The world is set to blow past the 1.5 degree Celsius warming limit that countries agreed to in the 2015 Paris climate accord. In a report released this week, the United Nations Environment Programme said overshoot is now unavoidable given current emissions trends, and it laid out the first detailed UN plan for how to come back below the threshold.
Overshoot, peak and decline
The report, titled Limiting Overshoot, describes an "overshoot, peak and decline" pathway as the best remaining option. Under this approach, countries would cut greenhouse gas emissions sharply to reach net zero, then move toward net-negative emissions, removing more carbon from the air than they add. That would eventually pull global temperatures back down. "The challenge has shifted from avoiding overshoot to navigating it," said Richard Betts, a report co-author from the University of Exeter. UN Secretary-General Antonio Guterres was direct: "We must make the overshoot above 1.5 degrees as small and short as possible."
The costs of going too far
Every extra year spent above 1.5 degrees raises the risk of irreversible damage, the report warns. The likelihood of crossing tipping points grows with the size and length of the overshoot: the destabilization of major ice sheets, the dieback of the Amazon rainforest and the disruption of the Atlantic ocean currents that help regulate the climate. During the overshoot period, more people will die or fall ill from worsening heat and extreme weather. Food production will drop, water shortages will worsen, and coral reefs will face extinction.
Adaptation has limits
The report calls for stronger international cooperation, more climate finance and inclusive governance. It says countries that have contributed most to climate change should act fastest. It also urges governments to prepare for abrupt, non-linear climate risks and to use measures such as nature-based cooling. But UNEP is blunt about the limits of adaptation: even if warming is eventually pulled back below 1.5 degrees, some losses and permanent changes will remain. "If we act now we can limit overshoot," the report says. "Not a safe world, but safer than remaining above 1.5 degrees for the long term."