The year 2024 saw the hottest year on record from June to August, continuing the stunning record global temperature streak that began in June 2023. This unexpected increase in temperature, described by climate scientists as humiliating and alarming, has led to intense research into the contributing factors.

Latest temperature trends

Between June and August 2024, global temperatures reached record levels, surpassing the same period in 2023. These extreme temperatures were not limited to the summer months; Global temperatures set new records for 15 consecutive months starting in June 2023 and ending in August 2024, according to NASA’s Goddard Institute for Space Studies (GISS).

While this prolonged heat wave is consistent with a broader warming trend caused by human activities, primarily greenhouse gas emissions, its intensity has puzzled climate scientists. GISS director Gavin Schmidt described the unexpected rise in temperatures at the end of 2023 as “humiliating” and “confusing”. Nature.

The charts on this page show how global temperatures in 2023 and 2024 differ from expectations based on NASA temperature records. Nearly a year later, Schmidt and other climate scientists are still trying to figure out why.

“The warming in 2023 was a head higher than in all other years and will be so in 2024,” Schmidt said. “I wish I knew why, but I don’t. We’re still in the process of evaluating what happened and whether we’re seeing changes in the way the climate system works.”

Prediction of climate fluctuations

The world’s air and ocean temperatures during a given year often reflect a combination of long-term trends, such as those associated with climate change, and short-term effects, such as volcanic activity, solar activity, and ocean conditions.

Schmidt used a statistical model to predict next year’s global temperatures at the end of 2022, as he has done every year since 2016. La Niña, which lowers sea surface temperatures in the tropical Pacific, was present in the first half of 2023 and should have affected global temperatures. Schmidt predicted that the global average temperature in 2023 will reach about 1.22 degrees Celsius above the baseline, making it among the three or four hottest years, but it will not be a record year. Scientists at the UK Met Office, Berkeley Earth and Carbon Brief have made similar predictions using different methods.

This chart shows Schmidt’s expectation of how much monthly temperatures between January 2023 and August 2024 will differ from NASA’s 1951-1980 baseline (also known as an anomaly). The expectation (shown by the dashed line in the chart) was based on an equation that calculated the global average temperature based on the rate of warming over the past 20 years (about 0.25°C per decade) and NOAA sea surface temperature measurements in the tropics. The Pacific accounts for the three-month delay in these temperatures affecting the global average temperature. The shaded area shows the range of variability (plus or minus two standard deviations).

“More complex global climate models are useful for predicting long-term warming, but statistical models like this help us predict year-to-year variability, which is often dominated by El Niño and La Niña events,” said climate scientist Zack Hausfather. . at the University of California at Berkeley. Hausfather helps create the Berkeley World Global Temperature Record and also creates annual estimates of global temperature change based on that data.

Exceeds expectations

Schmidt’s statistical model, which has successfully predicted global average temperature every year since 2016, underestimated the extreme heat in 2023, as did methods used by Hausfater and other climate scientists. Schmidt expected global temperature anomalies to peak in February or March 2024 as a delayed response to additional warming from El Niño. Instead, the abnormal warmth occurred well before El Niño reached its peak. The heat came with unexpected intensity; first in the northern part of the Atlantic Ocean, then almost everywhere else.

“The record was broken in September with a striking value of 0.5 degrees Celsius,” Schmidt said. “This hasn’t happened before in GISS records.”

The graph above shows how global temperatures calculated from January 2023 through August 2024 differ from the NASA baseline (1951-1980). Previous record temperature anomalies for each month (set in 2016 and 2020) are shown with a dashed red line. Since June 2023, monthly temperatures have exceeded previous records by 0.3-0.5°C. Although temperature anomalies in 2024 were close to past anomalies, they continued to break records until August 2024. The global average temperature in September 2024 was 1.26°C above NASA’s baseline; It was lower than September 2023, but still 0.3°C above any September on record. Until 2023.

To calculate global changes in Earth’s average temperature, NASA scientists analyze data from tens of thousands of weather stations on land as well as thousands of instruments aboard ships and buoys on the ocean surface. The GISS team analyzes this information using methods that take into account varying distances between temperature stations around the world and the effects of urban warming, which can skew the calculations.

Examination of unforeseen factors

Since May 2024, Schmidt has been conducting research on possible causes of the unexpected warmth, including changes in greenhouse gas emissions, solar radiation input, airborne particles called aerosols and cloud cover, as well as the effects of Hunga Tonga-2022. Eruption of Hunga Haapai volcano. But none of these factors provide what Schmidt and other scientists believe is a convincing explanation for the unusual warmth in 2023.

Greenhouse gas levels in the atmosphere continue to rise, but Schmidt estimates that the additional load from 2022 only causes additional warming of around 0.02°C. The Sun was approaching its peak in 2023, but its roughly 11-year cycle is well measured and insufficient to explain the temperature increase.

Large volcanic eruptions such as El Chichon in 1982 and Pinatubo in 1991 have caused brief global cooling in the past by releasing aerosols into the stratosphere. And a study published in 2024 showed that Tonga’s eruption had a net cooling effect in 2022 and 2023. “If that’s the case, there’s more warming in the system that needs to be explained,” Schmidt said.

Another possible factor is reducing air pollution. A research team led by Tianle Yuan, an atmospheric researcher at NASA’s Goddard Space Flight Center, found a significant decrease in aerosol pollution from shipping since 2020. The decline coincides with new international regulations on sulfur content in marine fuel and sporadic declines in shipping due to the coronavirus pandemic.

Sulfur aerosol emissions contribute to the formation of bright clouds that reflect sunlight entering space and have a clear cooling effect. Reducing this pollution has the opposite effect: it can warm the climate by reducing the likelihood of clouds forming. Although scientists, including Yuan, generally agree that the decline in sulfur emissions will cause net warming in 2023, the scientific community continues to debate the exact size of the impact.

“All of these factors explain perhaps a tenth of the warming,” Schmidt said. “Even if all plausible explanations are taken into account, the difference between expected and observed annual mean temperatures in 2023 remains approximately 0.2°C, roughly the difference between previous and current annual records.”

Facing new realities

Both Hausfater and Schmidt expressed concern that these unexpected changes in temperature could indicate a change in the functioning of the climate system. It could also be a combination of climate variability and changes in the system, Schmidt said. “It doesn’t have to be either-or.”

One of the biggest uncertainties in the climate system is how aerosols affect cloud formation, which in turn affects the amount of radiation reflected back into space. But one of the challenges facing scientists trying to piece together what happened in 2023 is the lack of updated global data on aerosol emissions. “Reliable estimates of aerosol emissions rely largely on a volunteer-led effort, and it may take a year or more before full data for 2023 is available,” Schmidt said.

NASA’s PACE (Plankton, Aerosol, Cloud, Ocean Ecosystem) satellite, which will be launched in February 2024, may help shed light on these uncertainties. The satellite will help scientists make a global assessment of the composition of various aerosol particles in the atmosphere. PACE data can also help scientists understand cloud properties and how aerosols affect cloud formation; This is important for creating accurate climate models.