NAEP Science Grade 12

Category - Earth Science

Approximately 14,000 years ago, warmer conditions began to melt the Pleistocene ice sheets. This warming was suddenly interrupted around 12,700 years ago by 1,000 years of below- average temperatures in northeastern North America and northern Europe. The onset of this sudden and prolonged cold snap, known as the Younger Dryas event, was caused primarily by which of the following?
  1. A reduction in tropical air masses moving north due to a shift in the position of the subtropical jet stream
  2. A change in the ocean currents in the North Atlantic caused by an increased input of freshwater.
  3. A decrease in the intensity of solar energy output due to changes in the Sun's magnetic field.
  4. A change in the timing of the seasons caused by variations in Earth's orbital parameters.
Explanation
Correct Response: B. As continental glaciers melted at the end of the Pleistocene epoch, vast amounts of freshwater flowed into the North Atlantic, altering the salinity of ocean water and consequently thermohaline ocean circulation. The Younger Dryas cooling event was caused by a change in northward flowing ocean currents connected to the larger thermohaline circulation of the Atlantic Ocean. The cooling of the Younger Dryas Event altered atmospheric circulation patterns and likely the position of the subtropical jet stream (A), but the Younger Dryas Event was not caused by those changes. Changes in the intensity of the solar energy output (C) were not involved in the Younger Dryas Event and although Earth's orbital parameters (D) were responsible for the major glacial cycles of the Pleistocene, they were not responsible for development of the relatively short-lived Younger Dryas Event.
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