Until recently, paleontologists assumed that the changes

Until recently, paleontologists assumed that the changes that brought the Permian period to an end, roughly 250 million years ago, took place gradually. However, the precise dating of the period's end, just recently established, has cast doubt on this theory by enabling a worldwide comparison of geological evidence. Comparing rock formations, researchers discovered a nearly identical and instantaneous global sequence of events at the exact close of the Permian. Black mud deposits, formed in the absence of oxygen, appeared in place of red, green, or gray rocks laid down when oxygen is present. Simultaneously, a drastic change in the concentration of atmospheric gases occurred, as evidenced by a sudden shift in the isotope ratios of rock carbon. These events coincided with a cataclysmic mass extinction which destroyed roughly 90% of the earth's species.

While some contend that the mass extinction was caused by a meteorite, further studies do not bear this out. A more plausible theory concerns huge volcanic eruptions in Siberia. Recent lava dating has shown that these explosions occurred at the same time as the extinctions. Researchers speculate that carbon dioxide released in the blasts increased the greenhouse effect, causing an increase in global temperatures that destabilized methane hydrate, a superconcentrated frozen gas. The methane release accounts for the shift in carbon isotopes and further enhanced the greenhouse effect, resulting in runaway global warming that contributed to the extinction. Studies of changes in oxygen isotope ratios have estimated the warming to be 6 degrees Celsius. Atmospheric scientists contend that a similar rise is not impossible by 2100. These findings therefore reinforce the urgency of addressing the problem of global warming in our time.

Q. Which of the following best characterizes the process that resulted from the Siberian volcanic eruptions as that process is described in the second paragraph?


a. A random interaction
b. A self-defeating mechanism
c. A positive feedback loop
d. A negative feedback loop
e. A pre-determined sequence of events