Unprecedented decline in photosynthesis caused by summer 2022 record-breaking compound drought-heatwave over Yangtze River Basin

Abrupt loss of species richness caused by ecosystem transition

Ecosystem transition occurs when ecological thresholds are crossed, causing ecosystems to irreversibly shift from secure to insecure states. However, how ecosystem transition exacerbates species richness loss remains poorly understood. This hinders the effective protection of species richness, which is an urgent global priority. In this study, we integrated multiple ecosystem variables to elucidate ecosystem transition and its impacts on species richness loss. Our findings reveal that species richness declines abruptly following ecosystem transition, as insecure ecosystems are characterized by reduced plant cover and productivity, intensified warming and drying, and diminished oxygen production. Insecure ecosystems imperil the survival of all species, including 39.4% of threatened birds and 29.2% of threatened mammal species. We project that by 2100, the ecosystem areas considered insecure will encompass 40.4% of the global land areas under the RCP8.5 scenario, contributing to 51.6% of species richness loss. In contrast, hyper-secure ecosystems are projected to account for 18.1% of species richness loss. This study identifies ecosystem transition as a critical driver of species richness loss that should be accounted for by policymakers in designing targeted conservation strategies.

Dissimilatory nitrate reduction pathways drive high nitrous oxide emissions and nitrogen retention under the flash drought in the largest freshwater lake in China

Flash drought (FD) events induced by climate change may disrupt the normal hydrological regimes of floodplain lakes and affect the plant-microbe mediated dissimilatory nitrate reduction (DNR), i.e., denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA), thus having important consequences for nitrous oxide (N2O) emissions and nitrogen (N) retention. However, the responses of the DNR pathways in the floodplain lake to the record-breaking FD in 2022 in Yangtze River of China, as well as the underlying microbial mechanisms and feedbacks to climate change remain poorly understood. Here, we collected exposed sediments and Carex cinerascens-associated soils in the littoral wetlands of Poyang Lake during 2022 FD and the dry seasons prior to and after this event. The potential DNR rates and the synergistic metabolism of microbial guilds involved in DNR were investigated using 15N isotope pairing technique, high-throughput and metagenomic sequencing. We found that the in situ N2O fluxes in the littoral wetlands were highest during the flash drought, especially in the exposed sediments. The potential DNRA rates were highest under flash drought conditions, and DNRA dominated the DNR for both exposed sediments (80.4 %) and Carex cinerascens-associated soils (57.5 %). Nutrients (i.e., N and P) and DNRA bacterial communities played a key role in producing the extremely high N2O fluxes from exposed sediments, which could be explained by the synergistic metabolism of DNRA bacteria and denitrifiers through the exchange of the key intermediates in DNR. Therefore, the climate change-induced flash drought promoted greater nitrous oxide emissions and N retention in the littoral wetlands of Poyang Lake, producing a greater flux of greenhouse gas emissions and elevating the risk of lake eutrophication. Hence, flash droughts reinforce a positive feedback between climate change and nitrous oxide emission from these aquatic ecosystems.

Compound hot drought events in the Mei-yu region: influences from polar and tropical regions

In the Mei-yu region, there has been noticeable increase in the occurrence of compound hot drought (CHD) events in recent years. However, the underlying causes of these occurrences remain poorly understood. To address this knowledge gap, we conducted a comprehensive study utilizing observational datasets, reanalysis datasets, and four numerical experiments to investigate the associated physical mechanisms. Our findings indicated that the prevalence of CHD events in the Mei-yu region is influenced strongly by two key factors: the decline in Barents Sea ice during February and the presence of a La Niña-like pattern of sea surface temperature (SST) in April. The decline in Barents Sea ice generates an anomalous Rossby wave in the Arctic that propagates southeastward. The La Niña-like SST pattern regulates a Rossby wave over western America, propagating along the subtropical jet stream. These two Rossby waves induce northward movement and strengthened intensity of the subtropical westerly jet in East Asia. The local circulation patterns in the Mei-yu region are influenced by the position and intensity of the subtropical jet, leading to downward motion in accordance with the secondary circulation theory for high-altitude jet streams. Consequently, these local circulation patterns might contribute to occurrence of CHD events. Moreover, our analysis revealed that the impact of Barents Sea ice and the La Niña-like SST pattern can explain approximately two-thirds of the mild CHD events in the Mei-yu region, and that the influence of each is relatively independent. This research underscores influences of polar-tropical systems on climate extremes in eastern Asia.

