Aerosol-weakened summer monsoons decrease lake fertilization on the Chinese Loess Plateau


The Chinese Loess Plateau (CLP) is the cradle of Chinese civilization, and environmental changes in this region have influenced the historical trajectory of ancient China7. The CLP is the largest loess region in the world in terms of extent, thickness and depositional sequence, and covers a total area of 640,000km2 (ref. 8). The more than 100 million people living on the CLP are faced with the worlds most serious erosion issues, particularly large losses in soil nutrients7, 8.

The climate of the CLP is largely influenced by the Asian monsoon circulation (Fig. 1a), and its location at the monsoon boundary zone makes the CLP particularly sensitive to global climate change9. More than 70% of the annual precipitation in the CLP falls in intense storms during the summer monsoons between June and September, and can cause extreme soil erosion8, 10 (Fig. 1b). This eroded surface soil delivers a massive amount of soil phosphorus (P) (40 million tons/year) into lakes, reservoirs and river systems such as the Yellow River (Fig. 1b), and ultimately into the marine ecosystem (Supplementary Fig. 1), resulting in severe eutrophication problems11. Importantly, the middle and lower reaches of the Yellow River are major sources of fresh water for about 107 million people, and often referred to as ‘the Mother River of China. The pollution and eutrophication of the Yellow River, whose sediment discharge from erosion exceeds the combined discharges of the Nile and Amazon rivers12, directly influences the livelihood of this large population11.

Figure 1: Lake Gonghai, located at the CLP, considered to be the most erodible area on Earth, is largely influenced by the Asian summer monsoon.

a, Climate system of the Asian monsoon area30. Summer monsoon rainfall and wind-dominated Asian summer monsoon region. b, Distribution of soil loss (ton km−2yr−1) on the CLP31: the erosion modulus of most regions on the CLP is above 8,000 tonkm−2yr−1. The blue dot indicates the location of Lake Gonghai. Also shown is the Yellow River system (blue line).

Recent studies have convincingly argued that increases in anthropogenic aerosols play an important role in affecting Asian summer monsoon intensity1, 3, 13, 14, 15, 16 that will have significant implications for more than a third of the worlds population. Changes to the monsoon system have tremendous impacts on agriculture, health, water resources, economies, and ecosystems throughout Asia, as monsoon rains provide up to 80% of the regions annual mean precipitation. However, it is unknown how reductions in monsoon intensity can affect freshwater ecosystems that are important to a large portion of the Asian population. Here, we use well-dated, high-resolution palaeolimnological records from a remote alpine lake (Lake Gonghai) on the CLP to compare lake responses to major periods of past and present climate warming over the past two millennia. This approach offers an excellent ‘time window for making comparisons between the complexities of recent anthropogenic climate change with well-documented past warm periods in this region17, both in terms of forcing mechanisms and the response of aquatic ecosystems.

Unlike major warming periods in the past, recent anthropogenic climate change is further complicated by interactions with multiple anthropogenic forcings, including greenhouse gases, anthropogenic aerosols, and land-use changes. Rapid economic growth, industrialization and urbanization in developing Asian countries has led to severe air pollution over the past few decades, resulting in Asia becoming a major source of aerosol emissions and an important contributor to global climate change18. This rapid and pronounced increase in anthropogenic aerosols has been linked to the widespread decrease in summer monsoon rainfall and wind intensity over Asia1, 3, 13, 14. Specifically, changes in incoming solar radiation forced by anthropogenic aerosols have reduced the thermal contrast between the Asian continent and the Indian and Pacific oceans, and thus the monsoonal circulation15, 16.

Lake Gonghai (38° 54′N, 112° 14′E; 1,840m above mean sea level) is a freshwater alpine lake located on the CLP (Supplementary Fig. 1a). This remote, high-elevation lake was strategically chosen to be representative of the region as it is a hydrologically closed, simple basin with a small, undisturbed catchment, and therefore an excellent passive monitor of environmental and climatic change. The lake has a surface area of ~0.36km2 with a maximum water depth of ~10m and a flat lake bed, making it an ideal palaeolimnological study site (Supplementary Fig. 1b, c). Importantly, the lake is remote and minimally affected by local human activities. Lake Gonghai in the CLP was also strategically chosen for this study as it is located within a region that is relatively low in atmospheric nitrogen deposition19. Furthermore, Lake Gonghai is currently oligotrophic (mean concentration values for total phosphorus (TP) and total nitrogen (TN) are 9.84 and 398μgl−1, respectively) and is strongly phosphorus limited (TN:TP mass ratio = 40.5). The lakes strategic location and limnological characteristics preclude it from being strongly affected by the recent increases in global atmospheric nitrogen deposition. The monthly mean temperature in this region ranges between −14°C and +23°C. The lake is currently typically ice covered from the middle of November to the end of April. The Lake Gonghai region has recently experienced large-magnitude increases in air temperature20 that are consistent with the amplification of warming reported in high-mountain regions21.

