Timescale-dependent responses of hydrological changes from global closed basins since the last glacial maximum

Water shortage has plagued the social development and human well-being of global closed basins. However, the hydroclimate research on different time scales in these regions remains inadequate at a global scale. In this paper, the hydrological responses from global closed basins to millennial-scale and centennial-scale cold/warm events since the Last Glacial Maximum were explored. Closed-basin lake records indicate that the westerlies-dominated closed basins are generally wetter during cold events than the corresponding warm ones on the millennial and centennial scales. In contrast, the monsoon-influenced closed basins prevail wetter climates during warm events. According to the hydroclimate simulations, precipitation seasonality plays a significant role in causing above spatial–temporal patterns. There is more winter rainfall in westerlies-dominated closed basins during cold events in the Last Glacial Maximum and Little Ice Age and more summer rainfall in monsoon-influenced closed basins during warm events in the mid-Holocene and Medieval Climate Anomaly. Under modern and future global warming, the hydroclimate changes in global closed basins show more regional differentiation, resulting in wetter mid-latitude Asian and low-latitude African closed basins but drier southwest North American and Australian closed basins.

Changes of lake organic carbon sinks from closed basins since the Last Glacial Maximum and quantitative evaluation of human impacts

Background Closed basins occupy 21% of the world’s land area and can substantially affect global carbon budgets. Conventional understanding suggests that the terminal areas of closed basins collect water and carbon from throughout the entire basin, and changes in lake organic carbon sinks are indicative of basin-wide organic carbon storages. However, this hypothesis lacks regional and global validation. Here, we first validate the depositional process of organic carbon in a typical closed-basin region of northwest China using organic geochemical proxies of both soil and lake sediments. Then we estimate the organic carbon sinks and human impacts in extant closed-basin lakes since the Last Glacial Maximum (LGM). Results Results show that 80.56 Pg organic carbon is stored in extant closed-basin lakes mainly found in the northern mid-latitudes. Carbon accumulation rates vary from 17.54 g C m−2 yr−1 during modern times, 6.36 g C m−2 yr−1 during the mid-Holocene and 2.25 g C m−2 yr−1 during the LGM. Then, we evaluated the influence by human activities during the late Holocene (in the past three thousand years). The ratio of human impacts on lake organic carbon storage in above closed basins is estimated to be 22.79%, and human-induced soil organic carbon emissions in the past three thousand years amounted to 207 Pg. Conclusions While the magnitude of carbon storage is not comparable to those in peatland, vegetation and soil, lake organic carbon sinks from closed basins are significant to long-term terrestrial carbon budget and contain information of climate change and human impact from the whole basins. These observations improve our understanding of carbon sinks in closed basins at various time scales, and provide a basis for the future mitigation policies to global climate change.

Synergy of the westerly winds and monsoons in the lake evolution of global closed basins since the Last Glacial Maximum and implications for hydrological change in central Asia

The monsoon system and westerly circulation, to which climate change responds differently, are two important components of global atmospheric circulation interacting with each other in the middle to low latitudes. Relevant research on global millennial-scale climate change in monsoon and westerly regions is mostly devoted to multi-proxy analyses of lakes, stalagmites, ice cores, and marine and eolian sediments. Different responses from these proxies to long-term environmental change make understanding climate change patterns in monsoon and westerly regions difficult. Accordingly, we disaggregated global closed basins into areas governed by monsoon and westerly winds, unified paleoclimate indicators, and added lake models and paleoclimate simulations to emphatically track millennial-scale evolution characteristics and mechanisms of East Asian summer monsoon and westerly winds since the Last Glacial Maximum (LGM). Our results reveal that millennial-scale water balance change exhibits an obvious boundary between global monsoon and westerly regions in closed basins, particularly in the Northern Hemisphere. The effective moisture in most closed basins of the midlatitude Northern Hemisphere mainly exhibits a decreasing trend since the LGM, while that of the low latitudes shows an increasing trend. In the monsoon-dominated closed basins of Asia, a humid climate prevails in the early to mid-Holocene, and a relatively dry climate appears in the LGM and late Holocene. In the westerly-wind-dominated closed basins of Asia, the climate is characterized by a humid LGM and mid-Holocene (MH) compared with the dry early and late Holocene, which is likely to be connected to precipitation brought by the westerly circulation. This study provides insight into the long-term evolution and synergy of westerly winds and monsoon systems as well as a basis for the projection of future hydrological balance.

Wet–dry status change in global closed basins between the mid-Holocene and the Last Glacial Maximum and its implication for future projection

Closed basins, mainly located in subtropical and temperate drylands, have experienced alarming declines in water storage in recent years. An assessment of long-term hydroclimate change in those regions remains unquantified at a global scale as of yet. By integrating lake records, PMIP3–CMIP5 simulations and modern observations, we assess the wet–dry status of global closed basins during the Last Glacial Maximum, mid-Holocene, pre-industrial, and 20th and 21st century periods. Results show comparable patterns of general wetter climate during the mid-Holocene and near-future warm period, mainly attributed to the boreal summer and winter precipitation increasing, respectively. The long-term pattern of moisture change is highly related to the high-latitude ice sheets and low-latitude solar radiation, which leads to the poleward moving of westerlies and strengthening of monsoons during the interglacial period. However, modern moisture changes show correlations with El Niño–Southern Oscillation in most closed basins, such as the opposite significant correlations between North America and southern Africa and between central Eurasia and Australia, indicating strong connection with ocean oscillation. The strategy for combating future climate change should be more resilient to diversified hydroclimate responses in different closed basins.

Synergy of the westerly winds and monsoons in lake evolution of global closed basins since the Last Glacial Maximum

Monsoon system and westerly circulation, to which climate change responds differently, are two important components of global atmospheric circulation, interacting with each other in the mid-to-low latitudes and having synergy effect to those regions. Relevant researches on global millennial-scale climate change in monsoon and westerlies regions are mostly devoted to multi-proxy analyses of lakes, stalagmites, ice cores, marine and eolian sediments. Different responses from these proxies to long-term environmental change make understanding climate change pattern in monsoonal and westerlies regions difficult. Accordingly, we disaggregated global closed basins into areas governed by monsoon and westerly winds and unified palaeoclimate indicators, as well as combined with the lake models and paleoclimate simulations for tracking millennial-scale evolution characteristics and mechanisms of global monsoon and westerly winds since the Last Glacial Maximum (LGM). Our results concluded that the effective moisture in most closed basins of the mid-latitudes Northern Hemisphere is mainly a trend on the decrease since the LGM, and of the low-latitudes is mainly a trend on the rise. Millennial-scale water balance change exhibits an obvious boundary between global westerlies and monsoon regions in closed basins, particularly in the Northern Hemisphere. In the monsoon dominated closed basins of the Northern Hemisphere, humid climate prevails in the early-mid Holocene and relative dry climate appears in the LGM and late Holocene. While in the westerly winds dominated closed basins of the Northern Hemisphere, climate is characterized by relative humid LGM and mid-Holocene (MH) compared with the dry early Holocene, which is likely to be connected with precipitation brought by the westerly circulation. This study provides insights into long-term evolution and synergy of monsoon and westerly wind systems and basis for projection of future hydrological balance in the low-to-mid latitudes.