Publications

A decadal chronology of 20th-century changes in Earth’s natural systems

Citation

Mantua, N.J.  2006. A decadal chronology of 20th-century changes in Earth’s natural systems. In Costanza, R., L.J. Graumlich, and W. Steffen (eds.), Sustainability or Collapse? An Integrated History and Future of People on Earth. Cambridge, Massachusetts: MIT Press.


Abstract

Human activities were a major driver for secular changes in Earth’s natural systems in the 20th century. Human-caused changes in the atmosphere included rapid increases in concentrations of greenhouse gases, aerosols (dust, sulfates, smoke, and soot), and low-level ozone, as well as late 20th-century decreases in stratospheric ozone.Arapid rise in global average temperatures has been attributed, in part, to the increased concentrations of greenhouse gases. As part of the climate warming, polar ice sheets and most glaciers shrank, and snow cover declined. The second half of the 20th century was especially notable for rapid ecosystem changes, including increases in the amount of freshwater impounded behind dams, increases in nitrogen and phosphorous used in fertilizers, conversion of forests to croplands, losses and degradation of mangroves and coral reefs, depletion in large marine predatory fish, and declines in global biodiversity.

Causes for decade- to century-scale variability in Earth systems can be traced to a variety of factors, many directly related to human activities, but also others arising from nonhuman aspects of the Earth system like natural climate variations. Decade- to century-scale variability in atmospheric circulation patterns contributed to prominent variability in regional climate and ecosystems. Decadal changes between pluvials (wet conditions) and drought were especially large in the Sahel and the continental United States. Prolonged regional drought was part of complex suites of changes in Earth systems that included vegetation change, increased dust loading in the atmosphere, and strong interactions between the land surface and atmosphere at regional to global spatial scales. Climate variations also contributed to variations in major marine ecosystems, especially in the North Pacific Ocean. In contrast, multidecade declines in Atlantic cod caused by overfishing were punctuated with a collapse in formerly productive cod fisheries in the early 1990s; however, these declines also coincided with major increases in the abundance of and fisheries for commercially valuable crab, shrimp, and lobster in many parts of the North Atlantic. Climate modeling studies attribute decade- to century-scale variability in the 20th century to a combination of subtle changes in the intensity of the sun, human-caused increased concentrations of greenhouse gases and sulfate aerosols, human-caused stratospheric ozone depletion, and subtle changes in ocean surface temperatures. In some situations, feedbacks between the atmosphere and the Earth’s surface (ocean temperatures and land surface characteristics that include snow, ice, and vegetation) appear to have amplified large-scale circulation-caused precipitation changes.