Introduction
The world is expected to exceed 1.5°C of warming above pre-industrial levels. A new United Nations Environment Programme (UNEP) report, Limiting Overshoot – Navigating exceedance of 1.5°C and pathways towards return, looks at what can still be done to limit how far temperatures rise above 1.5°C, reduce the time spent above that level and eventually bring temperatures down again.
UNEP says that even if current climate commitments and policies are implemented, warming is projected to reach at least 1.8°C. Other pathways could lead to higher temperatures. The report explains what higher warming means for people and nature, why both the highest temperature reached and the length of time above 1.5°C matter, how people can prepare for impacts that cannot be avoided, and what is needed to bring temperatures down.
Timeline
2015: The Paris Agreement sets the goal of keeping the increase in global average temperature well below 2°C above the pre-industrial level, while pursuing efforts to limit the increase to 1.5°C.
2024: The global average temperature was more than 1.5°C above the pre-industrial level for the first calendar year. UNEP points out that one calendar year above 1.5°C does not by itself mean that the long-term 1.5°C goal has been crossed. Scientists use longer periods to assess that goal.
2026: UNEP publishes Limiting Overshoot, examining how to limit warming beyond 1.5°C, keep the temperature peak as low as possible, shorten the period above 1.5°C and eventually bring temperatures down.
Q&A
1. What does “overshoot” in the report’s title mean?
UNEP uses the term “overshoot” for a period when global warming goes above 1.5°C and temperatures later come down. It is not simply the moment when the temperature crosses 1.5°C.
The report describes four stages: temperatures first rise above 1.5°C, then reach a peak, begin to decline and eventually stabilize. The height of the peak and the length of time spent above 1.5°C both affect the risks to people and nature.
2. Why is going beyond 1.5°C now expected?
UNEP says going beyond 1.5°C is now widely expected under current policies and other near-term pathways. But this does not mean that the amount of warming beyond 1.5°C is already decided.
The report looks at what can still be done to limit the rise. The aim is to keep the temperature peak as low as possible, spend as little time as possible above 1.5°C and then bring temperatures down.
3. How high could temperatures rise?
UNEP says that even if current climate commitments and policies are implemented, warming is projected to reach at least 1.8°C. Other pathways could produce higher temperature peaks.
This difference matters because the risks increase as temperatures rise. UNEP therefore says the temperature peak should be kept below 2°C, if possible, while the time spent above 1.5°C should also be kept as short as possible.
4. If warming approaches 2°C, how soon could that happen?
The IPCC says the world could cross the 2°C warming level as early as the 2030s or 2040s, depending on how quickly greenhouse gas emissions are cut.
Under a very high-emissions scenario, the 2°C level could be crossed around 2032–2051. With somewhat lower emissions, it could be crossed around 2037–2056 or 2043–2062.
If emissions remain relatively high, warming could be much higher by the end of the century. Under an intermediate emissions scenario, the IPCC projects 2.7°C of warming by 2081–2100, with a likely range of 2.1°C to 3.5°C, compared with 1850–1900.
The message is simple: the faster emissions are cut, the lower the warming and the later the 2°C level is likely to be reached.
5. Why is keeping the peak below 2°C important?
The higher the temperature peak, the greater the risks to people and nature. Higher warming means greater risks from heat, extreme weather, damage to ecosystems and other climate impacts.
A higher peak also increases the chance of crossing thresholds beyond which some changes may be difficult or impossible to reverse. This is why UNEP looks at both the highest temperature reached and the length of time temperatures remain above 1.5°C.
6. Why does the length of time above 1.5°C matter?
The temperature itself is only part of the issue. The amount of time spent above 1.5°C also affects the risks.
UNEP says a longer period of overshoot increases the chance of irreversible changes and of reaching limits to adaptation. A shorter period means less time for people, ecosystems and infrastructure to be exposed to higher temperatures and the risks that come with them.
7. What happens as warming rises above 1.5°C?
UNEP says climate impacts become more severe as warming increases. Some impacts can also increase disproportionately as warming rises, so the risks do not rise in exactly the same way for every additional fraction of a degree.
The impacts include more severe heat and extreme weather, damage to ecosystems and greater risks to human health, food and water supplies. Cities, infrastructure and economies can also face greater damage as warming increases.
8. What are the risks to ecosystems?
Higher temperatures can cause major changes in natural systems, some of which may not be reversible. UNEP points to risks such as damage or dieback of the Amazon rainforest, thawing of permafrost and destabilisation of major ice sheets.
The report also discusses the possible disruption of the Atlantic Meridional Overturning Circulation (AMOC). This is a large system of ocean currents that moves warm and cold water around the planet. A major change in this system could affect climate patterns.
