Fig. 25.1 shows a diagram of the feedback loop between Arctic ice melt and global warming. Human activities, particularly the burning of fossil fuels, have ...

(a) Explanation of the ice-albedo feedback loop [4]

1. As global temperature rises due to increased greenhouse gas concentrations, Arctic ice and snow begin to melt. [1]
2. This melting exposes darker surfaces underneath, such as ocean water and bare land, that were previously covered by white, highly reflective ice. [1]
3. White ice and snow have a high albedo (reflectivity), meaning they reflect most incoming solar energy back into space. Darker surfaces such as ocean water have a low albedo and absorb a much greater proportion of solar energy, converting it to heat. [1]
4. The extra absorbed solar energy causes further warming of the surface, which melts more ice, which exposes more dark surface, which absorbs more energy, creating a self-reinforcing cycle that accelerates the original warming. [1]

(b) Why this is described as a 'positive feedback' mechanism [2]

1. It is called positive feedback because the effect (warming) reinforces and amplifies the original cause (warming), rather than counteracting it. The word "positive" does not mean beneficial; it means the change feeds back to increase itself. [1]
2. Each stage of the loop makes the next stage stronger: more warming leads to more melting, which leads to more absorption, which leads to more warming. The process accelerates rather than reaching a stable equilibrium. [1]

(c) How permafrost melting could accelerate climate change [3]

1. Permafrost is permanently frozen ground, found mainly in Arctic regions, that contains vast quantities of trapped organic matter (dead plants and animals) accumulated over thousands of years. [1]
2. As global temperatures rise and permafrost thaws, this organic matter is exposed to decomposition by soil microorganisms, which release methane (CH₄) and carbon dioxide (CO₂) into the atmosphere. Methane is approximately 28 times more potent as a greenhouse gas than CO₂. [1]
3. These additional greenhouse gases trap more heat in the atmosphere, causing further warming, which thaws more permafrost, releasing more greenhouse gases. This creates another positive feedback loop that accelerates climate change beyond the rate caused by human emissions alone. [1]

(d) Two other effects of Arctic ice melting on the global environment [2]

1. Rising sea levels: Melting ice sheets (particularly the Greenland ice sheet) add water to the oceans, contributing to global sea level rise that threatens low-lying coastal communities, infrastructure, and small island nations worldwide. [1]
2. Loss of habitat: Arctic species such as polar bears, walruses, and seals depend on sea ice as a platform for hunting, breeding, and resting. The shrinking and thinning of sea ice threatens the survival of these species and disrupts the entire Arctic food web. [1]

(e) Why positive feedback mechanisms make it difficult to predict the exact rate of future climate change [2]

1. Positive feedback mechanisms can cause climate change to accelerate much faster than simple linear models predict, because each increment of warming triggers additional warming through multiple feedback pathways (ice-albedo, permafrost, water vapour). [1]
2. The exact thresholds at which these feedback mechanisms become dominant are not precisely known. Scientists cannot predict exactly when a tipping point will be reached, creating significant uncertainty in climate projections. Small differences in assumptions about feedback strength produce widely divergent long-term forecasts. [1]

(f) What is meant by a 'tipping point' in the context of climate change [2]

1. A tipping point is a critical threshold beyond which a small additional change in temperature or emissions triggers a large, abrupt, and potentially irreversible shift in a component of the climate system. [1]
2. Once passed, the change becomes self-sustaining through positive feedback and cannot be reversed even if emissions are subsequently reduced. Examples include the complete melting of the Greenland ice sheet (which would raise global sea levels by approximately 7 metres) or the dieback of the Amazon rainforest (converting it from a carbon sink to a carbon source). [1]