Transcript Document

New Developments in
Present-Day Sea Level Change
R. Steven Nerem
Colorado Center for Astrodynamics Research
Cooperative Institute for Research in Environmental Sciences
University of Colorado at Boulder
The Earth is Warming
Annual Global Mean Surface Temperatures
0.8
0.6
0.4
°C
0.2
0
-0.2
-0.4
-0.6
1880
1900
1920
1940
1960
Year
1980
2000
[Hansen et al., 2005]
What Causes Sea Level to Change?
The Bathtub Sea Level Model
Precipitation over Oceans
Runoff from Continents
+
Evaporation from Oceans
Precipitation over Continents
What is Global Mean Sea Level Change?
• “Global Mean Sea Level Change” is
•
•
•
the change in the average height of
the oceans over the entire globe at
a single point in time.
Sea level change at a specific
location in the ocean may be higher
or lower than the global mean
because of differences in ocean
temperature and other effects.
Usually measured in units of
millimeters (mm): 25.4 mm = 1 in
Does not include ocean tides, storm
surge
Land Movement Can Affect Sea Level Locally
InSAR Image of Subsidence in New Orleans
[Dixon et al., 2006]
mm/yr
Measurements of Sea Level Change
GRAVITY
Tide Gauge Sea Level Measurements
Tide Gauges with Greater Than 10 Years of Measurements
Tide Gauges with Greater Than 50 Years of Measurements
Tide Gauge Observations
150
3.2 mm/year
100
2.0 mm/year
MSL (mm)
50
0.8 mm/year
0
-50
Average Rate ~ 1.8 mm/year
-100
1880
1900
1920
1940
Year
1960
1980
2000
[Church and White, 2006]
Satellite Altimeters
TOPEX/Poseidon
1992-2005
Jason-1
2001 - ?
OSTM/Jason-2
2008
Global Mean Sea Level from Satellite Altimetry
Average Rate = 3.5 mm/year
(1993-2006)
1997-1998
El Nino
[Mitchum and Nerem, 2007]
Sea Level Change is Not the Same Everywhere
Average Sea Level Rise over 1993-2006
mm/yr
Sea Level Change Potential Contributions
Thermal Expansion: ~1 meter
Mountain Glaciers: 0.5 meters
Greenland Ice Melt: 7 meters
Antarctic Ice Melt: 60 meters
Land Water Storage: < 0.5 meters
Thermal Expansion: Contribution to Sea Level
20
15
Rate = 0.4 mm/year (1955-2004)
Rate = 1.2 - 1.6 mm/year (1993-2004)
MSL (mm)
10
5
Mt. Pinatubo
0
-5
-10
-15
1960
1970
1980
Year
1990
2000
[Levitus et al., 2005; Antonov et al., 2005]
Mountain Glaciers: Contribution to Sea Level
25
MSL (mm)
20
1961-2003: 0.5 mm/year
1994-2003: 0.9 mm/year
15
10
5
0
1960
1970
1980
Year
1990
2000
2010
[Dyurgerov and Meier, 2005]
Greenland Ice Mass Loss from GRACE
300
362 Gt = 1 mm GMSL
200
Gt/year
100
0
-100
-200
-300
-140 Gt/year = 0.4 mm/year GMSL
-400
2003.5
2004
2004.5
2005
Year
2005.5
2006
2006.5
Credit: Roger Braithwaite
[Luthcke et al., 2006]
Greenland Melt Extent
Antarctic Ice Mass Flux from InSAR
SLR 0.4 to 0.6 mm/yr
-37±20
-2
-4 km3/yr
km3/yr
-2 km3/yr
+5 km3/yr
km3/yr
-2 km3/yr
-3 km3/yr
-49±20 km3/yr
+48 km3/yr
-38 km3/yr
-4 km3/yr
-114 km3/yr
-22 km3/yr
-56 km3/yr
+33 km3/yr
-33 km3/yr
+21 km3/yr
+5 km3/yr
[Rignot, 2005]
Sea Level Budget (IPCC, mm/year)
1993-2003
1961-2003
Thermal Expansion
1.6 ± 0.5
0.4 ± 0.1
+
Mountain Glaciers
0.8 ± 0.2
0.5 ± 0.2
+
Greenland Ice Melt
0.2 ± 0.1
0.1 ± 0.1
+
Antarctic Ice Melt
0.2 ± 0.3
0.1 ± 0.4
?
?
Land Water Storage
= Total of Observed Contributions
2.8 ± 0.7
Observed Sea Level Change 3.1 ± 0.7
1.1 ± 0.5
1.8 ± 0.5
Is Sea Level Rise Accelerating?
• Short answer: probably
• The satellite sea level record is
•
•
too short (~14 years) to rule out
that the recent rise is due to
natural decadal variability.
This is only likely to be resolved
by having a longer satellite data
record (~30 years).
The decline in satellite programs
in recent years has put this in
jeopardy.
Sea Level Observations versus Predictions
Satellite Observations
Tide Gauge Observations
Climate Model Predictions
(IPCC 3rd Assessment, 2001)
[Rahmstorf et al., 2007]
What to Expect in the Future
• Future sea level
•
•
•
5.8°
predictions are uncertain
because of uncertainties
in the contributions of
Greenland and Antarctica.
1.4°
The IPCC 4th Assessment
projects 0.28 - 0.59 m of
sea level rise by 2100,
but did not exclude
higher rates.
In fact, a recent study
[Rahmstorf, 2007]
suggests higher rates
(0.5 - 1.4 m by 2100).
Even if greenhouse gases were stabilized now, substantial sea
level rise would continue for several centuries because of inertia
in the climate system (~ 0.1 - 0.25 m/century).
Summary
• Observations of sea level change are consistent with how
•
•
•
•
•
we expect sea level to respond in a warming climate.
Sea level rose faster in the last decade than the 20th
century average.
Whether the current rate of rise is accelerating can only
be resolved with longer satellite time series.
Presently, ocean warming, melting of mountain glaciers,
and melting of the polar ice caps are contributing in
roughly equal amounts to the observed rise.
The largest uncertainty in future sea level rise projections
is the contribution of Greenland and Antarctica.
Many of the remaining questions about sea level rise can
only be answered with continued satellite measurements,
which are in serious jeopardy.
Greenland Ice Mass Loss Estimates
Luthcke et al. [2006]
Krabill et al. [2004]
Zwally et al. [2005]
Rignot et al. [2006]
Velicogna and Wahr [2005]
Ramillien et al. [2006]
Chen et al. [2006]
Velicogna and Wahr [2006]
Mass
Loss
362 Gt = 1 mm GMSL
[Cazenave, 2006]
Antarctic Ice Mass Loss Estimates
Rignot and Thomas [2002]
Zwally et al. [2005]
Davis et al. [2005]
Velicogna and Wahr [2006]
Chen et al. [2006]
Ramillien et al. [2006]
362 Gt = 1 mm GMSL
[Cazenave, 2006]
Sea Level Budget (1993-2006)
Thermal Expansion: 1.2 - 1.6 mm/year
+
Mountain Glaciers: ~0.6 - 1.0 mm/year
+
Greenland Ice Melt: ~0.2 - 0.5 mm/year
+
Antarctic Ice Melt: ~0.2 - 0.4 mm/year
Land Water Storage: ?
=
Total: 2.2 - 3.5 mm/year
Observed by Satellites
Alaska Glacier Mass Changes from GRACE
Sea Level Contribution of 0.3 mm/year over 2002-2004
[Tamisiea et al., 2005]
Effects of Sea Level Rise
1 meter
2 meters
4 meters
8 meters
GFDL