9. What could happen to food and water supplies?
Higher temperatures, changes in rainfall, drought and other climate impacts can put pressure on food and water supplies. UNEP says global food production could decline by up to 14 per cent by 2050 without effective adaptation.
Changes in water availability can also affect farming and the natural systems on which food production depends. These effects can occur along with heat and other climate stresses.
10. What are “tipping points”?
A tipping point is a point after which a major change can become difficult to stop or reverse. UNEP discusses possible tipping points involving the Amazon rainforest, permafrost and major ice sheets.
The report also points to the possible disruption of the AMOC as a tipping risk. These risks are one reason UNEP says it is important to keep both the temperature peak and the period of overshoot as low as possible.
11. What does adaptation mean?
Adaptation means preparing people, infrastructure and natural systems for climate impacts and reducing the damage they may suffer.
It includes preparing for impacts that are already occurring, as well as risks that are expected but whose precise timing or severity is uncertain. But UNEP says adaptation has limits. As warming increases, some impacts can become too severe to adapt to, while some changes may be irreversible.
12. Why do emissions cuts and adaptation have to happen together?
Cutting greenhouse gas emissions limits further warming. Adaptation helps people and natural systems deal with the impacts that cannot be avoided.
UNEP says both are needed. Cutting emissions alone cannot prevent all near-term climate impacts, while adaptation alone cannot stop temperatures from continuing to rise. If warming becomes higher and lasts longer, some impacts can eventually go beyond the ability of people and ecosystems to adapt.
13. What happens after temperatures reach their peak?
After the temperature reaches its peak, the overshoot pathway enters a decline phase. Temperatures can eventually fall if emissions are reduced to net-negative levels, with carbon dioxide removal playing an important role.
But the effects of earlier warming do not disappear when temperatures begin to fall. Different parts of the climate system respond at different speeds, so some changes can continue for a long time.
14. What is net-zero?
Net-zero greenhouse gas emissions means that human-caused greenhouse gas emissions are balanced by removals.
UNEP says reaching net-zero can halt further warming. But bringing temperatures back below 1.5°C would require sustained net-negative emissions — removing more carbon dioxide from the atmosphere than is being released.
15. What is Carbon Dioxide Removal (CDR)?
Carbon Dioxide Removal (CDR) means taking carbon dioxide out of the atmosphere and storing it.
UNEP divides CDR into conventional and newer technological methods. Conventional methods include afforestation and improved soil management that increase carbon storage in ecosystems. Newer methods include Direct Air Capture, which uses technology to remove carbon dioxide directly from the air, along with other chemical systems.
16. How much more carbon removal would be needed?
UNEP says the amount of carbon dioxide currently being removed from the atmosphere would need to triple, and possibly quadruple, by 2050 to meaningfully contribute to limiting warming.
This larger amount of removal would be needed as part of a longer-term effort to bring temperatures down after they peak. But UNEP does not present carbon removal as a substitute for cutting emissions. Emissions still need to fall rapidly.
17. What are the limits and costs of carbon removal?
Conventional carbon removal methods such as planting trees require land. That land may also be needed for farming and other uses. Carbon stored in forests and soil can also be released again by heat, drought and wildfires.
Novel technological CDR methods are not yet proven at the scale that would be required and can be expensive. UNEP says some of these methods can cost up to US$600 for every tonne of CO₂ removed.
18. How much can carbon removal actually reduce warming?
UNEP says computer models show that Carbon Dioxide Removal could help reverse a few tenths of a degree of warming this century.
That can make a useful contribution to bringing temperatures down, but it cannot do the whole job. UNEP says rapid cuts in carbon dioxide, methane and other greenhouse gas emissions are still necessary. Carbon removal is therefore part of the longer-term pathway, not a replacement for cutting emissions.
19. Will the world return to normal if temperatures fall below 1.5°C?
Not necessarily. Bringing global temperatures below 1.5°C would not automatically undo all the changes caused by the period of higher warming.
Sea levels can continue to rise, ice can continue to melt and ecosystems can continue to change. Species may be lost, while food and water systems may also remain affected. Oceans, ice sheets and ecosystems respond slowly and can take decades or much longer to adjust.
UNEP also warns that some losses and changes may be permanent even if temperatures eventually return below 1.5°C. This means that returning to a lower global temperature does not mean returning every part of the climate system to its earlier condition.
20. What does UNEP say is needed to limit the overshoot?
UNEP says three things are needed. First, greenhouse gas emissions must fall quickly and stay low to keep the temperature rise as small as possible. Second, people and countries must continue preparing for climate impacts that cannot be avoided. Third, over time, sustained net-negative emissions, supported by carbon dioxide removal, will be needed to bring temperatures down.
The aim is to keep the temperature peak low, spend as little time as possible above 1.5°C and then bring temperatures down. UNEP also makes clear that removing carbon dioxide cannot replace cutting emissions. Both are needed.
