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For the first time, global warming has exceeded 1.5C over an entire year, according to the EU's climate service.
World leaders pledged in 2015 to try to limit the long-term rise in temperature to 1.5C, which is seen as essential to help avoid the most damaging impacts.
This first year-long breach does not break that historic "Paris Agreement," but it brings the world closer to doing so in the long run. Urgent action to cut carbon emissions could slow warming, scientists say.
"Going above 1.5C of warming in an annual average is important," says Prof Liz Bentley, chief executive of the Royal Meteorological Society.
"It's another step in the wrong direction. But we know what we have to do."
Limiting long-term warming to 1.5C above "pre-industrial" levels, before humans started burning large amounts of fossil fuels, has become a key symbol of international efforts to tackle climate change.
A landmark 2018 UN report said the risks from climate change, such as extreme heat waves, rising sea levels and the loss of wildlife, were much higher at 2C of warming than at 1.5C .
But temperatures have continued to rise at an alarming rate, data from the EU's Copernicus Climate Change Service shows from last year. The period from February 2023 to January 2024 reached 1.52C of warming. This year-long breach is no big surprise. January was the eighth warmest month in a row.
In fact, one science group, Berkeley Earth, says that calendar year 2023 was more than 1.5C above pre-industrial levels. Other scientific bodies, such as NASA, put the last 12 months at just under 1.5C of warming.
These small differences are largely due to the way global temperatures were estimated for the late 1800s, when measurements were rarer. But all major data sets agree on the recent trajectory of warming and that the world is in its warmest period since modern records began and likely for much longer.
And the world's sea surface is also at its hottest average temperature ever recorded, another sign of the pervasive nature of climate data. It's especially notable given that ocean temperatures don't normally peak for another month or so.
Why has 1.5C been broken over the past year?
The long-term warming trend is undoubtedly driven by human activities – primarily the burning of fossil fuels, which releases planet-warming gases like carbon dioxide. This is also responsible for the vast majority of the warmth over the past year.
In recent months, a natural climate warming phenomenon known as El Niño has also given air temperatures a further boost, although it would normally only do so by around 0.2C.
Average global air temperatures began to exceed 1.5C of warming almost every day in the second half of 2023, when El Niño began to kick in, and this has continued into 2024. This is shown where the red line is above the dashed line on the chart lower.
An end to El Niño conditions is expected within months, which could allow global temperatures to stabilize temporarily and then fall slightly, perhaps back below the 1.5C threshold.
But human activities mean that temperatures will eventually continue to rise in the coming decades unless urgent action is taken.
"Rapid reductions in greenhouse gas emissions are the only way to stop rising global temperatures," concludes Samantha Burgess, Deputy Director of Copernicus.
Is the world warming faster than expected?
Can we still limit global warming?
At the current rate of emissions, the Paris goal of limiting warming to 1.5C as a long-term average, rather than a single year, could be exceeded within the next decade.
This would be a hugely symbolic milestone, but researchers say it would not mark a climate cliff edge.
"It is not a threshold beyond which climate change will spiral out of control," says Prof Myles Allen of the University of Oxford and Gresham College, and a lead author of the landmark 2018 UN report.
However, the impacts of climate change will continue to accelerate, something extreme heat waves, droughts, fires and floods have given us over the past 12 months.
"Each tenth of a degree of warming causes more damage than the last," adds Prof Allen.
An additional half degree, the difference between 1.5 C and 2 C of global warming, also greatly increases the risk of passing "tipping points". These are thresholds within the climate system that, if crossed, can lead to rapid and potentially irreversible changes.
For example, if the Greenland and West Antarctic ice sheets passed a tipping point, their potentially runaway collapse would cause "catastrophic" rises in global sea levels over the following centuries, says Prof Bentley.
But researchers are keen to point out that humans can still make a difference to the trajectory of global warming. The world has made some progress, with green technologies such as renewables and electric vehicles flourishing in many parts of the world.
This means that some of the worst-case scenarios of warming of 4 degrees Celsius or more this century – thought possible a decade ago – are now considered much less likely, based on current policies and promises.
And perhaps most encouraging of all, it is still thought that the world will more or less stop warming once net zero carbon emissions are achieved. Effectively halving emissions this decade is seen as particularly important.
"This means that we can ultimately control how much warming the world experiences, based on our choices as a society and as a planet," says Zeke Hausfather, a climate scientist at the US-based Berkeley Earth Group.
"Punishment is not inevitable."
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