As expected, the World Meteorological Organization confirmed the news in early January: 2024 was the hottest year ever recorded. For the first time, over an entire year, the planet’s temperature was more than 1.5°C above the 1850-1900 period, the reference used by climate scientists to measure global warming.

The record-breaking temperatures had tangible consequences, as seen in last year’s heatwaves, floods, and droughts. However, they also marked the breach of a symbolic threshold set in the 2015 Paris Agreement, where nearly every country committed to limiting global warming well below 2°C and striving to stay within 1.5°C by the end of the century. This ambitious goal was championed by leaders of Pacific island nations, which are directly threatened by rising sea levels.

Does 2024 mark the failure of the Paris Agreement?

The Paris Agreement does not specify how to formally
acknowledge the breaching of its targets. While various proposals are on the
table, climate scientists agree on one point: “A single year with a
temperature above 1.5°C is not enough to confirm that the threshold has been
permanently exceeded,” explains Quentin Lejeune, a researcher at the
Berlin-based scientific institute Climate Analytics.

Typically, climate warming is assessed over 10- to 20-year
averages, which help filter out natural climate variations. The 2014-2023 average
is around +1.2°C. Phenomena like El Niño, which contributed to the sharp temperature rise in 2023 and 2024, can influence short-term fluctuations.
However, El Niño alone does not explain the full increase. “Some aspects
of this rise, including record temperatures in the North Atlantic, remain
poorly understood,” notes Pierre Friedlingstein, a researcher at the
University of Exeter in the United Kingdom.

Nonetheless, the overall warming trend is not surprising. “Current temperatures fall within the range predicted by models 40 years
ago,” said Frédéric Hourdin, research director at France’s National Center for Scientific Research and the
Dynamic Meteorology Laboratory at the Pierre-Simon Laplace Institute. In the
coming decade, Friedlingstein expected that “some years will exceed 1.5°C,
while others may fall below it.”

However, the prospect of permanently surpassing this
threshold is approaching. While 23 countries have reduced their CO₂ emissions
from oil, gas, and coal, global emissions continue to break records (+0.8% last
year). According to climate models synthesized by the Intergovernmental Panel
on Climate Change (IPCC), staying below 1.5°C would have required emissions to
peak around 2020, followed by a 45% decline by 2030. To remain under 2°C, most
scenarios suggest emissions should peak by 2025, followed by rapid reductions.
A UN report in November warned about the “lost time” over the past
four years—delaying emissions cuts only makes future reductions harder and more
disruptive for societies.

If current policies persist, the UN projects a 3°C warming
by the end of the century. While this is less dire than the 4°C warming
projected before the Paris Agreement, it would still represent a radical
transformation of the planet.

Can we reverse climate change?

Only half of every ton of CO₂ emitted is absorbed by natural carbon sinks such as forests and oceans. The “CO₂ problem” is one of accumulation—once in the atmosphere, emissions persist for thousands of years. In theory, stopping global warming would require reducing CO₂ emissions from 40 billion tons per year to zero.

In practice, this is unlikely, as some
emissions—particularly from aviation and agriculture—cannot be entirely
eliminated. This is why the goal of “carbon neutrality” has emerged:
drastically cutting emissions while removing remaining greenhouse gases from
the atmosphere. This could stabilize global temperatures and eventually lead to
cooling.

To achieve this, additional carbon sinks must be created,
such as new forests or carbon capture technologies—large industrial facilities
that act like “CO₂ vacuums.” However, reducing temperatures further
would require removing more CO₂ than we emit, which would be an enormous
challenge.

A recent study published in Nature estimates that to offset even a small overshoot of 0.1 to 0.3°C,
about 400 billion tons of CO₂ would need to be removed this century—the
equivalent of 10 years of current emissions. Currently, we remove only 2
billion tons annually, mainly through reforestation. However, tree planting has
limitations and is increasingly threatened by droughts and wildfires.
Meanwhile, technological solutions capture just 0.1% of the required amount.
Even if their capacity increases, technical, economic, and sustainability
challenges could limit large-scale deployment.

This reality has divided the scientific community. In 2023,
1,000 scientists from the group Scientist Rebellion urged acknowledging the
failure of the 1.5°C target. “It’s time to publicly admit there’s no
credible path to staying below 1.5°C, even if it remains geophysically possible,”
argued climatologist Peter Kalmus. Others advocate keeping 1.5°C as a political
goal since every fraction of a degree matters. “From a scientific
perspective, staying below 1.5°C is still feasible,” said Lejeune,
though he acknowledged that politically, temporary overshoot seems inevitable.

Are irreversible consequences
unavoidable?

Even if emissions drastically decline and CO₂ is removed
from the atmosphere, some climate impacts could persist for decades or
centuries.

Lower temperatures may reduce the frequency of extreme heatwaves, but other effects—such as changes to rainfall patterns—are less predictable. Ocean cooling would lag behind, meaning Europe, which is bordered by the Atlantic, could experience prolonged climate disruptions.

Some changes are already in motion: sea level rise, ocean
acidification, and melting ice sheets and glaciers may continue even if
CO₂ levels drop. Beyond 1.5°C, 70-90% of tropical corals are at risk of
extinction, rising to 99% at 2°C.

There is also the looming threat of climate tipping
points—major, irreversible shifts with global consequences. The Arctic
permafrost, for instance, stores vast amounts of greenhouse gases, and its
thawing could release emissions that accelerate warming.

“The exact thresholds and speed of these tipping
points remain uncertain,” noted James Dyke, a researcher at the University
of Exeter. “But the further we move beyond 1.5°C, the greater the risk of
triggering them.”

Recent studies have warned of potential collapses in the Atlantic
Meridional Overturning Circulation, a key ocean current regulating
European climate. Other concerns include accelerated ice loss in Greenland and drying in the Amazon rainforest.

While some uncertainties remain, one thing is clear:
climate change is already affecting ecosystems and human societies. “Every
degree of warming brings more suffering,” emphasizes Carl-Friedrich
Schleussner of Climate Analytics. “That should be our greatest
concern.”