There’s a great deal of permafrost on Earth. This perennially frozen ground, distributed across Arctic, Antarctica and high-altitude regions, covers about 18 million sq km – or roughly 12% of the planet’s land surface. Around 5% of global permafrost, or close to a million sq km – is vulnerable to abrupt thaw like thaw slumps, a landslide-like form of abrupt ground collapse caused by rapid thawing.
Permafrost is under intense pressure from climate change. When these frozen landscapes heat up quickly, the “permanent” freeze begins to fail. If a large amount of ice turns to water, the ground can collapse in a retrogressive thaw slump.
As with landslide, vegetation is stripped away and soil-stored carbon is exposed, releasing it into the atmosphere and further accelerating climate change.
“Permafrost acts like a giant freezer holding twice the carbon currently in our atmosphere. When it thaws, it releases trapped carbon dioxide and methane, directly accelerating global warming and affecting communities worldwide, including us living in Hong Kong,” says Professor Liu Lin of CUHK’s Department of Earth and Environmental Sciences.
We know how the land collapses – but until recently, we didn’t have any idea how long vegetation takes to recover or how quickly the area can regain its role in carbon recycling.
From physical observation to ecological insight
Professor Liu, one of CUHK’s leaders in polar research, traveled on China’s 41st Antarctic expedition in 2024 and 2025, and has been studying permafrost thaw for more than a decade. Initially, his focus was solely on the physical changes to the landscape wrought by climate change.
“This study started when we realised that while we understood the physical mechanics of these land collapses, the ecological impact remained a major mystery,” he says. “We saw these disturbances dramatically strip away vegetation and release soil carbon, but nobody had a clear timeline or pattern for how vegetation recovers across different regions. We need to bridge the gap between earth sciences and plant biology to understand how these massive ‘scars’ in the earth eventually heal and start absorbing carbon again.”
To address this gap, Professor Liu, together with Professor Mark Lara, a professor of plant biology at the University of Illinois Urbana-Champaign and Dr Summer Xia Zhuoxuan, Professor Lara’s postdoctoral fellow, and collaborators from Canada, Germany and Russia, travelled to eight permafrost regions, both Arctic and mountainous ones, in Alaska, Canada, Siberia and the Qinghai-Tibet Plateau, to study 137 thaw slumps, ranging from 0.04 to 8 ha in size.
“Both the Arctic and the Qinghai-Tibet Plateau are experiencing amplified warming,” says Dr Xia, who is also a CUHK PhD graduate. “Thaw slumps, as a consequence of permafrost degradation, are becoming more frequent and widely distributed across these regions.”
The team used a remote sensing-derived index (Normalized Difference Vegetation Index) and high-resolution imagery to monitor changes in surface greenness after the collapse. In addition, they tracked changes in plant types across thaw slumps from different years – the oldest of them dating back about 70 years.
The team’s findings show that recovery is neither uniform nor random. In low-Arctic regions, vegetation can return within a decade. In high-Arctic and high-mountain regions, by contrast, recovery can take many decades – or even more than a century.
A new tool to predict recovery
The researchers found that climate alone does not determine how quickly vegetation returns after a thaw slump. Instead, it is best predicted by a statistic known as gross primary productivity (GPP), which is the key finding of the study. GPP captures the combined influence of local and regional factors that shape recovery, including nutrient availability, soil moisture and plant diversity, reflecting the overall capacity of an ecosystem to convert carbon dioxide into plant biomass through photosynthesis.
“Ecosystem recovery is complex,” explains Dr Xia. “But we were surprised to find that this complexity can be simplified if we focus on the right key factor.”
To estimate GPP across vast and remote permafrost regions, the team used solar-induced chlorophyll fluorescence (SIF) to capture the faint glow emitted during the photosynthetic process of plants. By evaluating the 137 thaw slumps, as well as 4 additional sites for validation, GPP has proven to be very helpful in calculating how long the recovery of a particular thaw slump is likely to take. The team discovered a strong relationship between productivity and recovery time: areas with higher GPP (low-Arctic) recovered more quickly, whereas those with lower GPP (high-Arctic and high-mountains) took much longer to heal.
When vegetation does return, it often differs from the original cover and is unlikely to return quickly to its original state. Woody plants tend to recolonise thaw slumps first, which can help stabilise the soil and absorb some of the carbon released by thawing – though not enough for taking up all carbon that was released due to the thaw slump.
“At sites with rapid recovery, we observed the colonisation of erect shrubs. In contrast, at sites with slower recovery, erect shrubs were largely absent, and the landscape remained dominated by low-stature vegetation,” says Dr Xia. “However, this does not imply that no ecological changes occurred in these slow-recovering systems. Further detailed investigation is needed to better understand potential shifts in species composition.”
Ground and biosphere
“This study is a vital puzzle piece in understanding the Earth as an interconnected system,” says Professor Liu. “We often look at permafrost thaw as an isolated physical crisis driven by global warming, but this work highlights the dynamic relationship between the frozen ground and the living biosphere. It shows us that vegetation and natural processes interact with thawing permafrost in ways that might actually facilitate recovery.”
Professor Liu’s work is far from finished. Next, he aims to map the feedback loops between thawing ground, vegetation change and climate, improving the accuracy of global climate models.
