Joel A. Carr

Recent History and Geography of Virtual Water Trade

The global trade of goods is associated with a virtual transfer of the water required for their production. The way changes in trade affect the virtual redistribution of freshwater resources has been recently documented through the analysis of the virtual water network. It is, however, unclear how these changes are contributed by different types of products and regions of the world. Here we show how the global patterns of virtual water transport are contributed by the trade of different commodity types, including plant, animal, luxury (e.g., coffee, tea, and alcohol), and other products. Major contributors to the virtual water network exhibit different trade patterns with regard to these commodity types. The net importers rely on the supply of virtual water from a small percentage of the global population. However, discrepancies exist among the different commodity networks. While the total virtual water flux through the network has increased between 1986 and 2010, the proportions associated with the four commodity groups have remained relatively stable. However, some of the major players have shown significant changes in the virtual water imports and exports associated with those commodity groups. For instance, China has switched from being a net exporter of virtual water associated with other products (non-edible plant and animal products typically used for manufacturing) to being the largest importer, accounting for 31% of the total water virtually transported with these products. Conversely, in the case of The United states of America, the commodity proportions have remained overall unchanged throughout the study period: the virtual water exports from The United States of America are dominated by plant products, whereas the imports are comprised mainly of animal and luxury products.

Resilience and reactivity of global food security

Significance The past few decades have seen an intensification of international food trade and the increase in the number of countries that depend on food imports. As an effect of the associated globalization of food, local shocks in food production, combined with the adoption of new national or regional energy and trade policies, have recently led to global food crises. Here we develop a framework to investigate the coupled global food–population dynamics, and evaluate the effect of international trade on global food security. We find that, as the dependency on trade increases, the global food system is losing resilience and is becoming increasingly unstable and susceptible to conditions of crisis. The escalating food demand by a growing and increasingly affluent global population is placing unprecedented pressure on the limited land and water resources of the planet, underpinning concerns over global food security and its sensitivity to shocks arising from environmental fluctuations, trade policies, and market volatility. Here, we use country-specific demographic records along with food production and trade data for the past 25 y to evaluate the stability and reactivity of the relationship between population dynamics and food availability. We develop a framework for the assessment of the resilience and the reactivity of the coupled population–food system and suggest that over the past two decades both its sensitivity to external perturbations and susceptibility to instability have increased.

Dual role of salt marsh retreat: Long‐term loss and short‐term resilience

Two major causes of salt marsh loss are vertical drowning, when sediment accumulation on the platform cannot keep vertical pace with sea level rise, and horizontal retreat, associated with wave-induced marsh boundary erosion. Despite these processes having been extensively documented and modeled, it is unclear which loss modality dominates given a set of environmental parameters. A three-point dynamic model was developed to predict marsh loss as a function of sea level rise, allochthonous sediment supply, wind regime, tidal range, and marsh bank and mudflat erodability. Marsh horizontal and vertical evolutions were found to respond in opposing ways to wave-induced erosion processes. Marsh horizontal retreat was triggered by large mudflats, strong winds, high erodability of marsh bank and mudflat, whereas the opposite conditions acted to reduce the sediment supply to the marsh platform, promoting marsh loss to drowning. With low and moderate rates of sea level rise (∼5 mm/yr), retreat was found to be a more likely marsh loss modality than drowning. However, conditions associated with marsh retreat also increase the system resilience by transferring sediment on the marsh platform and preventing drowning. Our results suggest the use of a modular strategy for short-term marsh management: selectively protect extensive salt marsh regions by maintaining healthy vegetation on the platform, while allowing other areas to retreat, leveraging the natural resilience embedded in the lateral loss of marsh extent.

Tree-Grass Coexistence in the Everglades Freshwater System

Mosaic freshwater landscapes exhibit tree-dominated patches —or tree islands—interspersed in a background of marshes and wet prairies. In the Florida Everglades, these patterned landscapes provide habitat for a variety of plant and animal species and are hotspots of biodiversity. Even though the emergence of patchy freshwater systems has been associated with climate histories, fluctuating hydrologic conditions, and internal feedbacks, a process-based quantitative understanding of the underlying dynamics is still missing. Here, we develop a mechanistic framework that relates the dynamics of vegetation, nutrients and soil accretion/loss through ecogeomorphic feedbacks and interactions with hydrologic drivers. We show that the stable coexistence of tree islands and marshes results as an effect of their both being (meta-) stable states of the system. However, tree islands are found to have only a limited resilience, in that changes in hydrologic conditions or vegetation cover may cause an abrupt shift to a stable marsh state. The inherent non-linear and discontinuous dynamics determining the stability and resilience of tree islands should be accounted for in efforts aiming at the management, conservation and restoration of these features.

