According to the annual average global temperature analysis of the Goddard Institute for Space Studies, NASA announced that 2023 will be the hottest year in history. Scientists, who keep the temperature record starting in 1880, calculate a global temperature anomaly every year to determine how much the temperature has changed compared to the temperature of 1951-1980.

Every month from June to December 2023 was the hottest month on record. July was considered the hottest month in the entire history of observation. So why was 2023, especially the second half, so hot? Scientists also asked themselves the same question. Here’s a breakdown of the main factors scientists believe are responsible for the record heat.

Long-term increases in greenhouse gases are an important factor.

For more than 100 years, humans have burned fossil fuels such as coal, gas and oil to power everything from light bulbs to cars to factories to cities. These actions, along with changes in land use, have resulted in increased greenhouse gases in the atmosphere. Greenhouse gases act as a blanket that traps heat around the planet. The more you add, the thicker this blanket becomes, which warms the Earth further.

In May 2023, atmospheric carbon dioxide concentrations peaked at 424 parts per million at NOAA’s Mauna Loa Observatory in Hawaii. The annual peak has been increasing steadily since measurements began in 1958. (Other global carbon measurement projects have shown similarly high numbers.) Deepening the record further with ice cores, carbon dioxide concentrations reached the highest level in at least 800,000 years.

“We’re going to continue to break records because the base temperature continues to rise,” said Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies in New York. “This warming trend over the last 50 to 60 years is due to changes in greenhouse gas emissions, primarily carbon dioxide and methane.”

The return of El Niño further escalated the situation.

Besides the long-term trend of global warming, there are also natural fluctuations in the climate. One of the largest sources of this year-to-year variability is the El Niño Southern Oscillation (ENSO) that occurs in the tropical Pacific.

ENSO moves between three phases: El Niño, La Niña, and neutral or average. During El Niño, trade winds weaken; That is, the winds that normally blow from east to west in the tropical Pacific are weakening. The sea surface around the equator in the central and eastern Pacific near South America is also becoming warmer (and higher) than normal. El Niño often coincides with the warmest years on average on Earth.

During La Niña, the opposite occurs: trade winds increase and sea surface temperatures in the eastern Pacific Ocean are colder than normal. This could help offset temperature increases from long-term global warming.

La Niña was observed in the Pacific Ocean for three years, from 2020 to 2022. El Niño then returns starting in May 2023. This El Niño has not yet been as strong as 2015-2016 or 1997-1998; both caused large jumps in global average temperatures. But when you add this ocean warming to the long-term warming trend from greenhouse gases, the onset of El Niño helped temperatures rise high enough to set a new temperature record.

“It’s mostly us and El Niño,” said Josh Willis, a climate scientist at NASA’s Jet Propulsion Laboratory. “After all, humans are warming the planet and El Nino is dancing above our heads.”

On a global scale, long-term ocean warming and above-normal sea surface temperatures have played a significant role.

More broadly, the tropical Pacific wasn’t the only part of the ocean that was warmer than normal this year. Global sea surface temperatures set new records in 2023 as the North Atlantic and other parts of the ocean experience several marine heatwaves.

“Ocean temperatures are rising, just like global temperatures,” Willis said. “They’ve been growing for the past century or more and they’re not slowing down. On the contrary, they are accelerating.”

What’s behind the rise in ocean temperatures? Greenhouse gases warming the planet. Approximately 90 percent of the heat trapped by rising greenhouse gases is absorbed by the oceans. This means that as greenhouse gas emissions increase, the temperature of the oceans will also increase, which in turn will increase the temperature around the world.

There are fewer aerosols, so they no longer slow down the temperature rise.

Another global trend that scientists are following is the change in atmospheric aerosols. Aerosols are small particles in the air that can affect the climate, such as smoke, dust, volcanic gases, sea spray, air pollution or soot. Airborne particles can either reflect sunlight, causing the air to cool slightly, or absorb sunlight, causing the air to warm slightly.

Aerosols have decreased in most regions as governments make regulations to reduce air pollution and improve air quality. Many of these man-made particles are the type that cools the climate slightly, so having fewer of them in the air would result in minor warming. However, this contribution is quite small compared to the much larger warming resulting from the growth of greenhouse gases.

Scientists at NASA and around the world are investigating how reductions in aerosols due to new shipping regulations could potentially change the amount of solar energy reflected back into space. While these changes may be noticeable on a regional scale, their global impact will likely be small, Schmidt said.

Scientists found that the eruption of the Hunga Tonga-Hunga Haapai volcano did not contribute significantly to the record temperature.

In January 2022, the eruption of underwater volcano Hunga Tonga-Hunga Haapai launched an unprecedented amount of water vapor and fine particles, or aerosols, into the stratosphere. Water vapor, a greenhouse gas, can warm the atmosphere, so scientists studied the impact of the eruption on global temperatures. On the other hand, sulfate aerosols from explosions sometimes lead to global cooling.

A recent study found that volcanic sulfate aerosols reflected some of the sunlight coming from the Earth’s surface, leading to a slight cooling of less than 0.1 degrees in the Southern Hemisphere following the eruption. Warming, mainly due to an increase in water vapor in the stratosphere, was offset by cooling caused by volcanic sulfate aerosols, resulting in a slight cooling of the lower atmosphere. This means the eruption likely did not contribute to record temperatures in 2023.

“We’re very interested in the weather and extremes of any given year because that’s what affects us,” Schmidt said. “But the key difference between this decade and previous ones is that temperatures continue to rise due to our activities, primarily the burning of fossil fuels.”