Transcript Slide 1

Who Pulled the Plug?
The Mysterious Case of Disappearing Siberian Lakes
T.J. Bohn1, R. Schroeder2, E. Podest2, N. Pinto2, K.C.
McDonald2, and D.P. Lettenmaier1
1Dept.
of Civil and Environmental Engineering, University of
Washington, Seattle, WA, USA
2JPL-NASA, Pasadena, CA, USA,
UW-UBC Hydrology Symposium
Vancouver, BC, 2011-Sep-30
Disappearing Arctic Lakes!
Smith et al. (2005) Science
•Compared LANDSAT imagery over
W. Siberia from 1973 with 1997/1998
•Found 6% decrease in total lake area
•10% of lakes shrank to below 40ha
•1% of lakes completely disappeared
Hypothesized thawing permafrost as
the driver
This has been cited many times as
evidence of dramatic climate change
(Smith et al., 2005)
Why we should care
• Approximately the same amount of carbon
is stored in the world’s permafrost as is
currently in the atmosphere (> 500Gt,
Zimov et al, 2006)
• If permafrost thaws, soil carbon will
decompose to CO2 (or CH4) and escape
into the atmosphere
• Potential feedback to global warming
How are lakes involved?
• Much of the Arctic land surface is covered
by lakes, ponds, and wetlands
– Permafrost can
impede drainage of
water through soil
– Low summer ET
rates
– Flat topography
– Snowmelt can
inundate large
areas, seasonally
• Permafrost is sensitive to lakes
– Lakes have low albedo
– Ice floats and insulates the lake in winter
– Lake bottoms tend to resist freezing: “thaw bulb” or “talik”
• Lakes are sensitive to permafrost
•
•
•
•
As permafrost thaws, melt from
excess ground ice collects in
ponds/lakes – increase in lake
area
At same time, warming climate
warms the lakes and thaw bulbs
get deeper
Eventually, thaw bulbs may extend
completely through permafrost
If permafrost was impeding lake
drainage, lake can now drain –
decrease in lake area
Ice Cover (in winter)
Liquid Water
Thaw Bulb
Permafrost
Unfrozen Soil/Rock
Lakes are indicators of permafrost health
Breach
Smith et al’s Hypothesis
Lake area changes
showed distinct
geographic pattern:
• In continuous permafrost
zone, 12% increase in
total lake area
– Melting of excess ground
ice? (first phase of
permafrost degradation)
• In discontinuous, isolated,
and sporadic permafrost
zones, 5% to 9%
decrease in total lake area
– Lakes thawing through
permafrost? (second phase
of permafrost degradation)
(Smith et al., 2005)
Are *all* Arctic lakes thawing
through permafrost?
Lakes actively thawing through permafrost has big implications for
climate change projections
• Liberation of massive amounts of carbon stored in frozen soil, as CH4
(e.g. Walter et al. (2006))
Problems with Smith et al’s hypothesis:
• Only 1% of the lakes examined were found to vanish completely
– Can we assume that the other lakes are going to vanish?
• Observations had poor temporal coverage
– All LANDSAT images were from “summer” – presumably JJA
– Where multiple observations existed, the minimum lake area was taken to
be the “true” lake area (all extra assumed to be seasonal)
– Images only came from three years: 1973, 1997, 1998
• Can we draw conclusions based on a few snapshots?
Could some other process have caused lake areas to change?
Modeling Framework
• VIC hydrology model
– Large, “flat” grid cells (e.g.
100x100 km)
– On hourly time step,
simulate:
•
•
•
•
•
Soil T profile
Water table depth ZWT
NPP
Soil Respiration
Other hydrologic variables…
• Link to CH4 emissions
model (Walter & Heimann
2000)
VIC Dynamic Lake/Wetland Model
• Water & energy balance
model
• Includes mixing, ice cover
• Dynamic area based on
bathymetry
• Can flood surrounding
wetlands based on
topography
Special application: treat all
lakes, ponds, and inundated
wetland area as a single “lake”
Bowling and Lettenmaier, 2010
Lake Bathymetry/Topography
Lake size histograms
from GLWD (Lehner
and Doll, 2004) and
LANDSAT
Lake depths from
literature
•ILEC
•Arctic
Thaw Lakes
•Bog Pools
Lake storage-area relationship
SRTM and ASTER
DEMs for surrounding
topography
10
Depth(m)
LANDSAT
courtesy of E.
Podest and N.