Sea level driven marsh expansion in a coupled model of marsh erosion and migration

Coastal wetlands are among the most valuable ecosystems on Earth, where ecosystem services such as flood protection depend nonlinearly on wetland size and are threatened by sea level rise and coastal development. Here we propose a simple model of marsh migration into adjacent uplands and couple it with existing models of seaward edge erosion and vertical soil accretion to explore how ecosystem connectivity influences marsh size and response to sea level rise. We find that marsh loss is nearly inevitable where topographic and anthropogenic barriers limit migration. Where unconstrained by barriers, however, rates of marsh migration are much more sensitive to accelerated sea level rise than rates of edge erosion. This behavior suggests a counterintuitive, natural tendency for marsh expansion with sea level rise and emphasizes the disparity between coastal response to climate change with and without human intervention.

Upscaling carbon dioxide emissions from lakes

Quantifying CO2 fluxes from lakes to the atmosphere is important for balancing regional and global-scale carbon budgets. CO2 emissions are estimated through statistical upscaling procedures that ag ...

Reserves and trade jointly determine exposure to food supply shocks

While a growing proportion of global food consumption is obtained through international trade, there is an ongoing debate on whether this increased reliance on trade benefits or hinders food security, and specifically, the ability of global food systems to absorb shocks due to local or regional losses of production. This paper introduces a model that simulates the short-term response to a food supply shock originating in a single country, which is partly absorbed through decreases in domestic reserves and consumption, and partly transmitted through the adjustment of trade flows. By applying the model to publicly-available data for the cereals commodity group over a 17 year period, we find that differential outcomes of supply shocks simulated through this time period are driven not only by the intensification of trade, but as importantly by changes in the distribution of reserves. Our analysis also identifies countries where trade dependency may accentuate the risk of food shortages from foreign production shocks; such risk could be reduced by increasing domestic reserves or importing food from a diversity of suppliers that possess their own reserves. This simulation-based model provides a framework to study the short-term, nonlinear and out-of-equilibrium response of trade networks to supply shocks, and could be applied to specific scenarios of environmental or economic perturbations.

Historical trade-offs of livestock’s environmental impacts

Human demand for animal products has risen markedly over the past 50 years with important environmental impacts. Dairy and cattle production have disproportionately contributed to greenhouse gas (GHG) emissions and land use, while crop demands of more intensive systems have increased fertilizer use and competition for available crop calories. At the same time, chicken and pig production has grown more rapidly than for ruminants, indicating a change in the environmental burden per animal calorie (EBC) with time. How EBCs have changed and to what extent resource use efficiency (RUE), the composition of animal production and the trade of feed have played a role in these changes have not been examined to date. We employ a calorie-based perspective, distinguishing animal calorie production between calories produced from feedcrop sources—directly competing with humans for available calories—and those from non-feed sources—plant biomass unavailable for direct human consumption. Combining this information with data on agricultural resource use, we calculate EBCs in terms of land, GHG emissions and nitrogen. We find that EBCs have changed substantially for land (−62%), GHGs (−46%) and nitrogen (+188%). Changes in RUE (e.g., selective breeding, increased grain-feeding) have been the primary contributor to these EBC trends, but shifts in the composition of livestock production were responsible for 12%–41% of the total EBC changes. In addition, the virtual trade of land for feed has more than tripled in the past 25 years with 77% of countries currently relying on virtual land imports to support domestic livestock production. Our findings indicate that important tradeoffs have occurred as a result of livestock intensification, with more efficient land use and emission rates exchanged for greater nitrogen use and increased competition between feed and food. This study provides an integrated evaluation of livestocks impact on food security and the environment.

Past and present biophysical redundancy of countries as a buffer to changes in food supply

Spatially diverse trends in population growth, climate change, industrialization, urbanization and economic development are expected to change future food supply and demand. These changes may affec ...

Inequality or injustice in water use for food

The global distributions of water availability and population density are uneven and therefore inequality exists in human access to freshwater resources. Is this inequality unjust or only regrettab ...