A Study of the Impacts of the Spatial Differences in Climate Engineering Programs on the Intensities of Extreme High-Temperature Events in China Under A 1.5℃ Temperature Control Target

With the continuous development of global climate changes and the acceleration of urbanization, it has become increasingly difficult to achieve the temperature control target of 1.5℃ set by the Paris Agreement. At present, addressing the problem of global warming has become one of the important challenges facing mankind (IPCC AR5 ^[1]; IPCC SREX ^[2]; IPCC SR1.5 ^[3]). This is especially true for small island nations located in the oceans, because global warming is causing glaciers to melt, accelerating the rise of sea levels and water cycles around the world. These phenomena lead to a series of problems that directly threaten the survival of regional residents and also have a negative impact on social development (Morgan et al. ^[4]; Hegerl et al. ^[5]; Pierce et al. ^[6]). It is found that the frequent occurrence of extreme weather and climate events caused by global warming has brought serious challenges to both global and regional socio-economic development (Roache ^[7]; Dickinson ^[8]; Tilmes ^[9]).

In recent years, extreme high temperature events have frequently occurred all over the world (IPCC AR5 ^[1]). The heat island effect caused by rapid urbanization has been intensifying, especially in developing countries, which has led to increasingly frequent hot weather in the region (IPCC SREX ^[2]). Extreme high-temperature events have become a serious meteorological disaster, which will not only directly affect agricultural production and cause water supply and power shortages, but also directly endanger human health and seriously influence the living conditions and quality of life for affected populations (Pierce et al. ^[6]). The Intergovernmental Panel on Climate Change (IPCC) pointed out in the fifth global temperature assessment report that compared with the records from 1850 to 1900, the average global temperature increased by 0.78 ℃ from 2003 to 2012. It is estimated that by the end of the 21st century, the global average temperature in the RCP 8.5 scenario will rise by 2.6℃-4.8℃ when compared with that at the beginning of the 21st century, and the global average temperature in the Arctic region will rise by even more than 11℃. In contrast, the rises of the average temperatures in the low and medium emission scenarios are determined to be in smaller ranges of between 0.3℃-0.7℃ (RCP 2.6), 1.1℃-2.6℃ (RCP 4.5), and 1.4℃ - 3.1℃ (RCP 6.0), respectively (IPCC AR5 ^[1]). The Special Report for Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX) launched by the IPCC showed that by 2081-2100, in most low-latitude regions of the world, such as sub-Saharan Africa and northern and central South America, the maximum daily temperature will exceed 30 ℃ for more than 110 days each year (IPCC SREX ^[2]). As for China, from 1951 to 2007, the average annual temperature in China increased significantly, with a temperature change of 0.22 ℃ / year. The warming in China began in the 1980s and has displayed a gradual acceleration trend since then. At the same time, the average temperature of spring, summer, autumn, and winter in China is on the rise, especially in winter (Chen et al. ^[10]; Xin et al. ^[11]; Kong et al. ^[12]). In addition to direct warming, global warming and climate change may also have more complex potential impacts on the planet and humans. Rising temperatures lead to increased instability of the weather system, making natural disasters such as tropical storms and tornadoes more frequent (Xin et al. ^[11]). Also, the stability and distribution of the world's food production activities are expected to change dramatically. Extreme hightemperature events will lead to reductions or even terminations of crop production activities, as well as death of poultry and livestock caused by heatstroke. Unfortunately, extreme high-temperature weather events will have more widespread and serious impacts on human health in the future if no actions are taken. Some diseases which currently occur mainly in tropical areas, for example malaria, may spread to mid-latitudes as Earth's climate warms (IPCC SREX ^[2]). Although airconditioning and other cooling measures are becoming increasingly popular, and people have a better understanding of high-temperature events, there are still many people suffering from high-temperature weather (IPCC SR1.5 ^[3]).

