Extract from ABC News
While 2023 has already been called the world's hottest year, the full set of climate data up to December 31 shows global temperatures reached "exceptionally" high levels last year, according to the European Union's key climate service.
Overnight, the Copernicus Climate Change Service (CCCS) released its official summary of global climatic conditions in 2023.
It found Earth was 1.48 degrees Celsius warmer than pre-industrial levels, with temperatures during the year overtaking the previous record set in 2016 by a large margin.
It is also likely to have exceeded those of any period in at least the past 100,000 years, according to its deputy director Samantha Burgess.
"2023 was an exceptional year with climate records tumbling like dominoes," she said.
So, in an exceptional year, what did things look like on the ground?
Here's a glimpse of some of the defining events from the world's hottest year in pictures and charts.
World temperature anomaly
According to the CCCS, the annual average air temperatures were the warmest on record, or close to the warmest, over the majority of ocean basins and continents around the world, except Australia.
This map shows how much higher or lower surface air temperatures across each country were compared to the 1991-2020 average.
First signs in June
After a milder start to the year, the earliest signs of how unusual the year would become emerged in early June.
Every month since then — and nearly every day — has been its hottest on record, according to data from the CCCS, and the University of Maine's Climate Reanalyzer platform.
Similar records have played out in the world's oceans, beginning in March.
These unprecedented high sea surface temperature were a critical driver of the extreme air temperature in 2023, according to the CCCS.
Extreme temperature observations on the ground
The CCCS data is focused on the climate, rather than individual weather events.
But within the year there were several extreme heat events on the ground that formed part of the overall data.
Though records were set over various parts of the globe throughout the year, some of the most intense temperatures occurred during the Northern Hemisphere summer, particularly in July.
On July 16, China set a new national temperature record, reaching 52.2C at a weather station in the far north-western region of Xinjiang, according to state media.
Meanwhile, on the same day, temperatures at the Saratoga Spring station, in the Death Valley National Park, challenged all-time records, hitting 53.9C.
This was the highest temperature of 2023, according to independent climate scientist Jérôme Reynaud, who runs the GeoClimat website.
The extreme temperature was just a slice of the historical heatwave that stretched from California's desert into Arizona and Texas during the 2023 summer.
For 31 straight days, from the last day of June and through July, the south-west United States city of Phoenix hit 43.3C (110F), smashing the previous record of 18 consecutive days.
Thermal images show how much hotter the pavements and other urban surfaces were during that time, with medics reporting a spike in contact burns.
Europe also sweltered in July with an intense and prolonged heatwave that moved across Spain, Portugal, France, the United Kingdom, central Europe and Scandinavia.
During the event, the UK set a new national daily maximum temperature record of 40.3C in Lincolnshire on July 19 — the first time that the country ever recorded a temperature of over 40C.
Meanwhile in the Southern Hemisphere, Australia had its warmest winter since records began in 1910.
Globally, September was the month with a temperature deviation larger than any month in the ERA5 dataset, when compared with the 1991–2020 average.
That month Australia recorded its second hottest September on record for maximum temperatures, with unseasonable heatwaves coinciding with several major sporting events.
Extreme bushfires
The hot temperatures set the stage for severe wildfires in the Northern Hemisphere, which played out across Canada, Russia, the United States, and Europe.
But the most significant were in Canada.
From the beginning of March, the country faced a series of ongoing, record-setting wildfires, which charred forests from British Columbia in the west, to Quebec in the east.
According to the latest public records, the Canadian wildfires of 2023 razed 18.5 million hectares of land, and released copious amounts of carbon into the atmosphere.
The intense fires led to above-average global fire emissions overall, according to the Global Carbon Budget, but were offset by below-average fire emissions from major source regions, including the tropics.
Coral bleaching
Last year also saw widespread coral bleaching across the globe.
Mass coral bleaching occurred in at least 35 countries or territories, across five different oceans and seas, according coral reef ecologist Derek Manzello, the coordinator of NOAA's Coral Reef Watch.
In the Florida Keys area in July, surface ocean temperatures soared to hot-tub temperatures, hitting 38.4C in Manatee Bay.
The heat stress was so severe in Florida and the wider Caribbean that NOAA's Coral Reef Watch had to add additional alert levels.
Ian Enochs, the head of the Atlantic Oceanographic and Meteorological Laboratory's (AOML) Coral Program, said he had "never seen anything like" last year's bleaching event.
