Accelerated multiphase water transformation in global mountain regions since 1990

Changing


INTRODUCTION
The global hydrological cycle is a key component of Earth's climate system; it is also the main form of redistribution of energy on Earth.Understanding and quantifying the observed global water cycle changes is crucial for predicting the future climate.It is widely recognized that the warming of the global surface and lower atmosphere will strengthen the water cycle, [1][2][3] resulting in changes in water resource availability, frequency and intensity of floods and droughts, and amplification of warming through water vapor feedback and ecosystem services.An intensified global water cycle at a rate of 8 ± 5% per degree of surface warming was observed for the period 1950-2000 based on the global surface salinity changes. 4A water cycle amplification of 3.0 ± 1.6% /°C was further observed for the period 1950-2010 based on full depth salinity measurements. 5In addition, the enhanced atmospheric hydrological cycle in the Southern Ocean associated with the warming in the second half of the 20th century resulted in an increase in the Antarctic sea ice over the past three decades. 6It was also found that the acceleration of global water storage increased to 12 mm/a in regions such as Australia, Turkey, and Northern India over the 2003-2012 period with inter-annual and decadal climate variability.
Satellite observations have revealed that continental evaporation increased at rates consistent with expectations derived from temperature trends dominated by the El Niño-Southern Oscillation. 7The increasing intensity of the hydrologic cycle can also be proven by an increase of 5400 km 3 /10a in global freshwater discharge attributed to an increase in global ocean evaporation (768 0km 3 /10a) in the period 1994-2006. 8The global annual evapotranspiration increased on average by 7.16 mm/a/10a from 1982 to 1997, after which the increase seemingly ceased until 2008 owing to decreasing soil-moisture. 91][12][13] Therefore, the evidence for the hydrological cycle accelerating due to variations in evaporation has not been determined yet.Some studies have linked the recent global warming with increasing evaporation; whereas others have reported a decrease in pan-evaporation.However, a direct observational evidence of a positive trend in the global evapotranspiration is still lacking. 14nderstanding the observed changes in the acceleration of the global water cycle is crucial for predicting future climate changes and their impacts.While many datasets document crucial variables such as precipitation, ocean salinity, runoff, and humidity, most showed significant uncertainties related to determining long-term changes.Furthermore, the in-situ networks provided long time series over land but were sparse in many areas, particularly in mountainous regions.Satellite and reanalysis datasets provided global coverage, but their long-term stability was lacking.Therefore, the acceleration or intensification of the hydrological cycle with global warming is a longstanding paradigm in climate research, and the hypothesis that the water cycle has strengthened on a global land scale requires more evidence.Mountains are extremely important components of the Earth's surface.Although they occupy only about one-eighth of the world's land surface outside Antarctica, they are home to 10% of the world's population, and more than half of the global population is directly or indirectly dependent on mountain resources and services.Mountainous areas are often described as the world's "water towers;" they supply approximately 70% of the world's land freshwater resources. 15Mountain and hillside areas account for approximately one-third of the habitat for all terrestrial plant and animal species, providing a global barrier for ecological security and the focus areas for global biodiversity conservation.Mountains are also fragile environments that are sensitive to climatic changes; they are highly vulnerable to human and natural ecological imbalances. 16Under persistent warming, mountain disasters are expected to become increasingly more frequent with a growing list of environmental threats to humans, including floods, landslides, glacial lake bursts, mudrock flows, avalanches, and fires. 17The hydrological processes in mountain regions are core indicators of the global water cycle.The prominent hydrological features of mountainous regions can be characterized by the coexistence of multiphase waters and their transformations (Figure 1).This refers to the frequent conversions of bodies of water between solid (glaciers, snow cover, lake ice, and permafrost ground ice), liquid (rivers, lakes, marshes, soil water, plant water, and groundwater), and gas (local evapotranspiration vapor and advection vapor) states. 17Drastic multiphase water transformation (MWT) associated with climate warming has been occurring in the mountain regions comprising glaciers, snow, permafrost, lakes, and vegetation, which are crucial links in the water cycle.][18] However, the possible MWT changes and hydrological responses are poorly understood because of high meteorological variability, physical inaccessibility, and complex interplay between climate, cryosphere, and hydrological processes.Thus, a comprehensive study of MWT is urgently needed to provide a scientific foundation for predicting future water resources, ecological protection, disaster prevention, and risk management.It is important to examine the changes in hydrology in the context of climate change over the global mountain regions to understand the links between the MWT changes and hydrological responses and to develop a sustainable water resource strategy.
Therefore, based on extensive existing data (details regarding the data sources are shown in Support Information), in this review, climate driving for the MWT was analyzed for the global mountainous regions, using the year 1990 as the change point to divide the time series into two periods.The MWT processes related to changes in glacier and permafrost properties, snowfall, evapotranspiration, snow sublimation, lake ice phenology, frost days, and precipitation moisture recycling ratio have been explored in detail.Finally, changes in hydrological processes have also been discussed.This analysis is expected to provide a broad understanding of the frequency, intensity, and duration of MWTs and their hydrological effects.Understanding the potential additive and interactive effects of MWTs on hydrological processes is key to predicting and mitigating the consequences of climate warming in global mountain regions.Finally, we also discuss an increase in the frequency and intensity of natural disasters under MWT.In addition, the study also develops a new theoretical basis, approach, and direction for cold region hydrology.

