Table of contents

DOI: https://doi.org/10.58248/RR107 

Understanding the 1.5°C global warming level: Implications for policy and adaptation 

Recent assessments from the World Meteorological Organization (WMO) indicate that the world is approaching 1.5°C of global mean surface temperature warming faster than previously anticipated.  

Individual years above 1.5°C of warming have already occurred, and there is now a high probability that temporary exceedances will become increasingly common during the remainder of this decade (table 1). The focus of climate science is shifting from asking whether 1.5°C will be exceeded to:  

Current evidence suggests that sustained warming of 1.5°C above pre-industrial levels is likely to be reached during the late 2020s or early 2030s. Recent assessments indicate that human-induced warming has already reached approximately 1.37°C above 1850–1900 levels, increasing by 0.27°C (0.2°C to 0.4°C) from 2016 to 2025. This highlights the proximity of the 1.5°C threshold and the narrowing window for limiting the magnitude and duration of any overshoot. The possibility of renewed El Niño conditions during 2026–2027 could temporarily increase warming and thus the likelihood of record-breaking temperatures and extreme weather.  

This evidence suggests that 1.5°C warming should be considered a near-term planning condition, rather than a distant future scenario. 

What does 1.5°C of global warming mean? 

The Paris Agreement established a framework to limit the rise in global temperature to well below 2°C above pre-industrial levels and to pursue efforts to limit warming to 1.5°C. These limits are relative to global mean surface temperatures during the pre-industrial period (1850–1900). 

To distinguish long-term anthropogenic (human-made) warming from natural year-to-year variability, warming is assessed over multi-decadal timescales, typically using a 20-year average. The Intergovernmental Panel on Climate Change (IPCC) defines the crossing time of future global warming levels as the mid-point of the 20-year period. This means that just using observations of the previous 20 years, or even the previous 10 years, is automatically out of date because it does not allow the mid-point of the period to be the current year. A more up-to-date measure consistent with IPCC definitions can be obtained by combining observed temperatures from the previous decade with climate model projections for the following decade. This metric has been shown to perform well when applied retrospectively to the passing of previous global warming levels such as 1°C using information that would have been available at the time.  

A global average warming of 1.5°C does not mean that all regions will warm uniformly. Land generally warms faster than oceans, and increases in the intensity of extreme weather, such as heatwaves or unusually heavy rainfall, exceed changes to mean warming. Higher-latitude regions such as the Arctic are warming fastest of all, at nearly four times faster than the global average. The UK has already warmed by approximately 1.3°C relative to the period 1961 to 1990, and is at increased risks from heatwaves, heavy rainfall and sea-level rise. 

The 1.5°C global warming level should not be interpreted as a precise boundary between safe and dangerous climates. Rather, climate risks increase progressively with every increment of warming, and many impacts are already being experienced today. 

When might the world reach 1.5°C? 

According to the latest WMO Global Annual-to-Decadal Climate Update, there is a 91% probability that at least one year between 2026 and 2030 will exceed 1.5°C above pre-industrial levels and a 75% probability that the mean temperature across the period 2026 to 2030 will itself exceed 1.5°C.  

The first calendar year above 1.5°C occurred in 2024. Although a single year above this threshold does not imply that the Paris Agreement limit has been formally breached, recent research suggests that the first year above 1.5°C is likely to occur within the first 20-year period whose average warming reaches the same level.  

Taken together, the latest evidence suggests that sustained global warming of 1.5°C is likely to occur soon, during the late 2020s or early 2030s. Recent work has increasingly focused on the consequences of temporary overshoot, rather than on the exact timing of exceedance. 

Table 1: Temporary exceedance vs sustained warming 
Measure  Meaning 
Single month average >1.5°C  Weather and short-term variability 
Single year average >1.5°C  Early signal that long-term warming is approaching 1.5°C 
Five-year average >1.5°C  Increasing evidence of sustained warming 
Twenty-year average >1.5°C  Paris Agreement threshold reached 

Based on information from: IPCC (2021); WMO (2026); Bevacqua and others (2025) 

Does exceeding 1.5°C mean the Paris Agreement has failed? 

