Which of the following is an effect of the melting of glaciers and ice caps on ocean currents?

Question

BlackTom AI · Accepted Answer

Freshwater from melting glaciers and ice caps reduces surface salinity and therefore the density of seawater, which weakens the thermohaline (density-driven) component of ocean circulation.

1) Relevant knowledge points and concepts
- Thermohaline circulation is driven by differences in water density, which depend on temperature and salinity. Colder, saltier water is denser and tends to sink, driving deep ocean currents and global overturning.
- Upwelling and vertical mixing bring nutrient-rich deep waters to the surface; horizontal currents transport nutrients across regions.
- Large inputs of freshwater (from melting ice) lower surface salinity and can stabilize the water column (stratification), reducing vertical exchange.

2) Step-by-step reasoning
- Step 1: Melting ice injects significant freshwater into the ocean surface, especially at high latitudes.
- Step 2: Surface salinity decreases where freshwater inputs occur; lower salinity makes surface water less dense.
- Step 3: Reduced density contrast weakens deep-water formation because surface water no longer sinks as readily.
- Step 4: Weaker sinking reduces the strength of the thermohaline circulation (for example, a slowdown of parts of the AMOC).
- Step 5: A weaker circulation and increased stratification reduce vertical mixing and upwelling, and slow horizontal currents that transport nutrients.
- Step 6: With less vertical and horizontal transport, the supply of deep, nutrient-rich water to surface ecosystems declines, decreasing nutrient availability.

3) Calculations
- No numerical calculation is required; the argument is based on cause-and-effect in physical oceanography.

4) Why the correct answer is correct
Because freshwater input lowers salinity and density, it diminishes the driving force for major ocean currents, slowing them and reducing the volume and rate at which nutrients are carried from deep waters to productive surface zones.

5) Why other options are incorrect
- “It speeds ocean currents up” (both variants): incorrect because added freshwater reduces density contrasts that drive currents, so currents are expected to weaken, not strengthen.
- “...increasing the amount of nutrients they transport”: incorrect because weaker currents and increased stratification reduce upwelling and nutrient transport rather than increase it.

Conclusion: The correct choice is “It slows ocean currents down, decreasing the amount of nutrients they transport,” which matches the provided answer.

Which of the following is an effect of the melting of glaciers and ice caps on ocean currents?Single choice

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