“It’s very likely that the next big El Nino could take us over 1.5 degrees,” Professor Adam Scaife, the head of long-range prediction at Britain’s Met Office, told The Irish Times earlier this month.

Why 1.5 degrees matters

The target to stay within 1.5 degrees was embraced after frantic negotiations in the dying hours of the 2015 climate negotiations that resulted in the Paris Agreement. Climate-vulnerable nations used their leverage to extract agreements from global giants determined to get unanimous support for a climate deal from 195 nations.

Even then, the language was equivocal, with global leaders committing to adopting policies to hold warming “well below” 2 degrees and as close to 1.5 degrees as possible.

Since then, the target has been embedded in climate diplomacy, championed by those nations - and indeed those climate scientists - who agree that every tenth of a degree of warming will have direct implications on millions of lives.

There has also been real progress on climate since Paris. Where once we were on a trajectory for over 3 degrees of warming by 2100, the United Nations Environment Program estimates the world is on track to 2.8 degrees if governments pursue their current policies. Should all the promises made at climate talks be met, we are on track for around 2.4 degrees.

Given the way the ratchet mechanism of the Paris Agreement is supposed to work - where nations are expected to keep raising ambitions as targets are met - that mark should fall further.

But 2.4 degrees of warming is still catastrophic. Some low-lying countries, such as the Maldives, that could survive 1.5 degrees of warming do not expect to live through 2 degrees.

Today the world is now around 1.14 to 2 degrees warmer than the pre-industrial period, according to the World Meteorological Organisation (WMO). And the average temperature anomaly keeps growing.

Which brings us to the fears that 1.5 degrees is about to be breached.

Three key drivers

Obviously, the key driving force of heating is the ongoing and increasing emission of greenhouse gases.

Despite a global surge in the deployment of renewables, there has been a return to fossil fuels such as coal due to the energy crisis caused by Russia’s invasion of Ukraine.

The concentration of carbon dioxide in the atmosphere was 419.94 parts per million on January 24, up from 419.8 a year ago and from 400 parts per million a decade ago.

But greenhouse gas emissions are no longer the only problem.

Global meteorological data also shows that during La Niña years, global average temperatures are cooler than in those years when the system is not in place. Similarly, the El Nino weather pattern brings us hotter temperatures.

Despite these cyclical peaks and troughs, the past eight years have been the hottest on average globally. As a result of this trend 2022, which coincided with a rare three-year La Niña period, was not the hottest year in history, but the fifth or sixth hottest.

It is clear now that each peak and trough is growing warmer than those before it. (These regular dips in the ever-growing heat of the planet also explain why sceptics like to announce every couple of years that “global warming has halted”.)

The WMO El Nino/La Niña Update indicates there is about a 60 per cent chance that La Niña will persist until March 2023, and should be followed by neutral conditions (neither El Nino nor La Niña).

This suggests temperatures will increase. When a new El Nino takes hold, that temperature increase will be even higher.

In a paper for Columbia University last year, leading climatologist Professor James Hansen and colleagues wrote: “We suggest that 2024 is likely to be off the chart as the warmest year on record.

“Without inside information, that would be a dangerous prediction, but we proffer it because it is unlikely that the current La Niña will continue a fourth year. Even a little futz of an El Nino – like the tropical warming in 2018-19, which barely qualified as an El Nino – should be sufficient for record global temperature.

“A classical, strong El Nino in 2023-24 could push global temperature to about 1.5 degrees higher relative to the 1880-1920 mean.”

Perversely, the third driver of dangerously hotter conditions in the coming years is likely to be the world’s success in efforts to decarbonise our energy and transport systems.

Burning fossil fuels causes warming carbon dioxide to be emitted into the atmosphere, but it also releases larger particles into our skies. These cause smog and can be dangerous, but they also reflect some of sun’s heat back into space.

Some recent research suggests that man-made sulphates in the atmosphere are responsible for short-term cooling of about half a degree, and that as China and India rapidly clean their skies, we can expect to lose some of that cooling impact.

So, what does it mean?

“To be frank, I dread the next El Nino,” says Dr Martin Rice, research director with the Climate Council. “The Black Summer bushfires scarred so many of us.”

That catastrophe - and the record-breaking drought that preceded and accelerated it - followed an El Nino, the same weather pattern that helped make 2016 the hottest year on record.

For eastern Australia this will mean a return to intensely hot and dry conditions, which are also likely to afflict Papua New Guinea and Indonesia, says Rice. In the Americas the pattern will likely spur floods and storms in the west from California down to Peru.

But should an El Nino set in - which is by no means certain yet - and should it drive global average temperatures beyond the 1.5-degree mark, it does not mean we have breached the Paris Agreement temperature threshold, says Rice.

“Climate science works on decadal trends. Under IPCC and UN [Framework Convention on Climate Change] terms, the 1.5-degree marker is based on a 30-year average.”

But breaching the barrier for even a year is likely to have a profound impact on the climate movement, some elements of which are embroiled in an internal dispute over how frank they should be in public about how far and fast heating is progressing.

For Rice, it is further evidence of the staggering amount of work the world must do in the short term to stave off long-term calamity.

Writing in December, Gates said that reaching net zero in time to hold warming to even 2 degrees will be the hardest thing humans have ever done.

“We need to revolutionise the entire physical economy - how we make things, move around, produce electricity, grow food, and stay warm and cool - in less than three decades.”

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