Page 49 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 1 essentials of Geography 13
1800s, the unnaturally high elk population stripped many areas of natural vegetation. After the 1995 reintroduction of Canadian wolves into Yellowstone, elk numbers de- clined with wolf predation. Since then, aspens and willow are returning, improving habitat for birds and small mam- mals and providing other ecosystem benefits.
If feedback information encourages change in the system, it is positive feedback. Further production of positive feedback stimulates system changes. Unchecked positive feedback in a system can create a runaway (“snowballing”) condition. In natural systems, such un- checked system changes can reach a critical limit, lead- ing to instability, disruption, or death of organisms.
Global climate change creates an example of posi- tive feedback as summer sea ice melts in the Arctic Re- gion (discussed in Chapter 4). As arctic temperatures rise, summer sea ice and glacial melting accelerate. This causes light-coloured snow and sea-ice surfaces, which reflect sunlight and so remain cooler, to be replaced by darker-coloured open ocean surfaces, which absorb sun- light and become warmer. As a result, the ocean absorbs more solar energy, which raises the temperature, which, in turn, melts more ice, and so forth (Figure 1.8). This is a positive feedback loop, further enhancing the effects of higher temperatures and warming trends. For more on a positive feedback loop involving climate change and greenhouse gases that has serious consequences for the Arctic, see Chapter 11, Geosystems Now.
The acceleration of change in a positive feedback loop can be dramatic. Scientists have found that the extent of sea ice has decreased in area, and that the vol- ume has dropped at an accelerating rate. Volume, a bet- ter indicator than extent for existing sea ice, has dropped by half since 1980; however, the rate of decrease was 2.5 times faster during the decade from 2000 to 2012
than it was from 1980 to 1990. As the feedback loop ac- celerates, the possibility of complete summer ice melt in the Arctic may become reality sooner than predicted— September is normally the month for lowest sea-ice extent; in 2012 this happened in August.
System Equilibrium Most systems maintain structure and character over time. An energy and material sys- tem that remains balanced over time, in which condi- tions are constant or recur, is in a steady-state condition. When the rates of inputs and outputs in the system are equal and the amounts of energy and matter in stor- age within the system are constant (or more realisti- cally, fluctuate around a stable average), the system is in steady-state equilibrium. For example, river channels commonly adjust their form in response to inputs of water and sediment; these inputs may change in amount from year to year, but the channel form represents a sta- ble average—a steady-state condition.
However, a steady-state system may demonstrate a changing trend over time, a condition described as dynamic equilibrium. These changing trends may appear gradually and are compensated for by the system. A river may tend toward channel widening as it adjusts to greater inputs of sediment over some time scale, but the overall system will adjust to this new condition and thus main- tain a dynamic equilibrium. Figure 1.9 illustrates these two equilibrium conditions, steady-state and dynamic.
Note that systems in equilibrium tend to maintain their functional operations and resist abrupt change. However, a system may reach a threshold, or tipping point, where it can no longer maintain its character, so it lurches to a new operational level. A large flood in a river system may push the river channel to a threshold where it abruptly shifts, carving a new channel. Another example of such a condition is a hillside or coastal bluff
that adjusts after a sudden landslide (Figure 1.9c). A new equilibrium is eventually achieved among slope, materials, and energy over time. High- latitude climate change has caused threshold events such as the relatively sudden collapse of ice shelves surrounding a portion of Antarctica and the crack-up of ice shelves on the north coast of Ellesmere Island, Canada, and Greenland.
Also, plant and animal communities can reach thresholds. After 1997, warming conditions in oceans combined with pollution to accelerate the bleaching of living coral reefs worldwide— taking coral systems to a threshold. Bleaching is the loss of colourful algae, a food source for the coral, causing the eventual death of the coral colonies making up the reef. In some areas, 50% of regional coral reefs experienced bleaching. On the Great Barrier Reef in Australia, coral die-off
of up to 90% occurred during the worst years. Today, about 50% of the corals on Earth are ailing; more on this in Chapter 16. Harlequin frogs of tropical Central and South America are another example of species reaching
Ocean absorbs more heat
Temperatures rising
Reflectivity, or albedo, is altered (ocean reflects less sunlight)
Sea ice melts, exposes darker ocean surface
▲Figure 1.8 The Arctic sea ice–albedo positive feedback loop. Average ice thickness in the Arctic summer has dropped dramatically, leaving thinner ice that melts more easily. Since 2000, 70% of the Septem- ber ice volume has disappeared. If this rate of ice volume loss continues, the first ice-free Arctic September might happen before 2017. [NOAA.]