Impacts of Climate Change: Sea Level PDF

Impacts of Climate Change: Sea Level OCN-310 November 4, 2024  Kulp and Strauss, Nature Comm. 2019 300 million people living along the worl...

Impacts of Climate Change: Sea Level OCN-310 November 4, 2024  Kulp and Strauss, Nature Comm. 2019 300 million people living along the world's coasts could be hit by devastating flooding by 2050, about three times more than previously estimated. The figure could double to 630 million people affected by 2100 if little is done to rein in greenhouse gas emissions that continue to rise around the planet. Honolulu 2050 (Forbes) https://www.ssfm.com/hawaii-at-risk-from-coastal-flooding-linked-to- climate-change/ Key concepts Sea level, on a global average, is rising by a few mm per year Sea level change not the same everywhere, in fact some places may be seeing dropping sea level When discussing trends, it’s important to understand time range Need to keep in mind that acceleration of a trend is hard (impossible?) to quantify, and this may cause many of the different estimates Sea Level Usually measured with respect to land, but really should be w.r.t. some mean gravitational “distance” from the Earth’s center à geoid Can change for different reasons over different time and space scales Important to distinguish between ”sea level” and inundation Why does sea level change? Local changes (mostly): Tides Ocean circulation patterns Atmospheric pressure (1 mbar ~ 1 cm) Land changes Etc. Global changes: Density of sea water changes (e.g., thermal expansion) Total ocean mass changes (e.g., ice melt) Ocean basin volume changes (e.g., plate tectonics) How to compute “coefficient of thermal expansion of seawater” is 2.1 x 10-4 ºC-1 Assume average depth of ocean is about 4 km 4 km x 1000m/km x 1ºC x 2.1 x 10-4 ºC-1 = 0.84 m (per square meter) 1ºC rise in temperature leads to 84 cm rise in sea level (NOTE: assumes heating over entire ocean) Sea Level Through Geological Time Density of sea water changes (e.g., thermal expansion) Steric changes Total ocean mass changes (e.g., ice melt) Eustatic changes Ocean basin volume changes (e.g., plate tectonics) Isostatic changes Steric Sea Level Change Global/regional change - Warmer ocean = Higher sea level - Heating up the ocean makes the seawater less dense, and it expands to take up more volume - A few centimeters to meters variation Eustatic Sea Level Change Global change - Less ice on land = Higher sea level - Global warming and loss of ice sheets and land glaciers (decades to centuries) - Glacial cycles (100k years) 10 Isostatic Sea Level Change Local tectonic effect Land uplift à Lower relative sea level Land sinking à Higher relative sea level The IPCC Fourth Assessment Report found that thermal expansion accounted for about one-quarter of the observed sea-level rise for 1961–2003, melting of land ice accounted for about half For the last 10 years of that period (1993–2003), the IPCC estimated that thermal expansion and land ice melt each contributed about half to the total sea-level rise In a more recent estimate, for 1993–2008, the contribution from land ice increased to 68 percent, the contribution from thermal expansion decreased to 35 percent, I. Recent estimates of sea level How monitored/measured? What does the record show?  How measured/monitored In situ: Tide Gauges Tide gauge network University of Hawaii Sea Level Center (UHSLC) How measured/monitored Remote: Satellite Satellite altimeter Satellite-derived Sea Level Satellite altimeters have been used to measure the marine geoid TOPEX/Poseidon measured sea surface from 1992; follow-on mission to present Cover globe from 66ºS to 66ºN every 10 days Most changes however are short term Like the El Nino on the next slide What do these data show? Shorter term variability from satellites (last ~30 years) Tide gauges longer-term, in some cases almost 100 years https://uhslc.soest.hawaii.edu/ http://uhslc.soest.hawaii.edu http://uhslc.soest.hawaii.edu http://uhslc.soest.hawaii.edu https://www.psmsl.org/products/trends/ II. Longer-term Sea Level Changes Ice age progression over the last 100,000 years à changes to total mass of the ocean How measured? Using "sequence stratigraphy” scientists have