Understanding Climate Hazard Patterns and Urban Adaptation Measures in China

: Climate-related risks pose a great threat to urban safety, infrastructure stability and socioeconomic sustainability. China is a country that crosses diverse geomorphic and climatic regions in the world and is frequently affected by various climate hazards. In this study, we propose a comprehensive analysis on the spatial pattern of major climate hazards in China from 1991 to 2020, including rainstorms, droughts, heatwaves, coldwaves, typhoons, and snowstorms, and generate an integrated sketch map on multi-hazard zones. It is detectable that South of the Yangtze River is in danger of heatwaves, rainstorms, and typhoons, while the North China Plain is more likely to suffer droughts. Coldwaves, snowstorms, and freezing mainly affect Northeast China, Northwest China, and the Qinghai–Tibet Plateau. In the view of climate governance, cities are hotspots affected by intensiﬁed climate hazards in a warmer climate. There is an urgent need to incorporate a climate adaptation strategy into future city construction, so as to improve social resilience and mitigate climate impacts in rapid urbanization process. Speciﬁc adaptation measures have been developed from the perspectives of land-use planning, prevention standard, risk assessment, and emergency response to facilitate the understanding of climate resilience and urban sustainability. areas, collapsed buildings, and direct economic losses caused by climate hazards.


Introduction
Climate action has emerged as one of the defining issues attracting great attention from scientists, governments, and the public. A warming climate is believed to boost the frequency of extreme events and hence aggravate climate risks in the future, endanger the sustainable development of human society [1,2]. China, located in the southeast of the Eurasian continent and the west of the Pacific Ocean, is one of the countries with the most severe climate hazards in the world [3,4]. Climate hazards in China are characterized by various kinds, high frequency, long duration, distinct seasonality, and regional differentiations. In monsoon regions, the hazards of coldwaves, strong winds, and snowstorms concentrate in winter, while the hazards of rainstorms, heatwaves, droughts, and typhoons occur frequently in summer. Moreover, the interaction of compound hazards can lead to the nonlinear amplification in hazard intensity, resulting in more serious socioeconomic impacts [5,6]. Since the 21st century, climate risks in China remain high due to the elevated exposure caused by rapid economic growth and urbanization process, and climate-related economic losses have been climbing in recent years ( Figure 1).
Since the impacts of climate hazards are experienced locally, it is understandable that certain cities located in hazard zones have the needs to occupy a crucial position in adaptation agenda [7][8][9][10]. Allied to the urbanization trend in China, the pressing nature of adaption in cities becomes apparent. Cities create unique microclimates, complex topographies, and plentiful emission of heat and mass of buildings and allied with their heavy reliance on interconnected networked infrastructure, high population densities, and multifarious population constitutions, increase exposure to climate hazards, while poor governance structures or inadequate urban design exacerbate climate risks [11][12][13]. At present, actions on climate adaptation are mainly concentrated in national and supranational levels, while the potential for climate adaptation in urban level remains grossly underestimated [14]. Scholars points out that urban planning, as an important policy tool to optimize urban land-use and arrange public infrastructures, should become one of the main ways for implementing climate adaptation strategies [15][16][17]. However, due to some practical problems such as the inadequate understanding of climate hazards and the lack of technical standards, traditional urban planning in China has not played the leading role in climate actions. A literature review shows that many attempts have been made to focus on the impacts of climate hazards on urban areas [18][19][20] and incorporate climate adaptation an important part of urban planning and city expansion management [21][22][23][24].
Given that previous studies on climate hazards in China are scattered in a certain hazard type or a certain region, this study first conducts a comprehensive analysis on the spatial patterns of multiple climate hazards in China, then clarifies the difficulties and challenges that urban development faces in mitigating climate impacts. On this basis, specific suggestions on building climate-resilient cities in China are proposed from the perspectives of land-use planning, prevention standards, risk assessment, and emergency response in order to facilitate collaborations between economic, social, and climate policies and provide scientific reference for policy makers dealing with climate risks.