In view of this, climate engineering has become the most direct means of human intervention into climate warming and has been frequently mentioned in international climate negotiations in recent years. The subject of climate engineering even sparked a global discussion about the possibility of coping with climate warming trends (Hegerl et al. ^[5]; Pierce et al. ^[6]; Roache ^[7]; Dickinson ^[8]; Tilmes ^[9]). According to the definition of the IPCC, climate engineering, also known as geoengineering, can be divided into two categories. The first is carbon dioxide removal (CDR). The main principle and path of CDR is to reduce the concentration of greenhouse gases in the atmosphere through various carbon capture, storage and conversion technologies. The second category is solar radiation management (SRM), which mainly affects solar radiation entering the atmosphere to "directly cool" the earth. The IPCC held its first meeting to address global warming with the theme of "geo-engineering" in 2011. At the same time, the IPCC fourth and fifth assessment reports began to pay more attention to the use of climate engineering solutions to respond to global warming, and began to explore the technical and governance issues of using climate engineering technologies to respond to global warming (IPCC AR5 ^[1]; IPCC SREX ^[2]). At present, the research on geo-engineering focuses on policy and ethical issues, mainly discussing the potential for climate engineering to destroy regional weather and climate models and monsoon systems. This may pose severe challenges to agriculture and other sectors which are dependent on predictable seasonal cycles (Shi et al. ^[13]; Xin ^[14]; Chen ^[15]). In particular, it has been suggested that the sudden stop of climate engineering programs may lead to a retaliatory rebound in temperature, which could pose serious threats to natural environments and biodiversity (Chen et al. ^[16]; Xing ^[17]; Dong et al. ^[18]; Yin et al. ^[19]). Comparatively speaking, there are few studies on the quantitative diagnosis of the impact of climate engineering using numerical simulations, and China's research on climate engineering is still preliminary. There is a pressing need to strengthen the research on climate engineering to improve our understanding on it.

As mentioned earlier, based on the G4 climate engineering test data of the BNU-ESM model, this study evaluated the possible impacts of climate engineering on extreme high-temperature events in China before and after the implementation of the programs. Also, this study explored the regional differences in extreme hightemperature events at different stages of implementation under the scenarios of climate engineering and nonclimate engineering on Earth, and particularly investigated whether there would be a retaliatory rebound of extremely high temperatures after the climate engineering programs had ceased. On the one hand, the results of this study will likely expand and deepen the understanding and knowledge of the possible impacts of climate engineering on regional climates. On the other hand, it is expected that the results will attract the attention of climate engineering stakeholders and provide possible references for international climate negotiations and climate engineering governance in the future.

This study is based on the BNU-ESM (2.5 ° × 2.5 °) model scenario, using the estimated daily maximum and average temperature data for China's 0.5 ° × 0.5 ° non-climate projects from 2020 to 2099. Due to the low spatial resolution of the original data, in order to study the temporal and spatial variation of high temperature in China, we made statistical downscaling of the original data. Under the RCP4.5 scenario, the 125 × 105 grid data were obtained using statistical downscaling methods which had been corrected by bilinear interpolation and ISIMIP (Xin ^[14]; MIAO et al. ^[20]). The spatial coverage ranges were 73.25° to 135.25° E and 3.25° to 53.75° N, respectively. The observational forcing data of the model were the data obtained from January 1, 1970 to December 31, 1999. The daily maximum temperature prediction data of the climate engineering were sourced from the G4 tests of climate engineering conducted from January 1, 2020 to December 31, 2019. In other words, the sulfate aerosol injected into the stratosphere had reflected the solar radiation (SAI scenario) and reduced the global temperature. The injections was scheduled to stop in 2070. That is to say, the climate engineering programs would cease in 2070, while the model would continue to operate until December 31, 2099. Finally, the maximum temperature response following the completion of the climate engineering programs would be checked and analyzed (Keith et al. ^[21]; Scott et al. ^[22]). The GeoMIP plan includes four major simulation test schemes: G1, G2, G3 and G4. Among them, G1 and G2 study directly reduce the climate effect of solar radiation geoengineering (such as reducing solar radiation constant); G3 and G4 study the climate effect of stratospheric injection aerosol geoengineering. G3 test simulates aerosol injection into stratosphere to change radiative forcing. In order to get closer to the reality, the experiment assumes that based on the RCP4.5 scenario in CMIP5, a certain amount of aerosols will be injected into the stratosphere gradually from 2020 to balance the solar radiation forcing caused by the increased carbon dioxide of human activities. In order to achieve this goal, G3 experiment further hypothesizes that according to the needs of different balance carbon dioxide radiative forcing, appropriate amount of aerosols can be injected randomly to achieve a sustained and stable radiative forcing equilibrium state. The mechanism of G4 test is similar to that of G3 test, and it is also based on the scenario of RCP4.5 in CMIP5 to simulate the stratospheric injection of aerosols. However, the specific test assumptions are different from that of G3 test, which aims to achieve a sustained and stable radiation balance in the whole test cycle. G4 test assumes that from 2020, a fixed dose of aerosols will be continuously injected into the stratosphere every year. And then the corresponding climate impact is observed. BNU-ESM is one of the 15 participating models of the GeoMIP plan. The atmospheric model in BNU-ESM is CAM3.5, the ocean model is MOM4p1, and the land surface model adopts the common land surface process model (CoLM) independently developed by Beijing Normal University. The BNU-ESM simulation schemes all refer to the unified simulation return process, that is, the fifth global coupling model comparison program, and uniformly adopt the method of set simulation to remove the background interference of the model itself, and then conduct a comparative study of the simulation effects.

