Decline in seasonal predictability potentially destabilized Classic Maya societies.

Classic Maya populations living in peri-urban states were highly dependent on seasonally distributed rainfall for reliable surplus crop yields. Despite intense study of the potential impact of decadal to centennial-scale climatic changes on the demise of Classic Maya sociopolitical institutions (750-950 CE), its direct importance remains debated. We provide a detailed analysis of a precisely dated speleothem record from Yok Balum cave, Belize, that reflects local hydroclimatic changes at seasonal scale over the past 1600 years. We find that the initial disintegration of Maya sociopolitical institutions and population decline occurred in the context of a pronounced decrease in the predictability of seasonal rainfall and severe drought between 700 and 800 CE. The failure of Classic Maya societies to successfully adapt to volatile seasonal rainfall dynamics likely contributed to gradual but widespread processes of sociopolitical disintegration. We propose that the complex abandonment of Classic Maya population centres was not solely driven by protracted drought but also aggravated by year-to-year decreases in rainfall predictability, potentially caused by a regional reduction in coherent Intertropical Convergence Zone-driven rainfall.

A spatial total nitrogen budget for Great Britain.

Understanding nutrient budgets makes it possible to predict where and by how much nutrients are accumulating in the environment. Previous studies have considered this problem for nitrogen (N) but have limited themselves to reactive N species (i.e. excluding N2) or have considered total N (including N2) but have been limited to regional or national scales. In this study the spatially-distributed total nitrogen (N) budget of Great Britain (GB) was estimated at a 1 km2 grid scale. The inputs of N considered were: biological N fixation; atmospheric deposition; food and feed transfer; and inorganic synthetic fertilizer. The outputs of N considered were: atmospheric emission; terrestrial denitrification; fluvial loss from the soil; gaseous emissions from sewage treatment plants; direct sewage flux loss; and groundwater loss. All pathways were considered over a number of years. This study constructed a spatially-differentiated total N budget for GB, which not only includes all major N pathways but also distributes the N budget to various land uses with a 1 km2 spatial resolution. The results showed that both sink and source areas exist across GB, although the majority of 1 km2 grid squares were identified as sources. Based on a mass balance model calculated for 2015, total N exhibited a net flux of a source of -1045 (±244) ktonnes N/year. The spatial N budget across GB ranged from -21 (±3) tonnes N/year to 34 (±5) tonnes N/year, where 66% of grid squares were source areas and 34% were sink areas. Urban and arable land use were predominantly source areas: 97% of total urban land use and 98.5% of total arable land use. 65% of grassland was a sink area. The total amount of N released to the environment by human activity in 2015 was -16.65 kg N/ca/yr.

Intertropical convergence zone variability in the Neotropics during the Common Era.

Large changes in hydroclimate in the Neotropics implied by proxy evidence, such as during the Little Ice Age, have been attributed to meridional shifts of the intertropical convergence zone (ITCZ), although alternative modes of ITCZ variability have also been suggested. Here, we use seasonally resolved stalagmite rainfall proxy data from the modern northern limit of the ITCZ in southern Belize, combined with records from across the Neotropics and subtropics, to fingerprint ITCZ variability during the Common Era. Our data are consistent with models that suggest ITCZ expansion and weakening during globally cold climate intervals and contraction and intensification during global warmth. As a result, regions currently in the margins of the ITCZ in both hemispheres are likely transitioning to more arid and highly variable conditions, aggravating current trends of increased social unrest and mass migration.

Global analysis reveals climatic controls on the oxygen isotope composition of cave drip water.

The oxygen isotope composition of speleothems is a widely used proxy for past climate change. Robust use of this proxy depends on understanding the relationship between precipitation and cave drip water δ18O. Here, we present the first global analysis, based on data from 163 drip sites, from 39 caves on five continents, showing that drip water δ18O is most similar to the amount-weighted precipitation δ18O where mean annual temperature (MAT) is < 10 °C. By contrast, for seasonal climates with MAT > 10 °C and < 16 °C, drip water δ18O records the recharge-weighted δ18O. This implies that the δ18O of speleothems (formed in near isotopic equilibrium) are most likely to directly reflect meteoric precipitation in cool climates only. In warmer and drier environments, speleothems will have a seasonal bias toward the precipitation δ18O of recharge periods and, in some cases, the extent of evaporative fractionation of stored karst water.

Persistent northward North Atlantic tropical cyclone track migration over the past five centuries.

Accurately predicting future tropical cyclone risk requires understanding the fundamental controls on tropical cyclone dynamics. Here we present an annually-resolved 450-year reconstruction of western Caribbean tropical cyclone activity developed using a new coupled carbon and oxygen isotope ratio technique in an exceptionally well-dated stalagmite from Belize. Western Caribbean tropical cyclone activity peaked at 1650 A.D., coincident with maximum Little Ice Age cooling, and decreased gradually until the end of the record in 1983. Considered with other reconstructions, the new record suggests that the mean track of Cape Verde tropical cyclones shifted gradually north-eastward from the western Caribbean toward the North American east coast over the last 450 years. Since ~1870 A.D., these shifts were largely driven by anthropogenic greenhouse gas and sulphate aerosol emissions. Our results strongly suggest that future emission scenarios will result in more frequent tropical cyclone impacts on the financial and population centres of the northeastern United States.