From storm surges to seawater intrusion: What challenges does global warming bring to coastal cities?

Tuchong Creative

On October 21, seawater backflow suddenly occurred in the coastal areas of Dalian, Yingkou, Panjin, Jinzhou and Huludao in Liaoning Province, my country, causing some streets to be flooded and houses to be flooded, causing considerable losses. Although the area has experienced flooding during high tides before, this year's seawater backflow is more serious. This seawater backflow occurred during the astronomical high tide, and at the same time, affected by cyclonic activity, the wind speed reached 8-9 or above. Based on this, the Liaoning Provincial Marine Early Warning and Monitoring Center issued a Level IV blue alert for sea waves, predicting that there will be 2.5 to 3.5 meters of high waves in the Bohai Sea and the northern Yellow Sea. Therefore, seawater backflow is inevitable in the coastal areas of Liaoning.

It is worth noting that places where seawater backflow occurs generally have special terrain, such as bays and river estuaries concave on the coastline. In Panjin City, where seawater backflow occurred this time, the Daliao River gradually widened after flowing through Panjin City, forming a large trumpet-shaped mouth when it entered the sea. The width has reached more than ten times the width of the river in the land area. When the tide rushed inward from the estuary, the width continued to shrink, causing the tide to cross the embankment and flood the low-lying areas.

From a longer-term perspective, the sea level rise caused by global warming provides a favorable background condition. The sea level rise in the past century is supported by monitoring data. Reliable tide gauge data around the world can be traced back to 1880. However, there were relatively few early observation sites. To study the changes in the global average sea level, it is necessary to rely on reconstruction algorithms, which have certain uncertainties. In the past 30 years, there have been relatively continuous and reliable satellite data. According to the Special Report on the Ocean and Cryosphere in a Changing Climate (IPCC, 2019) released by the Intergovernmental Panel on Climate Change (IPCC) of the United Nations in 2019, the global sea level rose by about 15 cm in the 20th century. According to the 2021 IPCC Sixth Assessment Report of Working Group I (IPCC, 2021), the rate of global average sea level rise in the 20th century was the highest in the past 3,000 years, and the global sea level has risen by about 20 cm since the Industrial Revolution.

A 20-centimeter sea level rise is not a small number. Just like in some areas, 50 millimeters of rain a day may not seem like a large number, but these rains gather in low-lying areas, causing local floods, economic losses and casualties. Sea level rise is similar. Although the global average rise is only 20 cm, under favorable conditions, it will cause abnormally strong sea level rise in some areas, far exceeding the global average. Such favorable conditions include storm surges, astronomical tides, and trumpet-shaped terrain, which will cause water levels far exceeding normal values, leading to seawater backflow and seawater overflowing the embankment, thus causing coastal floods.

Coastal areas around the world are the center of economic, social and cultural development. According to the United Nations, 40% of the world's population lives within 100 kilometers of the coastline, which means that nearly three billion people around the world may be affected by sea level changes. Sea level rise is more urgent for low-lying coastal areas and small island countries, and the consequences are serious. my country has a long coastline, with many low-lying areas and terrain areas that are conducive to flooding from south to north, and frequent strong winds, temperature cyclones and typhoon activities, which will pose serious risks to coastal areas.

According to the IPCC assessment data in 2021, the rate of global average sea level rise in the 20th century was the highest in the past 3,000 years. And the global sea level rise is not uniform, but shows an accelerating characteristic. Between 1901 and 1990, the global average sea level rose by 0.12 meters, with an average rise rate of 1.35 mm per year. From 1971 to 2018, it rose to 2.3 mm per year, and from 2006 to 2018, it soared to 3.7 mm per year. According to the latest research results (Hamlington et al., 2024), since satellites began observing sea level height in 1993, by the end of 2023, the global average sea level has risen by 111 mm. Among them, from 1993 to 2002, the global average sea level rose by 2.1 mm per year. However, this rate has more than doubled in the past 10 years, reaching 4.8 mm per year from 2014 to 2023. According to IPCC estimates, the global average sea level will rise by 15 to 30 centimeters by 2050.

Current scientific research has proven that the root cause of sea level rise comes from global warming. Specifically, there are two main impact pathways: one is that global warming causes the melting of polar glaciers and ice sheets, and the other is the thermal expansion of seawater due to rising temperatures. From the 1970s to the early 21st century, these two pathways contributed roughly the same amount to sea level rise. However, in recent years, with the increase in greenhouse gas emissions, the average temperature of the earth has continued to rise, and the melting of polar ice sheets and mountain glaciers has accelerated, resulting in a large amount of fresh water being injected into the ocean. The first pathway has increased its contribution to sea level rise. Currently, Greenland and Antarctica melt about 270 billion tons and 150 billion tons of ice each year, respectively.

As one of the most sensitive regions to global climate change, China is particularly affected by rising sea levels. According to the China Climate Change Blue Book (2024) released by the National Climate Center, the rate of sea level rise in China's coastal areas was 4.0 mm/year from 1993 to 2023, which is higher than the global average for the same period. In 2023, the sea level in China's coastal areas was 72 mm higher than the average from 1993 to 2011. In addition to exacerbating storm surge disasters in coastal areas, rising sea levels will also damage coastal wetland ecosystems, exacerbate coastline erosion, and make seawater intrusion and land salinization increasingly serious.

Rising sea levels also pose a challenge to China's marine economy and the sustainable development of coastal cities. It is worth noting that as sea levels rise, when extreme weather such as severe drought occurs in low-lying coastal areas, the river and groundwater levels drop, and seawater advances upstream along tidal channels and backflows into the river estuary, forming a saltwater intrusion. In 2022, the drought in the entire Yangtze River basin and the saltwater intrusion affected the water intake of the Yangtze River Reservoir in Shanghai, with the longest lasting more than 90 days. In particular, when the typhoon "Meihua" was active, seawater backflow formed in the offshore area of ​​the Shanghai estuary, affecting the water intake of the Shanghai Yangtze River Reservoir, bringing the most severe challenge to the safety of Shanghai's urban water supply. There was a rush for water in major supermarkets in Shanghai, and everyone rushed to buy bottled water. Rising sea levels and extreme weather will cause even more serious impacts in the future.

Curbing the trend of sea level rise requires global joint efforts. First, reducing greenhouse gas emissions is the key. This requires the international community to take more stringent emission reduction measures, promote the transformation of energy structure, develop clean energy, and improve energy efficiency. Secondly, strengthen the protection and restoration of coastlines, such as building seawalls and restoring natural barriers such as mangroves to enhance the resilience of coastlines. In addition, raising public awareness of climate change and sea level rise and enhancing society's ability to adapt to climate change are also important ways to curb the trend of sea level rise.

This article is a work supported by the Science Popularization China Creation Cultivation Program

Author: Wei Ke

Reviewer: Dai Yunwei, Senior Engineer, China Meteorological Administration

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.