In this experimental study, the sequences of the temperatures exceeding 90% of the maximum daily temperature were defined as extreme high temperatures. The extreme high temperature of different cycle periods is used as the risk factor measurement of extreme high temperature events, and the calculation is carried out using Weibull distribution theory. The distribution of each grid sample set is fitted using the Weibull distribution theory. The probability density function of Weibull Distribution, as well as the recurrence period corresponding to an extreme maximum temperature under this distribution, were calculated as follows:

In the formula above, λ > 0 is a scale parameter; k > 0 is a shape parameter; f(x) represents the probability density function; F(x) is the cumulative density function; p indicates the recurrence period corresponding to the extreme maximum temperature; x denotes the extreme high temperature; and x_m is the extreme maximum temperature above x, corresponding to the 10, 20, 50, and 100-year recurrence periods. Based on the results of extreme maximum temperatures in different recurrence periods, this study compared the spatial pattern and difference characteristics of extreme high-temperature intensity in different recurrence periods under climatic engineering and non-climatic engineering scenarios. Also, the potential impact of different stages of climate engineering project implementation on extreme hightemperature intensities was compared.

From the perspective of the implementation of climate engineering programs from 2020 to 2069, this study first conducted a comparative analysis of spatial differentiation patterns of the extreme high temperatures with different recurrence periods under the scenarios of climate engineering and non-climate engineering for the period ranging from 2020 to 2069. The results are shown in Fig. 1.In the figure, the redder the color, the higher the temperature, and the bluer the color, the lower the temperature. Therefore, for the implementation of climate engineering in 2020 to 2069, the extreme high temperature intensities in both scenarios had approximately the same spatial differentiation characteristics. In other words, there was low intensity on the Qinghai-Tibet Plateau, and high intensity on the east, north, and northwest. It is noteworthy that the intensities of the extreme high temperatures with different recurrence periods under the non-climate engineering scenario were significantly higher than those under the climate engineering scenario. From the perspective of the different recurrence periods, the spatial patterns of extreme high-temperature intensities also displayed different characteristics. Under the climate engineering scenario, the extreme hightemperature intensity of the Qinghai-Tibet Plateau is lower than 30 ℃, and there are few areas above 42 ℃. The extreme high-temperature intensities in the eastern and northern sections of the Qinghai-Tibet Plateau had mainly ranged within 36 to 39℃. Meanwhile, the extreme high-temperature intensities in the eastern coastal areas were relatively low, falling within the range of 33 to 36℃ (Fig. 1a). In the non-climate engineering scenario, the extreme high-temperature intensities of the Qinghai-Tibet Plateau were still below 30℃. However, the distribution areas were less than those of the climate engineering on the edges of the plateau. It is worth noting that under the non-climatic engineering scenario, the extreme high-temperature intensities in the mainland of China, with the exception of the Qinghai-Tibet Plateau, increased with the increase of latitude. The extreme high-temperature intensities of eastern Xinjiang-western Gansu and northeastern Inner Mongolia-northwestern Heilongjiang were the highest and exceeded 54℃ (Fig. 1b). With the increases in the recurrence periods, the intensities of the extreme high temperatures in China gradually increased under the two scenarios (Fig. 1c-Fig. 1h). The study found that, under the climate engineering scenario, the number of areas with extreme high temperature intensity exceeding 39℃ in the east and west of the Qinghai-Tibet plateau continued to increase, and the majority of the areas were from 39℃ to 42℃ (Fig. 1c, Fig. 1e, and Fig. 1g). It was observed under the non-climate engineering scenario that with the increases in the recurrence periods, the range within the Qinghai-Tibet Plateau where the extreme high-temperature intensities were lower than 30℃ had continuously decreased, and the number of regions in northeast and northwest regions of China with extreme high-temperature intensities (exceeding 54℃) expanded continuously. The regions where the extreme high temperature extra-intensities (exceeding 54℃) with recurrence periods of 100 years had gradually expanded to the north of the Yellow River. At that time, extreme high-temperature weather events (over 54℃) began to appear over a wide range in Beijing, Tianjin, and Hebei (Fig. 1d, Fig. 1f, and Fig. 1h). At the same time, it is worth noting that the extreme high-temperature intensities in Hainan and southern Taiwan, which had recurrence periods of 100 years, were mainly lower than 39℃ in the climate engineering scenario, and higher than 54℃ in non-climate engineering scenario. In summary, in this study's comparison of the extreme high-temperature intensities in China under the two scenarios from 2020 to 2069, it was observed that the implementation of the climate engineering programs contributed to the reductions of extreme high-temperature intensities with different recurrence periods.

