Climate change is affecting the Earth's frozen ground. Frozen soils help to hold moisture and are impermeable. A thin layer of frozen soil prevents moisture in the layers below from evaporating. Hence frozen ground can help regulate the water cycle.
Frozen ground also affects the way carbon can cycle through an ecosystem. Soil typically releases carbon into the atmosphere; the carbon comes from decaying organic material in the soil. This process releases methane, carbon dioxide, and other greenhouse gases. If frozen soils are cold enough, they will not release carbon so carbon tends to stay trapped in frozen ground. Scientists predict that the quantity of carbon trapped in the frozen ground equals the amount of carbon already contained in the atmosphere. The additional gases could speed up the rate of global warming. Then even more permafrost could thaw, releasing even more gases. This is an example of a positive climate feedback cycle, which is very dangerous for the stability of the climate.
When permafrost thaws, the ice, which is frozen in the soil, converts to liquid water (although the soil itself doesn't melt, it just thaws). Some water remains and forms ponds, while some flows to rivers and oceans. If all the permafrost in the world thawed, it could release enough water to raise global sea levels by 3 cm to 10 cm. This rising sea level is enough that cities along coastlines might have to build walls to keep the sea out, or people would have to move to higher ground.
As the Earth's climate warms, the ground warms up. Permafrost and frozen ground around the world will eventually thaw and vanish. The amount and thickness of seasonally frozen ground would also decrease. As the active layer in the ground becomes thicker, the landscape will change, typically weakening the land. For example, in hilly areas, when the land thaws it can trigger landslides. In Russia and China, scientists have found that the active layer became thicker in the last 50 years. Specifically in Siberia, it has become 25 cm thicker than it was 50 years ago while in the Tibetan Plateau, it is up to 100 cm thicker. As the active layer becomes thicker, the permafrost layer underneath it gets thinner. Scientists have found that there is now 10% less frozen ground in the Northern Hemisphere than in the early 20th century. This 10% is equivalent to more than five million square kilometers, which is ~2/3 the size of Canada. Climate scientists believe land temperatures could increase 3°C to 5°C by the end of the 21st century. If that were to happen, frozen ground and permafrost that is between 0°C and -2.5°C would thaw, mostly impacting warmer permafrost found in high mountain ranges, the Tibetan Plateau, and the interior of Alaska.