It's a moment scientists have warned about for months: Earth has just ended its warmest year since people began keeping records, and scientists say it may have been the warmest in 125,000 years.

Even though the December data isn't yet official, the results were already "locked in" by mid-December, Gavin Schmidt, a scientist at the National Aeronautics and Space Administration, told USA TODAY.

Given the six consecutive months of extremely warm temperatures, it was virtually impossible for December to be cold enough to alter the final results.

"We are already beyond the point that any normal process would be able to keep 2023 from being the hottest year," Robert Rohde of Berkeley Earth, said in mid-December.

Official reports from organizations such as the Copernicus Climate Change Service in Europe, and U.S. agencies such as NASA and the National Oceanic and Atmospheric Administration are set to make their "warmest year on record" announcements over the next couple of weeks.

What's especially concerning, experts say, is that "the rate of warming over the past century has no precedent as far back as we are able to look, not only hundreds or thousands, but many millions of years," according to University of Pennsylvania meteorologist Michael Mann's book "Our Fragile Moment."

"We are engaged in an unprecedented experiment with our planet," Mann told USA TODAY. "There is still time to prevent devastating climate consequences, but the window of opportunity is shrinking."

2023 set to be the hottest year on record

Each year since 2014 stands among the top 10 warmest years since people began keeping track, based on increases in global average temperatures above the previous century, according to NASA and NOAA. The year 2016, which included the influence of a strong El Niño, was the warmest year on record. Then 2020 matched it.

Once a series of marine heat waves and another developing El Niño began influencing Earth's temperatures in 2023, it became increasingly clear to the world's scientists that the year was likely to see the biggest increases in average temperatures compared to last century.

By November, NOAA reported the January-November global surface temperature was 2.07 degrees Fahrenheit above the 1901-2000 average of 57.2 degrees, blowing past the 2016 average for the same period by more than a tenth of a degree.

The only question remaining is how much warmer 2023 will wind up being when the world's agencies complete their final analyses over the next two weeks.

Experts on the United Nations Intergovernmental Panel on Climate Change and at institutions around the world have warned the long-term global average temperature increase should be kept below 1.5 - 2.0 degrees Celsius to avoid catastrophic consequences, such as higher sea levels and an even greater number of intense heat waves that claim lives, devastate crops, kill wildlife and drive warmer temperatures.

Most records are measured in 'temperature anomalies.' What's that?

Calculating the global average temperature and how it's changing over time is challenging and complex. To do it with confidence, climate scientists use a combination of strategies to estimate the change.

For example, they use weather stations and ocean temperature readings and track how much warmer or colder those temperatures are compared to a set reference period in the past, Schmidt said. The resulting anomalies are then averaged over the globe. 

In a second method, satellite data that is close to surface air temperature is averaged together, then adjusted for gaps in resolution or data. The anomalies also are the most robust way to look at this data, he said. A third method reanalyzes by plugging satellite and other data into a weather model that calculates an average.

The best analyses use all three methods to examine the data, Schmidt said.

Global temperatures keep trending upwards. What next?

A host of international scientists have warned in one report after another that they expect global temperatures to continue on an upward trend. They say a combination of heat-trapping fossil fuel emissions (such as carbon dioxide), natural cycles and human development will fuel the trend.

Reports from the intergovernmental panel and others say the temperature increases can be reversed, but not without more significant effort to reduce greenhouse gas emissions and the levels of harmful gasses in the atmosphere.

Until then, scientists say more years will look like 2023.

And 2024 could be even warmer. Many climate scientists expect a strong El Niño to fuel an even hotter year globally, perhaps temporarily hitting the 1.5 degree Celsius threshold even more often.

"We expect two new temperature record-breaking years in succession," stated Nick Dunstone, with the Met Office, the national meteorological service for the United Kingdom.

The World Meteorological Organization estimates the five-year period 2023-2027 will almost certainly be the warmest five-year period ever recorded.

Month after month in 2023 broke records

Since June, every month has comfortably set an all-time temperature record and Earth has seen 547 consecutive months with temperatures above the 20th century average.

In May, carbon dioxide levels in the atmosphere reached a record high, the fourth-largest annual increase on record.

Among the other milestones:

  • During Earth’s single hottest month on record (July 2023), 81% of people on the planet experienced heat made more likely by human-caused climate change, according to Climate Central.
  • Heat waves driven by climate change affected parts of Europe, China, the U.S., northern Africa, South America, South Asia, and Madagascar. 
  • In the U.S., through November, four states on the southern coast had sweltered through their warmest year on record: Florida, Louisiana, Mississippi and Texas.
  • The planet hit a global average temperature of 2 degrees Celsius above pre-industrial temperatures on Nov. 17.

Air temperatures were hot in 2023. Water temps were even hotter.

Daily air temperatures 6.5 feet (2 meters) above the surface by years since 1979:

Ocean temperatures set new record highs for months in a row, causing widespread coral bleaching in the Florida Keys. At one point in July, the temperature of the ocean in the Keys surpassed 100 degrees, which may have been a world record.

Daily sea-surface temperatures by year since 1982:

Climate scientists say the world's oceans are hotter because they've absorbed the majority of the excess warming caused by an increase in greenhouse gas emissions over decades. Additionally this year, ocean heat was fueled in part by the June return of El Niño, a pattern of warmer-than-normal sea surface temperatures along the equator in the tropical Pacific Ocean, which affects weather around the world.

Scientists also are studying whether the warming was driven in part by the effects of shipping regulations that reduced the sulfur in shipping fuels and the temporary massive shot of water vapor to the Earth's stratosphere from the eruption of the Hunga Tonga - Hunga Ha'apai volcano in January 2022.

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