Figure 1.  Spatial patterns of extreme high-temperature intensities in China under the two scenarios during the implementation of climate engineering programs (2020 to 2069).

In this study, in order to compare the difference characteristics of extreme high-temperature intensities with different recurrence periods between the two scenarios during the implementation of climate engineering programs (2020 to 2069), the extreme hightemperature intensities with different recurrence periods in the climate engineering scenario were utilized and the extreme high-temperature intensities with corresponding recurrence periods in the non-climate engineering scenario were subtracted. The results are shown in Fig. 2. The extreme high-temperature intensities with different recurrence periods in the climate engineering scenario (Fig. 2) were determined to be lower than those in the non-climate engineering scenario. In the figure, the deeper the blue, the greater the temperature difference, and the deeper the red, the smaller the temperature difference. Therefore, it could be concluded from the results that, according to the comparison of the extreme high temperatures under the climate engineering scenario in China during the period ranging from 2020 to 2069 with that under the non-climate engineering scenario, the extreme high-temperature differences in the Chinese mainland under the two scenarios increased with the increases in latitude. Among those affected areas, the areas with temperature difference exceeding 18℃ were mainly concentrated in Hainan, Taiwan, northeast regions of northern China, and the border areas between Xinjiang and Gansu. Moreover, in the Chinese mainland, it was found that with the increases in the recurrence periods, the range of the regions with extreme high- temperature differences (exceeding 18℃) under the two scenarios gradually increased and expanded. Also, for the middle and lower reaches of the Yangtze River, the temperature differences of the extreme high temperatures with a 10-year recurrence period in those areas were mainly concentrated between 4℃ and 6℃ (Fig. 2a). It was observed that with the increases in the recurrence periods, the temperature differences continuously increased (Fig. 2b, Fig. 2c, and Fig. 2d). The extreme high-temperature differences with the 100-year recurrence period in the middle and lower reaches of the Yangtze River Basin increased by 6℃ to 8℃ in both scenarios. The extreme high-temperature differences in the Tibetan Plateau were found to have obvious regional characteristics under the two scenarios, in which the temperature differences in Tibet approximately ranged between 4℃ and 14℃, and had displayed minimum changes with the increases in the recurrence periods. However, the temperature differences in Qinghai area had increased from 4℃ - 14℃ to 6℃-18℃ as the recurrence periods increased.

Figure 2.  Differences in the characteristics of extreme high-temperature intensities in China under the two scenarios during the implementation of climate engineering programs (2020 to 2069).

According to the data of the number of grid points of extreme high-temperature differences under the two scenarios (2020 to 2069), the extreme high temperatures with different recurrence periods in the climate engineering scenario were generally lower than those in the non-climate engineering scenario, and the temperature differences were mainly within a 5℃ to 15℃ range (Fig. 3). It was observed that with the increases in the recurrence periods, the number of grid points within the 5℃ to 15℃ range gradually decreased, indicating that the longer the recurrence period, the larger the areas with increased extreme temperature differences under the two scenarios. In summary, the spatial patterns of extreme high temperatures under the two scenarios have not fundamentally changed during the implementation of climate engineering programs (2020 to 2069). However, it was found that the extreme high-temperature differences between the two scenarios have significant differences, and the longer the recurrence period of the extreme high temperatures, the larger the temperature differences. The results also indicated that the implementation of climate engineering programs had been helpful in mitigating extreme hightemperature intensities in China.

Figure 3.  Statistical calculation of the number of grid points of extreme high-temperature intensity differences under the two scenarios during the implementations of climate engineering programs in China (2020 to 2069).