Here we use high-resolution diatom records from two dated Lake Gonghai sediment cores to determine how recent aerosol-driven anthropogenic climate change compares with past episodes of natural warming in terms of summer monsoon intensity, lake fertilization, and aquatic ecosystem dynamics. We reconstruct past limnological conditions mainly using changes in diatom (Bacillariophyceae) assemblage composition. Diatoms are well-documented indicators of lake water nutrient levels22 and have also been widely used to track changes in climate-related lake properties, such as the extent of lake ice and thermal stratification6. By their nature, palaeolimnological records typically integrate ecological signals over several years within each sedimentary core interval, thereby incorporating within-lake seasonal changes, including fluctuations in nutrient distributions as a result of vertical mixing and thermal stratification. Here, we use a long (3.96m) sediment core (GH09B) dated using 14C geochronology (Supplementary Fig. 2; see Methods) to focus on diatom responses (~25 years/interval) to known warming periods over the past ~2,000 years (Fig. 2), whereas we use a shorter core (GH13F) with chronology established using 210Pb gamma spectroscopy (Supplementary Fig. 3; see Methods) to track high-resolution (~4 years/interval) diatom responses to current warming-related environmental changes within the context of the past ~175 years (Fig. 3).

Figure 2: Long-term trends (over the past ~2,000 years) in reconstructed air temperature and Asia summer monsoon rainfall compared with the Lake Gonghai sediment core trends for phosphorus-laden soil erosion, and shifts in dominance between oligotrophic (low nutrient) and eutrophic (high nutrient) diatom indicators.

Long-term trends (over the past [sim]2,000 years) in reconstructed air temperature and Asia summer monsoon rainfall compared with the Lake Gonghai sediment core trends for phosphorus-laden soil erosion, and shifts in dominance between oligotrophic (low nutrient) and eutrophic (high nutrient) diatom indicators.

a, Air temperature reconstructed from phenological observations recorded in Chinese historical documents from North China28. b, Reconstruction of Asian summer monsoon rainfall from speleothem δ18 O records from Wanxiang Cave and Huangye Cave (dark cyan line)24, 25 and the modelled Asian summer monsoon rainfall from the ECHO-G model simulation26 (blue line). c, Trends in the concentration of sediment phosphorus determined from the Lake Gonghai core. d, Changes in the relative abundances of the diatom taxon Stephanodiscus hantzschii, a well-documented eutrophic indicator. e, Relative abundances of Lindavia taxa (L. praetermissa and L. bodanica), diatoms characteristic of low nutrient concentrations. Grey shading corresponds to the three warm periods recognized over the past ~2,000 years in China including the Sui–Tang Warm Period (STWP), the Medieval Warm Period (MWP) and the Current Warm Period (CWP).

Figure 3: The major shift in diatom assemblage composition recorded in the Lake Gonghai sediment core (GH13F) compared to the global mean air temperature trend observed during the anthropogenic warm period.

The major shift in diatom assemblage composition recorded in the Lake Gonghai sediment core (GH13F) compared to the global mean air temperature trend observed during the anthropogenic warm period.

ac, The relative abundances of diatom taxa represented by a rise to dominance of Cyclotella ocellata (a) and Fragilaria tenera (b), well-documented indicators of warming (that is, decreasing ice cover, extended thermal stratification6), and a concurrent decline (c) in Lindavia taxa (L. praetermissa and L. bodanica), diatoms characteristic of low nutrients in alpine environments. d, Global mean temperature change9, which is strongly correlated with the temperature change over the past 55 years observed in the meteorological observation station in the Lake Gonghai region (R = 0.89; P < 0.001; d.f. = 55). The shaded area represents a pronounced increase in air temperature during the past few decades.

Palaeoecological records from the past ~2,000 years have clearly documented two warm periods throughout China, consisting of the Sui–Tang Warm Period (STWP, AD 541–760) during the Sui–Tang dynasties, and the Medieval Warm Period (MWP, AD 931–1320), which broadly corresponds to the Song–Yuan dynasties5. These past warm periods were associated with strong Asian summer monsoons with abundant rainfall and greater wind intensity, as warming typically results in a sharper land/ocean thermal contrast23, 24, 25, 26 (Fig. 2b). These warm periods with strong summer monsoons are clearly expressed in our diatom records from the CLP (Fig. 2 and Supplementary Fig. 4). For example, a sharp increase in temperature during the Sui–Tang Warm Period—STWP (AD 541–760) and the Medieval Warm Period—MWP (AD 931–1320) resulted in abrupt and nearly complete shifts in diatom species dominance from low nutrient Lindavia taxa (L. praetermissa and L. bodanica) to eutrophic Stephanodiscus hantzschii (Supplementary Fig. 4). S. hantzschii is an unambiguous indicator of high phosphorus (P) levels, whereas Lindavia taxa are often associated with lower nutrients in alpine environments27. Pronounced shifts in dominance between oligotrophic (Fig. 2e) and eutrophic taxa (Fig. 2d) are synchronous with temperature reconstructions in North China based on 2,000 years of historical documents of phenological observations, including variations in regional flowering5, 28 (Fig. 2a), and have strong temporal coherence with the summer monsoon rainfall record24, 25, 26 (Fig. 2b) and the sediment phosphorus record (Fig. 2c). These sedimentary records indicate that the STWP and MWP in the CLP were accompanied with strong summer monsoons and increased rainfall intensity that resulted in the transport of large quantities of phosphorus-rich soil eroded from the surrounding catchment, leading to marked natural lake fertilization.

