As
with all of Kibo's glaciers, the Northern Ice Field (NIF) is shrinking; margins are retreating
as the glacier thins. Although the two rates differ considerably, retreat is occurring around the entire perimeter - unlike most glaciers - and the surface mass balance is everywhere negative. As
illustrated by the two images above, NIF margin retreat from 2009 (lower) to
2015 (upper) was on the order of tens of meters. Thinning is less apparent in
these images, other than the development of holes, and for the same
period was roughly an order of magnitude less.
Ablation loss as a result of thinning is
what we measure continuously at the AWS, and periodically at stakes on the mountain's
other glaciers (i.e., specific mass balance).
Since installing our NIF AWS in
February 2000, ablation of the horizontal glacier surface has
required resetting the tower on 5 occasions. Until October 2011 when we reset the tower most recently, the interval between
lowerings has ranged from 12 to 51 months. Yet for 4 years now, the
horizontal mass balance has been more positive than negative, never lower in height than 15-20 cm below the Oct. 2011 datum, and currently only 5-10 cm below that level.
However, the rate of ablation on Kibo often begins to accelerate
during August, and considerable ice loss can occur before new snow
accumulates in association with the short rains (typically Nov/Dec).
During July we measured 23 cm of net ablation at the AWS. With another
10 cm, the dirty, low-albedo surface of the NIF will be exposed once
again. This dark surface - possibly centuries old - was briefly exposed
at the AWS in September 2013 before ablation was checked by a
snowfall
event which was followed by early arrival of the short rains. In
contrast, extended dry seasons in 2000, 2003, and 2008 had the
opposite effect, and all required lowering of the AWS shortly
thereafter. Albedo is the primary control on ablation here, by governing the receipt of net
radiation. (Note in the
upper image from last month that snowcover remains only on higher-elevation portions
of the NIF, maintaining higher albedo and reducing ablation.)
By the end of this month (August), we will have a better estimate of
when the tower will need to be reset. Fortunately, plans are
underway for fieldwork during September, when we can carry out the work if necessary.
Our 2015 fieldwork aspirations are ambitious. We will be an
international
group of 5 scientists, ably guided by the one-and-only
Simon Mtuy of
Summit
Expeditions (
SENE), and working for 6 days at the summit.
Annual service will be done on the AWS - in addition to possibly
resetting the tower - and a number of sensors will be swapped out for
calibration. We will also conduct the first thorough survey of NIF
ice thickness and stratigraphy, by a series of radar transects. A
small number of ice cores will be drilled and transported in frozen
state back to labs for additional radiocarbon dating, as well as
micrometer-resolution analysis of impurities. And as always, we will
continue documenting both retreat and thinning of all the mountain's
glaciers.
Collaborating Institutions for 2015 fieldwork:
*
Dept. of Geosciences, Univ. of Massachusetts Amherst
(USA) - lead
*
Institute of Environmental Physics, Univ. of Heidelberg
(Germany)
*
Climate Change Institute, Univ. of Maine (USA)
*
Alpine Cryosphere and Geomorphology Research Group,
Dept. of
Geosciences, Univ. of Fribourg (Switzerland)
*
Analytical Chemistry Group, Laboratory for Radiochemistry and
Environmental Chemistry, Paul Scherrer Institute (Switzerland)