Intensification of multiphase water transformation since 1960
Solid to liquid transformation.To quantify the intensification of multiphase water transformation during different periods, the variations in glacier areas for 45 glacier regions in the global mountainous regions (Alaska, western North America, northern Canadian Arctic North, southern Canadian Arctic, Iceland, Svalbard and Jan Mayen, Scandinavia, northern Asia, central Europe, central Asia, southwestern Asia, southeastern Asia, low-latitude regions, southern Andes, and New Zealand) were reanalyzed from previous literature, as shown in Supplementary Table 1 and Figure 2 and Figure 1). 202][23] Glacier melting is the main phase transformation from solid to liquid water, as indicated by the following three facts.The average glacier area retreat rate was about 73.8 km 2 /10a for the 42 glacier regions in the global mountain regions during 1960-2010s (Table 1).Notably, the largest glacier areas retreat occurred in High-Mountain Asia, low-latitude regions, southern Andes, and Alaska, whereas a relatively slow retreat occurred in the eastern Pamir, Kalakunlun and Kunlun (  (  (  ( ( ( (  ( ( ( ( 1), and all glaciers but 14 displayed a retreat in the global mountain regions (Supplementary Table 3).
Liquid to solid transformation.The variation of lake ice phenology reflected the liquid to solid transformation.The data for lake ice phenology was obtained from National Snow & Ice Data Center (http://nsidc.org/data/G01377.html),which contains freeze and thaw/breakup dates as well as other descriptive ice cover data for lakes and rivers in the Northern Hemisphere.This database includes water bodies distributed around the Northern Hemisphere and allows the analysis of broad spatial and long-term temporal patterns.In this study, one lake in Russia, three lakes in Finland, one lake in Switzerland, one lake in Canada, and 17 lakes in America, all of which are in the global mountainous regions are selected for analyzing the period from the 1960s to the 2000s (Supplementary Table 4 and Figure 1).As a sensitive indicator of climate change, the variation of lake ice phenology for 22 lakes reflected the liquid to solid transformation during 1960-2000 (Supplementary Table 4), and the complete freeze time (no liquid water left) was delayed by -23.10 d/10a, whiles the complete melting time was advanced by 1.70 d/10a, and the ice cover duration (liquid water) was also decreased by -4.80 d/10a in global mountainous regions (Table 1).
Liquid to gaseous transformation.The liquid to gas transformation was represented by evapotranspiration.The actual evaporation data in the global mountainous regions were obtained from the Global Land Evaporation Amsterdam Model Version 3.3a (https://www.gleam.eu/)(Miralles et al.,  2010; Martens et al., 2016).In this version, evaporation was estimated based on the reanalysis of net radiation and air temperature, satellite and gaugedbased precipitation, satellite-based VOD, and soil moisture.The dataset provides global actual evaporation during 1980-2017 with a spatial resolution of 0.25°.The enhanced liquid to gas transformation was confirmed by increased evaporation, and the actual evaporation increased by 2.60 mm/10a, respectively (Table 1).
Gaseous to liquid transformation.The transformation from gaseous to liquid was confirmed by moisture recycling.Precipitation over a land region is derived from two sources: (1) water vapor evaporated within the region and (2) water vapor evaporated outside of the region and later transported into it (Brubaker et al., 1993).Developed through the Copernicus Climate Change Service (C3S), ERA5 is the fifth major global reanalysis product produced by the European Center for Medium-Range Weather Forecasts.ERA5 is currently available for the period 1979 to the present (https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5).We used the total precipitation, evaporation, and vertical integral of the divergence of the mois-ture flux to calculate the precipitation recycling ratio.All reanalysis data were available within a 0.25° latitude × 0.25° longitude grid.The data covered the period from 1980 to 2017.The strengthened transformation from gaseous to liquid was confirmed by the increased moisture recycling ratio (Table 1), which includes contributions from terrestrial evaporation (on the surface of soil and water) and plant transpiration to precipitation, and the contribution rate from moisture recycling to precipitation was increased by 0.18 %/10a during 1980-2017 in global mountainous regions.
Gaseous to solid transformation.The frost phenomenon involves the transformation of water vapor to solid water when the air temperature falls to 0 °C.RClimDex (http://cccma.seos.uvic.ca/ETCCDI/software.shtml) was used to calculate frost days (FD) from the daily temperature data reported by Berkeley Earth.Frost is an adverse weather event of low-energy status, characterized by ice deposited over plants and objects exposed to an open sky condition. 24The frost phenomenon involves the transformation of water vapor to solid water when the air temperature falls to 0 °C and the lower atmosphere is humid.It can be quantified by the number of frost days owing to the difficulty in determining the amount of frost.The number of frost days decreased by -2.10 d/10a during 1960-2017 in global mountainous regions (Table 1).
Solid to gaseous transformation.Snow sublimation is a phase transformation from solid to gaseous water.7] Snow sublimation is the loss of water from the snowpack to the atmosphere, which is a phase transformation from solid to gaseous water that directly affects snow accumulation, which in turn affects ecosystem processes, soil moisture, soil porosity, biogeochemical processes, wildfires, and water resources. 28Snow sublimation had an increasing trend of 0.3 mm/10a during 1980-2017 in global mountain regions (Table 1).

