Sea Level and Its Rising

Warren Wiscombe, NASA Goddard

  with assistance fromBenjamin Chao, NASA Goddard

and Bruce Douglas, International Hurricane

Center &Florida International University, Miami

Apr 2005 Meto 401 2

Underwater stalactites are one of many kinds of evidence for big sea level changes over 1000s of years

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What has happened since end of last ice age?

Since 20K yr ago, sea level has risen over 120 m!

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Ice Sheet Coverage, Last Glacial Maximum

from Peltier (2000)

21,000 yrs ago

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Ice Sheet Coverage Today

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Estimates of global sea level change (m) over past 140,000 years show N. America is

biggest player

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interactive QT/VR movie

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Sea level rise has been much milder in the past few thousand years

Last 6000 yr: variations over a few hundred yr less than 0.3-0.5 m (most of ice has melted by this time)

1900s: 1-2 mm/yr (larger than in 1800s but no acceleration of rate)

2000: 2 mm/yr

But … best estimate is ~1 mm/yr

(hence the 2002 Physics Today article “The Puzzle of Sea Level Rise”)

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Movie: Sea Level Rise

Emphasizes Maldive Islands (S. of India) but most atoll-based islands face the same fate

Male, Maldives

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Tuvalu is also sinking

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Primary Impacts Of Sea Level Rise

• Increased flooding and storm damage

• Inundation of low-lying areas

• Higher water tables

• Salt intrusion into aquifers and surface waters

• Erosion of beaches and coastlines

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People huddle near the water...

• Two-thirds of the world's population lives within 500 km of a coastline.

• 80% of the world's population lives below 500 m elevation.

• This places an overwhelming majority of people at risk when sea levels rise.

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Last 300 yr (IPCC examples)

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Sea level is rising most

places, but not everywhere

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1900-1980 Sea Level averaged over 84 stations

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How does the IPCC* parse out the 2 mm/yr rise of the 1900s?

~1 mm/yr from ocean thermal expansion

0.2-0.4 mm/yr from melting ice– -0.2 to 0 from Antarctica (increasing precip)– 0 to 0.1 from Greenland (melting beats precip)– 0 to 0.5 from Antarc/Grnld due to past climate

changes

-1.2 to +0.4 mm/yr due to terrestrial water storage

“Unfortunately sufficient measurements exist to determine the mass balance for only a few hundred of the world’s 160,000 glaciers.”

*Intergovernmental Panel on Climate Change

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What does the IPCC project for the future?

Warming of ocean will continue even if greenhouse gases are stabilized; after 500 yr, this “steric” sea level rise still only half its ultimate value

Projections:1990-2100: 0.1 to 0.8 m risenext 500-1000 yr: 1-4 m rise for 4xCO2

W. Antarctic ice sheet in no immediate danger but if temperature rises enough to uncork its outflow glaciers, collapse could be more rapid

E. Antarctic ice sheet never melts in any scenario

Greenland or Antarctic melting would take milennia!

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Examples of sea level rises of 15 & 25 feet

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Climate modeling results from NCAR predict “committed sea level rise”

• Even if no more greenhouse gases were added to the atmosphere, globally averaged surface air temperatures would rise about 0.5C and global sea levels would rise another 11 cm from thermal expansion alone by 2100.

• “… we are committed to a significant amount of global warming and sea level rise because of the greenhouse gases we have already put into the atmosphere…”

• The projected sea level rise is twice the 5-cm rise that occurred during the latter half of the 20th century.

• These numbers do not take into account fresh water from melting ice sheets and glaciers, which could at least double the sea level rise caused by thermal expansion alone.

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Aerial photographs, Blackwater Wildlife Refuge, MD Eastern

Shore.

Note progressive drowning & loss of coastal marshes due to a higher than normal rate of sea level rise.

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In 1900, 15 families lived on the Poplar islands in Chesapeake Bay. Will be gone in a few years.

FDR and Truman were visitors in its days as a hunting club.

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This former Civil war prison camp is now largely underwater

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Bruce Douglas, sea level expert

“The rate of sea level rise in the mid-Atlantic states is 3.5–4 mm/yr (about 2x the global average) because of post glacial rebound.

So if we want to see what a doubled rate of sea level rise would do to beaches and wetlands elsewhere..., we have only to look at the mid-Atlantic.

Louisiana is seeing rates of relative sea level rise as high as 10 mm/yr, and loses wetlands and beach at the rate of tens of acres/day.”

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Glacial Rebound is a local see-saw effect that can cause sea level to rise or fall

Maryland

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East Coast Relative Sea Level Rise (mm/yr), vs Latitude

Note that approaches 2 mm/yr at low latitudes

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Causes of sea level change?

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from IPCC Report

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Cause 1: Volume Change at Constant Mass

• “steric” (thermal + salinity)

• Local atmosphere-ocean thermal interactions make maps non-uniform

– El Nino, North Atlantic Oscillation, & other vacillations alter the pattern greatly

• Deep currents and thermohaline (vertical pump) circulations move heat around in 3D

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Summary: IPCC “Indicators of Warming”

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Temperature Change, last 1000 yr(the Mann “Hockey Stick” curve)

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The Surface Warming Evidence – IPCC

In some places there is a cooling! Why might this be?