Advancing diurnal analysis of vegetation responses to drought events in the Yangtze River Basin using next-generation satellite data

Extreme climate events, particularly droughts, pose significant threats to vegetation, severely impacting ecosystem functionality and resilience. However, the limited temporal resolution of current satellite data hinders accurate monitoring of vegetation's diurnal responses to these events. To address this challenge, we leveraged the advanced satellite ECOSTRESS, combining its high-resolution evapotranspiration (ET) data with a LightGBM model to generate the hourly continuous ECOSTRESS-based ET (HC-ETECO) for the middle and lower reaches of the Yangtze River Basin (YRB) from 2015 to 2022. This dataset showed strong agreement with both ground-based and satellite observations. Utilizing the SPEI, we identified the significant drought period: September to November 2019 and August to September 2022. By integrating hourly Solar-Induced Chlorophyll Fluorescence (SIF) data, we observed that during drought period, the typical afternoon peak in SIF was absent. In contrast to non-drought period, morning photosynthesis and SIF-based Water Use Efficiency (WUESIF) anomalies were primarily driven by high Vapor Pressure Deficit (VPD), while the afternoon reductions were influenced by both high VPD and low Soil Moisture (SM) as the drought progressed. Our simulated HC-ETECO data revealed that ET in the middle and lower reaches of the YRB was consistently lower than normal during drought period. Attribution analysis indicated that this reduction was primarily driven by midday temperature increases and high VPD, suggesting that vegetation in the region copes with drought stress predominantly by limiting water loss. These findings highlight the utility of the generated high-resolution ET dataset in advancing our understanding of vegetation dynamics under drought climate conditions. This work provides critical insights for enhancing climate adaptation strategies and enhancing ecosystem management practices in the face of increasing climate variability.

Influence of extreme 2022 heatwave on megacities' anthropogenic CO2 emissions in lower-middle reaches of the Yangtze River

An unprecedented heatwave hit the Yangtze River Basin (YRB) in August 2022. We analyzed changes of anthropogenic CO2 emissions in 8 megacities over lower-middle reaches of the YRB, using a near-real-time gridded daily CO2 emissions dataset. We suggest that the predominant sources of CO2 emissions in these 8 megacities are from the power and industrial sectors. In comparison to the average emissions for August in 2020 and 2021, the heatwave event led to a total increase in power sector emissions of approximately 2.70 Mt CO2, potentially due to the increase in urban cooling demand. Suzhou experienced the largest increase, with a rise of 1.12 Mt CO2 (12.88 %). Importantly, we observed that changes in daily power emissions exhibited strong linear relationships with temperatures during the heatwave, albeit varying sensitivities across different megacities (with an average of 0.0076 ± 0.0075 Mt d-1 °C-1). Conversely, we find that industrial emissions decreased by a total of 8.45 Mt CO2, with Shanghai seeing the largest decrease of 4.71 Mt CO2, while Hangzhou experienced the largest relative decrease (-21.22 %). It is noteworthy that the majority of megacities rebounded in industrial emissions following the conclusion of the heatwave. We convincingly suggest a tight linkage between the reductions in industrial emissions and China's policy to ensure household power supply. Overall, the reduction in industrial emissions offset the increase in power sector emissions, resulting in weaker emissions for majority of megacities during the heatwave. Despite remaining uncertainties in the emissions data, our study may offer valuable insights into the complexities of anthropogenic CO2 emissions in megacities amidst frequent summer heatwaves intensified by greenhouse warming.

Key microbial taxa play essential roles in maintaining soil muti-nutrient cycling following an extreme drought event in ecological buffer zones along the Yangtze River

Climatic extremes, especially extreme droughts, are occurring more frequently and profoundly impacting biogeochemical processes. However, the relative importance of microbial communities on soil nutrient cycling and community maintenance under natural extreme drought events remains elusive. During a record-breaking drought in the Yangtze River Basin (YRB) in the summer of 2022, we collected ambient soils and drought-affected bare and vegetated soils in ecological buffer zones from two sites with similar soil and vegetation characteristics along the YRB, and examined the relative contribution of soil bacterial communities in supporting multi-nutrient cycling index (MNCI) involving carbon-, nitrate- and phosphorus-cycling and their associations with microbial network. Extreme drought decreased (p < 0.05) bacterial α-diversity but increased MNCI in vegetated soils at both sites, while both remained unchanged (p > 0.05) in bare soils, possibly as a result of vegetation releasing rhizodeposits under drought which selectively recruited bacterial communities. Bacterial community compositions were shifted (p < 0.05) only in vegetated soils, and they exerted more influence than α-diversity on soil MNCI. Notably, the Anaerolineae, identified as a biomarker enriched in vegetated soils, had close associations with enzyme activities and soil MNCI at both sites, suggesting their potential recruitment by vegetation to withstand drought. Furthermore, key ecological clusters (Module 1) in bacterial co-occurrence networks at both sites supported (p < 0.05) higher MNCI, despite no substantial variation in network structure due to drought. Specifically, the most important taxa within Module 1 for predicting soil MNCI revealed by random forest modeling analysis (R2 = 0.44 - 0.63, p < 0.001), such as B1-7BS, SBR1031 and Nocardioides, could be deeply involved in soil nitrogen-cycling, suggesting an essential role of specialized interactions of bacterial communities in maintaining soil multifunctionality. Overall, this study demonstrates that changes in biomarkers and functional taxa under extreme drought may better reflect the biological mechanisms involved in microbial communities impacting ecosystem function, which may aid in forecasting the ecological consequences of ongoing climate change in the ecological buffer zones along the YRB.