Accelerated intensification of multiphase water transformation since 1990
Glacier melting has accelerated since 1990 (Figure 2), and the average glacier area retreat rate after 1990 is 1.3 times that before 1990, which increased by 21.50 km 2 /10a for the 42 glacier regions (Table 1).Specifically, the glacier area retreat rate increased by 25.60 km 2 /10a for 37 glacier regions, whereas it decreased by 331 km 2 /10a for the other five glacier regions during the same period (Supplementary Table 1).The average negative glacier mass balance after 1990 is 3.1 times that before 1990, and it increased by 376 mm for the 45 glaciers (Table 1).Further, it increased by 388 mm for 44 glaciers from the period before to that after 1990, while only one glacier in Scandinavia decreased by 124 mm during the same period (Supplementary Table 2).The average glacier length retreat rate after 1990 is 1.5 times that before 1990 for 414 glaciers, and it increased by 60 m/10a (1.5 times that before 1990) from before to after 1990 (Table 1).There are three  1, Supplementary Table 3).Moreover, the change in lake ice phenology also accelerated since 1990 in global mountainous regions (Table 1, Supplementary Table 4, Figure 2).The delaying trend of complete freeze time increased by 31.20 d/10a after 1990, to 5.2 times that before 1990.The advancing trend of complete melting time increased by 5 d/10a during the same period (7.3 times that before 1990).The decreasing trend for ice cover duration also increased by 3.90 d/10a during the same period (2.4 times that before 1990).Meanwhile, the increasing evaporation rate has also accelerated since 1990 (Table 1), and the variation trend of actual evaporation increased by 1 mm/10a after 1990, or 1.2 times that before 1990 (Figure 2).More importantly, the contribution ratio from moisture recycling to precipitation increased by 0.4 %/10a during 1991-2017 (Table 1) to 12.0 times that before 1990 (Figure 2).Furthermore, the decreasing trend for frost days also increased by 0.3 d/10a after 1990 (Table 1), to 1.2 times that before 1990 (Figure 2).Finally, changes in snow sublimation also accelerated, and the trend increased to 2.40 mm/10a after 1990 (Table 1), which is 4.4 times that before 1990 (Figure 2).