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Show movie SST2001

SST (Sea Surface Temperature) varies a lot! Helps make sea level vary a lot from place to place, and makes regional predictions of sea level change difficult.

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Ocean heat

content is rising

change in global ocean heat content from the 1950s to the 1990s is equivalent to a net downwards surface heat flux of 0.3 W/m2 over the whole period

top 300 m of ocean

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Thermal-Expansion Sea Level Trends, 1955-96

From Cabanes, et al., 2001

upper 3000 m

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Cause 2: Mass Change

• Imbalance of precip (P) and evaporation (E) over ocean (P–E not zero)

• Land Hydrology — changes in– ice sheets, glaciers, permafrost– human-built reservoirs!– lakes, inland seas, wetlands, rivers– soil moisture, surface snow– groundwater

Hard to distinguish Cause 1 from Cause 2

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Greenland is thinning near the coast, thickening on the ice sheet

more or less what models would predict.

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More information on Greenland from NASA Wallops observational program

Greenland’s ice sheet is rapidly thinning near the coasts (Krabill, Science, July 2004) — in some places, at a rate of more than 1 m per year. (NASA Wallops data)

“A conservative estimate indicates a net loss of approximately 51 km3/yr of ice from the entire ice sheet, sufficient to raise global sea level by 0.005 inches per year, approximately 7% of the observed rise.”

“The thinning cannot be accounted for by increased melting alone. It appears that ice must be flowing more quickly into the sea through glaciers.”

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East Coast Inundation if Greenland Melts

Red: land submergence for an estimated 5-m sea-level rise (equivalent to melting all Greenland ice).

It would take 1000 yr for Greenland to melt in place! Fast outflow glaciers could speed the process.

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Cause 3: Ocean Basin Shape Change due to Solid Earth deformations

• Post-glacial rebound

• Tectonic

• Loading effects (tidal, atmospheric, oceanic, hydrological)

• Local deformation

– seismic

– groundwater/oil extraction)

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Cause 4: Dynamic Changes

• Waves

• Tides

• Currents:– Wind-driven, – Pressure-driven, – Density-driven (thermohaline)

• Geoid change-induced

• of peripheral concern for long time and/or space averages

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Dynamic changes overwhelm the small changes in sea level we are seeking to measure

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IPCC Ranges of Uncertainty in Factors Contributing to Sea Level Rise, 1910–90

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Why the change in rate?

Sea Level at Amsterdam

Holland is highly motivated to measure sea level accurately since it’s partly

below sea level !

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Relative Sea Level Rise (mm), New York

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Sea Level Trend (mm/yr), New York City

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Annual Relative Sea Levels, Bermuda

Cabanes et al., 2001

fit to red points under-estimates longer-term trend by a factor > 2!

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Sea Level Trend (mm/yr) vs. Record Length

b in (SeaLev = a + bt + ct2)

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Acceleration of Sea Level Rise

c in (SeaLev = a + bt + ct2)

all atmos-ocean climate models show an acceleration in 1960-90 compared to whole century

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What kinds of observations are/will be used?

• Direct -- “geodetic”– Tide gauges/GPS (long history)– Radar altimetry (TOPEX & Jason satellites)– Laser altimetry (IceSAT satellite)– Time-variable gravity (GRACE satellite)– SAR/InSAR (Synthetic Aperture Radar; new)– GPS buoys (experimental)– GPS bistatic reflection (experimental)

• Sea surface temperature

• XBTs (eXpendable BathyThermographs) — dropped from ships; ocean temperature & salinity profiles

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Sea Level from Space

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Here’s what one swath of Jason altimeter data looks like

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The latest sea level data from the Jason satellite altimeter

high=warm,low=cold.composited over a 10-day period (no-El Nino).

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Ocean dynamic topography is caused by winds and temperature

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Show movie OceanTopoAndDynam

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Global Sea Level Rise From Topex/Poseidon

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New satellite measurements would add ancillary information to the sea

level problem

• Mixed layer depth (no ideas yet)

• Ocean salinity (long wavelength radar)

• Soil moisture, snow depth (also long wavelength radar)

• (all very difficult observations by remote sensing)

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What’s Happening to Coastlines and Beaches?

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100:1 Response Of

Beach To Sea Level

Rise

1 cm of sea level rise translates to 1 m of beach loss

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Erosion Rate vs Sea Level Rise Rate

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From Galgano (1998)

State-by-state Summary Of Shoreline Behavior

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Future Sea Level Rise and Erosion

• Global sea level rose at ~ 2 mm/yr in 20th century,

• ...but perhaps 1 mm/yr equivalent has been stored in reservoirs, hiding a...

• 50% increase in rise rate since WWII.

• Ergo...global sea level rise and erosion rates could go up 3 or 4 times in the 21st century!

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Sea Level Monitoring Issues

• Has the rate of sea level rise increased in recent decades?

• How can the mass balance of the great polar ice masses be measured?

• Can we develop an erosion monitoring program to detect an increase?

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Backup

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Southern Oscillation Index, 1935–2000

Note the change of state after 1980.