Higher sea levels caused by a warmer climate will continue threatening coastal cities in a slow but sometimes catastrophic way and China has reacted in many ways to counter the impacts, reports said.

Extreme sea-level events that used to occur once a century are projected to occur annually by 2100.The rise will increase the frequency and severity of coastal flooding in low-lying areas and coastal erosion, according to an assessment released by an Intergovernmental Panel on Climate Change working group.

Over the past 120 years, the global mean sea level rise has increased from 1.4 millimeters a year before 1990 to 3.3 mm annually after 1993, the China Blue Book on Climate Change published by the China Meteorological Administration said.

That occurred amid an increase in global temperatures. The report said that the global surface temperature increase from 1850-1900 to 2011-20 was 1.09 C.

"Heating of the climate system has caused global mean sea levels to rise, mainly through ice loss on land and thermal expansion from ocean warming," the report said.

"When thermal expansion happens, sea volume increases as its temperature rises. Ice mass loss comes from melting glaciers and ice sheets," said Zhai Panmao, a Chinese climatologist and one of the co-chairs of the working group.

"Although climate change affects every region across the globe, impacts differ according to local geographical conditions."

He said that rising sea levels will pose higher long-term risks to China, a populous country with its wealthy provinces along coastal areas.

"In coastal cities, the extreme sea level events due to sea level rise and storm surges, in combination with extreme rainfall and river flooding, will bring significant risk for buildings and infrastructure, and may even inundate an entire city," Zhai said.

In August last year in Jiangmen, Guangdong province, high sea levels blocked the discharge of floodwater and worsened the disaster caused by Typhoon Higos. Economic losses reached more than 150 million yuan ($23 million), the 2020 China Sea Level Bulletin issued by the Ministry of Natural Resources said.

Rising seawater eroded more coastal areas in Liaoning, Jiangsu and Fujian provinces last year than in 2019, the bulletin said.

Erosion can coarsen sand, damage soil, destroy infrastructure and cause economic losses, said an essay published by the ministry.

High sea levels will also cause salty water to threaten fresh water supplies, and affect agricultural and industrial production, the bulletin said.

"Our crops and drinking water will be ruined immediately if they get soaked by saltwater," Zhai said.

"We can't reverse some of the changes, such as an increase in sea levels, but by reducing emissions of greenhouse gases, we can slow the process in some respects while controlling the level of warming. It is not the responsibility of any particular country, but of all the countries around the globe who should work together to cope with climate change."

Reduction measures include replacing fossil fuels with clean energy and using energy more efficiently, he said.

China has set up observatories to measure greenhouse gas emissions so that policymakers can formulate reduction plans. Seven such ground-based observatories are in the provinces of Qinghai, Zhejiang, Heilongjiang, Hubei and Yunnan, the Xinjiang Uygur autonomous region and Beijing, the China Meteorological Administration said.

Adaptation to climate change is also very important as we face more risks in the future, Zhai added.

"Warning systems for extreme weather or other disastrous events and better urban planning are needed for adapting to climate change and protecting people from risks," he said.

Shanghai, a coastal metropolis, began controlling its underground water in 1965, replenishing it with surface water to stop its ground from subsiding, said a 2018 report from the China Water website operated by the Ministry of Water Resources.

The measures aimed to prevent the city from flooding caused by rising sea levels.

A document released by the city said it had managed to limit subsidence to under 6 mm annually by the end of 2018.

In coastal areas that are vulnerable to rises in sea levels, authorities should improve infrastructure to prevent floods and enhance defensive facilities. They also need to restore wetlands and forests eroded by salt water, the China Sea Level Bulletin suggested.