Author(s): Niu Yuhan

The Mogao caves: Preserving cultural heritage in a changing climate

Source(s): China Dialogue
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On a scorching July morning, dazzling summer sun shines on the cliffs of Gansu province’s Mogao caves, as buses packed with tourists arrive. 

Situated at a strategic point along the Silk Road, the nearly 500 cells and cave sanctuaries in Mogao are famous for their statues and wall paintings, spanning 1,000 years of Buddhist art.

The UNESCO World Heritage Site is extremely popular. In August 2023, visitor numbers were 30% up on pre-pandemic 2019, according to Xinhua News. At the start of the summer holidays, the caves’ administration committee announced the tourist limit was being reached every day, and reminded visitors to book tickets in advance.

Li Ping, deputy head of the committee, told The Paper how large numbers of visitors in a short time can pose a serious threat to the fragile frescoes and painted sculptures. Bodies and breath sharply raise the temperature, humidity and CO2 concentration within the caves.

Climate change is also impacting the frescoes. Recently, Greenpeace and Huafeng Meteorological Media Group, a subsidiary of the China Meteorological Administration, issued a joint report on its impacts to the ecology, industry, and cultural heritage of north-west China. With a particular focus on Gansu, the report showed how climate change is threatening the conservation of cultural heritage sites.

Warm and wet, or warm and dry?

Many researchers believe that China’s arid northwest is becoming warmer and wetter. In the second half of the 20th century, temperatures in the region increased by 1C, while rainfall has also been increasing, particularly since the 1990s.

Frequent temperature changes contribute to fresco damage, and many people are concerned about the Mogao paintings. But the north-west region is vast, containing several climate systems, and making it difficult to generalise a “warmer and more humid” regional trend. In the vicinity of the Mogao caves, precipitation is increasing, but this does not necessarily mean humidity is, too.

Dunhuang Academy and other organisations analysed temperature and precipitation data collected from 1990-2020 by a meteorological station situated atop the Mogao caves. A report of the results showed that the caves’ average temperature has increased by 0.46C per decade, while average annual precipitation increased at a slower rate of 2.21mm per decade. The fast-rising temperature means greater evaporation and an overall trend of warming and drying. The report showed that such changes are, in fact, “conducive to the preservation of cultural relics” in the caves.

“The Mogao caves lie deep in the hinterland of the Gobi Desert, where the air is dry and rainfall scarce,” said Su Bomin, president of the academy, during a seminar. “It’s just such an arid, typically continental climate, which provides good atmospheric conditions for the long-term preservation of the frescoes.”

Extreme weather a wake-up call

Although the overall drying trend may be good for the Mogao artworks, disasters caused by extreme rainfall are a pressing problem that has been exacerbated by climate change.

The site of the caves is arid, averaging 20 rainy days per year, with annual rainfall of about 40mm. However, the number of days with rainfall exceeding 10mm has increased significantly since 1961, the Greenpeace report showed.

“The most direct impact of extreme precipitation is the increasing humidity in the caves,” Li Chao, senior researcher at Greenpeace and lead author of the report, told China Dialogue. Much of the fresco damage has been related to changes in temperature and humidity, whether due to tourists or heavy rain. The harm includes efflorescence (movement of salt to the surface), flaking, and detachment of whole paintings from their walls.

The frescoes are composed of three basic parts: the base rock, a plaster layer which acts like a “canvas”, and the pigment layer. When torrential rain falls, the relative humidity within some of the caves can reach as high as 60-65%. The soluble salts in the rock and plaster dissolve and move when exposed to this level of moisture, accumulating in the plaster. Frequent changes in humidity causes the salts to repeatedly crystallise, contract, redissolve, and recrystallise, which leads to disruption and flaking. Coupled with increasingly warm temperatures, this degradation will only get worse, and could even result in large sections of murals falling off. 

Extreme weather also challenges the integrity of the caves. In June 2011, a downpour caused a leak onto the eaves of the Mogao caves’ iconic nine-storey wooden building. The wall surface collapsed, allowing rainwater to seep into the grotto of the giant Buddha. The damage took over seven months to repair.

