The Paris Agreement threshold of 1.5 degrees Celsius above pre-industrial levels is a small-sounding number with big implications for people and nature.

It is a marker of risk, with more warming from planet-heating emissions linked to worsening floods, heatwaves and storms, as well as gradual effects like sea level rise and species extinctions.

"Every fraction of a degree beyond this level translates into more extreme weather, biodiversity loss, and human suffering," said William Ripple, Professor at Oregon State University.

But when can we say 1.5C is officially crossed?

That is the focus of two studies published Monday in the journal Nature Climate Change.

Scientists sought to square the long-term Paris goals with the extraordinary heat seen in 2024, the first full calendar year above 1.5C, according to the World Meteorological Organization.

A single year above the limit does not mark a breach of the Paris deal, which is measured as a rolling average over 20 or 30 years, to smooth out year-on-year temperature variability.

By that measure the world has so far warmed roughly 1.3C, warmer than it has been for the last 125,000 years, scientists say.

- 'Call to action' -

One team based in Germany and Austria used observational data and computer modelling to assess whether crossing 1.5C over one year might represent an "early warning" that the long-term limit at risk.

They found that historical patterns suggested the first single year that crosses a particular temperature threshold tends to fall within the longer-term 20-year period where that global warming level is reached.

"The calendar year of 2024 was announced as the first above 1.5C warming and, therefore, it signals that most probably Earth has already entered a 20-year period at 1.5C warming," the authors said.

An official breach of the Paris goal would happen at the mid-point of the period -- so within the next 10 years -- unless "stringent" efforts are made to slash greenhouse gas emissions, they said.

"A year above 1.5C is not the time for despair, but a call to action," the authors said.

In the second study, Alex Cannon of the Canadian Ministry of the Environment and Climate Change used a slightly different methodology, but reached a similar conclusion.

Under a middle of the road scenario, a short term crossing of 1.5C in 2024 would suggest "long-term crossing would probably occur before 2029", he said, but stressed that real-world factors in the coming years would play an important role in the timing.

The UN's IPCC climate experts have said that there is a 50/50 chance of passing 1.5C warming in the early 2030s.

- 'New era' -

The new research comes as most countries missed a deadline to submit emissions-cutting plans for the decade ahead that align with 1.5C.

Containing global warming to 1.5C rather than "well below" 2C -- the less ambitious Paris goal -- would significantly limit its most catastrophic consequences, the IPCC says.

Last year was the first taste of a 1.5C world, with soaring temperatures unleashing deadly and costly storms, floods and fires.

Reinsurance giant Swiss Re estimated natural disasters caused $310 billion in damage in 2024.

This year, the damage and economic loss from fires in Los Angeles alone could top $250 billion, according to estimates by private meteorological firm AccuWeather.

And the heat shows no sign of abating.

Europe's Copernicus monitor has said last month was the hottest January on record, surprising climate scientists who had expected cooler La Nina conditions to ease the warm-streak.

In a world that has warmed 1.5C, coral reefs are projected to decline 70 to 90 percent, the IPCC says, while some 14 percent of terrestrial species will face an extinction risk.

Warming between 1.5C and 2C could push Arctic sea ice, methane-laden permafrost, and ice sheets with enough frozen water to lift oceans by a dozen metres beyond points of no return.

"Unless rapid action is taken, 2024 will be remembered not as an anomaly, but as the beginning of a new climate era -- one defined by escalating risks," said Ripple.

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SWISS RE AG