The Earth was due for another year of record warmth but this warm?

Earth is ending its warmest year in the past 174 years, very likely in 125,000 years.

Extreme heat waves scorched Phoenix and Argentina. Wildfires are raging across Canada. Floods in Libya killed thousands. Winter ice cover in the dark seas around Antarctica was at unprecedented lows.

Global temperatures this year didn't just surpass previous records. They left them in the dust. From June to November, the mercury spent month after month rising off the charts. December temperatures remained largely above normal: Most of the Northeastern U.S. is expecting spring conditions this week.

That's why scientists are already examining the evidence – from the oceans, volcanic eruptions, and even pollution from cargo ships – to see whether this year might reveal something new about climate and what we do with it.

One hypothesis, and perhaps the most alarming, is that the Earth's temperature is accelerating, and that the effects of climate change are coming our way more quickly than ever before. “What we are looking for, in fact, is a body of corroborating evidence that all points in the same direction,” said Chris Smith, a climate scientist at the University of Leeds. “Then we look for causation. That will be really interesting.”

Although the temperatures were extreme this year, they did not surprise researchers. Scientists' mathematical models provide a range of projected temperatures, and 2023's temperature is still broadly within that range, albeit at a maximum.

Andrew Dessler, an atmospheric scientist at Texas A&M University, said that one exceptional year would not, by itself, be enough to indicate that something was wrong with the computer models. Global temperatures have long oscillated up and down around a consistent warming trend due to cyclical factors such as El Niño, a weather pattern that appeared in the spring and has intensified since then, potentially signaling more record heat to come in 2024.

“Your default position should be: The models are correct,” Dr. Dessler said. “I'm not ready to say we've 'climate broken' or that anything strange is going on until more evidence comes in.”

One thing researchers will be watching is whether something unexpected might happen in the interaction between two major climate forcings: the heating effect of greenhouse gases like carbon dioxide and the cooling effect of other types of industrial pollution.

For most of the past 174 years, humans have been filling the sky with greenhouse gases and aerosols, or with small particles from smokestacks, tailpipes and other sources. These particles are harmful to the lungs when inhaled. But in the atmosphere, they reflect solar radiation, partially offsetting the heat-trapping effect of carbon dioxide.

But in recent decades, governments have begun to reduce aerosol pollution for public health reasons. This has already caused an acceleration in temperature rise since 2000, Scientists estimate.

In a much-discussed report last month, climate researcher James E. Hansen says scientists have greatly underestimated how much the planet will warm in the coming decades if countries clean up aerosols without cutting carbon emissions.

Not all scientists are convinced.

Arguments like Dr. Hansen's have been difficult to reconcile Patterns “Climate change has occurred in recent decades,” said Reto Knuti, a climate physicist at the Swiss university ETH Zurich. In recent years, scientists have also discovered that global warming is shaped not only by the amount of heat trapped near the Earth's surface, but also by how and where that heat is distributed across the planet.

This makes it more difficult to conclude with confidence that global warming is about to accelerate, Dr. Knuti said. Until the current El Niño event ends, he said, “it is unlikely that we will be able to make specific claims.”

Determining the exact scale of the aerosol effect has also been difficult.

Part of how aerosols cool the planet is by making the clouds brighter and deflecting more solar radiation. Clouds are very complex, coming and going and leaving little traces for scientists to examine, said Tianli Yuan, a geophysicist at NASA and the University of Maryland, Baltimore County. “That's basically why it's such a difficult problem,” he said.

This year, aerosols were particularly important due to a 2020 international regulation that restricted pollution from ships. Dr. Yuan and others are trying to determine how regulation has affected global temperatures in recent years by reducing aerosols that reflect sunlight.

Dr. Hansen's argument for faster temperatures is based in part on reconstructing climate shifts between ice ages over the past 160,000 years.

Using Earth's distant past to make conclusions about climate in the coming years and decades may be difficult. However, the planet's deep history highlights how extraordinary the current era is, says Barbel Hoenisch, a scientist at the Lamont-Doherty Earth Observatory in Columbia.

For example, fifty-six million years ago, geological upheavals added carbon dioxide to the atmosphere in amounts similar to what humans add today. Temperatures jumped. The oceans became acidic. The species died out en masse.

“The difference is that it took 3,000 to 5,000 years to get there” at that time, compared to a few centuries today, Dr. Hoenisch says.

It then took Earth longer to neutralize the excess carbon dioxide: about 150,000 years.

Nadia Popovic Contributed to reports.