Vigilance is particularly necessary for caves carved into the cliff face of the west bank of the Daquan River, which face threats of rainfall-induced floods. In 2012, severe local flooding caused varying degrees of damage to the infrastructure and cultural relics of the Mogao caves, the Western Thousand Buddha caves, and the Yulin grottoes.

Li Chao said that dealing with the impacts of climate change on ancient buildings mainly involves daily maintenance, environmental monitoring, and post-disaster renovation. After so many years of protective work, the Mogao caves now benefit from a relatively sophisticated arsenal of countermeasures.

Mogao at the vanguard

In 1998, the Dunhuang Academy cooperated with the US-based Getty Research Institute to install an automatic weather-monitoring system in Cave 85, which contained severely damaged frescoes. This gave access to real-time data, including ambient and wall temperatures, relative humidity, and CO2 concentration. The two parties also collaborated on an environmental monitoring project, establishing a correlation between cave air temperature, humidity changes, and fresco damage. The results served as the model for monitoring dozens of caves.

Since 2013, a comprehensive monitoring system has been in operation, allowing staff to monitor cave conditions in real time. Should conditions exceed safe levels, the system issues a warning and staff can immediately make adjustments, such as modifying tourist routes, evacuating visitors, or closing cave doors to shut out moist air, explained Li Chao.

When frescoes show signs of degradation or cracks, experts draw up a specialised restoration plan. Once approved by the National Cultural Heritage Administration, a qualified engineering team carries out the plan.

Since Dunhuang established a preservation agency in 1944, generations of researchers have used scientific methods to partially restore over 5,000 square metres of murals in more than 280 damaged Mogao caves. This conservation regime is often referred to as a “model of success” by the media.

In recent years, flooding of the Daquan River has become increasingly common. After the flood of 2012 seriously affected the caves, better flood-control measures were implemented. These included reinforcing the riverbanks and dredging the riverbed. Around 20 million yuan (US$2.7 million) has been invested in these projects.

The National Cultural Heritage Administration has approved 51 restoration projects since 2012, including cliff reinforcement and environmental remediation, according to Deng Chao, director of the administration’s Department of Cultural Relics and Monuments. Projects such as reinforcing thin-roofed caves and installing cave doors have comprehensively solved the problem of rainwater ingress, Deng said. The administration and the local meteorological department have established a heavy rain and flooding early-warning system, as well as a “flash flood emergency command platform”. This allows them to gather data on meteorological changes in advance, and carry out emergency measures.

According to a China News report, the National Development and Reform Commission had approved a feasibility report as early as 2007 on protecting the Mogao caves using engineering. Construction began the following year with a total investment of 261 million yuan (US$35 million). In 2014, a Digital Exhibition Center opened 16km from the caves, to allow tourists to see projections of the murals without having to enter the caves and risk damaging them. At this point, the six-year conservation strategy was complete. Construction of phase two of the Digital Exhibition Center, which aims to accommodate larger numbers of visitors, began in 2022, with a total planned investment of about 290 million yuan. It is expected to be completed and operational by June 2024.

Sites beyond Mogao

Many other caves in north-western China are affected by climate change. The Greenpeace report stated that Gansu has 219 grotto temple sites and 236 cultural relic sites, including 229 grotto temples and seven stone statues carved into cliffs or boulders, also known as “cliff statues”. Not all of these sites have the resources available to the Mogao caves.

The Dunhuang Academy is using science and tech to protect some of the other caves. In 2022, it launched the first phase of the Cave Temple Monitoring and Early Warning System. Part of this project is a platform that monitors 115 of Mogao’s caves, and has access to some data from five other cave temples in Gansu that are overseen by the academy: the Yulin Grottoes, Western Thousand Buddha Caves, Maijishan Grottoes, Bingling Temple, and North Grotto Temple.

