Scientific news sheet # 258 ( pdf , 130 Ko )

Sheet n°258 - February 2007
Himalayan glacier melting
observed from space
© IRD/ Yves Arnaud
T
he Himalayan glaciers
are melting under the
effect of global warming.
However, the extent of this
melting remains difficult to
assess from ground surveys
owing to the great number
of glaciers, the difficulty of
access and the vastness
of the mountain chain. IRD
and CNRS scientists from
the research units Great Ice
and Legos (1) have overcome
these difficulties by using
satellite imagery. This
technique enabled them to
compare different glacier
topographies established
first in February 2000 then
in November 2004 on a 915
km2 test area in the Himalaya.
A complex data extraction
process to separate reliable
information from noiseaffected glacial images was
elaborated. It was found that,
over the period 2000-2004,
thinning of 8 to 10 m occurred
at altitudes below 4400 m and
a weaker but still marked
wasting of about 2 m in the
highest areas above 5000 m.
Earlier the same team had
validated the method on the
well-mapped Alpine massifs,
with the support of LGGE (2) .
It is the first time that such a
technique has been applied
on a geographical zone for
which no reliable ground
survey data is available as
control, and therefore no
reference point for accurate
positioning of the images.
This evaluation is the
first step of a much more
extensive project: evaluation
and surveillance of the whole
system of Himalayan glaciers
spread over a territory as
large as France.
DGPS positioning of an ablation marker on the tongue of Chhota Shigri Glacier at 4400 m
The Himalaya, the “Roof of the World”,
source of the seven largest rivers of Asia
are, like other mountain chains, suffering
the effects of global warming. To assess
the extent of melting of its 33 000 km2
of glaciers, scientists have been using a
process they have been pioneering for
some years. Satellite-imagery derived
glacier surface topographies obtained at
intervals of a few years were adjusted
and compared. Calculations indicated
that 915 km2 of Himalayan glaciers of
the test region, Spiti/Lahaul (Himachal
Pradesh, India) thinned by an annual
average of 0.85 m between 1999 and
2004. The technique is still experimental,
but it has been validated in the Alps and
could prove highly effective for watching
over all the Himalayan glacier systems.
However, the procedure for achieving a
reliable estimate must overcome a number of sources of error and approximation
inherent in satellite-based observations.
The researchers started by retrieving
satellite data for two periods, 2000 and
2004. A digital field model was extracted for each of them, representing the
topography of a ground reference point
in digital form and therefore usable in
computerized processing. The earliest
topography of the area studied was provided by NASA which observed 80% of
the Earth’s surface during the Shuttle
>>
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Sheet n°258 - February 2007
For futher information
CONTACTS :
YVES ARNAUD
IRD UR 032, Glaciers et
Ressources en Eau d'Altitude
- Indicateurs Climatiques et
Environnementaux,
LGGE, Saint Martin d'Hères
+ 33 (0)4 76 82 42 73
[email protected]
ETIENNE BERTHIER
CNRS, LEGOS
Toulouse
[email protected]
PRESS OFFICE :
+33 (0)1 48 03 75 19 ;
[email protected]
INDIGO BASE, IRD PICTURE LIBRARY
+33 (0)1 48 03 78 99 ;
[email protected]
www.ird.fr/indigo
IRD AUDIOVISUEL
+33 (0)1 48 02 56 24 ;
[email protected]
www.audiovisuel.ird.fr/
Radar Topographic Mission of February
2000. Then, in November 2004, two 2.5
m resolution images of the same area
taken at two different angles were acquired especially by the French satellite
Spot5 in the framework of an ISIS (CNES)
project. Comparison of these two images
has helped build a field model, a Digital
Elevation Model (DEM), by stereoscopic photogrammetric techniques (3). The
DEM model reveals that NASA radar
data underestimate values at high altitudes and overestimate them at lower
altitudes. And the Spot satellite produces
an uncertainty of +/- 25 m in the horizontal positioning of images. Moreover, as
the authorities of the major Himalayan
countries (India, Pakistan, China) do not
permit public access to detailed topographic maps or aerial photographs of
these sensitive cross-border regions, no
reference is available for satellite observation error assessment and correction.
It is therefore by comparing the SRTM
and SPOT5 topographies using stable
non-glaciated areas around glaciers that
researchers have been able to adjust for
the deviations and superimpose the two
digital field models. These comparisons
gave the bases for a map of glacier elevation (and hence thickness) variations for
altitude intervals of 100 m over the period
2000-2004.
The results show clear regression of
the large glaciers whose terminal tongues reach the lowest levels (about 4000
m) with a thinning of 8 to 10 m below 4400
m. Such loss is 4 to 7 m between 4400
and 5000 m, passing to 2 m above 5000
m. The satellite image evaluation yields
an average mass balance of –0.7 to –0.85
m/a water equivalent for the 915 km2 of
glaciers surveyed, a total mass loss of 3.9
km3 of water in 5 years. In order to check
these results and validate the procedure,
the satellite-derived results were compared with the mass balance for the small
glacier Chhota Shigri (15 km2) determined
from the field measurements and surveys,
performed between 2002 and 2004 by the
Great Ice research unit and its Indian partners. The mass balance determined from
these field data and that calculated from
satellite data agree. For both evaluation
methods, Chhota Shigri glacier appears
to have lost an average of a little over 1 m
of ice per year.
These results are in line with global
estimates for glacier made for the period
between 2001 and 2004. The approach is
therefore being extended to other areas of
the Himalaya in order to gain more information on the still poorly known changes
taking place in the region’s glaciers, which
are a water resource on which tens of
millions of people depend.
(1) Great Ice: ‘Glaciers et ressources en eau d'altitude - Indicateurs climatiques et environnementaux’
IRD Research Unit 032.
Legos: ‘Laboratoire d'études en géophysique et
océanographie spatiales’, Joint Research Unit
UMR 065 IRD, CNRS, CNES, Université Paul
Sabatier, Toulouse & Pôle Terre Vivante et Espace
(Toulouse).
(2) Laboratoire de Glaciologie et Géophysique
de l'Environnement, University Joseph Fourier
(Grenoble), CNRS, Observatoire des Sciences de
l’Univers de Grenoble.
(3) Technique involving the calculation of a threedimensional image from two photos of the same
place taken from different angles. In this way the
relief can be estimated and a reconstruction made.
REFERENCE :
BERTHIER ETIENNE, ARNAUD
YVES, KUMAR RAJESH, AHMAD
SARFARAZ, WAGNON PATRICK,
CHEVALLIER PIERRE - Remote
sensing estimates of glacier mass balances in the
Himachal Pradesh (Western
Himalaya, India), Remote
Sensing of the Environment,
in press.
Accessible on line since 28
December 2006 on
www.sciencedirect.com/
DOI : 10.1016/
J.RSE.2006.11.017
Rédaction – IRD : François Rebufat
Translation : Nicholas Flay
KAY WORDS :
HIMALAYA, GLACIER, SATELLITE
IMAGES, GLOBAL WARMING
Location map of the survey area
Marie Guillaume-Signoret, coordinatrice
Délégation à l’information et à la communication
Tél. : +33(0)1 48 03 76 07 - fax : +33(0)1 40 36 24 55 - [email protected]