"When we jumped in at the site [Cheeca Rocks], it was completely bleached, and in fact, you could see it from the surface," Dr Enochs said.
"Absolutely everything was bleached, stark white, severely paled.
"And those were just the hard corals, the soft corals, the sea fans ... they hadn't just bleached, they had disintegrated."
But he said some corals were "thankfully" showing signs of recovery already.
"The fact that we have corals there still and that some of them are recovering, I find encouraging, in the midst of a really depressing, very difficult, very hard to observe summer," he said.
There was no bleaching in Australia in 2023, but Dr Manzello said the Great Barrier Reef and other parts of Australia face risk of bleaching over the next month or so.
Extreme rainfall
Several record-setting rainfall events also took place during the year.
This included the extreme rainfall triggered by powerful Typhoon Doksuri in China during July, which killed dozens and left a trail of destruction as it stretched as far north as the Chinese capital Beijing and the surrounding province of Hebei.
Georgia Institute of Technology climate scientist Annalisa Bracco said it was "very plausible" that the intensely warm ocean temperatures aided several of these events, although formal studies would be required to confirm it.
Five-sigma event in Antarctica
In its summary, the CCCS also noted how "remarkable" 2023 was for Antarctic sea ice, which reached record low extents for eight of twelve months of the year.
It included an all-time low in February, and a record low winter maximum in September.
In July, physical oceanographer Edward Doddridge described the unfolding event as "gobsmacking".
"For those of you who are interested in statistics, this is a five-sigma event. So it's five standard deviations beyond the mean. Which means that if nothing had changed, we'd expect to see a winter like this about once every 7.5 million years," he said.
In December, however, the daily sea ice extent shifted closer to average which was described as "really good news" by CCCS deputy director Dr Burgess.
But she said it remained to be seen whether this was a short or long term trend.
Hot year surprising to climate scientists
The fate of 2023 was known for some time before the year's end, with climate agencies around the world reporting in October that 2023 would very likely become Earth's warmest on record.
However, Berkeley Earth climate scientist Zeke Hausfather said 2023 was not initially expected to be so hot.
"The beginning of the year was on the cool side due to a lingering and rare triple dip La Niña event, and a developing El Niño later in the year was expected to impact 2024 more than 2023," he said.
Dr Hausfather said the extremities of temperatures witnessed had also been surprising, with its exact causes uncertain.
"We are not sure why we have seen such exceptional temperatures in the summer of 2023 – temperatures that would be not unexpected after the peak of a strong El Niño event, but occurred while the El Niño was still in its earlier stages," he said.
He said either the El Niño was behaving differently to recent analogues potentially because of its rapid transition out of extended La Niña conditions, or there were other factors at play.
"That being said, the other factors that we know of [such as] the Tonga eruption in 2022, the marine sulphur fuel phase-out in 2020, an uptick in the 11-year solar cycle, are all far too small in effect to explain the anomaly this year," he said.
"Of course, the level of heat we are seeing this year would be effectively impossible without the approximate 1.3C of warming the world has experienced to-date due to human emissions of CO2, other greenhouse gases, and aerosols."
CCCS director Carlo Buontempo said unless the concentration of greenhouse gases in the atmosphere was stabilised, we could not expect a different result in future years from those we'd seen in recent months.
"On the contrary, following the current trajectory, the record-breaking year of 2023 will probably we remembered as a cool year."
The top 10 warmest years on record have all occurred since 2010.
What could be in store for 2024?
What exactly is in store for 2024 is still unknown.
Dr Hausfather said, all things being equal, climate scientists would expect 2024 to be warmer than 2023, based on the global temperatures analogues of previous El Niño events.
However, he said the past year had been so anomalous that he was less certain that would play out.
"I find myself agreeing with [climate scientist] Gavin Schmidt's assessment that 2024 is slightly favoured to edge out 2023 but hardly certain," he said.
Dr Bracco said she also expected 2024 to see continued warmth into 2024.
"We were coming from a series of La Niña years, and unfortunately a 'jump' up had to happen. It was stronger than I would have liked," she said.
"Hopefully, it has helped realising we must do something quickly."
For the world's coral reefs, researchers believe the widespread marine heatwaves from 2023 will likely be a precursor to a global mass bleaching and mortality event over the next 12 to 24 months.
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