Melt ponds cause the Arctic sea ice to melt more rapidly
Melt pond on Arctic sea ice, Photo: Stefan Hendricks, Alfred Wegener Institute |
The Arctic sea ice has not only declined over the past decade
but has also become distinctly thinner and younger. Researchers are now
observing mainly thin, first-year ice floes which are extensively covered with
melt ponds in the summer months where once metre-thick, multi-year ice used to
float. Sea ice physicists at the Alfred Wegener Institute, Helmholtz Centre for
Polar and Marine Research (AWI), have now measured the light transmission
through the Arctic sea ice for the first time on a large scale, enabling them
to quantify consequences of this change. They come to the conclusion that in
places where melt water collects on the ice, far more sunlight and therefore
energy is able to penetrate the ice than is the case for white ice without
ponds. The consequence is that the ice is absorbing more solar heat, is melting
faster, and more light is available for the ecosystems in and below the ice.
The researchers have now published these new findings in the scientific journal Geophysical Research Letters.
Melt
ponds count among the favourite motifs for ice and landscape photographers in
the Arctic. They are captured glistening in a seductive Caribbean sea blue or
dark as a stormy sea on the ice floe. "Their colour depends entirely on
how thick the remaining ice below the melt pond is and the extent to which the
dark ocean beneath can be seen through this ice. Melt ponds on thicker ice tend
to be turquoise and those on thin ice dark blue to black," explains Dr.
Marcel Nicolaus, sea ice physicist and melt pond expert at the Alfred Wegener
Institute.
In
recent years he and his team have observed a strikingly large number of melt
ponds during summer expeditions to the central Arctic. Virtually half of the
one-year ice was covered with melt ponds. Scientists attribute this observation
to climate change. "The ice cover of the Arctic Ocean has been undergoing
fundamental change for some years. Thick, multi-year ice is virtually nowhere
to be found any more. Instead, more than 50 per cent of the ice cover now
consists of thin one-year ice on which the melt water is particularly
widespread. The decisive aspect here is the smoother surface of this young ice,
permitting the melt water to spread over large areas and form a network of many
individual melt ponds," explains Marcel Nicolaus. By contrast, the older
ice has a rougher surface which has been formed over the years by the constant
motion of the floe and innumerable collisions. Far fewer and smaller ponds
formed on this uneven surface which were, however, considerably deeper than the
flat ponds on the younger ice.
The
growing number of "windows to the ocean," as melt ponds are also
referred to, gave rise to a fundamental research question for Marcel Nicolaus:
to what extent do the melt ponds and the thinning ice alter the amount of light
beneath the sea ice? After all, the light in the sea -- as on the land --
constitutes the main energy source for photosynthesis. Without sunlight neither
algae nor plants grow. Marcel Nicolaus: "We knew that an ice floe with a
thick and fresh layer of snow reflects between 85 and 90 per cent of sunlight
and permits only little light through to the ocean. In contrast, we could
assume that in summer, when the snow on the ice has melted and the sea ice is
covered with melt ponds, considerably more light penetrates through the
ice."
To find
out the extent to which Arctic sea ice permits the penetration of the sun's
rays and how large the influence of the melt ponds is on this permeability, the
AWI sea ice physicists equipped a remotely operated underwater vehicle (ROV
"Alfred") with radiation sensors and cameras. In the summer of 2011
during an Arctic expedition of the research ice breaker POLARSTERN, they sent
this robot to several stations directly under the ice. During its underwater
deployments, the device recorded how much solar energy penetrated the ice at a
total of 6000 individual points all with different ice properties!
A
unique data set was obtained in this way, the results of which are of great
interest. Marcel Nicolaus explains: "The young thin ice with the many melt
ponds does not just permit three times as much light to pass through than older
ice. It also absorbs 50 per cent more solar radiation. This conversely means
that this thin ice covered by melt ponds reflects considerably fewer sun rays
than the thick ice. Its reflection rate is just 37 per cent. The young ice also
absorbs more solar energy, which causes more melt. The ice melts from inside
out to a certain extent," says Marcel Nicolaus.
What
might happen in the future considering these new findings? Marcel Nicolaus:
"We assume that in future climate change will permit more sunlight to
reach the Arctic Ocean -- and particularly also that part of the ocean which is
still covered by sea ice in summer. The reason: the greater the share of
one-year ice in the sea ice cover, the more melt ponds will form and the larger
they will be. This will also lead to a decreasing surface albedo (reflectivity)
and transmission into the ice and ocean will increase. The sea ice will become
more porous, more sunlight will penetrate the ice floes, and more heat will be
absorbed by the ice. This is a development which will further accelerate the
melting of the entire sea ice area." However, at the same time the
organisms in and beneath the ice will have more light available to them in
future. Whether and how they will cope with the new brightness is currently
being investigated in cooperation with biologists.
Source: Helmholtz Association of German
Research Centres
Leave Your Comments!
Share What’s Going on
in your brain about the Topic. We need Your Response . Feel free to leave comments!
Posted by Unknown
on Saturday, January 19, 2013.
Filed under
Earth And Climate
.
You can follow any responses to this entry through the RSS 2.0