Industrial Revolution of 1850–1900. Sea surface temperatures reached 21.07℃, slightly surpassing the previous record of 21.06℃ set in February 2024.

Climatologists around the world are paying close attention to the global average temperature trend, particularly over the past year, as they debate whether global warming is accelerating faster than expected. Carbon Brief, a UK-based website dedicated to climate change science and policy, provides some background.

One of the triggers was a pre-peer-reviewed manuscript published in December 2022 by former NASA scientist Dr. James Hansen and colleagues (later peer-reviewed and published in 2023), in which they predicted the rate of global warming over the next 30 years to increase between 0.27℃ and 0.36℃ per decade, which is 50–100% faster compared to the trend since the 1970s. These projections, along with the unusually high temperatures observed in 2023, have sparked debate among scientists and the general public about the possibility of accelerated warming.

In fact, between 1970 and 2008, the world experienced a temperature increase of 0.18℃ per decade. In recent years, however, the rate of temperature increase has exceeded this long-term trend, with observational data for the past 15 years suggesting that the rate has doubled since the 1970s, with temperatures rising by 0.3℃ every decade.

Scientists are trying to figure out what contributed to the increased rate of warming. Some researchers pointed to a strong El Niño event, while others theorized that it was due to the elimination of air pollution over the past few decades, the reduction in atmospheric aerosols due to tighter regulations on sulfur content in marine fuel oil in 2020, the solar cycle peak every 11 years or so, and the impact of the Hunga Tonga volcano eruption in 2022.

Dr. Zeke Hausfather of Carbon Brief warns that even if we exclude natural factors such as El Niño/La Niña, volcanic eruptions, and solar cycle variations, it would be hasty to draw conclusions about the rate of warming based on 15 years of observations. In fact, during the 15-year period from 1998 to 2012 when global warming was observed to have paused, contrary to climate model predictions, researchers and the media were involved in investigating the causes. Multiple causes were pointed out for the discrepancy between climate model predictions and observations, including natural variability factors (e.g., heat absorption by the oceans), changes in observation methods (automatic observation buoys from ship engine rooms), and incomplete comparisons that excluded areas such as the Arctic, where the density of observation points is low.

While it is important to avoid concluding that global warming is accelerating based on short-term observations, there is evidence that the acceleration of global warming currently experienced worldwide cannot be explained solely by natural variability. Satellite observations of ocean heat content and the global heat budget suggest accelerated warming.

Indeed, this recent warming is largely consistent with climate change model predictions. For example, the IPCC AR6 SSP2-4.5 scenario (SSP2-4.5: Climate policy under moderate development, with a temperature increase of about 2.7℃ by the end of the 21st century) projects a warming of 0.24℃ (0.17℃–0.34℃) per decade from 2015–2050, which is 26% faster than the rate of warming since the 1970s. This demonstrates the credibility of climate models that attribute global warming to anthropogenic activities.

However, this does not mean that there will be no years with colder temperatures in the future, and Dr. Hausfather pointed out that the year 2025 could be colder than the average temperatures recorded in recent years due to the La Niña phenomenon.

Contributor: IIYAMA Miyuki (Information Program)