In climate change, a feedback loop is the equivalent of a vicious or virtuous circle – something that accelerates or decelerates a warming trend. A positive feedback accelerates a temperature rise, whereas a negative feedback decelerates it.
Scientists are aware of a number of positive feedbacks loops in the climate system. One example is melting ice. Because ice is light-coloured and reflective, a large proportion of the sunlight that hits it is bounced back to space, which limits the amount of warming it causes. But as the world gets hotter, ice melts, revealing the darker-coloured land or water below. The result is that more of the sun’s energy is absorbed, leading to more warming, which in turn leads to more ice melting – and so on.
The Dust Feedback Loop
As the ice sheet melts around its edges, dust captured in eons of snow becomes exposed. Most of this dust does not get washed away by melt, partially because much of the ice loss is from sublimation, the process by which ice evaporates directly instead of changing into water. But much of the dust also stays in place on the ice simply because the flow speed of meltwater is very low except where it concentrates into what the scientists call supraglacial stream flow, or simply melt streams on the ice.
As more ice melts, more dust gets left behind on the ice. This has radical implications for increased melt because dust absorbs far more sunlight than ice. An example is that ice reflects up to 90 percent of sunlight back into space where dirt, rocks, plants and open water absorb up to 90 percent of sunlight and change it into heat.
Since 2000, the amount of dust in the ice sheet has increased 5 percent, or more accurately, the brightness has decreased by 5 percent. (7) The National Snow and Ice Data Center, 2014 Melt Season Review says:
The darker snow absorbs more sunlight, leading to earlier melt onset and overall more melting, even if air temperature conditions are similar to previous years (as was the case in northwestern Greenland in 2014). Darker snow is a result of increased soot, dust, and even microbes in the snow, and the general trend of warmer summer conditions.
The melt season length has increased an astonishing 70 days in the last 35 years, significantly more in the fall than in the spring. Another study referenced by the National Snow and Ice Data Center says that new-fallen snow reflects 2 percent less sunlight than it did in 2003 (from 12 percent to 10 percent). This small amount of light energy is equal to 15 to 20 percent more heat.
The amount of energy from the sun that the snow absorbs increases, leading to earlier melt, more pronounced snow and ice evaporation (sublimation), melt run-off, and later re-freezing of the ice sheet surface.
In 2009, an abrupt shift in sunlight absorption was reported by researchers at Météo-France and other French climate science institutions. Their research says this is “due to a persistent increase in the amount of soot or dust in new-fallen snow over the island.”
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