1Sea level change• Important processes– Global vs. local– Geologic– Modern• Current rates of rise– Observations, calculations, and models• Lessons from the past• Projected future changesLast 20,000 years of ice sheet changeHow much did sea level change?http://earth.rice.edu/mtpe/cryo/cryosphere/topics/ice_age.htmlGlacial sea level• This coral (Acroporapalmata) alwaysgrows in shallowwater (<5m)• Cores taken throughBarbados reef haveA. palmata at manydifferent depthsSea level - fromseveral sites• Tahiti, New Guinea,Barbados, Vanuatu forcomparison• Why need many sites?• Is sea level risesteady?• Meltwater “pulses”evident duringdeglaciation• A valid analog forfuture?2Global sea level changes• Geologic time (100my): up to 100m higher• Ice ages: 125m range lower• Very small increases (0.1-0.2mm/yr) or stable inHolocene (post-ice age)• Mid-19th century: global rise begins• 20th century: Rise intensifies as world warms• Prediction from IPCC AR4: < 1m in this century– Conservative: Explicitly ignores ice sheet melting (!!)Sea level measurement• Tide gauges– Local factors = bigcomplications!• Satellite altimetry– Global picture, shortrecords (decade-scale)• Long tide gauge records are sparse• Decadal variability in all records is significant• Trends vary widely depending on interval over which calculatedLocal sea level changes• Weather/climate: wind, barometric pressure, storm surges• Land subsidence: compaction from pumping water or oil• Sedimentation• Changes in water storage on land: reservoirs, deforestation• Vertical motion of land: earthquakes, isostatic rebound• Ocean circulation3Why care about 21st-century sea level change?• Small island states and low-lying coastalregions could be inundated• Potential for increased coastal erosion• Intensification of storm surges• Saltwater incursion into freshwater aquifers• Coastal wetlands threatened - often littlechance for migration inland• All made more problematic by especially rapidpopulation growth in coastal regionsCauses of modern sea level change•Changes in water storage–Melting glaciers, ice sheets–Groundwater extraction–Reservoirs–Deforestation–Permafrost•Ocean processes–Thermal expansion–Circulation•Local complications20th Century Rates of Sea Level Rise(IPCC TAR 2001)• Observe from coastal tidal gauges: 0.6 - 1.8mm/yr• Calculate from summing relevant hydrological factors: bestguess ~ 1mm/yr• Ocean Model: 0.2-1.0 mm/yrAre these discrepancies important?What’s right?Reconcile sea level change estimates? Problem: tide gauges say up to 2mm/yr; models andcalculations say 1mm/yr• Are tide gauges representative?• Do we know all of the factors that add into the hydrologic calculations?(esp. large ice sheet behavior)• Do we have full information on temperature for thermal expansioncalculations?• How well do models simulate future changes?Satellite data promise to help….-Short record length to date (since 1993-Noisy-No historical info4Processes of sea level rise(Cabanes et al., 2001, Science)• Satellite data: 1993-98 rise = 3.2 mm/yr– TOPEX/Poseidon measures sea surface height– Short interval, scatter• Temperature data from top 500m: thermal expansionshould be 3.1 mm/yr• Important: the actual values of trends over such a shortinterval must be used with care!• Thermal expansion is the a primary source of variation.– How reasonable is this?Sea level rise from satellite data• Satellite measurements agree with thermal expansion calculated fromtemperature data• Trends are 3.1-3.2 mm/yr - not reliable (yet)Cazenave and Nerem 2004Sea level anomaly (mm)Rate = 2.8 ± 0.4 mm/yr• Update from tide gaugedata• Best fit includesacceleration• Acceleration begins inearly 20th century• Rates peak above 3mm/yrbut vary• Leads to total rise of~300mm in 2100• Will this change?Is sea level rise accelerating?DataDecadal rateCauses of observed SL change(blue = 1961-2003; brown = 1993-2003)= “ice sheets”Current rate of sea level rise is about 3mm/yr.Evidence supports acceleration, but data are noisyIPCC AR4, chapter 55Observed & projectedsea level rise• About 200mm since 1880• Lots of decadal variability• IPCC prpojection is 250-500 by 2100• Last IPCC: 0.1-1.0m• What’s up? Are estimatesreally decreasing?• NO…IPCC (2007) estimates of futuresea level change• Use models to include thermal expansion, glacier melting,some terrestrial hydrology• Prediction: sea level rise of 0.2-0.6m by 2099– Very poor understanding of how large ice sheets fit in here! Theyare explicitly excluded.– Alternate predictions suggest greater increase• Sea level will continue to rise after that, due to thermalequilibration with atmosphere• Ultimate rises will be 0.5-2m for doubled CO2 and 1-4mfor quadrupled CO2 - could take 100’s of years to achieve(why?)• Big uncertainties: what will large ice sheets (Greenland,Antarctica) do?Model uncertainty is large(note regional differences among models)From Church, 2001• Models run out to 3000years• CO2 increased by 1% peryear to doubled (top) andquadrupled (bottom)• Note that CO2 changesare finished within acouple centuries• It takes many centuries toreach the full sea levelresponse to warming• These do not includelarge ice sheetsFrom IPCC 20012XCO24XCO205m02.5m6What about those ice sheets?Melt area on Greenland is expandingGreenland melt• Melt is contributing0.5mm/yr to SL rise• Melt is accelerating: 3xfaster today than 5 yrs ago• Data from GRACE satelliteproject (gravity data): Chenet al. 2006, ScienceThe Greenland Ice Sheethasn’t always been liketoday…The Last Interglacial - sl was 4-6m higherThe Greenland Ice Sheet (GIS) 130,000 years agoK. Cuffey and S. Marshall, Nature 404, 2000 Greenland130kyrs agoAt the beginning of thelast interglacial (130,000years ago):• global sea level was 4to 6 meters higherthan today due at leastpartially,to melting ofGreenland Ice Sheet• How do we know?- ice cores, or lack thereof- raised shorelines and coral reefs etc.Camp CenturyDye 3Summit7Today 125,000 years agoImage from Bette Otto-Bliesner, National Center for Atmospheric ResearchChanges in Greenland ice sheetsimulated by climate-ice sheet model130,000 yrs agoAD 2100Change in summer temperature: Last Interglacial vs AD 2100Overpeck et al. 2006, Science(model results, but consistent with data)130,000 yrs ago AD 2100Change in annual; snow accumulation: