Global action to curb emissions has not been enough to avoid a rise in temperature exceeding 1.5°C above pre-industrial levels "in the next few years", and governments must plan for an "overshoot, peak and decline pathway", the UN Environment Programme (Unep) said today.
"The policy challenge has shifted from avoiding overshoot to navigating it", Unep said in its Limiting overshoot report.
The Paris climate agreement seeks to curb the global rise in temperature to "well below" 2°C above the pre-industrial era and pursues a 1.5°C limit. This remains the "central benchmark for climate ambition", Unep said.
Under current policies, the world is on track for a rise in temperature of 2.6°C above pre-industrial levels by 2100, and has already warmed by around 1.4°C. The most optimistic scenario, which would require full implementation of all countries' climate plans, and meeting additional net-zero targets, puts an expected peak temperature increase at 1.8°C, Unep said.
Despite the "inevitable" overshoot of the 1.5°C target, "the goal remains limiting warming to this level… the goal must now be approached from above while the world adapts to intensifying climate impacts", Unep executive director Inger Andersen said.
If "unprecedented action" was taken globally, it would be possible to return to a 1.5°C limit, the report found. An overshoot, peak and decline pathway would require immediate and sustained cuts to greenhouse gas (GHG) emissions and the use of carbon removals to offset remaining GHGs. Alongside this, action to adjust to current and expected effects of climate change — known as adaptation — would be needed, Unep said.
"Net-zero is a necessary milestone to returning below 1.5°C", the report found. Governments and businesses should avoid "carbon lock-in" such as power and transport reliant on fossil fuels, and ramp up renewables, it said.
Carbon dioxide removals (CDR) will be needed as well as emissions reductions, but the technology has limits, the report noted. CDR "can only contribute credibly to a return to below 1.5°C within the 21st century if peak warming remains well below 2°C", it found. Climate change will curb CO2 removal and retention rates for some nature-based forms of CDR. And "even optimistic CDR scaling is unlikely to deliver a temperature reversal beyond a few tenths of a degree this century", Unep said.
Some impacts will continue, even if temperatures are stabilised, and some will be irreversible, Unep said.
"Sea levels will continue to rise for centuries in response to past warming. Overshoot pathways commit the world to long-term increases in coastal water levels", the report found.
Reversing global warming is possible but the pace is uncertain and constrained by earth system responses, Unep said. These include tipping points — a critical threshold beyond which a system reorganises. Extreme weather is intensifying and "limits to adaptation are being reached", it said.
Governments and institutions should also take into account that many existing planning assumptions "no longer hold", the report found. But "limiting the magnitude, duration and consequences of overshoot, while preserving the possibility of bringing temperatures back down, is the best remaining option", Andersen said.
