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Feedback Loops, Ocean Acidification, and Invasive Species

You’ll be able to

Positive feedback amplifies; negative feedback stabilizes

The words do not mean good and bad — they describe direction. A positive feedback amplifies the original change; a negative feedback counteracts it and stabilizes the system. The climate examples worth having ready are all positive, which is why they matter. Ice-albedo: warming melts ice, exposing dark ocean or land that absorbs more sunlight than the white ice reflected, causing more warming. Permafrost: warming thaws permafrost, releasing stored methane and CO₂, causing more warming. Water vapor: warming increases evaporation, and water vapor is a greenhouse gas, causing more warming. Forest dieback: warming and drought kill forests, releasing stored carbon and removing a sink. A negative feedback for contrast: higher CO₂ can increase plant growth, which removes some CO₂ — real, but far too small to offset emissions.

Ocean acidification, which is a chemistry problem rather than a temperature one

This is a separate consequence of CO₂ from warming, and the exam rewards treating it as such. The ocean absorbs roughly a quarter to a third of emitted CO₂. Dissolved CO₂ reacts with water to form carbonic acid, which dissociates and releases hydrogen ions, lowering pH — surface ocean pH has fallen from about 8.2 to about 8.1 since pre-industrial times, which on a logarithmic scale is roughly a 30% increase in hydrogen ion concentration. Those hydrogen ions then combine with carbonate ions, reducing the carbonate available for organisms to build shells and skeletons from calcium carbonate. The organisms affected are corals, molluscs, echinoderms and calcifying plankton such as pteropods and coccolithophores — and because calcifying plankton sit at the base of marine food webs, the consequences propagate upward. Corals face a double pressure: acidification weakens skeletons while warming causes bleaching, the expulsion of symbiotic zooxanthellae.

Invasive species: why they win

An introduced species becomes invasive when it spreads and causes ecological or economic harm, and the reasons are consistent. No natural predators, parasites or diseases in the new range, because those co-evolved elsewhere. Broad tolerance and generalist diet, so it exploits many conditions. Rapid reproduction. And native species with no evolved defenses, since they have never encountered it. Standard cases to name: zebra mussels clogging intakes in the Great Lakes; kudzu smothering vegetation in the American South; cane toads in Australia, introduced deliberately for pest control and now poisoning predators that eat them; Burmese pythons in the Everglades; Asian carp threatening the Great Lakes. Control options in increasing cost and decreasing success: prevention through ballast-water regulation and inspection, by far the cheapest; early detection and rapid response; mechanical or chemical removal; and biological control, which is effective when it works and carries the real risk that the control agent itself becomes invasive — the cane toad being the standard cautionary case.

Ocean acidification chemistry
CO₂ + H₂O → H₂CO₃ → H⁺ + HCO₃⁻; the added H⁺ consumes CO₃²⁻, reducing carbonate for shell building
A chemistry consequence of CO₂, not a temperature one — which is why it would occur even without warming.
Worked example

Explain the ice-albedo feedback and state whether it is positive or negative, then give one contrasting negative feedback.

  1. 1.Start with the change: rising temperature melts sea ice and snow cover.
  2. 2.Identify the property that changes: ice and snow have high albedo, reflecting most incoming solar radiation, while the ocean and bare land beneath have low albedo and absorb it.
  3. 3.Trace the consequence: replacing a reflective surface with an absorbing one means more solar energy is retained as heat.
  4. 4.Close the loop: that additional heat raises temperature further, melting more ice — the change reinforces itself, so the feedback is positive.
  5. 5.Contrast: increased CO₂ can stimulate plant growth, which removes some CO₂ from the atmosphere and opposes the original change, making it a negative feedback — though far too small to offset emissions.
Answer: The ice-albedo feedback is positive. Warming melts high-albedo ice, exposing low-albedo ocean or land that absorbs more solar radiation, which warms the system further and melts more ice — the effect amplifies its own cause. A contrasting negative feedback is CO₂ fertilization of plant growth, which removes some carbon and opposes the warming, though at a scale far too small to counteract emissions.
Watch out

"Positive feedback" does not mean beneficial. It means the effect reinforces its own cause. Every major climate feedback that concerns scientists is positive precisely because amplification is what makes a system hard to stabilize.

Checkpoint

Ocean acidification harms coral primarily because

Checkpoint

The most cost-effective approach to invasive species is generally

Answer the 2 checkpoints as you read.

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