**1.1 The concepts of vulnerability to climate change**

Climate change mainly includes temperature, precipitation, solar radiation, wind speed, humidity, and air pressure. It represents a typical multi-scale global change problem, characterized by the infinite diversity, multiple pressures and time scales [1]. There is much research on vulnerability caused by climate change, and also has gradually become a forward position field of global environmental change and sustainable development. Vulnerability includes the fields of disasters research (delineated into human ecology, hazards, and the 'Pressure and Release' model) [2], which is the core of climate change. It can include the vulnerability of natural systems, such as the impact of floods and hurricanes caused by climate change on agricultural production and human life; it also includes economic and social vulnerability [3], emphasizing risks and uncertainties come from economic and social environment change. In social vulnerability, it is often measured as a function of the socioeconomic conditions of the communities [4].

### **1.2 The identification of megalopolis vulnerability**

A city is a complex giant system, and urban systems have been growing exponentially in size and complexity [5]. While a megalopolis is a complex coupled giant system. Spatial scales about connection relationships and measurement indicators in quite different ways. The energy use of industry, infrastructure (water, electricity, gas), land and buildings in cities are the importance of factors in climate change [6]. Megalopolis is core to cities, which the highly integrated connectivity networks formed by developed infrastructure networks such as transportation, information and communications. With the industrial division, factor flow, and the integration of cross-regional infrastructure in megalopolis, there are more connectedness in cities, while the more vulnerability increased due to the effects of climate change.

The concept of urban vulnerability evolved from vulnerability and refers to the ability affecting by adverse events. It was first proposed by the United Nations Development Program in 1999, and specifically includes sensitivity and capacity of response [7]. The reasons for urban vulnerability include the natural environment and external shocks related to trade and diplomacy. At present, the research on urban vulnerability in China mainly focuses on the resource-based cities (such as mining cities and ocean cities).

There are more than 660 cities in China, including 7 megalopolises with urban populations of more than 10 million, 9 megacities with populations of 5-10 million, and 124 large cities with populations of 1 to 5 million. The five major urban agglomerations (the Pearl River Delta, the Yangtze River Delta, the Beijing-Tianjin-Hebei, the middle reaches of the Yangtze River, and Chengdu-Chongqing) gather 55% of the country's total economic output and 40% of the total population. In 2018, the domestic product of the Yangtze Delta megalopolis reached about 2.7 trillion US dollars, and the economic scale surpassed that of Italy, which is the eighth largest economy in the world, and the Pearl River Delta megalopolis was about 1.2 trillion US dollars, which is equivalent to Mexico. However, in general, the internal links are not connected between China's megalopolises and cities, it should consolidate cooperation in different sizes of cities. The urban commuter railway mileage in central cities such as Beijing and Shanghai is less than other international metropolises. Core cities, surrounding areas, and peripheral cities were of inferior spatial connectivity, and even some core cities have "fault zones" with surrounding areas.

China's megalopolises are the spatial carriers that form economies of scale and scope, they are also a crucial support for improving economic efficiency, resilience, and resistance to external uncertain risks. In particular, as the construction of new infrastructure will enter a stage of rapid growth, which represented by 5G technology, artificial intelligence and data centers. It will be beneficial to the sustainability of megalopolises and the adaptability to vulnerabilities and risks, for enhancing the connectivity, the flexible layout adjustment of cities's internal structure.

This chapter firstly identifies the vulnerabilities of Guangdong-Hong Kong-Macao, Yangtze River Delta and Beijing-Tianjin-Hebei, and establishes the sensitivity index and capacity of response index for the three megalopolises under the effects of climate change. Secondly, it assesses the impact of urban vulnerability, and finally put forward response the measures for China's megalopolises that are climate-adaptive.
