If you’ve always dreamed of seeing the awe-inspiring glaciers in Yosemite and Yellowstone, you’d better move fast.

A new report from the United Nations warns that the glaciers at the vulnerable World Heritage sites will be gone within decades, almost completely gone by 2050.

The UN report says that they will almost completely disappear without further reductions in greenhouse gas emissions.

But it’s not all bad news. If humanity can limit global temperature increase to 1.5 degrees Celsius (2.7 degrees Fahrenheit – which doesn’t sound like a lot, but is on a climatological scale) above preindustrial levels, two-thirds of the World Heritage glaciers could survive.

That 1.5 degree goal matches the one set by the Paris Agreement, which world leaders signed in 2015 at the UN Framework Convention on Climate Change.

Daniel Farinotti, co-author of the new report and professor of glaciology at ETH Zurich in Switzerland, told Live Science this week, “The changes are rapid and really alarming. If we want to do something about it, we need to reduce global emissions of greenhouse gases, and we need to do that now.”