What we don't know can still hurt us very much indeed. 2023's climate is a testament to that fact.

Climate scientists were always expecting last year to break heat records. One analysis predicted there was a 99 percent chance that 2023 would be the hottest year since records began.

But the reality turned out to be much, much worse, triggering historic droughts, floods, storms, and wildfires around the world that many nations were entirely unprepared for.

NASA's leading climatologist, Gavin Schmidt, was "frankly astonished" by what happened.

Schmidt told the AFP 2023 wasn't simply a record-breaking year, it was more like a "record that broke the previous record by a record margin."

The vast majority of climate scientists agree that the long-term trend is tied to global warming, but as Schmidt puts it, "what happened in 2023 was that, and then plus something. And that 'plus something' is much larger than we expect, or as yet can explain."

The mysterious specifics at play could be tied to aerosols, El Niño, events in the Antarctic or North Atlantic, or Earth's energy imbalance, among many other hypotheses.

Scientists are now desperately trying to figure out what they are missing.

For the very first time, all 365 days in 2023 exceeded 1 °C above pre-industrial levels. Scientists say the year's climate sat uncomfortably close to 1.5 °C above pre-industrial level, threatening to break the Paris Climate Agreement's goals.

And yet the year before, ten different climate prediction centers foresaw a very low likelihood that the annual temperature for 2023 would be as high as it turned out to be.

El Niño was building up last year, but it seemed to do so in a weaker way than it has in the recent past. This strongly suggests that the record temperatures seen in 2023 were not just due to climate change and El Niño working in combination.

Other contributing factors are likely at play. Last year, for instance, Antarctic sea ice hit the lowest it's been in 45 years – a change that scientists say goes well beyond 'unprecedented'. This southernmost land mass is melting in a way that our climate models simply never predicted.

"It's five standard deviations beyond the mean," physical oceanographer Edward Doddridge explained to ABC News Australia in 2023.

"Which means that if nothing had changed, we'd expect to see a winter like this about once every 7.5 million years."

If enough sea ice melts in Antarctica, scientists worry it could trigger a positive feedback loop, making the ice sheet even more vulnerable to wind and waves.

Apart from what's going on in the Antarctic, climate scientists are also exploring changes in volcanic eruptions, changes in aerosol emissions, and the solar cycle as possible explanations for the 2023 climate anomaly.

"It matters why 2023 was the way it was, because does that mean it's going to continue?" wonders Schmidt.

"Does that mean the impacts are going to start to accelerate? We don't know! And that's problematic."

Earth scientist Robert Rohde from Berkeley University says we just have to wait and see if 2023 was an outlier or the beginning of accelerated climate changes.

"With greenhouse gas emissions continuing to set record highs," he predicts, "it is likely that the climate will regularly exceed 1.5 °C in the near future."

Buckle up.