The world is likely to cross the 1.5°C long-term global warming threshold within the next few years but returning below it remains possible. Every fraction of a degree and every year spent above it will determine how much damage is done, according to a new United Nations Environment Programme (Unep) report.
Even under an optimistic scenario in which governments fully implement their climate plans and meet additional net-zero commitments, global warming is expected to peak at about 1.8°C.
Unep’s Limiting Overshoot: Navigating Exceedance of 1.5°C and Pathways Towards Return report says the world has entered a new phase of the climate crisis.
The focus can no longer be solely on preventing 1.5°C from being breached. Governments must prepare for the possibility of temperatures exceeding the threshold, limit how far warming rises, adapt to the resulting impacts and ultimately bring temperatures back below 1.5°C.
“The global goal remains limiting long-term average warming to 1.5°C,” the report says.
But if the threshold is temporarily exceeded, it must be approached “from above”, while governments simultaneously accelerate efforts to cut emissions and adapt to climate impacts.
Unep says an “overshoot, peak and decline” pathway is the best remaining option. Under this scenario, global temperatures would be pushed to the lowest possible peak before declining towards 1.5°C by the end of the century.
The agency stresses that this is not an acceptable alternative to avoiding 1.5°C altogether but rather, the least damaging pathway left if the threshold is breached. The higher temperatures rise and the longer they remain above 1.5°C, the greater the risks of long-lasting and irreversible damage.
“The duration and severity” of the overshoot will depend largely on how quickly emissions are cut and how rapidly countries strengthen their ability to adapt, the report says.
Reducing methane and other short-lived climate pollutants will be particularly important in the immediate term, while adaptation will require greater investment, stronger institutions and better access to technology and finance.
The higher temperatures rise and the longer they remain above 1.5°C, the greater the risks of long-lasting and irreversible damage.
Even if the world succeeds in bringing warming back below 1.5°C, Unep warns that this will not undo all the consequences of overshoot.
Risks include faster sea-level rise, a greater chance of ecosystem collapse, including coral reefs, more extreme heat and wildfires and profound changes to the cryosphere: ice sheets, sea ice, glaciers, permafrost and snow.
Glaciers, for example, could lose more than a quarter of their mass by 2100, potentially contributing 9cm to 12.5cm to sea-level rise under warming of up to 3°C and permanently altering water availability in many places.
There is also a greater risk of irreversible tipping points involving systems such as the West Antarctic and Greenland ice sheets, the Atlantic Meridional Overturning Circulation and the Amazon rainforest, changes that Unep says could reshape the world permanently.
While there is uncertainty over the timing and severity of some of the impacts and tipping points, their consequences will be felt by people, societies and economies around the world, particularly in Small Island Developing States and low-lying cities, some of which face partial or complete submersion.
Food security and water supplies will come under increasing pressure. Without effective adaptation, global food production could decline by up to 14% by 2050, the report says.
More frequent and intense heatwaves, nutrition-related diseases and greater risks of pathogen spillover will increase illness and deaths.
Cities and infrastructure will face greater risks from prolonged heat and flooding, while warmer, more acidic and increasingly oxygen-depleted oceans will put further pressure on marine life and the communities that depend on it.
Rising energy demand, higher consumer costs and growing stress on insurance and financial systems will create further risks for countries.
For Africa, the challenge is particularly acute. Unep identifies the continent among the regions facing some of the most severe adaptation gaps, with many vulnerable countries lacking the resources and capacity needed to prepare for escalating climate impacts.
Africa, too, faces a fundamental problem in planning for a changing climate: there are too few long-term observations to produce reliable climate projections.
Across the continent, there are only about 200 long-term temperature stations and 800 monthly rainfall stations, compared with roughly 5 000 rainfall stations in the US alone, the report says.
The impacts on Africa’s ecosystems are varied but the overall direction is clear.
Half of Africa’s mountain plant species are projected to experience contractions in their ranges even under more sustainable development pathways, rising to three quarters under high-emissions scenarios.
Climate shifts at higher elevations could occur at up to three times the global average, potentially outpacing the ability of many species to move into suitable habitats.
Southern African savannahs and grasslands are also experiencing the expansion of woody vegetation, with knock-on effects for rural ecosystem services.
In tropical oceans, species richness around the equator has been declining since at least the 1970s, while subtropical areas have gained species. Tropical marine biomass is projected to decline more broadly.
Many of the ecological changes could persist even after temperatures begin to fall.
Species displaced by changing conditions may not readily return, while some losses could be effectively irreversible on timescales relevant to policy and conservation, the report says.
Climate change will also increasingly challenge the effectiveness of protected areas.
Under 2°C of warming, the report projects that 24% of climates found in protected areas globally could disappear from the network, while 36% of protected areas could develop climates that have not previously been protected anywhere.
Climate change could increasingly push species beyond the borders of the protected areas and countries where they are conserved. In Africa, more than half of protected-area climate equivalents are projected to cross national borders, creating new challenges for conservation.
The prospect of overshoot has also raised concern among climate-vulnerable countries and campaigners, who fear that accepting a temporary breach of 1.5°C could become an excuse for weaker action.
For the Alliance of Small Island States, 1.5°C must remain the “North Star”. The group has warned that overshoot cannot justify inaction, particularly when vulnerable island states are experiencing severe climate impacts.
Climate Analytics has similarly warned that the language of overshoot could become “a call to apathy rather than a call to arms” if it distracts from the need for a rapid phase-out of fossil fuels and stronger action on mitigation, adaptation and loss and damage.
Environmental campaign group 350.org put the choice more starkly: “Delay is not a neutral act, it is a decision.”
Unep’s report makes a similar argument. “Every fraction of a degree matters,” it says, warning that shortening the period of overshoot reduces exposure to climate risks, while every investment in resilience helps preserve options for the future.
“Rapid, collective and global action is vital,” Unep says, calling for stronger political will, international cooperation, finance, technology transfer and capacity-building.