Data and Method
Daily observations of 2419 national meteorological stations in China from 1 January 1991 to 31 December 2020 are adopted in the research. The datasets are compiled and issued by the Meteorological Information Center (MIC) of the China Meteorological Administration (CMA), and it can be accessed from http://data.cma.cn (28 October 2021). The preliminary quality control has been conducted by the MIC, through checking spatial consistency, temporal consistency, and internal consistency and adjusting the suspicious records [25]. The variables used include daily precipitation, daily mean temperature, daily maximum/minimum temperature, and weather phenomena. There are some missing records in the dataset. To obtain reliable climatic statistics, two steps are processed. First, the annual mean value is taken as a missing one when the total missing records exceed 20% of a year. Second, the station with the consecutive annual mean values less than 30 years is removed from the calculation.
The daily records of weather phenomena are used to identify the snowfall day. The CMA TC database is also adopted in the analysis [26]. The TC best-tracks are applied to derive major typhoon tracks affecting China.
The hazard data from 2001 to 2020 are collected from "Annuals of Meteorological disaster (2020)", including direct economic losses, affected population, mortality, affected croplands, crop failure areas, and collapsed buildings.
In this analysis, a hot day is defined when the daily maximum temperature reaches 35 • C and above. A rainstorm is identified with the daily precipitation reaches 50 mm and above. The scientific basis of these definitions are from Warning Signals for Meteorological Hazard issued by the CMA. Drought is defined based on the meteorological drought composite index (MCI) [27,28], which is adopted to monitor drought operationally by the CMA. The MCI is calculated by precipitation and mean temperature in each station. A snowstorm day is identified when the daily snowfall reaches 10 mm and above. Figure 2 shows the spatial pattern of the major climate hazards in China from 1991 to 2020. It is detectable that Southeastern China and Northwestern China are the two hazard zones that are frequently affected by heatwaves (Figure 2a). The number of annual hot days reaches 20 to 30 in the south of the Yangtze River, Hainan, and Chongqing. They are even greater in southern Xinjiang, Junggar Basin, and Western Inner Mongolia, being generally 30 to 50 hot days on annual average. Turpan Basin is the region with the highest temperature in China, with more than 60 hot days on annual average. In addition, a historical extreme temperature of 49.0 • C has been recorded in Turpan city, Xinjiang. The heatwaves in China usually occur from May to September. It has negative effects on human health and agriculture production, while long-lasting heatwaves can also strain energy supplies by leading to a surge in demand for water and electricity. Due to accelerated climate warming, the areas affected by heatwaves in mainland China expanded from 468 km 2 in the 1990s to 515 km 2 in the 2010s (Figure 3a), which indicates that the heatwave risk will continue to aggravate in the future.  China's rainfall is greatly influenced by the East Asian monsoon. With the northward movement of the East Asian monsoon, the monsoon rain belt experiences three notable stationary stages and forms the pre-summer rainy season in South China, the Meiyu in the Yangtze River, and the rainy season in North China [29][30][31][32][33][34][35]. Accompanying the southward retreat of the East Asian monsoon, a flood season caused by tropical cyclones affects South China again [36,37]. Rainstorms occur frequently in the rainy seasons and tend to result in floods. As shown in Figure 2b, the annual number of days with rainstorm decreases from southeast to northwest in China. There are generally four to eight rainstorm days in the south of the Yangtze River and more than eight days in coastal South China, but it is almost rare in Western China. Compared with the 2010s and 1990s, the areas affected by rainstorms in mainland China expanded from 361 km 2 to 382 km 2 (Figure 3b), indicating the intensification of rainstorm hazard in nearly 30 years.

