Residents of Juneau, Alaska, have been urged to evacuate after an overflowing glacial lake sent water surging down the Mendenhall River.

Forecasters said river levels could break a record between 8 a.m. and noon local time on Wednesday, threatening homes and roads in parts of the state capital, which has a population of more than 30,000.

Such floods have been a recurring problem in Juneau since 2011, but recent years have seen record-setting surges as rising temperatures cause glaciers in the area to melt more rapidly. Alaska has warmed faster than the global average, and the fastest of any state, according to the Environmental Protection Agency.

Flooding from the glacial lake in Juneau last August inundated several hundred homes with four to six feet of water, although no deaths or injuries were reported. The city put up a temporary levee along the river in response.

Here’s what to know about these floods.

What’s a glacial lake outburst flood, or GLOF?

As glaciers melt, they tend to retreat uphill, leaving an empty bowl at the bottom of the valley where the ice once sat. Meltwater from the glacier starts pooling in this bowl, and over time a lake forms.

But the sides of the lake are fragile. They might be formed of loose dirt and rock or ice. If one day an avalanche or a landslide occurs, or a piece of a nearby cliff plunges into the water, the disturbance can cause the sides of the lake to collapse. In a flash, most of the lake’s water might cascade down the valley, threatening towns and cities below.

Glacial lake outburst floods can be catastrophic because, by the time the water reaches downstream settlements, it has picked up huge amounts of sediment and boulders along the way, turning it into a thick slurry that can knock down buildings.

In 2023, a GLOF in northern India killed at least 55 people and destroyed a hydropower dam. All in all, 15 million people around the world live within 50 kilometers, or 31 miles, of a glacial lake and less than a kilometer from the potential path of a GLOF, scientists estimated in a 2023 study.

How common are they in Alaska?

The glacial lake that is overflowing this week in Alaska sits at the foot of the Suicide Glacier, an ice mass north of Juneau. Decades ago, the Suicide Glacier flowed into a much larger river of ice, the Mendenhall Glacier. But as the Suicide melts and shrinks, a steep gap has opened up between it and the Mendenhall. This gap is now called Suicide Basin.

(Experts have proposed renaming Suicide Basin to Kʼóox Ḵaadí Basin, which in the Tlingit language translates to “Marten’s Slide Basin.” A marten is a lithe, weasel-like animal found in the area.)

Snowmelt and rain accumulate in the basin, and when the water is high enough, it starts draining through cracks in the Mendenhall Glacier before flooding the Mendenhall River.

The first time this happened was in July 2011, and it took downstream communities by surprise. The basin has since filled and drained at least 39 times, according to the National Weather Service.

Early Wednesday, as the basin drained once more, the Mendenhall River crossed into major flood stage, the National Weather Service said, indicating water levels of above 14 feet. The river was expected to crest at 16.75 feet on Wednesday.

The glaciers in this region are part of the Juneau Ice Field, a sprawling area of interconnected ice that is melting twice as quickly as it did before 2010, scientists reported last year. More of the area’s glaciers are detaching from one another, the researchers also found, which can lead to the formation of lakes like Suicide Basin.

Accelerated melting is producing more water to fill these lakes and hence more water that eventually floods neighborhoods downstream, said Bob McNabb, a glaciologist at Ulster University who has studied the Juneau Ice Field. “As you get more and more melting coming down, that will fill up the basin a bit more each time,” Dr. McNabb said.

How is climate change affecting GLOFs?

The world’s high mountains are warming more quickly than Earth as a whole. That is causing thousands of glaciers to shrink and new lakes to form beneath them. Since 1990, the number, area and volume of glacial lakes around the world have all grown by roughly 50 percent, scientists estimated in a 2020 study.

But bigger lakes don’t directly translate into greater GLOF hazards. Each glacial lake and valley has distinct features that influence how likely it is to burst, and what the consequences would be if it did. So predicting future flood risks is “very complex,” Dr. McNabb said.

In the Mendenhall Valley, for instance, rising temperatures are the reason the Suicide Glacier has withered away and Suicide Basin has formed. But as the planet warms further, the Mendenhall Glacier might melt by so much that the flood threat actually decreases. The reason? There would no longer be enough ice at the side of Suicide Basin to trap large amounts of meltwater. Instead, the water would just empty into the valley gradually.

Scientists in Alaska have predicted that this could come to pass within the next decade or two. Until then, the people of Juneau will continue to live with the dangers from the warming landscape just a few miles to their north.