WHAT THE SCIENTISTS SAY

The trees
that lean.

In parts of Alaska and Siberia, whole stands of spruce tilt in all directions at once.

Scientists call them drunken forests. The cause lies underground: ice-rich frozen soil that has started to thaw. This independent guide explains, from university and NASA sources, what permafrost is, how a thaw slump forms, and why the same process bends roads and cracks houses.

Questions this page answers

What is a drunken forest?
What is permafrost?
What is a thermokarst?
How much of the world has permafrost?
Can a tilted tree straighten up again?
Choose a type of ground

A boreal forest of thin black spruce trees leaning at different angles across boggy ground under a pale northern sky — AI-generated image, not a photograph of a real person or place.

Ice holds the ground
Thaw makes it sag
Shallow roots tip over
A slow change you can see from the road

THE SHORT ANSWER

It's the ground,
not the trees.

Trees grow upright on frozen soil for decades.
When the ice below melts, the surface sags and the trees go with it.

THE LOOK

Leaning every which way

The Geophysical Institute in Alaska describes drunken forests of black spruce that tilt in all directions. Melting permafrost is the most common cause.

Check the ground

THE CAUSE

A slump in the soil

When ice-rich permafrost thaws, the ground surface sags into a depression called a thermokarst. Trees with wide, shallow roots bow towards it.

See the numbers

THE KNOCK-ON

Roads and houses tilt too

NASA says frozen permafrost is harder than concrete, but thawing ground can damage houses and roads as it shifts. In interior Alaska, roads dip into a familiar roller coaster.

How it unfolds

A QUICK GROUND CHECK

How long does
it stay frozen?

Pick a type of frozen ground to see how scientists define it. The bar shows how long the ground stays frozen compared with the others.

Frozen ground check

Choose a type of groundIntermittently frozen groundSeasonally frozen groundThe active layerPermafrostAncient deep permafrost

Intermittently frozen ground1–15 days a year

Near-surface soil that freezes for between one and 15 days a year.

Seasonally frozen groundOver 15 days a year

Freezes for more than 15 days a year. NSIDC says about half of the Northern Hemisphere's land surface is covered by it.

The active layerThaws each summer

The top layer above permafrost, which thaws in summer and refreezes in autumn. NASA says it can be just 10 to 15 cm thick in the coldest places and several metres in warmer ones.

Permafrost2 years or more

Soil, sand, sediment or rock that stays at or below 0 °C for at least two years. It is defined by temperature alone, and can be over 30 percent ice.

Ancient deep permafrostThousands of years

Much permafrost has existed for many thousands of years. It ranges from under 1 metre to more than 1,500 metres thick.

Sources: NSIDC, Frozen ground and permafrost; NASA Science, What is permafrost?

Definitions from the US National Snow and Ice Data Center (NSIDC) and NASA.

TWO NUMBERS WORTH KNOWING

How much frozen ground,
and how slow the tilt.

One number for the whole hemisphere,
one for a single spruce tree.

≈ 25%

Exposed Northern Hemisphere land on permafrost

About 23 million square kilometres, roughly 65 percent of it in Eurasia and 35 percent in North America and Greenland. It reaches as far north as 84°N in Greenland and as far south as 26°N in the Himalaya.

EXTENT · NSIDC

≈ 50 years

Years trees may grow normally first

Spruce can grow upright for perhaps 50 years before the permafrost warms enough to thaw and form a thermokarst. One curved spruce began righting itself after a slump formed 120 years earlier.

TIMING · GEOPHYSICAL INSTITUTE, ALASKA

Estimates of permafrost extent depend on definition and method: NASA's education pages give a figure of around 15 percent of Northern Hemisphere land. Both describe a very large frozen area.

FOUR STEPS TO A DRUNKEN FOREST

From frozen floor
to tipsy trees.

The process the Geophysical Institute describes takes decades, not days.

Seeds land on frozen ground
Spruce germinate on a surface that looks solid, because ice-rich permafrost lies just below.
Roots spread wide and shallow
With frozen soil underneath, trees adapt wide, shallow root systems to hold on to the thin soil above the permafrost.
The ice thaws and the ground sags
Warming, disturbance or even a well-used animal trail can let the permafrost thaw. The surface slumps into a thermokarst.
Trees tilt, and sometimes recover
Trees bow towards the new hollows. Some grow back towards the sky, leaving a curved trunk that records when the ground moved.

A gravel road running through northern spruce forest, its surface rising and dipping in gentle waves — AI-generated image, not a photograph of a real person or place.

WHY IT MATTERS BEYOND THE FOREST

Carbon in
the freezer.

A sunken, water-filled hollow in a spruce forest, with the trees around its edge tipping inwards — AI-generated image, not a photograph of a real person or place.

NASA explains that permafrost soils near the surface hold large amounts of organic carbon from dead plants that could not rot in the cold. When the ground thaws, microbes start breaking it down and release carbon dioxide and methane, which warm the planet further.

That is why scientists watch permafrost from space as well as on the ground. NASA's SMAP satellite measures moisture in the top 5 cm of soil everywhere on Earth and can tell whether that water is frozen or thawed.

Questions

Questions people ask

What is a drunken forest?

A stand of trees, often black spruce, that lean in all directions. The Geophysical Institute at the University of Alaska Fairbanks says melting permafrost is the most common cause, though landslides and earthquakes can also create one.

What is permafrost?

Ground, including soil, sand, sediment or rock, that stays at or below 0 °C for at least two years. It is defined by temperature alone and can contain more than 30 percent ice.

What is a thermokarst?

The scientific name for a ground slump caused by melting permafrost. When ice-rich permafrost thaws, the surface sags and nearby trees tilt towards the depression.

How much of the world has permafrost?

NSIDC estimates about 23 million square kilometres, or about 25 percent of exposed land in the Northern Hemisphere. Most of it lies north of 60°N in Russia, Canada and northern Alaska.

Can a tilted tree straighten up again?

Sometimes. Researchers in Alaska found a spruce with a bow-shaped trunk whose rings showed it began righting itself after a thermokarst formed 120 years earlier.

Sources

The trees
that lean.