Question

Historians studying the mid-sixth-century cooling event (536–550 CE) long debated whether solar minima or volcanic activity triggered the severe global temperature drop recorded in tree-ring anomalies. Recent ice-core analyses from Greenland and Antarctica revealed synchronized spikes in volcanic sulfate deposition dating precisely to 536 CE and 540 CE, leading most paleoclimatologists to conclude that consecutive stratospheric sulfur eruptions were the sole primary driver of the decade-long cold spell. Proponents of this volcanic hypothesis point out that high-latitude volcanic plumes reflect solar radiation back into space, thereby inducing rapid, multi-year hemispheric cooling.

However, planetary geochemists note that sulfate deposition in polar ice sheets merely indicates the magnitude of atmospheric sulfur aerosol loading, not the geographic elevation or latitude of the eruption source, nor the duration of tropospheric residence. Crucially, tree-ring density data across Northern Hemisphere boreal forests show that growth suppression persisted for nearly fourteen years after 536 CE—a duration nearly double the typical residence time of volcanic aerosols in the stratosphere before gravitational settling occurs. To account for this discrepancy, some researchers contend that an initial volcanic cooling pulse triggered a self-sustaining oceanic-sea ice feedback loop in the North Atlantic, wherein expanding sea ice increased planetary albedo and weakened heat transport from the tropics, thus prolonging cold conditions long after the volcanic aerosols had cleared.

Which of the following is an assumption on which the researchers' explanation for the prolonged duration of the cooling event depends?