River deltas constitute a quintessential example of how human-dominated ecosystems are evolving over the Anthropocene. Sitting at the interface of land and ocean, deltas owe their existence to a fundamental interplay between natural terrestrial and oceanic processes. At the same time, these coastal landforms have long served as prime locations for human settlement due their inherent capacity to support agriculture, gas and oil extraction, fisheries, and trade. These benefits have been accompanied by elevated risk arising from a broad array of human actions: local land use for agriculture; urbanization; hydrocarbon extraction; groundwater overuse; upstream water management that reduces essential flows of water and sediment needed to nourish these land forms; coastal engineering to ensure the utility of these systems for society; and, the specter of sea level rise from global climate change, accompanied by changes in storm and flood surge exposure. Many of these factors are interconnected, operate far beyond the boundaries of the deltas themselves, and generate substantial risk to the half-billion people who live there. The long-term integrity of these systems has thus been called into question.
We developed a global, systematic assessment of how delta risk is increasing due to sea-level rise and anthropogenic drivers of delta land subsidence. Rates of change of risk vary greatly across deltas (Panel A):
Risk is expected to increase greatly due to relative sea-level rise in deltas such as the Krishna (India), Ganges (Bangladesh) and Limpopo (Mozambique). Relative sea-level rise (the combination of offshore sea-level rise and coastal land subsidence) will have a large impact on coastal communities due to coastal flooding.
Today, some delta systems, notably the Mississippi (US), Rhine (Netherlands), Pearl (China), and Yangtze (China), are able to offset the geophysical and anthropogenic sources of risk by leveraging their wealth. Rich countries can construct high quality infrastructure, including large-scale engineered coastal defense systems. Additionally, wealth increases the capacity of society to prepare for and recover from hazardous events.
IMPLICATIONS FOR SUSTAINABLE DEVELOPMENT
The capability for wealthy and developing nations to reduce the overall risk to their deltas through risk-reducing defensive investments is not guaranteed into the future. The cost-effectiveness of these defenses is reduced both by increasing energy costs, as well as increasing sea levels. Both trends increase the overall cost for a given level of coastal protection. We utilize decade-to-century economic trend forecasts to estimate the varying impact on delta risk across each delta system. Certain delta systems, in particular the Mississippi (USA), the Rhine (the Netherlands), the Yangtze (China), and the Chao Phraya (Thailand), are more sensitive to changes in economic-trends given their underlying geophysical and environmental characteristics (Panel B). Efforts to address the root causes of land subsidence in the near-term are critical for long-term sustainability.