农民影视免费观看VIP在线观看 农民影视免费观看VIP无 农民影视免费观看VIP在线观看 农民影视免费观看VIP无 ,柳萱岳风刚刚最新章节在线观看 柳萱岳风刚刚最新章节 柳萱岳风刚刚最新章节在线观看 柳萱岳风刚刚最新章节 ,一下子就弄进去了岳在线观看 一下子就弄进去了岳无 一下子就弄进去了岳在线观看 一下子就弄进去了岳无

发布日期:2021年12月05日
wechat_bg

2020-12-31 10:43:29

In July of 1949, a historically strong typhoon coincided with high tides to push seawater off the coast of low-lying Shanghai to about 2 meters above their normal levels. Levees, damaged by war, were breached. There were no walls to contain the Huangpu River, which cuts through the city center, and Shanghai’s antiquated drainage system proved no match for the water. As a result, the city flooded, houses and farmland were destroyed, and the groundwater became saline. With more than 1,600 people dead, it was one of the most destructive storms in China’s modern history.

In the 70-plus years since, the Chinese government has tremendously improved the country’s defenses against the seas. Nowhere more so than in Shanghai, which is now surrounded by hundreds of kilometers of storm walls. But as climate change is pushing the world’s oceans ever higher, China’s current defenses will be rendered inadequate. Protecting China’s low-lying coast, home to much of the country’s population and wealth, is inevitably going to become harder and more expensive.

The world’s seas are on the rise because of climate change, which causes glaciers and ice sheets to melt, as well as ocean water to heat and expand. The most recent report on the issue from the United Nations’ Intergovernmental Panel on Climate Change, published in 2019, estimated that by the year 2100, sea levels will rise by 0.39 meters to 0.84 meters, depending on future emissions and other factors. But since that report came out, scientists have pointed out that its estimates could be too low, given the uncertainty about how quickly polar ice sheets will melt in the hotter decades to come.

Though a global phenomenon, there are regional variations in how much coastal waters will rise. China’s annual sea level report from 2019 shows that, with an average rate of 3.4 millimeters per year over the past 40 years, sea levels in China’s coastal areas have risen faster than the global average over the same period.

Rising water, sinking cities

Sea-level rise is only half the story, according to Gao Shu, director of the State Key Laboratory of Estuarine and Coastal Research at Shanghai’s East China Normal University. China’s coast is also at greater risk of flooding due to increasingly strong storms. From 2005 to 2015, more than 200 storm surges hit China, according to 2016 research noting that the storms’ frequency and intensity have been increasing as a result of climate change.

The potential damage from floods has also escalated. Beijing-Tianjin-Hebei, the Yangtze River Delta, and the Pearl River Delta, the country’s three major economic hubs, have a combined population of about 126 million people living on low-lying, vulnerable land. Cities from those regions, namely Guangzhou, Shenzhen, Tianjin and Shanghai, are among the top major cities projected to suffer the most — or facing the highest increases in — economic losses from flooding in 2050, according to a 2013 report co-authored by researchers at the World Bank that analyzed 136 seaport cities with populations of over 1 million and that accounted for flood defences.

We should choose to pay attention to climate change ... a passive response will result in greater losses.

Already, China has experienced several effects of sea level rise. Shorelines have eroded, drinking water sources have come under threat, and storm surges have devastated coastal areas. Between 2014 and 2017, some stretches along the coast in Jiangsu province, north of Shanghai, eroded by more than 100 meters. In 2019, super typhoon Lekima struck China’s east coast, causing floods and deadly landslides in Zhejiang province, south of Shanghai. In the Pearl River Delta, this year’s dry season saw saltwater intrude to the point of emergency efforts being needed to safeguard the region’s water supply.

And as seas go up, land goes down. About 32 Chinese coastal cities face land subsidence, with Tianjin and Shanghai, as well as Panjin in the northeastern Liaoning province, facing an accumulated land subsidence of more than 2 meters, mainly due to groundwater exploitation and high-rise construction. In some coastal areas, subsidence is so serious, protective levees are shrinking by up to 20 centimeters each year. As a result, the relative sea-level rise is even higher.

Central Shanghai had sunk by up to 3 meters in the 19th century, making the city center noticeably concave with an altitude of only 2 meters, says Wen Jiahong, head of the disaster risk assessment center at Shanghai Normal University. 

During a 1997 storm surge, a Shanghai hydrological station saw a record-breaking, 5.72-meter-high storm tide. Taking into account the subsidence that has since occurred, such a tidal level would put waters at nearly 3 meters above Shanghai’s city center, says Wen. “It looks like a ship, and the surrounding water is higher than the bottom of the ship,” he says. “This will give huge pressure to flood control and drainage.”

To control land subsidence, the city government carried out measures such as groundwater recharge. The land subsidence rate slowed down, though it did not stop. In 2018, Tianjin subsided 17 millimeters on average, while Shanghai subsided 5.1 millimeters on average.

Waves batter a sea wall in Dongying, Shandong province, Oct. 27, 2020. People Visual

Waves batter a sea wall in Dongying, Shandong province, Oct. 27, 2020. People Visual

Protection

Shanghai’s sea walls are designed to withstand flood risks occurring once every 200 years and Category 12 typhoons — the highest standard in the country. But as seas rise, worst-case scenarios previously projected to happen just once every two centuries are becoming more likely. Their expected frequency will rise to once every 30 or even 20 years in the second half of the 21st century, says Gao, of the State Key Laboratory of Estuarine and Coastal Research. Current design standards will fall short of future needs.