As for the period after the implementation of climate engineering programs (2070 to 2099), this study further compared the extreme high-temperature intensities with different recurrence periods under both the climate engineering and non-climate engineering scenarios. The results are shown in Fig. 4. The legend in Fig. 4 has the same meaning as that in Fig. 1. It can be seen that under the climate engineering scenario from 2070 to 2099, China's extreme high temperature intensity had obvious spatial differentiation characteristics. Among those characteristics, the extreme high-temperature intensities in the Qinghai-Tibet Plateau were observed to be the lowest and generally below 30℃, indicating little change occurred with the recurrence period (Fig. 4a, Fig. 4c, Fig. 4e, and Fig. 4g). The extreme high-temperature intensities reached the highest in the southern and northern sides of Tianshan Mountains in Xinjiang. It was observed that with the increases in the recurrence periods, the extreme hightemperature areas (within 45 to 48℃) continued to expand up to the 100-year recurrence period. The extreme high temperature of these areas was basically in the range of 45-48℃, except for the Tianshan Mountains. The extreme high-temperature intensities in eastern China were also found to have obvious regional characteristics. The majority of the coastal areas were in the range of 36 to 39℃, while the adjacent inland areas were in the 39 to 42℃ range. It was observed that with the increases in the recurrence periods, the inland areas with extreme high-temperature intensities between 39 and 42℃ had gradually expanded to the coastal areas. Meanwhile, the coastal areas with extreme hightemperature intensities within the 36 to 39℃ range had continued to shrink. It is noteworthy that the extreme high temperatures in Hainan and Taiwan did not change very much with the increases in the recurrence periods under climate engineering scenarios. However, the intensities of the extreme high temperatures in the Beijing, Tianjin, and Hebei regions increased from 39 to 42℃ with a recurrence period of 10 years, to the range of 45 to 48℃ with a recurrence period of 100 years. In the non-climate engineering scenario, the extreme hightemperature intensities with different recurrence periods in China during the period ranging from 2070-2099 displayed obvious regional characteristics. Among them, the Qinghai-Tibet Plateau had the lowest intensity, the northwest and the area north of the Yellow River the highest, and the area south of the Yellow River intermediate (Fig. 4b, Fig. 4d, Fig. 4f, and Fig. 4h). Among them, the temperature in the Qinghai-Tibet Plateau is generally lower than 30℃. However, with the increase of the recurrence period, the area with temperature below 30 ℃ decreases, while the area with temperature within the range of 30 ~ 36 ℃ increases. The area of extreme high temperature intensity (over 48 ℃) gradually expanded from the northwest, northeast and north to east and south of China, with a recurrence period of up to 100 years, and then to the north of Huanghuai. It was found that the majority of the extreme high-temperature intensities in southern China were higher than 48℃. As can be seen from the above results, during the period from 2070 to 2099, climate engineering still helped to alleviate the extreme hightemperature intensities in China. Generally speaking, the mitigation effects were found to more obvious in eastern China with the increases in latitude.

Figure 4.  Spatial patterns of extreme high-temperature intensities in China under the two scenarios following the implementation of climate engineering programs (2070 to 2099).

In order to compare the characteristics of the extreme high-temperature intensities with different recurrence periods under the two scenarios in the period ranging from 2070 to 2099, this study used the extreme high-temperature intensities with different recurrence periods under the climate engineering scenario to subtract those under the non-climate engineering scenario. The calculation results is shown in Fig. 5. As can be seen in Fig. 5, the extreme high-temperature intensities with different recurrence periods under the climate engineering scenario were also lower than those in the non-climate engineering scenario. The legend in Fig. 5 has the same meaning as that in Fig. 2. It was obvious from the calculation results that the extreme high temperature differences in China under the two scenarios from 2070 to 2099 had regional differentiation (Fig. 5a, Fig. 5b, Fig. 5c, and Fig. 5d), and the temperature differences in western China displayed little change with the increases in the recurrence periods. The extreme high-temperature differences in eastern China were found to have the obvious spatial differentiation characteristics of "higher in the north and lower in the south". It was observed that with the increases in the recurrence periods, the areas with extreme hightemperature differences (over 18℃) in the north continued to expand. Also, up to a 100-year recurrence period, those areas could potentially expand to Beijing, Tianjin, and Hebei. In the middle and lower reaches of the Yangtze River, the regions with extreme hightemperature differences (within 2-6 ℃) were continuously shrinking. In conclusion, after the implementation of climate engineering, the extreme high temperatures in China from 2070 to 2099 were still significantly different under the two scenarios. This was fIn summary, it was found in this study that with the implementation of climate engineering programs, extreme high-temperatures in northwestern, eastern and southern China, northwestern Yunnan and Heilongjiang showed the highest degree of mitigation, and the extreme high-temperatures in Beijing, Tianjin and Hebei were relieved to a certain extent. Therefore, the climate engineering programs still contributed to the mitigation of the extreme high-temperature intensities.

Figure 5.  Difference characteristics of the extreme high-temperature intensities in China under the two scenarios following the implementation of climate engineering programs (2070 to 2099).

