The icealbedo feedback

This is probably the best known of feedbacks. If the climate warms, the extent of snow and ice cover will decrease. The Earth's surface albedo decreases, meaning greater absorption of solar radiation at the surface. The climate hence warms more, melting more snow and ice, and so on. The mechanism can work in reverse, whereby cooling allows more snow and ice to remain on the surface, increasing the Earth's albedo, causing further cooling. The feedback is positive in that, through the chain of events, the initial perturbation (warming or cooling) is reinforced. The ice-albedo feedback is an important process in simulations of global warming in response to increased concentrations of greenhouse gases (see Chapter 11) and to simulations of global cooling during Ice Age maxima (see Chapter 10).

Although most attention has been paid to the feedback process associated with changes in the areal extent of snow cover and sea ice, there are also feedbacks associated with processes within the snow and ice zones. This is illustrated conceptually (Figure 5.14) for the case of the sea ice cover. The outer loop represents the

Positive Feedback Mechanism

Figure 5.14 Schematic of the ice-albedo feedback mechanism using the framework of Kellogg (1973). The direction of the arrow indicates the direction in the interaction. A + indicates a positive interaction (an increase in the first quantity leads to an increase in the second). A — indicates a negative interaction (an increase in the first quantity leads to a decrease in the second quantity). A +/— indicates that the sign of the interaction is uncertain or that the sign changes over the annual cycle (from Curry et al., 1996, by permission of AMS).

Figure 5.14 Schematic of the ice-albedo feedback mechanism using the framework of Kellogg (1973). The direction of the arrow indicates the direction in the interaction. A + indicates a positive interaction (an increase in the first quantity leads to an increase in the second). A — indicates a negative interaction (an increase in the first quantity leads to a decrease in the second quantity). A +/— indicates that the sign of the interaction is uncertain or that the sign changes over the annual cycle (from Curry et al., 1996, by permission of AMS).

feedback associated with areal extent whereby an initial perturbation (e.g., increased or decreased ice extent) instigates a chain of events amplifying the perturbation (this loop also holds with respect to areal changes in terrestrial snow cover). The interior part of the figure illustrates processes occurring within the multiyear ice pack. Warming leads to more melt ponds, and more melt ponds will reduce the albedo. Similarly, warming results in less snow cover, which reduces surface albedo. This also means thinner ice, reducing the albedo. Albedo is also reduced with higher temperatures due to an increase in lead fraction.

However, there are competing interactions. Although warming leads to more melt ponds, reducing albedo, less snow cover means fewer melt ponds, which increases albedo. The sign of other interactions is poorly understood or changes seasonally. Sorting out the sign and relative magnitude of the different linkages represents a major challenge to the modeling community. The study by Curry et al. (1995), using a one-dimensional sea ice model, suggests that the magnitude of the positive ice-albedo feedback mechanism is increased by the inclusion of melt ponds and diminished by the inclusion of ice thickness distribution and ridging (see Chapter 7 for a discussion of sea ice characteristics).

0 0

Post a comment