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An Overview of the Observations An Overview of the Observations of Sea Level Changeof Sea Level Change
R. Steven NeremUniversity of Colorado
Department of Aerospace Engineering SciencesColorado Center for Astrodynamics Research
Cooperative Institute for Research in Environmental Sciences
NSCE 11th Conference on Science, Policy, and the Environment:
Our Changing Oceans
The Earth is WarmingThe Earth is Warming
Why does sea level change?Why does sea level change?The Bathtub ModelThe Bathtub Model
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Precipitation over OceansRunoff from Continents
Evaporation from OceansPrecipitation over Continents
Thermal Expansion
Ice Melt
What is Global Mean Sea Level What is Global Mean Sea Level Change?Change?
• ““Global Mean Sea Level ChangeGlobal Mean Sea Level Change”” is is the change in the the change in the averageaverage height height of the oceans over the entire of the oceans over the entire globe at a single point in time.globe at a single point in time.
• Sea level change at a specific Sea level change at a specific location in the ocean may be location in the ocean may be higherhigher or or lowerlower than the global than the global mean because of differences in mean because of differences in ocean temperature and other ocean temperature and other effects.effects.
• Does not include ocean tides, Does not include ocean tides, storm surgestorm surge
• Does not include effects of land Does not include effects of land subsidence, which locally can subsidence, which locally can exacerbate the effects of sea level exacerbate the effects of sea level rise.rise.
GRAVITY
How do we measure sea level change?How do we measure sea level change?
Argo
What do the tide gauges tell us?What do the tide gauges tell us?
Average Rate ~ 1.8 mm/year
3.2 mm/year
[Church and White, 2006]
What do the altimeterWhat do the altimetersatellites tell us?satellites tell us?
TOPEX/Poseidon
Jason 1 & 2
Trend = 3.3 ± 0.4 mm/year
What do satellite gravity measurements tell us?What do satellite gravity measurements tell us?
Rate of Ice mass change:
All Greenland: -239 Gt/yr
South Greenland: -162 Gt/yr
North Greenland: -77 Gt/yr
Total Greenland ice volume
April, 2002 – June, 2010
-239 Gton/yr = 0.66 mm/yr
sea level rise
(1 Gton = 1 km3 of water)
Greenland Ice Mass Changes from Greenland Ice Mass Changes from GRACEGRACE
[Wahr, 2010]
Greenland Ice Mass Changes from Greenland Ice Mass Changes from GRACEGRACE
All Antarctica: -143 Gton/yr
West Antarctica: -155 Gton/yr
East Antarctica: +15 Gton/yr-143 Gton/yr = 0.40 mm/yr sea level rise
Total Antarctic ice mass April, 2002 – June, 2010
Antarctica Ice Mass Variations from Antarctica Ice Mass Variations from GRACEGRACE
[Wahr, 2010]
Is Antarctic Ice Mass Loss Holding Is Antarctic Ice Mass Loss Holding Steady?Steady?
[Wahr, 2010]
IceSat Surface Elevation ChangesIceSat Surface Elevation Changes
[Pritchard et al., 2009]
Rate of mass change between April, 2002 and May, 2010
Rate of mass change: -55 Gton/yr = 0.15 mm/yr sea level rise.
Alaskan Glaciers from GRACEAlaskan Glaciers from GRACE
[Wahr, 2010]
The Argo ArrayThe Argo Array
Current Sea Level BudgetCurrent Sea Level Budget
Thermal Expansion: ~ 1 mm/year
Mountain Glaciers: ~ 1.1 mm/year
Greenland Ice Melt: ~ 0.6 mm/year
Antarctic Ice Melt: ~ 0.4 mm/year
Land Water Storage: ?
Total: ~ 3.1 mm/year
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Spatial Variations in Sea Level Rise 1993-Spatial Variations in Sea Level Rise 1993-20102010
Western Pacific Sea Level ChangeWestern Pacific Sea Level Change
Merrifield (2010)
Predicted Regional PatternsPredicted Regional Patternsof Sea Level Change from Ice Meltof Sea Level Change from Ice Melt
[Bamber, 2009]
What Will Happen in the Future?What Will Happen in the Future?
Coupled climate models IPCC, 2007
Empirical projections Rahmstorf, 2007
1 Meter of Sea Level Rise – Gulf Coast1 Meter of Sea Level Rise – Gulf Coast
1 Meter of Sea Level Rise - Florida1 Meter of Sea Level Rise - Florida
6 Meters of Sea Level Rise - Florida6 Meters of Sea Level Rise - Florida
It can happen – because it has happened beforeIt can happen – because it has happened before
120 80 40 0Age (x10 3 yr)
-120
-80
-40
0
Se
a Le
vel (
m)
[Clark, 2009]
The last time the Arctic was 3 to 5°C warmer than present, global sea level was
~6 meters higher than present
SummarySummary• Observations of sea level change are consistent with how Observations of sea level change are consistent with how
we expect sea level to respond in a warming climate.we expect sea level to respond in a warming climate.• Sea level rose twice as fast in the last decade than over Sea level rose twice as fast in the last decade than over
the last half of the 20the last half of the 20thth century. century.• Presently, ocean warming, melting of mountain glaciers, Presently, ocean warming, melting of mountain glaciers,
and melting of the polar ice caps are contributing in and melting of the polar ice caps are contributing in roughly equal amounts to the observed rise.roughly equal amounts to the observed rise.
• The large uncertainty in future sea level rise projections is The large uncertainty in future sea level rise projections is due mainly to the uncertain contributions of Greenland due mainly to the uncertain contributions of Greenland and Antarctica, which appear to be accelerating.and Antarctica, which appear to be accelerating.
• Whether the current rate of rise is accelerating can only Whether the current rate of rise is accelerating can only be resolved with longer satellite time series.be resolved with longer satellite time series.
• Many of the remaining questions about sea level rise can Many of the remaining questions about sea level rise can only be answered with continued satellite measurements, only be answered with continued satellite measurements, which have an uncertain future. which have an uncertain future.
Thanks!Thanks!