noted off-shore shifts of shorelines associated with a later recovery. The largest of these sedimentary cycles can in some cases be correlated around the world with great confidence. Also use oxygen isotope analysis from sediments 28 Post-Glacial Rise in SL Several times in Earth history that there have been major ice sheets at high latitudes Most recent was in the Quaternary (1.6 Ma ago) and it may not be over yet Within this period there have been several glaciations The most recent from 120,000 to 20,000 years ago So there may be another one on the way  Phanerozoic Eon Paleozoic Mesozoic Era Era Cenozoic Era Tertiary Period Quaternary Period Sea Level Through Geological Time Sea level today is at a relative "high stand" within the Quaternary glacial cycles because of rapid late-Pleistocene and early-Holocene de- glaciation. The ancient shoreline of the last glacial period is now under approximately 120 meters of water.  Last Glacial Maximum: 20,000 years ago Laurentide Ice Sheet, 3-4km thick All this ice caused a EUSTATIC sea level drop of 125m How do we know this? Aerial view of glaciated Bylot Island, Canada U-shaped valley Glacial Striations lo w i alF G lac 34 Last Glacial Maximum Melt-water Pulses (MWP) MWP are “melt-water pulses”, caused by rapid release of water into the ocean from the collapse of continental ice sheets. MWP1A, occurred between 13,500 and 14,700 years ago and global sea level rose between 16 and 25 meters in about 400–500 years, roughly 40–60 mm/yr Longer term changes (tectonics)  The Quaternary sea level fluctuations of 100 m and more were the result of about 50 x 106 km³ of water being alternately withdrawn from and returned to the oceans There remains about 30 x 106 km³ of ice in the polar icecaps Total melting would lead to a further increase of 60 m in sea level  Summary of spatio-temporal scales of sea level change (D) Plate Tectonics 100 m (C) Melting of ICE MSL (meters) Load from ice sheets deforms crust 10 m Thickness and area of continental crust Thermal state (age) of crust (A) Exchange of water with continents sediment loading (Groundwater, Lakes, etc.) (B) Thermal expansion 1m NOTE: A,B,C à change in volume of water D à change in shape of container 1 cm 1 day 100 1000 100 Ka 10 Ma 100 Ma TIME (years) Other processes complicating the study of mean sea level (ice or sediment loads): Post Glacial Rebound Implications: Future Sea Level At present sea level is rising by 2 mm per year. IPCC AR-5 Sea level has exceeded 5 m above present (very high confidence) when global mean temperature was up to 2°C warmer than pre- industrial (medium confidence). Transition in the late 19th century to the early 20th century from relatively low mean rates of rise over the previous two millennia to higher rates of rise (high confidence).  Ocean thermal expansion and glacier melting have been the dominant contributors to 20th century global mean sea level rise. There is high confidence in projections of thermal expansion and Greenland surface mass balance, and medium confidence in projections of glacier mass loss and Antarctic surface mass balance  The sum of thermal expansion, glacier mass loss, and estimates of land water storage explain 65% of the observed global mean sea level rise for 1901–1990 and 90% for 1971–2010 and 1993–2010 (high confidence).  It is virtually certain that global mean sea level rise will continue beyond 2100, with sea level rise due to thermal expansion to continue for many centuries. The amount of longer term sea level rise depends on future emissions. The available evidence indicates that sustained global warming greater than a certain threshold above pre-industrial would lead to the near-complete loss of the Greenland ice sheet over a millennium or more, causing a global mean sea level rise of about 7 m.  It is very likely that in the 21st century and beyond, sea level change will have a strong regional pattern, with some places experiencing significant deviations of local and regional sea level change from the global mean change. It is very likely that there will be a significant increase in the occurrence of future sea level extremes in some regions by 2100, with a likely increase in the early 21st century.