Pinto of NASA/JPL
5
0
0
Cumulative Area
100
(km2)
Arctic Application
• Model simulates permafrost
– Validation of active layer depths & soil temperatures is underway
• Model simulates heat flux below lake, and reproduces
thaw bulb under lake
• However, lake does *not* require presence of permafrost
to exist
– Soil under lake is not permeable enough for lake to drain through
soil very quickly
– Soil under lake = wetland soil = peat
– Soil’s baseflow rate was calibrated – we might experiment with
this
• Lake dynamics dominated by snowmelt, ET, and surface
drainage (via outlet channel)
• Outlet channel width calibrated to match remotelysensed inundation extent product
AMSR/QSCAT-Derived Inundation
Annual Max – Min Fractional Inundation
•Daily, for snowfree days
•2002-2009
•25km resolution
Max % - Min %
> 15 %
Courtesy R. Schroeder, NASA/JPL
< 3%
Comparison with AMSR
•For each grid cell, calibration iterates over
lake outlet width until annual average
simulated lake area is unbiased with
respect to AMSR inundation product
Irrigated Crops
•Calibrations do this iteration for a range of
values of wetland microtopography and soil
baseflow parameters
•The combination having lowest MSE/VAR
metric is taken ( = 1 – Nash-Sutcliffe Eff.)
•MSE is of monthly simulated lake area vs.
AMSR/Quickscat product, 2002-2007
•VAR is variance of monthly
AMSR/Quickscat product, 2002-2007
•MSE/VAR of 1.0 can be achieved with a
straight line through the mean, i.e. we want
to get scores substantially closer to 0.0 than
to 1.0 for a good fit
Tundra / Cont.
Permafrost
Uvaly
Hills
Simulated Changes in Lake Area
• Simulation gives similar
magnitude & pattern of
change, for the same
approximate snapshot as
Smith et al. (2005)
Percent change in lake area
1973-06 to 1997-06
• Break into 4 latitude
bands:
– 67-72 N (continuous
permafrost)
– 64-67 N (discontinuous)
– 61-64 N (isol./sporadic)
– 57-61 N (non-permafrost)
– All bands cover 65-90E
-30 -20 -10
0
10 20 30
% change
60-year Timeseries
• Average monthly lake
fraction from:
– Continuous permafrost (6772N)
– Discont. permafrost (64-67N)
– Isolated/Sporadic permafrost
(61-64N)
– Non-permafrost (57-61N)
• Seasonal cycle is larger
than interannual variability
0.15
0.10
0.05
0.0
0.15
0.10
0.05
0.0
0.15
0.10
0.05
0.0
Shifts in Seasonal Cycle: Lake Area
Fract. Area
Fract. Area
Cont Perm
Fract. Area
• Monthly average
lake area fraction, by
decade, 1967-2006
• All regions show
increase in peak
lake area, 1970s2000s
• In north, peak occurs
in mid-June; peaks
occur progressively
earlier as we move
south
• Between 1970s and
1990s, JJA lake
areas increase 10%
in north; decrease 210% elsewhere
• Could it be that what
really happened was
a simple shift
forward in the snow
melt pulse (plus
larger snow packs)?
Approx. 10% increase,
1970s-1990s
Discont Perm
Isolated Perm
Approx. 10% decrease,
1970s-1990s
Non Perm
Month
Melt begins/ends
earlier as we move N
to S
•
Peak lake area falls
generally in 2nd
month of melt
•
All regions show
increase in peak
SWE, 1970s-2000s
– Corresponds to
increases in peak
lake area
•
Shifts in melt timing
are difficult to see
here at monthly
resolution
SWE (mm)
•
Cont Perm
Discont Perm
SWE (mm)
Monthly average
SWE
Isolated Perm
SWE (mm)
•
SWE (mm)
Shifts in Seasonal Cycle: SWE
Non Perm
Month
Shifts in Snow Melt Dates
No perm.
1997-06
1977-86
Cont. perm.
1967-76
1997-06
1977-86
1967-76
Discont.
perm.
1987-96
– Likely due to NOAA’s snow
absence threshold being 50%
coverage, compared to VIC’s 0
mm SWE threshold.
• This shift is a plausible cause of
the shift towards earlier dates
of peak inundation and lower
midsummer lake extent
Sporadic and
isolated perm.
1987-96
• Both observed (NOAA) and
simulated (VIC) snow melt has
shifted 3-10 days earlier in the
3 latitude bands 61-64N, 6467N, and 67-72N.
• VIC gives a consistently later
snow melt date (1-7 days) than
NOAA S of 67N
Conclusions
• Permafrost degradation is not the only hypothesis that
explains the changes in lake area observed by Smith et
al (2005)
• The bulk of the “changes” in lake area could also be
caused by:
– shift towards earlier snowmelt
– larger spring snow packs
• The observed geographic pattern in lake area changes
could be the result of:
– Summer sampling in the north falls around the peak in lake area
– Summer sampling in the south falls mainly in the recession
following the peak
Conclusions
• Assessment of more remote sensing imagery,
spanning more years and a longer portion of the
year, would help solve this puzzle
• Model limitations:
– We should double-check model performance in
tundra region
– We should try to simulate lakes actively thawing
through permafrost – any differences?
• How many lakes are actively thawing through
permafrost has big implications for feedbacks to
global warming
Thank You