Question

In the Arctic tundra, permafrost—soil that remains frozen year-round—acts as a massive carbon sink, locking away the decomposed remains of ancient vegetation. However, rising global temperatures have initiated a feedback loop that threatens to accelerate warming. As the atmosphere warms, the upper layers of permafrost begin to thaw. This thawing exposes long-dormant organic matter to microbial decomposition. In saturated, oxygen-depleted soils, anaerobic microbes digest the organic material, producing methane gas ($CH_4$), a greenhouse gas significantly more potent than carbon dioxide at trapping heat in the short term. The released methane escapes into the atmosphere, where it absorbs thermal radiation that would otherwise escape into space. This trapped heat further raises global temperatures, which in turn hastens the thawing of deeper permafrost layers. Consequently, what began as a localized thawing process transforms into a self-reinforcing global climatic driver, demonstrating how minor shifts in high-latitude soil stability can propagate widespread environmental changes.

Based on the passage, arrange the following events in the correct causal sequence, starting with the initial climatic trigger and ending with the self-reinforcing feedback effect.