According to the number of grid points of the extreme high-temperature differences under the two scenarios from 2070 to 2099, the extreme high temperatures with different recurrence periods under the climate engineering scenario were found to be generally lower than those under the non-climate engineering scenario. The temperature difference between the two scenarios is mainly in the range of 3 ℃ and 15 ℃ (Fig. 6). Moreover, the temperature differences with shorter recurrence periods were relatively concentrated. It was found that with the increases in the recurrence periods, the temperature differences increased. Furthermore, the number of grid points of the larger temperature differences also increased. As can be seen by comparing the results in Fig. 5 with those in Fig. 3, the extreme temperature differences under the two scenarios were generally smaller in the period ranging from 2070 to 2099 following the implementation of climate engineering programs. In conclusion, it was found to be conducive for mitigating extreme high temperatures both during and after the implementation of the climate engineering programs. Also, the degree of mitigation of extreme high-temperature intensities during the implementation was found to be higher than the degree of mitigation after the implementations were complete.

Figure 6.  Statistics on the difference in the number of grid points of extreme high-temperature intensity in the two scenarios after China's implementation of climate engineering projects (2070-2099).

In this study, to compare the regional differences of the extreme high temperatures in China under the climate engineering scenarios both before and after the implementation, the extreme high temperatures within different recurrence periods during the implementation of climate engineering in the period ranging from 2020 to 2069 were utilized in order to subtract the extreme high temperatures in the period ranging from 2070 to 2099(the period after the implementation of climate engineering). The calculation results are shown in Fig. 7. As can be seen in Fig. 7, the extreme high-temperature intensities in China had been effectively alleviated to varying degrees during the implementation of the climate engineering programs when compared with those after the implementation. The intensity of the blue in the figure indicates that those areas were more conducive to the mitigation of extreme high-temperature intensities during the implementation of climate engineering programs. Meanwhile, the darker the red in the figure, the smaller the degree of mitigation. It can be seen from the figure that during the implementation of climate engineering programs, the mitigation of extreme high temperature was most obvious in Xinjiang, central Qinghai-Tibet Plateau, eastern and southern Yunnan, and northwestern Heilongjiang, with a drop in temperature exceeding 2℃. However, with the increases in the recurrence periods, almost no changes were observed in the above areas. It is worth noting that the extreme hightemperature differences both before and after the implementation of the climate engineering programs in the Beijing - Tianjin-Hebei region increased with the increases in the recurrence periods. In summary, it was found in this study that with the implementation of climate engineering programs, extreme hightemperatures in northwestern, eastern and southern China, northwestern Yunnan and Heilongjiang showed the highest degree of mitigation, and the extreme hightemperatures in Beijing, Tianjin and Hebei were relieved to a certain extent.

Figure 7.  Difference characteristics of the extreme high-temperature intensities in China before and after the implementation of climate engineering programs.

In this study, in accordance with the data of the number of grid points of extreme high-temperature differences, both before and after the implementation of climate engineering programs, the extreme hightemperatures during the implementation of climate engineering programs were mainly reduced by 0.5- 3.0℃ when compared with that after the implementation of climate engineering programs. It was observed that during the implementation, the temperature difference is generally between 0.5-1.5 ℃; after the implementation, the temperature difference range is between 1.5-3.0 ℃ (Fig. 8). It was found that with the increases in the recurrence periods, the grid points of the extreme hightemperature cooling range tended to gradually disperse, and the coefficient of variation of the extreme hightemperature differences with the recurrence periods of 10, 20, 50, and 100 years (before and after the implementation of climate engineering) tended to increase. These results indicated that before and after the implementation of climate engineering programs, the temperature range of extreme high temperatures tend to increase. That is to say, after the implementation of the climate programs, the mitigation effect on extreme hightemperatures is higher than that after the implementation of the climate programs.

Figure 8.  Statistical data of the number of grid points of the extreme high-temperature intensity differences in China both before and after the implementation of the climate engineering programs.