Spatial Pattern of Multiple Climate Hazards in China
Meteorological droughts refer to surface water shortage due to the imbalance between evapotranspiration and precipitation in a certain period, which has serious impacts on crop growth and even causes crises of water resources. Seasonality and regionality features the meteorological droughts in China. It mainly occurs in late spring, summer, and autumn in North China, in autumn and winter in South China, and in winter and spring in Southwest China. Generally, meteorological droughts mostly occur in North China, Huang-Huai Plain, Eastern Inner Mongolia, and Southwestern China, with an annual number of drought days being more than 60 days (Figure 2c). From 1991 to 2020, the areas affected by droughts in China shows a weak upward trend, shrinking from 218 km 2 in the 1990s to 165 km 2 in the 2010s (Figure 3c). However, an increase in drought occurrence are observed in North China, eastern Northwest China, and eastern Southwest China (Figure not shown).
The winter climate in China is dominated by the East Asian winter monsoon [38][39][40][41]. The strong East Asian winter monsoon leads the active cold air generating in the polar area to China and results in chilly weather, strong winds, snowstorms, and ice freezing, etc. The cold air is usually active in late autumn, winter, and early spring, which could lead to damages in houses and infrastructures and can adversely affect agriculture, transportation, livestock, and fishery production. The cold air breaks out southward along four main paths, including the west path, the middle path, the west path, and the concurrent eastwest path [42]. The active cold air tends to cause snowfall. Heavy snowfall, especially snowstorms, has a great impact on agriculture, animal husbandry, communication, energy supply, and traffic. Figure 2d shows the climatic distribution of annual snowy days. Snowfall mostly occurs in the northern Xinjiang autonomous region, Northeast China, the Tibetan Plateau, and Inner Mongolia, with the annual snowfall days reaching 30 days and above. The annual snowfall days appear the most in Southern Qinghai and Eastern Inner Mongolia, being over 60 days, and these areas are also prone to snowstorm [43,44]. Compared with the 2010s and 1990s, the areas affected by snowstorms in China shrank from 319 km 2 to 253 km 2 but varies widely on an inter-annual scale (Figure 3d).
China is heavily affected by the tropical cyclones generated in the northwest Pacific Ocean and the South China Sea. Typhoons not only bring wild winds and huge waves but are also accompanied by heavy rainfall and storm surges, causing serious socioeconomic impacts. Typhoons occur from April to December, especially from July to September, and are observed with three main moving paths to affect China: the northwest path, the westward path, and the offshore turning path. Recent decades have witnessed an average of 26 typhoons generated in the northwest Pacific Ocean every year, and about 7 of them land in China. The average length of typhoon season in China is 104 days and shows a shortening trend in recent years, but the intensity and duration of landing typhoons are increasing.
The disastrous impacts of climate hazards in China during 2001-2020 are estimated from six aspects, including affected population, mortality, affected cropland, crop failure areas, collapsed buildings, and direct economic losses ( Figure 4). In terms of affected population, floods and droughts account for a high proportion, 31.9% and 34.4%, respectively. The highest proportion of deaths is caused by rainstorms and floods at 52.1%, followed by severe convective weather at 36.2%. Drought is the dominant hazard to agricultural production, accounting for 48.1% of total affected croplands and 46.2% of total crop failure areas. Rainstorms and floods cause the majority of collapsed buildings among all hazards, occupying 73.8% of the total. In terms of direct economic losses, the highest proportion of 43.1% is caused by rainstorms and floods, followed by droughts and tropical cyclones, and comparatively limited losses can be seen in snowstorms and freezing. To learn the integrated spatial pattern of climate hazards in China, a comprehensive hazard map is generated combining heatwaves, rainstorms, droughts, snowstorms, as well as the main moving paths of coldwaves and typhoons during 1991 to 2020. The hazard zones are identified with comparative thresholds that are at least one standard deviation above the spatial average of certain indices in China (refer to Figure 2). Specifically, the heatwave hazard zone refers to an area with more than 20 hot days per year, the rainstorm hazard zone refers to an area with more than 4 rainstorm days per year, the drought hazard zone refers to an area with more than 45 drought days per year, and the snowstorm hazard zone refers to an area with more than 40 snowfall days per year.
As shown in Figure 5, most areas of China are affected by different types of climate hazards. Northern China is most susceptible to coldwaves and snowstorms. Droughts dominate central-eastern China and coldwaves also have widespread impacts in this re-gion. Heatwaves, rainstorms, and typhoons superimpose in Southeastern China, and coldwaves may also reach south of the Yangtze River. It is obvious that there are various types of climate hazards in China with broadly negative impacts. Southeastern China, in particular, is heavily affected by multiple hazards simultaneously, and the dense population and concentrated economic activities are expected to further amplify climate-related socioeconomic risks.