By the end of 2015, flood defences in most rural regions along China’s coast were inadequate, designed to only withstand extreme weather occurring once every twenty years, while only major cities have sea walls able to resist high tides occurring once every 50 years, according to a government report. This prompted China to issue a sea wall construction plan along the coast in 2017, aiming to expand China’s coastal sea walls to 15,000 kilometers in length, with 57% meeting standards by 2027, up from 14,500 kilometers and 42.5% in 2017.

The higher the sea wall is, the more difficult it is to maintain.

But upgrading sea walls is a costly proposition. Shanghai has invested 50 million yuan ($7.7 million) for every kilometer of its 510-kilometer sea walls. “The higher the sea wall is, the more difficult it is to maintain it,” says Gao. “The cost growth is exponential. The price for constructing a 2-meter-high sea wall, compared with that of a 1-meter-high one, is not double, but tenfold.”

Given that sea levels rise at a very slow pace, most of China’s levees and sea walls are built for current-day flood and tide control or for land reclamation, rather than for future extremes created by sea level rises, says Li Guosheng, researcher at the Chinese Academy of Sciences who studies climate change impacts in coastal regions.

“Taking into account the probability of extreme water levels, as well as the maintenance costs of the sea wall project, it is not recommended to raise the sea wall standards to an ultra-high level for the prevention of low-probability storm surge events in 2100,” he wrote in his research paper on storm surge defenses in the Pearl River Delta in 2019. 

According to Li, major cities in river deltas tend to invest more in defensive infrastructure, while rural areas such as in Guangxi, a relatively undeveloped region bordering Vietnam, have fewer funds to renovate sea walls. Rural coastal areas need more attention in the face of future risks, but there’s a lack of research resources and limited awareness of climate change from the local government, says a Shandong-based researcher who is not at liberty to talk to the media.

Sea walls are also disruptive to ecosystems. There are efforts to construct so-called bio-sea walls and living breakwaters, such as mangroves, marshes, or artificial reefs. These buffer the tides, with ecological benefits and lower costs. This year, two ministries published a five-year plan for planting and restoring 18,800 hectares of mangroves along China’s southern coast. Zhejiang province, prone to typhoons, announced this year that it would invest 350 billion yuan into building a 1,800-kilometer-long ecological coastal belt including remediating coastal wetlands and mangrove forests.

Gao also studies growing oyster reefs near the Shanghai coast. “Together with the salt marshes, they can form a line of defense for protection,” says Gao. Such reefs grow outside the sea walls, replacing traditional breakwater stones to buffer tides and dissipate wave energy, thus protecting the coast. But he also points out that the low salinity of the water in the Yangtze River estuary is an unfavorable factor for the growth of reefs, and so more research into new technologies is required.

Workers collect oysters by the sea in Weihai, Shandong province, Dec. 25, 2020. Wang Wenmao/People Visual

Workers collect oysters by the sea in Weihai, Shandong province, Dec. 25, 2020. Wang Wenmao/People Visual

Adaptation

Besides defensive infrastructure, governments should consider climate change risks in urban planning and land use, says Fang Jiayi, assistant researcher of coastal disasters at East China Normal University, adding that other measures to increase coastal resilience include raising living areas of coastal houses or carrying out planned retreats if necessary, though such practices are not common in China. “We should choose to pay attention to climate change, and take appropriate measures to adapt in advance. Otherwise, a passive response will result in greater losses,” she says.

In 2016, China issued the Action Plan for Urban Adaptation to Climate Change, setting a target for 2020 to incorporate climate change-related indicators into urban and rural planning systems, construction standards, and industrial development plans. In the next year, 28 cities, including coastal Dalian in Liaoning province and Haikou in Hainan province, were selected as pilot areas to carry out climate adaptation measures. 

The key is to improve resilience.

Li Huimin, associate professor of climate policy at Beijing University of Civil Engineering and Architecture, tells Sixth Tone that meteorological and emergency response departments have conducted more research into climate change to improve their disaster warning, alleviation, and responses, but that it’s not enough. 

“The key is to improve resilience,” Li Huimin says. A good example is China’s “sponge cities” initiative that focuses on coping with heavy rainfall and alleviating flood risks. However, this doesn’t officially fall under climate change adaptation, which hasn’t received much attention in government documents, he says. Many local officials routinely confuse adaptation with mitigation, such as with reducing emissions.

Fang has cooperated with foreign scholars to study the differences in response to climate change in various coastal countries. In her experience, the Chinese government encourages institutions such as universities and think tanks to research climate change, while low-level governments lack awareness of the issue. “In China, (the approach) is top-down.”

International climate change initiatives encourage various stakeholders to participate. But given that China has a comparatively dominant state sector, the role of nongovernmental organizations is weak, says Wen, of the disaster risk assessment center at Shanghai Normal University. “There is a lot of domestic research on climate change, but most of it is academic.” He calls for local governments’ urban planning and water resources departments to have more collaboration with researchers.

Though some effects of climate change, such as sea level rise, might not become acute for a while, Wen warns it’s a “boiling frog” issue that should be addressed sooner rather than later: “There seems to be no obvious impact in the short term. But its long-term accumulation will leave a great and serious impact on the entire society and ecosystem.”

Editor: Kevin Schoenmakers.

(Header image: A view of the coastline in Qingdao, Shandong province, Dec. 26, 2020. People Visual)