This study analyzed the spatial differences of China's average temperature under the two scenarios, and the results were shown in Fig. 9. In the figure, the stronger the blue, the lower the average temperature in the climatic engineering scenario compared to the average temperature in the non-climate engineering scenario. In addition, compared with non-climate engineering scenarios, the stronger the red, the higher the average temperature of the climate engineering scenario. Therefore, it can be seen from the figure that during the implementation of the climate engineering program (2020-2069), the average temperature under the Chinese climate engineering program was generally lower than the average temperature under the nonclimate engineering program. Among those, it was determined that the temperature differences in the western Huanghuai area, central and western Inner Mongolia, northern Gansu, central and northern Xinjiang, northern China, southern Qinghai, central and northern Guangxi, and so on, were the largest, reaching over 1℃ (Fig. 9a). The temperature differences in northeastern and eastern Inner Mongolia were observed to be the smallest, ranging from 0.5℃ to 0.75℃. It was also found that the temperature differences in other parts of the country mainly ranged between 0.75℃ and 1℃. Therefore, it was determined in this study that the implementation of climate engineering effectively alleviated global warming and also contributed to the realization of the temperature control target of between 1.5℃ and 2℃ set in the Paris Agreement. It was found that following the implementation of climate engineering program (2070 to 2099), the impacts of the climate engineering programs on the majority of China were mainly cooling effects, when compared with the non-climate engineering scenario. However, the range of the cooling effects was been significantly reduced when compared with those during the implementation of the climate engineering programs (2020 to 2069), as shown in Fig. 9b.

Figure 9.  Difference characteristics of the mean temperatures in China under the two scenarios at different stages of the climate engineering implementation.

This study also compared the different characteristics of China's average climate both before and after the implementation of climate programs under the climate engineering scenario. The results are shown in Fig. 10. In the figure, the darker blue color indicates the area that is more conducive to reducing the average temperature during the implementation of the climate engineering project than after the completion of the implementation. Similarly, the more intense the red, the less favorable the region is for the average temperature to fall.Therefore, as can be seen from Fig. 10, compared with the temperature after the completion of the program, the average temperature in China during the implementation of the climate program has dropped significantly. It was found that in the majority of the areas of eastern China, the cooling rate had reached 1.50℃ and above. It can be seen in the figure that only in regions of southern China, southwestern China, Tibet, and eastern northeast China had the range of cooling been the smallest, ranging from 1.25℃ to 1.50℃. In conclusion, when compared with the non-climate engineering scenario, it was determined in this study that the climate engineering scenario had effectively reduced the mean temperatures in China, and the cooling range during the implementation of the climate engineering programs was significantly higher than that after the implementation was complete.

Figure 10.  Difference characteristics of the mean temperatures in China prior to and after the implementation of climate engineering programs.

It is currently believed that climate engineering can significantly reduce global temperatures and effectively mitigate global warming. In this study, we compared the extreme high temperatures during different periods in climate engineering and non-climate engineering scenarios. The results obtained in this study indicated that the implementation of climate engineering could effectively alleviate the extreme high-temperature intensities in China. On the basis of this study's findings, the following aspects were considered to be worthy of further discussion.

The BNU-ESM model itself has high reliability, and its parameterization scheme has been adopted and recognized by the GeoMIP plan. Existing research results show that the prediction data of this model can reflect the climate characteristics well and have been proved to have high reliability (Xin ^[14]; Burger et al. ^[23]; Vassiliki et al. ^[24]).

At the same time, a limitation of this study is that the data only come from a single model, which has a certain degree of uncertainty. It is urgent to adopt the multi-mode set average method to reduce the uncertainty of climate engineering prediction in the future (Aswathy et al. ^[25]). Besides, a single model of climate engineering may lead to high uncertainty in simulation results due to errors in the input data and errors in the model itself. The resolution of the BNU-ESM model is obviously low, which to some extent will cover up the difference of the influence of geoengineering on sub regions. For example, the temperature, precipitation, long and short wave radiation, wind speed and other forced data used by BNU-ESM are extremely complex and interrelated at macro and micro scales. Although the deviation correction is carried out, there are still some differences with the actual situation.

At present, there are only a few researches on extreme weather and climate events in China based on the quantitative research of climate models, and these researches are still in the initial stage. This study summarized previous climate negotiations and concluded that the initial climate science issues would gradually evolve into political confrontations and international governance issues. Therefore, in order to promote the international governance of climate engineering, it is necessary to strengthen the research and experimental exploration in the field of climate events.

Moore et al. found that during the implementation of the G3 and G4 geo-engineering simulations (2020 to 2069) (Moore et al. ^[26]), the global average temperature increases were only half of those under the RCP4.5 scenario, which had significantly reduced the probability of coastal areas affected by hurricanes and floods. In another related study, Yu et al. compared and examined the four geo-engineering experiments of G1, G2, G3, and G4, and found that their effects on temperature were much greater than those on precipitation, and the cooling efficiency of the simulated solar radiation which was controlled in the geo-engineering scenarios (G1 and G2) was higher than that in the geo-engineering scenarios of stratospheric sulfur dioxide injections (G3 and G4)(Yu et al. ^[27]). Additionally, Irvine et al. obtained similar simulation results in their research. The results suggest that to prevent sea level rise, geoengineering through solar radiation management would need to reach the cooling levels at the current maximum warming rate. It was indicated that at the end of geo-engineering scenario the warming rate would be 30% (Irvine et al. ^[28]) higher than the current warming level. Therefore, the above studies all confirm the conclusion of this study to some extent.

