PERITO MORENO GLACIER, Argentina — A deep cracking noise from within the ice warns of an impending dramatic collapse. Moments later, a massive ice block about 70 meters tall — roughly the height of a 20-story building — breaks away from the face of the Perito Moreno glacier and plunges into the clear turquoise water below.

This spectacular event has drawn visitors to Argentina’s most famous glacier for years. Tourists stand on viewing platforms, eagerly awaiting the next crack to echo through the crisp Patagonian air. However, recently the size of the ice chunks breaking off—a process known as "calving"—has begun to raise concerns among local guides and glaciologists. These experts are already worried about a gradual retreat of Perito Moreno, which had previously defied the global trend by maintaining its mass even as rising temperatures accelerated glacial melting elsewhere.

"Ice calving events this large have been uncommon at Perito Moreno over the past two decades," said Pablo Quinteros, an official tour guide at Los Glaciares National Park in Santa Cruz province. "Only in the last four to six years have we started to witness icebergs this enormous," he told Reuters during an April visit. The glacier’s front, flowing from the Andes down to Lake Argentina, had remained relatively stable for decades, sometimes advancing, sometimes retreating. But over the past five years, the retreat has become more noticeable.

"For about 80 years, it stayed mostly in the same position, which is unusual," explained Argentine glaciologist Lucas Ruiz of Conicet, a state science agency. Ruiz studies the future of Patagonian glaciers amid climate change. "Since 2020, though, we’ve observed signs of retreat in parts of Perito Moreno’s face." While the glacier might recover as it has before, it is currently losing between one and two meters of water equivalent annually. If this trend continues, the rate of loss could accelerate.

A 2024 government-backed report co-authored by Ruiz, presented to Argentina’s Congress, indicated that although Perito Moreno’s overall mass has been stable for fifty years, since 2015 it has experienced its fastest and longest-lasting decline in 47 years, losing an average of 0.85 meters per year. Worldwide, glaciers are vanishing faster than ever, with the last three years marking the greatest glacial mass loss on record, according to a UNESCO report released in March.

"You can’t fully grasp the scale of it," said Ruiz. His team’s monitoring instruments have recorded a temperature rise of about 0.06 degrees Celsius per decade in the region and decreasing precipitation, which means less snow and ice accumulation. "What’s unique about Perito Moreno is that it took some time to show the effects of climate change," Ruiz noted. Now, the glacier’s ice accumulation at the top is being outpaced by melting and calving at the base.

"The balance has shifted. The glacier is losing both thickness and area." Despite these changes, the glacier remains a breathtaking attraction. Visitors board boats to witness the calving events and the huge icebergs drifting on the lake up close. "It’s insane. The most incredible sight I’ve ever experienced," said Brazilian tourist Giovanna Machado aboard one of the boats, which must navigate carefully around sudden ice falls. "Even in photos, you can’t truly appreciate its vastness. It’s perfect, amazing. Everyone should see this at least once in their lifetime."