sea level change recent past, present, futurecci.esa.int/.../5_esa_cci_colmeet_4feb2014_bis.pdf ·...
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Sea level change recent past, present, future
Anny Cazenave LEGOS-CNES
Toulouse, France
CCI_Colocation meeting, ESA/ESRIN
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Global mean sea level rise during the 20th century
(red : tide gauges; blue : altimeter satellites)
Source :LEGOS
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1.7 +/- 0.3 mm/yr 3.2 +/- 0.4 mm/yr
Sea level rise : 20th century and past 2 decades
1993-2012
Tide gauges Satellite altimetry
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Question • Is sea level rise accelerating? Yes! Church et al. (2013) concludes that « the increased rate of sea level rise since 1990 is not part of a natural cycle but a direct response to increased radiative forcing on the climate system » • Is the current rate of rise unusual? Yes and no!
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Rates of sea level rise on different time scales
0
4
8
12
16
20
24
28
32 mm/yr
Mean rate of
Sea level rise
during last
deglaciation
Last 2000 years
Deglaciation MWP-1A -14 000 yrs ago (duration ~300 years) ! 40 mm/yr
20th century Last 2 decades
-20 000 to -10 000 years 6
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Question Can we close the sea level budget?
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Sum Contrib. Observed Residual
2.8 +/- 0.5 mm/yr 3.2 +/- 0.4 mm/yr 0.4 mm/yr
residual
Sum of
contributions
Rate of rise
Observed sea level budget during the altimetry era (1993-2010) IPCC AR5
Observed
IPCC AR5
Thermal expansion Land ice Land waters
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The ocean heat content is increasing
IPCC AR5
A R G O
X B T
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Heat content (1022 J)
5
10
15 Oceans
Atmosphere Land
Thermal budget of the climate system (last 50 years)
IPCC AR5
>90% of (anthropogenic) heat excess of the climate system is
stored in the oceans
ΔH = Mo Co ΔT
Ocean heat content
Ocean mass
Heat capacity of water
5x1022 Joules =
Energy released if all known remaining fossil resources (oil, gas, coal) were burnt
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Thermosteric contribution to sea level rise
Contribution of thermal expansion to sea level rise (1993-2010):
1.1 +/- 0.3 mm/yr
IPCC AR5
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Since the early 1990s, ice sheet mass change measured from space
Greenland « GRACE »
space gravimetry
Radar & laser altimetry
Greenland
Antarctica
Ice sheet
Since 2002
Radar Interferometry
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Source: J. Wahr
Ice mass loss of the Greenland ice sheet (blue-violet-black) between 2003 and 2012 observed by GRACE
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Ice mass loss from Greenland and Antarctica measured by space techniques
since 1990 (in Gt) ! mass loss acceleration since 10-15 years
Antarctica + Greenland Antarctica Greenland
Shepherd et al., 2012 IPCC AR5
Rate of ice sheet mass loss Greenland : 34 +/- 40 Gt/yr (1992-2001); 215 (+/- 60) Gt/yr (2002-2011) Antarctica : 30 +/- 67 Gt/yr (1992-2001); 147 (+/- 74) Gt/yr (2002-2011)
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IPCC AR5 IPCC AR5
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Land water storage change
16
2 contributions: - Natural climate variability - Anthropogenic changes (dam building & groundwater extraction)
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Land water storage change due to natural climate variability
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Sea level trend (1993-2012) : 0.08+/- 0.02 mm/yr
El Nino La Nina
Sea level trend : 0.08 +/- 0.02 mm/yr
El Nino La Nina
Global land water storage change due to climate variability based on the IBSA-TRIP hydrological model (MétéoFrance)
Equivalent sea level
El Niño
La Niña
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Dam building along rivers :
Chao et al., 2008
Sea level drop
Cum
ulat
ive vo
lum
e of w
ater
stor
ed b
ehin
d da
ms (
km3 )
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Wada et al., 2012
the Global Mean Sea Level
Sea level rise Past and future ground water depletion
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Y. Wada (personal communication) Anthropogenic land water storage (1993-2012) Dam building : -0.3 +/- 0.03 mm/yr Ground waters : + 0.56 +/- 0.1 mm/yr Net effect : +0.26 +/- 0.11 mm/yr Natural land water storage : 0.08 +/- 0.02 mm/yr Total: 0.34 +/- 0.15 mm/yr
IPCC AR5 –anthropogenic only- (1993-2010) : 0.38 +/- 0.12 mm/yr
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37% Ocean
warming
13%
Thermal Exp Land Ice Land Waters
50%
Causes of sea level rise (altimetry era: last 2 decades) Individual contributions (in % of the observed rate of rise)
Land ice melt
IPCC AR5
Land waters
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Regional variability
