Why 1.5°C warming arrived early – and what it means for the future

A new study published in npj Climate and Atmospheric Science reveals that only models with the highest near-term warming rates accurately captured this early arrival, highlighting how natural climate variability and recent reductions in aerosol cooling may be accelerating transient warming. The findings suggest that while crossing the 2°C mark by mid-century is difficult to avoid, immediate emission cuts are the only way to prevent far more dangerous warming in the future.

The 1.5°C and 2°C thresholds refer to the goals set in the Paris Agreement, which aims to limit global warming to well below 2°C and pursue efforts to stay within 1.5°C compared to pre-industrial levels.

Why record hot years matter

Record global temperatures are not just numbers. Each additional fraction of a degree of warming increases the risk of damaging heatwaves, heavy rainfall, drought, wildfire conditions, ecosystem disruption and other climate-related impacts.

Recent record-warm years also sharpen concern about climate tipping points, where parts of the Earth system could undergo large and potentially irreversible changes. In that sense, the first year above 1.5°C is not an isolated anomaly, but a warning that the world is moving deeper into a zone of rising damage and risk.

A reality check for 2024 and beyond

The 1.5°C milestone reached in 2024 served as a global wake-up call. Researchers analysed a large number of climate models and found that this first crossing was projected to happen only between 2027 and 2031. The study suggests that 2024’s early arrival was likely driven by natural climate variations – such as El Niño – superimposed on an accelerating human-induced warming trend. Crucially, the models that did get the 2024 timing right were those that account for a faster pace of warming, potentially due to reduced aerosol cooling from lower shipping emissions and recent rapid emission cuts in East Asia, which have unmasked more greenhouse gas–driven warming.

Looking ahead, the research offers a reality check regarding the first crossing of the 2°C target. Due to the long-lasting impact of past greenhouse gas emissions, a first-time breach of the 2°C limit between 2040 and 2050 is now very difficult to avoid even under the most ambitious plans to cut emissions.

According to the study, a likely exceedance of 2°C before mid-century highlights the committed warming from past emissions. At the same time, the results underline that mitigation remains highly effective, as present-day choices will determine the magnitude of long-term warming.

What emission reductions mean for future warming

While near-term temperature changes are partly constrained by past emissions, future warming will still depend on the speed and scale of emission reductions. The study highlights that emission reductions have a major impact on higher warming levels.

If the world rapidly transitions to a low-emission pathway, most climate models show that we can avoid crossing the devastating 3°C threshold. On the other hand, staying on our current or a higher emission pathway would cause warming to spiral in the second half of the century, rapidly crossing 4°C or even 5°C limits with severe consequences for humanity and the planet.

“Ultimately, the study calls for a shift in how we view climate goals. Action is no longer just about staying under a specific limit but about aggressively cutting emissions to steer away from the most dangerous trajectories and buying precious time for societies to adapt to a changing world,” says Kalle Nordling, first author of the study and researcher at the Finnish Meteorological Institute.

The study was conducted by researchers from the Finnish Meteorological Institute and the Institute for Atmospheric and Climate Science at ETH Zurich.

Further information

Researcher Kalle Nordling, Finnish Meteorological Institute

Head of group Joonas Merikanto, Finnish Meteorological Institute

Email addresses are in the format firstname.lastname@fmi.fi.