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Overshoot explained: Five things to understand about a world beyond 1.5°C – EnviroNews
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In 2024, the world experienced its first year in which the average global temperature exceeded 1.5°C above pre-industrial levels.
While scientists assess climate change over decades rather than calendar years, the signal was clear: we’re heading for a future warmer than the Paris Agreement goal. Indeed, if all climate commitments and policies are implemented, warming is projected to reach 1.8°C at a minimum.
A new United Nations Environment Programme (UNEP) report, Limiting Overshoot, confronts this reality head-on. Its findings call for the world to face the inevitability of exceeding global warming of 1.5°C. The priority, it says, must be to keep the exceedance and peak in warming as short-lived and as low as possible, while working toward bringing temperatures back down thereafter. This is not the preferred path, says the report, but it’s the one we’re on and need to prepare for.
Other UNEP reports have already identified many solutions to minimise overshoot. Limiting Overshoot highlights what’s at stake if the solutions aren’t rapidly implemented and scaled.
What does this mean in practice? Here, UNEP breaks down “overshoot” into five key things to understand.
- Overshoot is a pathway, not a moment
Temperature change at a global scale is not something that occurs suddenly. It’s a multidecadal unfolding. Ideally, if we can limit the peak temperature and then bring it back down, overshooting 1.5°C becomes not a vertical line but a curve, which the team of authors have coined the “overshoot peak and decline pathway,” or the “overshoot pathway” for short. The pathway describes a trajectory of four stages, from temperature exceedance (going beyond 1.5°C), to a peak (ideally kept below 2°C ), a decline, and finally, stabilization. However the pathway is not predetermined; each decision and action implemented determines its trajectory.
The shape of the curve is crucial for assessing risk. The duration of exceedance and the magnitude of the peak directly influence how badly the world will be impacted by climate change. Important to understand is that the downward side of the curve bringing us back to 1.5°C – and, better yet, below – is not a given. It hinges on the mitigation actions we take now and continue into the future. In other words, the flatter and shorter the curve, the better – and this is up to us.
- Going above 1.5°C must not mean abandoning the goal
Article 2 of the Paris Agreement commits nations to keep global average temperatures well below 2°C above pre-industrial levels, and ideally capped at 1.5°C. But 1.5°C was never considered “safe.”
Even now, we are already experiencing increasingly severe heatwaves, fires, droughts, extreme weather and other risks for people and ecosystems. As we go beyond 1.5°C, climate impacts will intensify non-linearly and compound over time. This could lead to irreversible tipping points like the dieback of the Amazon rainforest, permafrost thaw and the collapse of ice sheets. Costs of mitigation will skyrocket as we’re forced to increasingly rely on carbon dioxide removal rather than just emissions cuts. Climate conditions will put caps on adaptation, raising dilemmas of human justice – like forced migration, the submersion of Small Island Developing States, and that global food production could decline 14 per cent by 2050.
The immediate priority therefore remains rapid and sustained emissions reductions. Discussing overshoot is not an alternative to prevention; it is part of preparing responsibly for a risk that is becoming increasingly difficult to ignore.
“Decisions, science and, most importantly, mindsets need to shift to seeing 1.5°C as a goal we’ll be approaching from above rather than below,” says Mirey Atallah, Chief of UNEP’s Adaptation and Resilience Branch and coordinator of the report. “This is the starting point for realizing just how imperative it is to massively reduce emissions and minimize the severity of the overshoot pathway we’re on.”
- Adaptation and mitigation must happen hand-in-hand
The report urges that we can no longer afford to approach mitigation and adaptation as separate buckets of action. They must be accelerated simultaneously, at scale, with raised ambition, and in an interconnected manner. Achieving net-zero emissions might be enough to halt warming, but re-stabilizing the climate back below 1.5°C requires us to be net-negative.
The report also illustrates just how interdependent mitigation and adaptation are. For example, adaptation now will help preserve our ability to mitigate in the future. Future adaptation will be shaped by evolving risks and conditions, based on how much we mitigate. Insufficient mitigation could lead to adaptation limits, when climate risks become so profound that adaptation is no longer an option.
- Carbon dioxide removal (CDR) is necessary, but it’s not a silver bullet
CDR is the process of pulling carbon dioxide out of the atmosphere and storing it safely. The report acknowledges that even pathways with the least amount of overshoot require total CDR to increase.
The report follows preexisting classification of CDR into two categories. There’s conventional CDR, which comprises ecological-based practices like afforestation and soil management that boost carbon storage in ecosystems. And there’s novel CDR, which uses technology like direct air capture and chemical systems to remove carbon dioxide from the air.
Conventional CDR is the most feasible. However, it requires land and would need the world to rethink how we use the land that’s available, particularly concerning agriculture and livelihoods. It also doesn’t guarantee safe storage. Temperature rise, drought and wildfires can release sequestered carbon from forests and landscapes.
Novel CDR has different limits. The technology is not proven and is substantially more expensive than conventional CDR (costing up to US$600 per tonne of carbon dioxide sequestration). To meaningfully mitigate, the amount of carbon dioxide it currently sequesters would need to triple – if not quadruple – by 2050, according to the report. Governance and implementation capacity on CDR is also limited.
Models find that CDR can only contribute up to a few tenths of a degree of warming reversal this century – a crucial contribution, but insufficient on its own. It “is not a substitute for rapid emission reduction” of carbon dioxide, methane and other greenhouse gases, says the report.
- Falling temperatures will not bring the world back to “normal”
How will the world look when temperatures decline and the climate restabilizes? We don’t know, says the report.
By surpassing 1.5°C, enormous changes to ecological and human systems are unavoidable. But, because planetary changes unfold at different speeds, what exactly will change, when, and at what scale are huge “unknowns” – sea level rise, the melting of the cryosphere, species extinctions, ecosystem changes, human displacement, changes to food and water systems, and the actions and reactions of governments.
“The pace of change of these systems is very slow, and very differentiated,” explains Atallah. “The climate system behaves like an enormous boiler. Cutting emissions turns down the flame; reaching net-zero stops adding to the heating. But the heat already accumulated does not disappear. The oceans, ice sheets and ecosystems take decades, or much longer, to respond, and bringing temperatures back down requires more than simply turning off the gas. This makes the future very difficult to predict.”
It also makes the decline period of the overshoot pathway a crucial scientific frontier, and the report’s authors hope their work will trigger new research into how the planet might change as temperatures fall.
The overshoot pathway also raises profound questions of governance and justice. Who decides what gets protected, rebuilt or left behind? What happens in places where adaptation limits are reached? What becomes of places that become uninhabitable? Who bears costs?
The report’s central message is to limit the size of these question marks by keeping peak warming as low as possible, shortening the duration of exceedance and acting now to preserve as much flexibility as possible for the future.
“Every fraction of a degree avoided reduces risks. Every year by which overshoot duration is shortened reduces exposure. Every investment in resilience preserves future options,” the report says.
