By Peter de Kruijff for the ABC
Last week, global temperatures appeared to momentarily breach a threshold set by world governments to try and avoid widespread climate devastation.
On 17 and 18 November, the world was, on average, 2 degrees Celsius warmer than pre-industrial levels for the first time in modern recorded history.
The pre-industrial period was a time before widespread fossil fuel use.
Burning fossil fuels has pumped substances like carbon dioxide (CO2) into the atmosphere, trapping heat at the planet's surface as part of the greenhouse effect.
The world's governments came up with the Paris Agreement in 2015 to keep global average warming to well below 2C, and ideally lower than 1.5C.
Breaching the 2C threshold for a few days is not the end of the world, experts say, but it is just another sign of how much the planet is warming.
And it comes at a time when countries are preparing to meet at the United Nation's 28th Climate Change Conference (COP28) to discuss fossil fuel reduction.
A complex topic when coal production is expected to increase to 2030, and oil and gas production to 2050 and beyond.
How do we know we went over 2C?
The figure was calculated by the European Centre for Medium-Range Weather Forecasts through one of its observational programmes, the Copernicus Climate Change Service.
The service uses a model, known as ERA5, which provides hourly estimates of atmospheric, land and ocean data from 1940 to the present.
The centre came up with the 2C figure by comparing last week's surface air temperatures from ERA5 to those from 1850 to 1900.
Although international agreements refer to "pre-industrial" temperatures, there is no agreement on the exact period this covers.
But the UN's Intergovernmental Panel on Climate Change (IPCC), which is the leading body for informing international climate policy, has used 1850 to 1900 as a baseline for its reports.
Australian National University's Mark Howden, who is the director of the Institute for Climate, Energy and Disaster Solutions, said the dataset was reliable and had a slight "cold bias".
"But over the more recent years, they have all converged and so there is now very little difference."
ANU climate lecturer and associate investigator at the ARC Centre of Excellence for Climate Extremes Joëlle Gergis said scientific rigour rested on reproducing the results across other datasets.
"[ERA5] starts in 1940, so it is only reporting anomalies from a relatively short period," Gergis said.
The result would be more significant, she added, if reproduced by datasets that extend into the 19th century.
What difference does a day make?
A day or two of above-average temperatures does not mean we have breached Paris Agreement targets.
Gergis said people should be careful not to over-interpret a single day, week or even monthly temperature anomaly.
"There is a lot of natural variability and random noise in all datasets," she said.
"What is more important is the consistent, long-term trend which is typically measured in 30-year base periods.
"The Paris Agreement refers to sustained temperature anomalies about 2C, not just single day or monthly temperature departures."
So while the planet in the 1950s may have first reached a day where it was 1C warmer than pre-industrial times, it was not until 2017 when the IPCC says the Earth was on average 1C warmer.
Currently the planet is about 1.1C warmer than pre-industrial times.
The past year has been about 1.3C warmer than before the world started widespread burning of fossil fuels.
University of NSW Climate Change Research Centre director Katrin Meissner warned that there was nothing magic about 1.5C or 2C as a threshold.
She said every 10th of a degree of warming would be dangerous.
"There is nothing 'special' about 2C from a climate science point of view; the warmer it gets, the more dangerous it gets," Meissner said.
"What we see in this record is what we have been predicting for years. Temperatures are rising and there is natural climate variability on top of that.
"With an El Niño underway, I would expect temperatures to break records now and in the coming summer."
2023 shaping up as hottest on record
A report from the UN's World Meteorological Organization said earlier this year that 2015 to 2022 were the eight warmest years on record, despite the cooling influence of a La Niña event for three of those years.
Earlier this month it also noted that greenhouse gases reached a record high, stating: "The last time the Earth experienced a comparable concentration of CO2 was 3-5 million years ago, when the temperature was 2-3 degrees Celsius warmer and sea level was 10-20 metres higher than now."
Meissner, Howden and Gergis all said this year is likely to be the hottest on record, surpassing the previous high point in 2016.
Ocean temperatures also reached new heights earlier this year.
Meissner said warming must be reduced as soon as possible.
"The only way to do this is to stop emitting fossil fuels," she said.
Howden said the public should take the latest observations as signs that the global climate is changing, and it is happening more quickly and severely than previously forecast.
"To head off further change, we need to start reducing rapidly and significantly greenhouse gas emissions," he said.
Global community to meet next week
Countries will get their chance to try and address CO2 emissions at the COP28, which starts next week.
The gathering in Dubai will include the first "global stocktake", which is a system to keep track of whether Paris Agreement nations are making progress on tackling climate change.
The stocktake results could influence an acceleration in decarbonisation for the next set of global climate action plans due in 2025.
A pair of studies published earlier this year showed the world had already chewed through half of its remaining climate budget to keep warming below 1.5C.
One estimated there was only six years left in the budget at the current rate of emissions.
Both studies also say that the world must reach net zero emissions by 2035 to have a 50 per cent chance of keeping warming below 1.5C.
Australia and New Zealand plan to reach net zero by 2050.
*This story was first published on the ABC.