D4.3 Climate change

This question tests the albedo-based positive feedback cycle. A positive feedback loop amplifies the original change rather than counteracting it.

Albedo is the proportion of incoming solar radiation that a surface reflects. Sea ice and snow have high albedo (~0.8–0.9), reflecting most incoming radiation, while open ocean water has low albedo (~0.06), absorbing most radiation.

Initial warming → ice melts → dark ocean exposed → more solar radiation absorbed → further warming → more ice melts. Each step amplifies the original warming, confirming this is a positive feedback loop.

The correct answer is: "Open ocean water absorbs more solar radiation than ice, raising ocean temperatures further and accelerating further ice loss." This accurately describes the mechanism (low albedo → greater absorption → more warming → more melting).

The question asks which option correctly explains why ice loss constitutes a positive feedback cycle — meaning it must describe a self-amplifying warming mechanism.

"Open ocean water reflects more solar radiation than ice" is factually incorrect. Ocean water has lower albedo than ice — it reflects less radiation, not more. Eliminate.

"Increased ocean salinity enhances thermohaline circulation" is inaccurate in this context. Melting ice actually reduces salinity by adding freshwater, and while thermohaline circulation is relevant, this does not describe a warming positive feedback. Eliminate.

"Increased phytoplankton growth absorbs CO₂" describes a negative feedback effect (cooling), not a positive feedback cycle. It also incorrectly links ice loss to nutrient upwelling as the primary mechanism here. Eliminate.

The remaining option — open ocean absorbs more radiation, raising temperatures and accelerating ice loss — correctly describes the albedo-driven positive feedback cycle. Select this option.

The greenhouse effect hinges on a critical distinction: incoming solar radiation is short-wave (visible light and UV), while radiation emitted by Earth's warmed surface is long-wave (infrared). This wavelength shift is what allows selective trapping.

Step 1: Short-wave solar radiation passes through the atmosphere and is absorbed by Earth's surface. Step 2: The warmed surface re-emits energy as long-wave (infrared) radiation directed toward space. Step 3: Greenhouse gas molecules absorb this outgoing infrared radiation. Step 4: Absorbed energy is re-emitted in all directions, including back toward Earth. Step 5: Net result is warming of the lower atmosphere and surface.

Only the option "Short-wave solar radiation reaches Earth's surface → surface re-emits long-wave infrared radiation → greenhouse gases absorb and re-emit it in all directions, including downward → surface warms" correctly describes all steps in the proper sequence.

The question tests knowledge of the full mechanistic sequence of the greenhouse effect, particularly the correct types of radiation at each stage.

"Long-wave solar radiation strikes Earth's surface" is incorrect at the first step. Solar radiation reaching Earth is predominantly short-wave (visible and UV), not long-wave. Eliminate.

"Short-wave solar radiation is absorbed by greenhouse gases in the upper atmosphere" is incorrect. Most incoming short-wave solar radiation passes through the atmosphere relatively easily and is absorbed by Earth's surface, not primarily by greenhouse gases in the upper atmosphere. Eliminate.

"Long-wave solar radiation passes through the atmosphere" incorrectly labels solar radiation as long-wave. Furthermore, greenhouse gases absorb and re-emit infrared radiation — they do not simply reflect it. Eliminate.

The correct option accurately states: short-wave radiation from the Sun → absorbed by Earth's surface → re-emitted as long-wave infrared → absorbed and re-emitted by greenhouse gases → warming. This is the only complete and accurate description.