Impacts of Climate Hazards on Urban Development
Climate hazards have negative impacts on urban development. Cities, where humans gather and economic activities are concentrated, have been rapidly expanding during Chinese urbanization in recent decades and hence have become hotspots affected by climate hazards [45][46][47]. Since climate changes are expected to drive the intensification of climate hazards, it is imperative to build climate-resilient cities to mitigate climate risks. In order to facilitate decision making on urban risk management, some issues and solutions have been discussed as follows based on the climate hazard patterns in China.
Climate hazards are proposed to be taken into consideration in urban planning from two aspects: climate change and extreme events. Climate change affects urban development through long-term changes such as rising sea levels, environmental aridification, and the intensification of urban heat/rain island effects, while extreme events have an immediate impact through heavy rainfalls, typhoons, and heat/cold waves. From the perspective of climate impacts, extreme-temperature-related impacts include increased summertime strain on materials, peak electricity loads in summer (conversely, reduced heating requirements in winter) [48][49][50]. Extreme-precipitation-related impacts include increased flooding (street, basement, sewer) and reduction in water quality [51,52]. Sealevel-rise-related impacts include inundation of low-lying areas, expansion of wetlands, increased structural damage, and impaired operations [53][54][55]. In warmer climate, heatwaves are projected to increase in frequency, intensification, and duration; inland flooding induced by precipitation extremes are likely to exacerbate, while flash droughts are also simulated to intensify in the future. Under this trend, climate risks are expected to bring greater challenges to urban development in the future.
The potential changes in climate hazards increase the complexity of urban planning for policymakers. For example, sea-level rise may cause the low-lying coastal areas, flood plains, and steep slope areas to become unaccommodated for residence; an increase in drought may result in the underground and drinkable water to be in short supply. Policymakers should fully understand the hazard exposure of residents and urban systems and consider how to reach the balance between urban developments and climate risks. Given that building of climate-resilient cities in China has a high priority, we have developed a strategic framework to achieve urban sustainability, shown in Figure 6.

Adaptive Capacity-Based City Expansion Management
City adaptive capacities are defined as the ability to absorb and recover from climate impacts [7]. Factors determining adaptive capacity include but are not limited to the following: income levels and Gross Domestic Product (GDP), natural resource availability and distribution, levels of public cognition on climate risks; the availability of technological capacity and adaptation options, the availability and quality of environmental factors (e.g., land, water, raw materials, biodiversity), infrastructure quality and provision, ability to act collectively to develop and implement adaptation responses, and public education as well as emergency skills.
As the expansion of cities aggravates the overwhelming energy consumption, transportation systems, and drainage systems, it is necessary to restrict the extent of cities according to environmental conditions. Moreover, the integrated assessment of climate risks, vulnerabilities, and adaptive capacities may provide a solution on city expansion. The types of climate hazards, levels of vulnerability and capacity, and socio-economic characteristics should be taken to promote the efficiency of adaptation planning and policy making in each city. The probability of climate extremes in the present and the foreseeable future can be estimated by climate models, and socioeconomic exposure as well as vulnerabilities are always changing with time, resulting in certain uncertainties of climate risks. Hence, the urban development path should be matched with the enhancement of adaptive capacity.

Improving Technical Standards for Hazard Prevention
Improving technical standards is good practice for preventing climate hazards at hardware level. It is necessary to develop risk assessment models for city construction based on scenario simulations. To date, urban climate assessment mainly relies on historical events or trend extrapolation but lacks the understanding of potential or unprecedented risks. In particular, specific industries and regions are rarely focused [56]. With the advances in numerical simulation, scientists obtained climate projection by using a combination of dynamic and statistical methods. Regional climate models can further improve the temporal and spatial resolution of climate simulations and enhance the performance of urban extreme weather and climate events. Given the differences in geographical location, economic level, and urban adaptability, each city in China should independently estimate the potential risks based on climate simulations and then formulate the specialized urban development plans.
The technical standards involve various industrial sectors, such as road traffic, water supply and drainage, electricity, communications, gas, flood control, and greening space. At present, climate change is limitedly considered in the technical standards referring to urban construction. Moreover, the infrastructure standards are far below the requirements of defense of climate hazards in coastal China [57,58]. The increased occurrence of compound hazards in coastal areas highlights the necessity of cross-department collaboration. For example, the prevention standards for coastal flooding caused by river flood, extreme rainstorm, and storm surge need to be jointly designed by meteorological, hydrological, and oceanic departments [5]. In the future, the formulation of technical standards will no longer be a simple technical issue but require more consideration of compound or cascading effects of multi-hazards on urban operations through multi-industry modeling and public participation [59][60][61].

