Glaciers in nearly one-third of UNESCO World Heritage sites will vanish in less than 30 years, a report by the UN organization warned Thursday. In addition to their obvious grandeur and beauty, glacier loss can cause serious water supply disruptions for millions who rely on their steady meltwaters. “They provide water resources to at least half of humanity,” lead author of the report Tales Carvalho Resende told the Washington Post.

“As water melts, this water will accumulate in what we call glacial lakes; and as water comes, these glacial lakes might burst,” he told CNN. “And this outburst can create catastrophic floods, which is something we can see very recently in Pakistan.” More hopefully, however, the report says swift and ambitious action to limit global warming to 1.5°C (2.7°F) can still save glaciers in most UNESCO sites.

Among the glaciers that will be lost by 2050 are all of Africa’s last remaining glaciers, including those on Mount Kenya and Mount Kilimanjaro, as well as glaciers in Yellowstone and Yosemite National Parks. The report comes just ahead of COP27, where the issue of how wealthy nations should compensate developing nations for the losses and damages caused by climate change will be a major topic of negotiation. (CNN, Washington Post $, NPR, Axios, CBS, The Hill, Gizmodo; Climate Signals background: Glacier and ice sheet melt)