In addition to increased P supplied from eroded catchment soil, an increase in summer monsoon wind strength during the STWP and MWP would also increase the duration and strength of water column mixing, enhancing internal nutrient recycling29 and favouring dominance by eutrophic diatom taxa (Fig. 2d). In contrast, periods of dominance by oligotrophic diatom taxa (Fig. 2e) during cooler periods, such as the Little Ice Age, are associated with weaker summer monsoon rainfall intensity and wind strength, resulting in diminished delivery of phosphorus-rich soil, and thus lower water nutrient concentrations.

In contrast to past warm periods experienced in the CLP, anthropogenic warming over the past half century has initiated vastly different but equally pronounced responses that are well expressed in our palaeolimnological records (Fig. 3 and Supplementary Fig. 5). The most recent warm period, commencing around the 1960s, tracks a nearly complete shift in species dominance from large-celled and heavier Lindavia taxa (L. praetermissa, L. bodanica) (Fig. 3c) to the abrupt arrival and dominance of more buoyant Cyclotella ocellata and Fragilaria tenera (Fig. 3a, b) for the first time in the ~2,000-year sedimentary record (Supplementary Fig. 4). These shifts among diatom taxa with similar nutrient optima, but different ecophysiological traits, are consistent with the rise in mean temperatures (Fig. 3d) and concomitant increases in thermal stability and attendant changes in resource availability6. The most recent diatom shifts are indicative of a fundamentally different climate mechanism and biological response from the previous STWP and MWP warm episodes.

The acceleration of aerosol emissions, as a result of rapid industrialization in Asia over the past few decades, compounded by a lack of strict air pollution control, has resulted in Asia being a major contributor to twentieth-century anthropogenic climate forcing. Anthropogenic aerosols have increased cloud albedo and heightened scattering of incoming solar radiation back to space, resulting in slower surface warming over land and a reduction in the thermal contrast between the Asian continent and the adjacent ocean basins at decadal timescales15, 16 (Fig. 4). As such, anthropogenic aerosol forcings have been shown to weaken Asian summer monsoon rainfall and wind intensity over the past 50years1, 13, 14, 15, 16 (Fig. 2b), which explains why the Current Warm Period (CWP) is warmer than the MWP but precipitation is lower17. We demonstrate that this has resulted in a marked decline in the delivery of phosphorus-rich soil from the CLP (Fig. 2c) that is consistent with the decreasing trend in inter-annual rainfall variability10 and, as recorded in the diatom assemblages, with a fundamental ecological change in aquatic ecosystems that includes a decline in lake nutrients and an increase in thermal stability.

Figure 4: Schematic diagram illustrating the different ecosystem responses to recent anthropogenic and past warming in monsoon regions.

Schematic diagram illustrating the different ecosystem responses to recent anthropogenic and past warming in monsoon regions.

a, Recent anthropogenic warming period: aerosol-weakened summer monsoons decrease lake fertilization. Atmospheric aerosol loading decreases the amount of incoming solar radiation by scattering/absorbing the solar radiation in the atmosphere, resulting in slower surface warming over land and a reduction in the land–sea temperature contrast, which thus weakens the strength and extent of the overall monsoon circulation. This results in decreased monsoon rainfall and wind intensity from the ocean to land, which lessens erosion and decreases lake fertilization. b, Previous warming periods: warming led to strengthened summer monsoons, resulting in increased lake fertilization. Natural warm periods in the past were associated with a higher land/ocean thermal contrast, resulting in stronger summer monsoon circulation with an increase in summer monsoon rainfall and wind intensity that led to increased lake fertilization.

Anthropogenic aerosols have fundamentally reversed the key limnological effects of warming on summer monsoon intensity and aquatic ecosystem properties (trophic status, mixing), highlighting that past climate is a poor analogue for recent warming and future climate trajectories in this economically and environmentally important region. The aerosol-affected period of recent anthropogenic warming has resulted in markedly different (albeit predictable) responses in lake ecosystem properties, hitherto not recorded in the previous two millennia. The consequences of these changes will undoubtedly cascade throughout the ecosystem. Ironically, continued environmental efforts to decrease anthropogenic aerosols in Asia, whilst global warming continues, will lead to the return of severe eutrophication, further impairing the already stressed freshwater supply of the region.