Geoscience
Intensification of multiphase water transformation The times for variation rate of MWT after 1990 of that before 1990 in the global mountainous region: ①the enhanced solid to liquid transformation evidenced by 42 glacier regions for area retreat, 45 glaciers for negative mass balance and 414 glaciers for glacier length retreat; ②the reduced liquid to solid transformation reflected by changes in complete freeze time (no liquid water), complete melting time (no ice), and ice cover duration of ice phenology for 22 lakes;③the increased liquid to gas transformation indicated by enhanced actual evaporation; ④the strengthened gaseous to liquid transformation evidenced by the increased moisture recycling ratio, which is defined as the ratio of precipitation contributed by water vapor evaporated within the region to the total precipitation;⑤the reduced gaseous to solid transformation evidenced by decreased number of frost days; ⑥the increased solid to gaseous transformation evidenced by enhanced snow sublimation.

IMPLICATIONS
This study has identified an accelerated intensification of the water cycle as evidenced by the accelerating MWT in the global mountainous regions under global warming.5][46][47] Therefore, most mountainous regions will require strong adaptation efforts to reduce the risk of disasters that can affect developing as well as developed countries under the accelerating MWT.
The findings from this study offer a global systematic assessment and improve our understanding of the acceleration of the water cycle in global mountainous regions.Moreover, the study contributes to a more accessible quantification of the apparent changes in the complex system of the water Index 1960-1990  1991-2017  1960-1990  1991-2017  1960-1990  1991-2017  1960-1990  1991-2017  1960-1990  1991-2017  1960-1990  1991-2017  1960-1990  1991  1960-1990  1991-2017  1960-1990  1991-2017  1960-1990  1991-2017  1960-1990  1991-2017  1960-1990  1991-2017  1960-1990  1991-2017 cycle that will facilitate research into water resource change and its influence.Considering the threat from the acceleration of MWT and the associated future trends in global mountainous regions, we offer the following recommendations: Firstly, an improved in situ monitoring network for MWT is a crucial for predicting the future of the water cycle and associated hazards, including their impacts on regional water, ecosystems, and food security, especially in high-altitude areas (>4000 m) where observations are sparse.Secondly, future research should focus on quantifying the hydrological effects of the accelerating MWT in global mountainous regions, which will elucidate the impacts on the world's "water towers."These changes pose a serious challenge to ecological security, biodiversity patterns, and ecosystem services.Therefore, it is critical to develop scientific strategies to control or reduce potential risks to environmental security under the accelerating MWT.Thirdly, as mountainous areas are typically economically underdeveloped, the international community should work closely with stakeholders to intensify their assistance in building the resilience of mountainous regions, as well as improving disaster prevention capacity and scientific forecasting to address the risks and challenges posed by the accelerating MWT.

Data and materials availability
A 250 m Hammond landform dataset was provided by the United States Geological Survey Land Change Science Global Ecosystems (https://wildfire.cr.usgs.gov/arcgis/rest/services/gmek3/MapServer),which has been used to extract data for the global mountainous regions.The daily gridded land surface temperature dataset from Berkeley Earth was used to examine climate warming trends and to calculate extreme climate indices (http://berkeleyearth.org/data/).The data for glacier areas and glacier lengths were established in this study and were used to calculate the glacier area retreat rates and average retreat rates of glacier lengths (Supplementary Table 1, Supplementary Table 3).The glacier mass balance data for 42 glaciers in global mountainous regions were provided by the World Glacier Monitoring Service (https://wgms.ch/), whereas the data for the other 19 glaciers were collected during this study to calculate the average glacier mass balance during different periods (Supplementary Table 2).The data for lake ice phenology were obtained from the National Snow & Ice Data Center in order to calculate the variation trends (http://nsidc.org/data/G01377.html,Supplementary Table 4).The actual evaporation and snow sublimation data in global mountainous regions were obtained using the Global Land Evaporation Amsterdam Model Version 3.3a for calculating the variation trends (https://www.gleam.eu/).The local moisture recycling ratio was calculated using ERA5 data produced by the European Center for Medium-Range Weather Forecasts (https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5).

