Shifting Perspectives on Global Temperature Thresholds
For over a decade, the goal of limiting global warming to 1.5 °C above pre-industrial levels has served as a cornerstone of international climate policy, representing a critical threshold beyond which climate risks are expected to escalate significantly. Recent findings from the United Nations Environment Programme (UNEP) indicate that, under current policies and short-term trajectories, surpassing this limit is now deemed inevitable.
The report titled “Limiting Overshoot,” published on 2nd September, suggests that the planet is likely to exceed the 1.5 °C mark in the coming years. The most optimistic scenario anticipates a peak temperature rise of around 1.8 °C, while other projections suggest spikes exceeding 2 °C. UNEP introduces a pathway of “overshoot, peak, and decline,” aiming to limit how much temperatures rise, reduce the duration above 1.5 °C, and eventually bring temperatures back below this threshold.
This warning signifies a pivotal shift in the global response to climate change. The 1.5 °C target remains a central tenet of the Paris Agreement, which seeks to keep global temperature increases well below 2 °C while striving to limit the rise to 1.5 °C. The new scenario emphasises the extent to which this threshold will be breached and the duration of the excess.
Record Temperatures and Their Implications
In 2024, the global average temperature was approximately 1.55 °C above the 1850-1900 average, according to the World Meteorological Organisation (WMO). This marked the hottest year in 175 years of recorded data and likely the first complete natural year to exceed 1.5 °C.
This outcome reflects both long-term trends driven by climate change and natural variations that can temporarily elevate or diminish temperatures. The assessment of global warming is typically conducted over longer periods; therefore, the WMO clarified that the 2024 figure does not indicate a sustained breach of 1.5 °C.
The UNEP report specifically focuses on the scenario of sustained warming. The term “overshoot” is used to describe a trajectory in which temperatures temporarily exceed a specific level, reach a peak, and subsequently begin to decline. The magnitude and duration of this overshoot will largely depend on greenhouse gas emissions in the coming years. These factors will be critical in determining the intensity of impacts faced by societies and ecosystems.
The Consequences of Incremental Warming
The effects and risks associated with warming intensify as temperatures rise. A world experiencing 1.6 °C, 1.8 °C, or more than 2 °C will encounter varying levels of risk, underscoring the importance of limiting each additional fraction of a degree.
UNEP warns that every fraction of a degree above 1.5 °C exacerbates the risks and impacts of climate change. The Intergovernmental Panel on Climate Change (IPCC) has previously concluded that damages increase with both the magnitude and duration of the overshoot. Greater warming also raises the likelihood of crossing tipping points, which could trigger profound and, in some cases, irreversible transformations.
The duration of the overshoot also affects adaptation possibilities. UNEP cautions that prolonged and heightened warming will intensify impacts on health, food security, water supply, urban areas, infrastructure, and economies, while pushing certain systems closer to their adaptation limits.
Some consequences may persist even if temperatures eventually fall back below 1.5 °C. The report anticipates irreversible losses and limits to adaptation, particularly in scenarios where warming reaches elevated peaks.
Strategies for Temperature Reduction
The initial step towards reversing temperature increases involves a rapid and sustained reduction in greenhouse gas emissions to achieve net-zero emissions. Achieving net-zero CO₂, alongside significant reductions in other greenhouse gases, could halt the rise of global warming.
To effectively lower temperatures, it would be necessary to achieve negative net emissions, meaning that more carbon dioxide is removed from the atmosphere than is emitted. UNEP considers CO₂ removal to be essential for any pathway aimed at returning temperatures below 1.5 °C after an overshoot.
These strategies encompass natural processes, such as reforestation, and technologies specifically designed to capture and store CO₂ over prolonged periods. The scale of these efforts will depend on how much temperatures have risen beforehand: a greater overshoot would necessitate larger carbon removals. The IPCC has also cautioned that a heavier reliance on such solutions increases the challenges of feasibility and sustainability.
Consequently, UNEP places immediate emission reduction at the heart of its proposal. The report advocates for a transition towards net-zero emissions, accelerating the shift from fossil fuels to renewable energy sources, reducing methane emissions, and enhancing adaptation measures.
The breach of the 1.5 °C threshold ushers in a new phase of the climate crisis. Decisions made in the coming years will determine how high temperatures will rise and for how long they will remain above this critical level. A lower peak and shorter duration would reduce risks to people, economies, and ecosystems. In this context, each fraction of a degree in temperature matters significantly.