At present, the actions of the international community on climate engineering is still immature, and many policy issues and possible implementation effects of climate engineering are still widely discussed in the academic community. However, quantitative analysis of the impact of climate engineering from a data perspective is still in its infancy. As a low-cost and effective means for the global society to directly participate in the control of climate change, climate engineering is characterized by short-term and emergency response. Under the current global economic development trends, global warming issues are not being effectively addressed. Therefore, it is of forward-looking scientific significance to theoretically explore the potential impacts of climate engineering on climate warming using numerical model methods. These studies will also help strengthen its voice in international negotiations on climate engineering.

This study quantified the potential impacts of climate engineering on extreme high-temperature intensities in China based on the BNU-ESM model data, which will help China in seizing strategic opportunity and in gaining a voice in international climate negotiations and climate engineering governance in the near future. It is worth noting that the research regarding climate engineering based on ensemble models is still developing, and the potential impact of climate engineering on extreme weather and climate events based on the results of a single climate model can be uncertain, particularly in terms of the physical parameters of individual patterns themselves that reflect reality in incomplete or unknown ways. The results of previous studies have shown that the impacts of climate change on high-altitude and high-latitude areas are significantly greater than the impacts on low-altitude and low-latitude areas. In this study, the responsiveness of the Qinghai-Tibet Plateau to climate engineering was determined to be potentially higher than that of other regions. This may have been due to the fact that the roles of climate engineering programs in high-altitude areas were more obvious than those in the low-altitude areas. These findings support the aforementioned conclusions from the aspect of excluding the problems of the data and method themselves. However, whether or not there were uncertainties in the scheme of the physical parameters of the model itself could not be directly demonstrated, and remained to be validated by the ensemble model. The processes and mechanisms of climate engineering programs on high-altitude and highlatitudes areas are considered to have a great influence on the accuracy of the models. At the same time, climate change is a multiple factor comprehensive change process(IPCC AR5 ^[1]; IPCC SREX ^[2]; IPCC SR1.5 ^[3]).In the future, there is an urgent need to carry out climate engineering research on the spatial and temporal changes of rainfall and other factors to further deepen the impact and reliability evaluation of climate engineering.

The following conclusions were reached in this experimental study.

(1) During the implementation of climate engineering programs in China (2020 to 2069), the spatial differentiation characteristics of extreme high temperatures with different recurrence periods under the two scenarios were found to have some similarities. For example, the characteristics of "lower in Qinghai-Tibet Plateau, and higher in the east and northwest". It was found that with the increases in the recurrence periods, the extreme high temperatures in China under the climate engineering scenario did not change significantly. However, in the non-climate engineering scenario, the extreme high temperatures increased significantly, especially in northern China. Under the two scenarios, the temperature differences of extreme high temperatures with different recurrence periods in China increased with the increases in latitude. Therefore, the implementation of climate engineering (2020 to 2069) would potentially contribute to the mitigation of extreme high temperatures in China.

(2) In the modelling process, it was found that after the implementation of climate engineering (2070 to 2099), the spatial differentiation characteristics of the extreme high temperatures with different recurrence periods in China under the two scenarios were similar to those during the period from 2020 to 2069. The retaliatory rebound of extreme high temperatures did not occur during the period ranging from 2070 to 2099 after the implementation of climate engineering programs had been completed, which was helpful in alleviating the extreme high temperatures with different recurrence periods in China. In this study, when comparing the results before and after the implementation of climate engineering programs under the two scenarios, we found that although both the implementation period and postimplementation period of climate engineering were helpful for the mitigation of extreme high temperatures, the mitigation of the extreme high-temperature intensities during the implementation period was higher than that after the implementation.

(3) By comparing the extreme high-temperature intensities both before and after the implementation of climate engineering under the climate engineering scenario, we found the extreme high-temperature intensities were alleviated during and after the implementation of climate engineering. Furthermore, the alleviation effects on the extreme high-temperature intensities during the implementation period we higher than those after the implementation. At the same time, it was found that the climate engineering programs also helped reduce the mean temperatures in China. The range of temperature reduction in most part of China could potentially be more than 1.25℃, and that in eastern China could be as high as 1.5℃. These findings indicate that the implementation of climate engineering could effectively alleviate global warming and contribute to the realization of the temperature control target of 1.5℃ set in the Paris Agreement.