This solution is far from comprehensive, however, and Gansu has many province-level cave temples whose cliffs have yet to be reinforced. It also has 187 small- and medium-sized cave temples at county-level that have still not been classified. More resources are needed to properly manage, protect, and restore these cultural treasures in the face of new crises.

There is still a dearth of domestic experts in the conservation of cultural sites in the face of climate risks, as well as specialists engaged in restoration work, said Li Chao. Researchers from Dunhuang Academy generally need to provide support for sites beyond Mogao too, and everyone is under great pressure from the workload, she added.

A good example is Gansu’s Jinta Temple Grottoes. In late July this year, a reporter from Caixin magazine visited the grottoes and discovered that the interior “monitoring system” was just a common household thermo-hygrometer – measuring temperature and humidity – which the staff hung below a camera to facilitate all-weather “readings”. These grottoes are located in an area with abundant rainfall and the cliff face is of red sandstone, which is not very water tolerant. They are therefore under much more threat than the Mogao caves and the monitoring equipment is not fit for purpose.

Dunhuang Academy has helped the Jinta Temple Grottoes design restoration plans and  digital experiences to minimise tourist damage. However, when asked about the difficulty of bringing the Mogao Cave learnings to the Jinta Temple, Wang Weidong, deputy director of the Zhangye Cultural Relics Protection Institute, told Caixin that they lacked the funds for conservation, personnel, as well as tech and equipment support. “It comes down to human and financial resources,” Wang said.

Global cultural heritage

“World Heritage and Tourism in a Changing Climate”, a report jointly released in 2016 by UNESCO and the UN Environment Programme, pointed out that climate change has already become one of the most important factors threatening cultural sites globally.

The damage caused to world heritage conservation by climate change is slow but significant. In July, an editorial in Ecological China, a media platform managed by the Ministry of Natural Resources, stated:

“Cultural relics in different regions face different troubles. Beijing’s frequent sandstorms mean that, for the Forbidden City’s interior furnishings, wooden articles, murals, and so on, it’s necessary to draw up countermeasures against sand and desiccation; some ancient buildings in the south need to worry about the threat posed by termites, microorganisms, moisture, and mould. The darkening of the Leshan Giant Buddha [in Sichuan province] is because the rainy weather creates a hospitable environment for microorganisms to multiply on the surface of the statue.”

Li Guanghan, assistant director of the UNESCO World Heritage Institute of Training and Research in the Asia and Pacific Region (Beijing), told China Dialogue that not many projects in China are like the Mogao caves, having historical climate data and the ability to provide long-term protection against climate change. Most are about reactive conservation – repairing damage after a climate-related disaster, she added.

In August this year, China held the inaugural International Forum on the Protection of Cave Temples. It issued a “Declaration on Cave Temple Conservation in the Context of Climate Change”, and proposed to “explore preventative protection” and “focus more on the systematic protection of small- and medium-sized grottoes”.

Li Guanghan said that the unequal allocation of resources for cultural heritage protection was a “cruel reality”, and that heritage determined to be of “higher value” would always receive more resources. The social and cultural impacts of climate change on heritage sites were often overlooked, she added. For example, climate change may force traditional villagers to stop farming, change the way they do, or even leave their village permanently.

“On the one hand, climate change has a huge impact on tangible and intangible cultural heritage, and needs people’s attention,” she said. “On the other, protecting agricultural landscapes, traditional villages, and the traditional farming relationship between people and the land, actually reduces changes in the way land is used, and assists with climate adaptation.”

In her view, China’s efforts in cultural heritage conservation are gradually growing thanks to increased governmental attention and the country’s growing status in the field. Cultural heritage is a public good, she said. In China, as elsewhere in the world, government departments usually take the lead in conservation efforts, with close involvement from universities and research institutions. However, China still needs to establish a mechanism for civic involvement and growing public awareness, she added. This could, for example, be by mobilising residents of ancient villages to maintain the landscape of their own community, enabling multiple parties to work together to protect cultural heritage in the face of a changing climate.

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