Daily dynamics of ground-dwelling invertebrate communities during and following an extreme high-temperature event in summer 2022, China

The recent increase in the frequency of extreme weather events and declining soil biodiversity in global agricultural ecosystems make it crucial to assess the daily dynamics of soil communities in fields. To elucidate the daily dynamics of ground-dwelling invertebrate communities, their communities were monitored temporally using infrared camera traps in field farmland during and following an extremely high-temperature (EHT) event in summer 2022 in Ningbo City, China. Nine taxa and 1,147 individuals of the ground-dwelling invertebrate community were photographed in the 176,256 images. There were no significant differences in the taxonomic richness and abundance of the total ground-dwelling invertebrate communities during and following the EHT event. The abundance of ants was significantly decreased following the EHT event, whereas the abundance of other taxa was not. Significantly daily dynamics and obvious differences between each day in taxonomic richness, abundance of ground-dwelling invertebrate community, and abundance of each taxon were not observed during and following the EHT event. The results of this study showed that the daily dynamics of richness and abundance of the ground-dwelling invertebrate community and the abundance of each taxon were not significant during and following the EHT event. Overall, this study provides a useful monitoring method to observe the daily dynamics of ground-dwelling invertebrates in field farmlands and suggests that the daily dynamics of soil fauna communities should be further studied when assessing the effects of climate change on soil biodiversity.

Influence of climate extremes on long-term changes in cyanobacterial blooms in a eutrophic and shallow lake

Climate change and human activities have crucial effects on the variations in phytoplankton blooms in lakes worldwide. A record-breaking heatwave and drought event was reported in the middle and lower reaches of the Yangtze River during the summer of 2022, but only little is known about how cyanobacterial blooms in lakes respond to such climate extremes. Here, we utilized MODIS images to generate the area, occurrence, and initial blooming date (IBD) of cyanobacterial blooms in Lake Chaohu from 2000 to 2022. We found that the area and occurrence of cyanobacterial blooms were largely reduced. At the same time, the IBD was delayed in 2022 compared with the previous 20 years. The annual occurrence and mean area of cyanobacterial blooms in 2022 were 17 % and 23.1 km2, respectively, which were the lowest reported levels since the 21st century. The IBD in 2022 was four months late compared with the IBD in 2020. The high wind speed in spring delayed the spring blooms in 2022. The record-breaking heatwaves and drought from June to August reduced the blooms by influencing the growth of cyanobacteria and reducing the flow of nutrients from the watershed into the lake. This study highlights the compound impact of heatwave and drought climate events on reducing cyanobacterial blooms in a long-term period, enhancing additional understanding of the changes in phytoplankton blooms in lakes.

Traditional Meiyu-Baiu has been suspended by global warming

More than 1000 years, the Meiyu-Baiu have shaped the uniqueness of natural resources, civilization and culture in the Yangtze River Basin of China and the main islands of Japan. In recent decades, frequent rainstorms and droughts have seemingly diminished the misty features of traditional Meiyu-Baiu rainfall. However, there is still no consensus on whether their traditional nature is suspended. In this study, we quantitatively demonstrate that the Meiyu-Baiu almost completely lost their traditional features during 1961-2023, ∼80% of which can be attributed to anthropogenic warming. Furthermore, in a warmer future, the traditional Meiyu-Baiu will be more unlikely to appear. This study underscores the urgency in adapting to climate shift because destructive extremes are measurably taking the place of mild and maternal rains.

Weak correlations among leaf thermal metrics, economic traits and damages under natural heatwaves

The frequency and intensity of heatwaves are increasing around the world, causing severe damages to plants, but whether leaf thermal metrics is in line with leaf economic spectrum is still controversial. Here, we measured leaf damage ratio, leaf thermal metrics (tolerance and sensitivity) and economic traits of 131 woody species across five cities along the Yangtze River after a two-month natural extreme temperature event. We found that leaf thermal sensitivity but not thermal tolerance was correlated with leaf damage ratio, and the relationships between leaf thermal metrics and economic traits were weak, indicating that leaf thermal adaptation may be independent from leaf carbon construction. This study suggests a potential indicator for predicting plant survival under heatwaves, urging future research to explore more physiological traits to comprehensively understand plant heat responses and adaptations.