As United Nations climate talks began on June 8 in Bonn, Germany, government representatives met with scientists to examine a question that is rapidly moving from theory to political reality: What happens to the world’s glaciers, ice sheets and oceans if global temperatures exceed 1.5°C and then return to lower levels?
The annual June climate negotiations, known as the Subsidiary Body (SB) meetings, serve as the primary mid-year negotiation session within the U.N. framework as countries prepare for the Conference of the Parties (COP) each fall. Climate negotiators meet to discuss issues ranging from mitigation and adaptation to finance, technology and capacity-building, while preparing decisions that may later be adopted at COP31 in Antalya, Türkiye this November.
The SB meetings are more than a technical bridge between COPs. As Maria Antonia Tigre, director of global climate change litigation at the Sabin Center, an affiliated center of the Columbia Climate School, told GlacierHub, they are “where the legal architecture for accountability actually gets negotiated,” serving as a key forum for advancing international climate action.
Increasingly, conversations at these negotiations look ahead to the impacts of exceeding 1.5°C above pre-industrial levels, the lower temperature limit of the tightly negotiated 2015 Paris Agreement. Despite continued efforts, current national commitments place the world on track for 2.7 to 2.9°C of warming by 2100.
In a U.N. side event organized by the International Cryosphere Climate Initiative, researchers and experts discussed how exceeding 1.5°C will bring irreversible damage from the loss of global snow and ice regions. The scale of that loss depends on both how long temperatures stay above 1.5°C and just how high they rise. Speakers also offered an early preview of scientific findings from a UNEP Overshoot Spotlight Report to be released later this year, and a companion Cryosphere and Overshoot Report to come soon after.
“For Iceland, changes in the Antarctic ice sheet and the North Atlantic Ocean are not a distant environmental issue. They are a matter of national security,” said Helga Barðadóttir, Iceland’s lead negotiator at the meetings. She raised growing concerns over the possible collapse of the Atlantic Meridional Overturning Circulation (AMOC)—a major system of ocean currents that circulates water within the Atlantic Ocean—whose weakening could trigger cascading impacts on climate stability.
The scientific presentations at the side event illustrated her point. From Antarctic sea ice to mountain glaciers and freshwater supplies, speakers described how the cryosphere is already responding to warming, with losses that will continue long after global temperatures peak.
All emissions pathways now considered plausible involve at least a temporary exceedance of 1.5°C, which overshoots the lower temperature limit of the Paris Agreement, raising urgent questions about which impacts can be reversed and which will persist for centuries or tens of thousands of years, even if temperatures eventually fall.
Florence Colleoni of the Scientific Committee on Antarctic Research outlined the implications of overshoot for the Greenland and Antarctic ice sheets, which can lock in multi-meter sea-level rise. The best way to minimize sea-level rise from the two ice sheets is minimizing scale and duration of overshoot.
“We are now seeing a dramatic loss of Antarctic sea ice,” said Petra Heil, science director of the British Antarctic Survey, describing record-low sea ice extent in addition to intensifying marine heatwaves, ocean freshening and acidification across polar regions. Those changes are occurring alongside growing concerns about the weakening of the AMOC, which helps regulate climate across Europe and the North Atlantic.
Heil also highlighted mounting evidence that disruptions to ocean circulation could have far-reaching consequences for weather patterns, marine ecosystems and economies across the planet, not just the North Atlantic.
The impacts of overshoot are perhaps most visible in the world’s glaciers. Lilian Schuster of the University of Innsbruck presented modelling showing that glacier losses continue long after temperatures have peaked. Glaciers will respond to even brief overshoot for hundreds of years.
Comparing warming pathways, Schuster demonstrated that every tenth of a degree matters, particularly in vulnerable regions such as the European Alps, the Caucasus and Central Asia. In the European Alps alone, roughly 40% of glacier ice has already been lost in recent decades. Under current warming levels, around half of the glacier mass present in 2020 is expected to disappear. Under higher overshoot scenarios peaking at 3°C, many glaciers in the region vanish entirely. Even when temperatures later decline, much of the lost ice does not return.
Limiting warming to lower temperatures slows glacier loss and provides much-needed time for adaptation.
Yet the discussion was not solely one of irreversible decline.
Bill Hare, CEO and founder of Climate Analytics, shared how global temperatures could still be brought back below 1.5°C by the end of the century through a combination of rapid fossil-fuel phaseout, accelerated electrification, steep methane reductions and large-scale carbon dioxide removal. Under these “highest possible ambition” pathways, warming peaks at roughly 1.7°C before falling to around 1.2°C by 2100.
But Hare cautioned against interpreting that possibility as a reason for complacency. “Overshoot is dangerous,” he said. “It’s dangerous for the cryosphere, dangerous for many places in the biosphere and dangerous for human development.” Current warming trajectories will dramatically accelerate glacier loss and sea-level rise, reshaping coastlines, water systems and ecosystems for centuries to come.
In a policy panel, Ambassador Dinara Kemelova of the Kyrgyz Republic warned that glacier loss is rapidly becoming a development challenge for Central Asia, citing projections that more than half of the region’s glaciers could disappear by mid-century. Delegates from coastal nations, including Bangladesh, emphasized the growing risks posed by accelerating sea-level rise and loss and damage.
A recurring theme was that climate impacts once viewed as future risks are increasingly becoming present realities. As UNEP’s Mirey Atallah summarized: “There is no such thing as benign overshoot. Any degree of overshoot brings irreversible risks.”
Overshoot would bring consequences that extend beyond physical hazards. “The human and social consequences of overshoot, including displacement, food insecurity, mounting strain on humanitarian systems, remain far less understood, even as they unfold on equally irreversible, multi-decadal timelines,” Joshua Fisher, director of the Advanced Consortium on Cooperation, Conflict and Complexity at the Columbia Climate School, told GlacierHub. “Policymakers need parallel investment in understanding and preparing for social risks and appropriate governance infrastructure so that climate action addresses both the physical and natural systems that are jeopardized.”
The importance of the 1.5°C limit was reaffirmed in 2025 when the International Court of Justice ruled that countries are legally obligated to pursue the highest possible ambition to keep global warming within 1.5°C. The historic verdict established that weak national commitments to emissions reductions breach international law.
Overshoot is becoming a central theme for the science and policy world in 2026. It is expected to feature prominently in discussions at COP31 this November. Earth’s frozen regions do not respond on political timescales; they lock in major loss for future generations. These impacts will continue for centuries, with irreversible changes unfolding as temperatures tick higher. The opportunity to change course has not yet disappeared. For the scientists and policymakers gathered in Bonn, the challenge they face today is raising climate action to match the scale and magnitude of the climate crisis.
Amy Imdieke is global outreach director at the International Cryosphere Climate Initiative. Her work in science communication helps policymakers at all levels of government, as well as across U.N. processes, understand the urgency of rapid emissions reductions to limit the global impacts of snow and ice loss.
