Keeping peak global warming to around 1.8° Celsius may be necessary if the world is to retain a realistic prospect of returning to the Paris Agreement’s 1.5° C. limit this century, according to a major new assessment that sharply narrows the remaining room for delay.
The U.N. Environment Program report concludes that exceeding 1.5° is now expected, likely within the next few years. But how far temperatures rise beyond that threshold could determine whether bringing them back down remains achievable.
Most pathways capable of returning warming to 1.5° by 2100 reach a peak of no more than about 1.8° to 1.9°, the report finds. Because of delays in cutting global emissions, recent studies put the lowest peak warming now considered feasible at around 1.7° to 1.8°.
“Humanity is speeding past the limit to avoid climate catastrophe,” U.N. Secretary-General António Guterres said as the report was released Wednesday. “But we can still bring warming back below 1.5° by the end of the century.”
Current government commitments already leave little margin for keeping peak warming near 1.8°. Even an optimistic scenario in which countries fully implement their national climate plans and additional net zero pledges puts expected peak warming at around 1.8°. Under existing policies, warming is projected to reach about 2.6° by 2100.
The findings change the practical challenge facing international climate policy. For years, negotiations have been organized around preventing warming from exceeding 1.5°. Governments must now consider how to limit a period above that threshold while preventing temperatures from rising so far that returning becomes increasingly difficult.
“It changes the pathway: from avoiding exceedance altogether to limiting how high and how long temperatures rise above 1.5° while preserving the possibility of returning below it,” the report says.
The 1.5° goal emerged from the 2015 Paris Agreement, which committed governments to hold warming well below 2° while pursuing efforts to limit it to 1.5°. The lower threshold became particularly important to countries highly vulnerable to climate change, including small island nations.
Exceeding it does not mean that 1.5° ceases to matter, UNEP argues. Instead, the objective becomes limiting both the magnitude and duration of what climate scientists call “overshoot.”
“There is no benign overshoot pathway, but its magnitude, duration and consequences are not predetermined,” the report says. Every fraction of additional warming and every year spent above 1.5° increases exposure to climate hazards and the likelihood of irreversible losses.
That leaves governments confronting three problems simultaneously: rapidly reducing emissions to limit peak warming, adapting societies and economies to more severe climate impacts, and eventually removing enough carbon dioxide from the atmosphere to reduce global temperatures.
The last task may prove particularly difficult. Returning from an overshoot requires sustained net-negative carbon dioxide emissions — meaning more CO₂ must be removed from the atmosphere than continues to be emitted.
Governments must speed and deepen emissions cuts
UNEP estimates that about 220 billion metric tons of net-negative CO₂ emissions would be needed to lower global temperatures by around 0.1°. Even if net removals eventually reached 10 billion metric tons annually, roughly a quarter of current global CO₂ emissions, temperatures might decline by only around 0.05° per decade.
Existing methods cannot easily provide removals on that scale. Establishing forests in areas where there was no forest before and forest management currently remove about 2.2 billion metric tons of CO2 annually. Even if fossil fuel emissions fell to zero and deforestation ended, maintaining removals at that rate for about a century would lower temperatures by only around 0.1°, according to the report.
Technological approaches involving geological carbon storage face their own constraints. UNEP warns that maintaining net zero while large residual CO₂ emissions continue could eventually consume estimated geological storage capacity, because those emissions would have to be continually offset by removals.
Overall, the report concludes that even optimistic expansion of carbon removal is unlikely to reverse warming by more than a few tenths of a degree this century. That makes the height of the temperature peak crucial.
“Beyond around 1.8°, decline to 1.5° during the 21st century becomes increasingly challenging,” the report says. Carbon removal can credibly contribute to a return below 1.5° only if peak warming remains well below 2° and remaining emissions of carbon dioxide, methane and other greenhouse gases are also reduced.
The report rejects using future carbon removal as a substitute for immediate emissions cuts. The higher temperatures rise before net zero is reached, the greater the subsequent removal burden — and the greater the risk that the necessary capacity will not exist. It also changes the significance of net zero.
In an overshoot world, reaching net zero would no longer be the endpoint of climate policy. It would mark the point at which warming could stop rising — and the much slower task of trying to bring temperatures back down could begin. Governments would need policy frameworks and institutions capable of managing carbon removal obligations and investment over periods extending well beyond conventional political cycles.
The implications of overshoot cannot be reversed simply by cooling the atmosphere later. Sea levels will continue rising, some ecosystems could cross irreversible thresholds and communities may be forced to relocate. UNEP says many countries and communities would face “irreversible losses and permanently changed conditions” even if warming eventually returns below 1.5°.
For small island developing nations, that could include territory becoming uninhabitable or submerged and unresolved questions over statehood and maritime rights. UNEP asks what international arrangements will be needed to ensure continued recognition of their sovereignty and maritime entitlements as those changes occur.