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Sea level trend patterns from satellite altimetry (1993-2012)
mm/yr
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Regional variability of the rates of sea level change over 1993-2012 (global mean rise removed)
Observed sea level by satellite altimetry
Observed thermal expansion + salinity changes 24
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Future sea level rise
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Warming scenarios considered by IPCC AR5
à Representative Concentration Pathways (RCPs)
à 4 RCP scenarios defined by their total radiative forcing by 2100:
- RCP2.6 (2.6 Wm-2) - RCP4.5 (4.5 Wm-2) - RCP6.0 (6.0 Wm-2) - RCP8.5 (8.5 Wm-2)
Radiative forcing : an energy imbalance imposed on the climate system either externally or by human activities; ususally reported as a change in energy flux at the top of the atmosphere and expressed in units of watts per square meters (Wm-2)
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4 scenarios for future greenhouse gas emissions considered
by IPCC AR5 for the 21st century
RCP 2.6
RCP 4.5
RCP 6.0
RCP 8.5
IPCC AR5
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IPCC-AR5 projections of Global Mean Sea Level Rise during the 21st century under two warming scenarios
Sea Level Rise
Pessimistic scenario (RCP 8.5)
Optimistic scenario (RCP 2.6)
50 cm-100 cm
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IPCC AR5
Climate related contributions to future sea level rise (RCP6.0)
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Ensemble mean projections of regional sea level rise by the end of the 21st century (regional variability due to non uniform thermal expansion & salinity
+ solid Earth effects )
metres
Global mean sea level rise of 75 cm
(high warming scenario)
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In addition to steric effects, there are other causes of regional variability à Large-scale water mass redistributions due to land ice melt à Deformations of ocean basins (because the Earth mantle is viscous and the crust is elastic ) + changes in mutual gravitational attraction between water & ice masses
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Regional sea level change due to solid Earth’s deformations in response to ongoing & future ice sheet melting
(Greenland : 0.5 mm/yr; west Antarctica : 0.5 mm/yr)
Antarctica melting
Greenland melting
<0
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Amplification of the rate of sea level rise in the tropics by 20%-30% IPCC AR5
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Regional variability by 2100 Percentage of deviation from the global mean rise
(same values for all scenarios)
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Long-term climate change Many aspects of climate change will persist
for many centuries even if emissions of greenhouse gases are stopped!
• 20% of emitted CO2 will remain in the atmosphere more than 1000 years
• Sea level will continue to rise for many centuries in response to deep ocean warming and associated thermal expansion • Ice sheet mass loss may become irreversible (Greenland) ! sustained warming above a certain threshold may lead to near-complete loss of the Greenland ice sheet over a time scale of 1000 years (! 7 m of sea level rise)
IPCC AR5
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Conclusions - Current global mean sea level rise is likely accelerating - The sea level budget (altimetry era) almost closed - Global mean sea level rise: very likely a consequence of
anthropogenic global warming - Regional variability (spatial trend patterns) : still
dominated by natural (internal) modes of climate variability
- The global mean sea level will continue to rise during the 21st century in response to global warming (values by 2100 in the range 50 cm-1 m NOT unlikely)
- The regional variability will amplify the global mean rise by 30%-40% in the tropics
- Even if GHG emissions stop tomorrow, sea level will continue to rise during several centuries
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Grand challenges for the coming years - Insure continuity of space-based & in situ observing systems (+ new observations) ! satellite altimetry (sea level & ice sheets) ! space gravimetry (GRACE Follow On) ( ice sheet mass balance, glaciers, land waters) à Argo + Argo deep ocean à Tide gauges + GPS - Improve past sea level reconstructions & OGCMs - Reprocess long time climatic time series (e.g., ESA CCI for ‘Essential Climate Variables’) ! climate services - Perform systematic comparisons between observations and climate model hindcasts…….
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Thanks for your attention
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Post Glacial Rebound
Models of Post Glacial Rebound
(also called Glacial Isostatic Adjustment
or GIA)
+
Deglaciation history Models of Earth’s viscosity structure 38