Specifying Climate Risk Assessment as a Mandatory Part of Urban Planning
As the application of climate risk assessments in China's urban planning is not mandatory, the potential impacts of climate risk are significantly underestimated [62,63]. This neglect can directly affect the normal operation of the social activities, as the climate hazard can damage the critical infrastructure in cities (e.g., power transmission tower, signal tower, water storage and supply system, railway stations, hazardous substance installations). Appropriate climate risk assessments can minimize the negative impacts of climate hazards and protect human life, health, and property.
The expanding urban area, increasing populations and growing economic status have elevated exposure of cities. Meanwhile, warming temperatures have intensified climate hazards in urban areas. This double pressure makes it necessary to consider climate risk assessment in urban planning, and due to the close and complicated connections of infrastructure in different sectors, it is not wise to consider risk management when the construction is already completed. Oppositely, the best timing of risk assessment is the preliminary stage of urban planning. To set climate risk assessment as a mandatory issue in urban planning has been approved an effective and economic way to enhance the climate resilience in many newly developed cities around the world.

Strengthening Emergency Management for Extreme Climate Hazards
As mentioned above, many cities are vulnerable to facing future climate changes. In order to mitigate climate risks, emergency management for extreme climate events should be strengthened. Both direct and indirect interventions contribute to the improvement of emergency management capabilities. Direct intervention is to improve the existing emergency management methods. For example, flood protection strategies should be adjusted according to the precipitation intensity. Indirect intervention includes reducing the vulnerability of cities through strategic spatial planning. For example, public transport networks should be designed or improved for easier access and then to reduce the exposure to high temperature in heatwave events.
Urban emergency infrastructures are also crucial to respond to extreme climate hazards, serving as a lifeline for people exposed to extreme events. Routine maintenance and stress testing of emergency infrastructures should be strengthened to maximize rescue functions. Since emergency management cannot take effect simply through the command of planning strategies, increasing public knowledge and awareness of extreme climate events is essential for vulnerability reduction and risk mitigation. Administrative management strategy should also combine with public initiatives, and the important role of scientific researches in supporting decision making should be constantly strengthened and verified in practice.

Conclusions and Discussion
China is one of the countries with the highest climate risks in the world due to its monsoon phenomenon and complex climate patterns. Here, we systematically reveal the spatial pattern of multiple climate hazards in China, including heatwaves, rainstorms, snowstorms, droughts, as well as the moving paths of coldwaves and typhoons. On this basis, we summarize the impacts of climate hazards on urban development, and further propose to incorporate the strategic goals of climate adaptation into urban development of China, hoping to mitigate climate risks and enhance urban sustainability.
The Southeastern China and Northwestern China are two hazard zones with more than 30 hot days annually, and the extent of hazard zone has been expanding in recent 30 years. Cities located in the heatwave hazard zone face great risks to human health and energy supply. South of the Yangtze River, especially in coastal South China, belongs to the hazard zone of rainstorms. Cities located in rainstorm hazard zone face great risks of urban waterlogging, river flooding, and triggered geological hazards. Drought mostly occurs in North China, the Yellow River-Huaihe River basin, eastern Inner Mongolia, and Southwestern China, with more than 45 drought days annually. Cities located in the drought hazard zone face great risks of water supply and agriculture production. Coldwaves, accompanied by strong winds, snowstorms, and ice freezing, frequently invade northern Xinjiang, Northeast China, Tibetan Plateau, and Inner Mongolia, threatening public infrastructures, transportation, and energy supply in cities along the way. Cities located in southeast coastal China should be prepared for typhoon system that bring wild winds, heavy rainfall, and storm surges in summer half year.
Given that cities are hotspots affected by intensified climate hazards in a warmer world, some specific adaptation measures have been developed to facilitate the construction of climate-resilient cities. Specifically, city scale and land-use pattern should be formulated by urban carrying capacity assessment. Technical standard of public infrastructures should be improved to resist intensified climate hazards. Local risk assessment should become a mandatory part of urban planning to guide future development away from hazard zones. Emergency response capabilities should be strengthened by stress testing to reduce the disastrous impacts of extreme hazards.
Due to the current underestimations of climate impacts on urban development, there is an urgent need to improve climate adaptation. If climate-related policies and actions are still marginalized, the steady deterioration of urban sustainability could lead to irreversible economic and social issues in the foreseeable future. Several actions proposed in this study could provide a guideline for building climate-resilient cities that are better able to address climate change in the coming future.

Conflicts of Interest:
The authors declare no conflict of interest.