Trends for climate warming and change point detection
The Mann-Kendall trend estimation method 48 was used to detect the linear trend before and after the change point for the variables used in this study, such as climate variables, extreme climate indices, and evaporation.This method has been recommended by the World Meteorological Organization for the trend analysis of meteorological variables.The Mann-Kendall trend test examines nonparametric monotonic trends for time-series data.It is insensitive to outliers and is widely used in climate variable trend testing.The magnitude of the trend was calculated using the Theil-Sen method. 49ClimDex (http://cccma.seos.uvic.ca/ETCCDI/software.shtml) was used to calculate climate indices from the daily data reported by Berkeley Earth.The expert team of the Climate Change Detection and Indices coordinated a suite of 11 precipitation and 16 temperature indices adopted by the Intergovernmental Panel on Climate Change Fourth Assessment Report (AR4).A bootstrap procedure was implemented to ensure that percentile-based temperature indices do not have artificial jumps at the boundaries of the base and outof-base periods. 50Indices not relevant to the study region were omitted, leading to a final selection of 11 temperature indices: frost days (FD), ice days (ID), TNx (annual highest daily minimum temperature), TXn (annual lowest daily maximum temperature), TXx (annual highest daily maximum temperature), TNn (annual lowest daily minimum temperature), GSL (growing season length), TX10 (cold day frequency), TN10 (cold night frequency), TX90 (warm day frequency), and TN90 (warm night frequency).
The temperature change point was examined using TAVG from the Berkeley Earth data.Considering the uncertainties in change point detection caused by different methods, we chose three widely used methods to detect change points in the TAVG, namely Mann-Kendall method, 48 Pettitt method, 51 Buishand U test, 52 and standard normal homogeneity test (SNHT). 53The Pettitt and Buishand U test yielded a TAVG change point in 1987, while the SNHT method yielded a TAVG change point in 1993 and the Mann-Kendall method yielded a change point in 1996.Combining the results of the three methods and the length of the data time series, we used the year 1990 as the change point to divide the time series into two periods.

1 .
The glacier mass balance data for 42 glaciers in the global mountainous regions were provided by the World Glacier Monitoring Service (https://wgms.ch/) (WGMS, 2017).Glacier mass balance data relevant for longer periods for the Hailuogou Glacier and Baishui Glacier No. 1 in China were obtained from Li et al. (2010), 19 while short-term data for the other 17 glaciers in China were acquired from Yao et al. (2015) (Supplementary Table

*
22 lakes for ice phenology Y 45 glaciers for mass balance variation

(Figure 1 .
Figure 1.Global mountainous region: above and below 3000 m Above Sea Level (m) spatial distribution of 42 glacier regions for area retreat, 45 glaciers for mass balance variation, 414 glaciers for glacier length retreat, and 22 lakes for ice phenology in the global mountain regions

6 5 Figure 2 .
Figure2.The times for variation rate of MWT after 1990 of that before 1990 in the global mountainous region: ①the enhanced solid to liquid transformation evidenced by 42 glacier regions for area retreat, 45 glaciers for negative mass balance and 414 glaciers for glacier length retreat; ②the reduced liquid to solid transformation reflected by changes in complete freeze time (no liquid water), complete melting time (no ice), and ice cover duration of ice phenology for 22 lakes;③the increased liquid to gas transformation indicated by enhanced actual evaporation; ④the strengthened gaseous to liquid transformation evidenced by the increased moisture recycling ratio, which is defined as the ratio of precipitation contributed by water vapor evaporated within the region to the total precipitation;⑤the reduced gaseous to solid transformation evidenced by decreased number of frost days; ⑥the increased solid to gaseous transformation evidenced by enhanced snow sublimation.

Table 1 .
Changes in multiphase water transformation during different periods