No.  

Exceeding 1.5°C does not imply that climate change mitigation efforts have failed. Climate risks, including the potential to pass climate tipping points, increase progressively with every increment of warming, and the scale and duration of any overshoot still strongly depends on future greenhouse gas emissions. 

Limiting warming to 1.5°C rather than 2°C would still avoid substantial impacts and reduce risks to ecosystems, infrastructure and societies. Consequently, the IPCC states that every tenth of a degree of avoided warming remains important. 

The role of El Niño 

Natural climate variability remains an important influence on year-to-year temperatures. One of the most significant sources of variability is the El Niño–Southern Oscillation (ENSO). 

Historically, strong El Niño events have contributed to record-breaking global temperatures, including those observed in 1998 and 2016. El Niño conditions during 2023–2024 also contributed to the exceptional warmth experienced globally. 

El Niño conditions are now developing during 2026, with forecast models indicating the 2026 El Niño event will be much stronger than anything observed in the last 150 years; models give a 91% chance of a record peak.  

The strong ‘Godzilla’ El Niño that is forecast could temporarily increase global temperatures by 0.1°C to 0.2°C above the underlying long-term trend, causing temporary overshoot of 1.5°C and making 2027 the warmest year on record by a considerable margin. It could also increase the likelihood of additional record-breaking global mean surface temperature years, with more frequent exceedances of 1.5°C. The increased warming will intensify heatwaves and droughts, cause more severe flooding and other climate extremes, with impacts on food security, ecosystems and infrastructure systems worldwide. 

However, El Niño should not fundamentally alter the long-term trajectory of climate change. The warming associated with El Niño is superimposed upon the continuing increase in temperatures caused by anthropogenic greenhouse gas emissions. 

Implications for policy and adaptation 

The latest evidence suggests that the policy challenge is increasingly shifting from one of avoiding 1.5°C altogether to one of managing the risks associated with approaching and potentially exceeding this level of warming. 

Importantly, climate impacts do not suddenly appear at 1.5°C. Risks progressively increase with every increment of warming, and could cause irreversible tipping points, and some consequences – including sea-level rise and ecosystem losses – are not reversible. 

Adaptation planning should assume that a 1.5°C world represents a near-term reality and that the climate conditions of the 20th century no longer exist. Infrastructure and planning decisions made today will remain in place for decades and should be able to withstand increasing risks associated with:  

  • heatwaves and overheating  
  • drought and water scarcity  
  • flooding  
  • sea-level rise and coastal change  
  • ecosystem degradation  
  • cascading failures across interconnected systems, such as:  
  • failures in power systems affecting rail infrastructure  
  • data centres going down (like they did in 2022), affecting the health system and hospitals 

The prospect of renewed El Niño conditions reinforces the need for resilience to both climate variability and long-term climate change. 

At the same time, the increasing probability of exceeding 1.5°C does not imply that higher levels of warming are inevitable. Continued emissions reductions can limit climate risks and avoid more severe warming, as future emissions pathways will determine the scale and duration of any overshoot. 

Recent research has highlighted concerns that increasing warming may raise the likelihood of crossing critical thresholds – tipping points – in components of the Earth system, including ice sheets, ocean circulation and ecosystems. Although large uncertainties remain regarding the timing of such changes, risks generally increase with increasing temperature and underscore the importance of limiting both the magnitude and duration of any overshoot.  

Acknowledgements 

Professor Hayley Fowler FRS is Professor of Climate Change Impacts in the School of Engineering at Newcastle University and a Director of the Centre for Climate and Environmental Resilience. 

Professor Richard Betts MBE is Chair in Climate Impacts at the University of Exeter and Head of Climate Impacts Research at the Met Office Hadley Centre. 

POST is grateful to the authors for kindly giving their time to produce this briefing and the Royal Society for identifying possible authors. 

Questions about this briefing should be referred to Jonathan Wentworth (post@parliament.uk), who acted as parliamentary lead for this work.