Presentation on theme: "Announcements For Thursday: – Section in IPCC report. For Box 3.1, read first and last paragraphs only. ~3 pages – Meehl et al – read all – Chen and Tung,"— Presentation transcript:
Announcements For Thursday: – Section in IPCC report. For Box 3.1, read first and last paragraphs only. ~3 pages – Meehl et al – read all – Chen and Tung, Read all except: Observed SST and OHC: Can skip the second half of this section (from p. 900, begin skipping with the paragraph that begins, "Over the Whole globe"). Discussion: read only first three paragraphs – Summaries for both of the peer reviewed papers.
Warming at depth only big at high Northern latitudes More warming right at sea ice line in Atlantic (ice-albedo feedback) Can see evidence of shifts in storm tracks/western boundary currents in mid-latitudes. Temperature trends 1971-2010 in top 700m: Unsurprising fact of the day: Global oceans are warming.
Let’s talk about a related ocean change: sea level rise.
20th Century Sea Level Rise Sea level rise since 1870 has been around 12-22 cm (5-9”) red = “reconstructed” from tide gauges and other sources blue = tide gauges black = satellite altimetry
What affects sea level rise? These don’t contribute to sea level rise: – Sea ice – Ice shelves (these are connected to ice sheets but floating on ocean) These contribute to sea level rise: – Thermal expansion of sea water Water expands when it warms This is the main contributor to sea level rise so far – Mountain glaciers – Ice sheets (Greenland and Antarctica) Contribute only a tiny amount: – Permafrost – Snow cover – Land use change (ex: ground water depletion for irrigation)
“The Day After Tomorrow” had it wrong: a disintegrating floating ice shelf has no effect on sea level. And the cooling that followed should have caused sea level to drop!
Mean Sea Level - The average height of the ocean at a given location What controls it? Thermal expansion of the ocean The exchange of seawater/ice with lake/soil water or land ice Tides Ocean currents Atmospheric winds Tsunamis Tectonic activity Some locations are rising/falling ex: recovery from the weight of all that ice during the last Ice Age, called “isostatic rebound” The first two vary with global warming These influences + spotty data means it’s been tricky to track global sea level accurately!
How do we measure sea level? Tide gauges are measuring sticks or sometimes floats in wells Always coastal Few long records
Instruments emits a short radar flash and measures the time-of-flight of its reflection from earth. 1,000 times per second. TOPEX-Poseidon Radar Altimetry Measures sea level and ice sheet height very accurately. How do we measure sea level?
Sea level rise from TOPEX-Poseidon Radar Altimetry Most accurate satellite measurements: 5.5 cm rise in for 17 years 1994 – 2010
Sea level doesn’t change uniformly everywhere Sea level anomalies from tige gauge records in each ocean basin. Can you see the sloshing of water back and forth like a bath tub? ex: high in UK when low in USA. These data have been adjusted to omit “glacial isostatic adjustment.” What’s that?
Isostatic adjustment Once the ice is gone, ground level rebounds. The Ground rises underneath the ice sheet, and sinks at the edges of the ice sheet.
NYC sea level rise 12” in 20 th Century! That’s 2x the global rate! What’s going on?
Regions under the ice sheet 20,000 years ago are rising. Regions at the edge of the ice sheet 20,000 years ago are sinking.
IPCC AR4 Figure 5.21 – no new version of this figure in AR5. Contributions to sea level rise over the observational record 1961 to 2003 (blue) 1993 to 2003 (brown) ~60% ~25% ~15% Mostly thermal expansion so far. Range of uncertainty ~.7mm/yr since AR4 ~.5mm/yr since AR4
Thermal Expansion Thermal expansion is primary contribution so far to sea level rise (60%) Due to ocean heat content increases – Water expands as it warms Ocean heat content has been steadily increasing in recent decades How do we know this?
Monitoring the ocean Many instruments are used to measure ocean heat content
Expendable Bathyermographs (XBT) - About 70 Voluntary ships toss them overboard - 14,000 each year (they are cheap, even these figures are ugly) -measure down to 1500 m, in use since 1962 -Not super accurate depth recordings, so data isn’t great. Monitoring the ocean heat content
Monitoring the ocean heat content Argo floats, since ~2000, reached full capacity at 2005, measure to 2000m depth
Ice Contributions to Sea Level Rise Mountain glaciers currently contribute ~25% to rising sea levels Greenland and Antarctica currently contribute ~20% to rising sea levels.
What will sea level be by the end of the 21st century? Model estimates from IPCC AR5 IPCC AR5 Figure 13.27 RCP8.5 RCP2.6 Paleo sea level data, tide gauge data, and altimeter data Our uncertainty in the science (shaded regions) is at least as big as our uncertainty in emissions. Very different story than for temperature.
2007 (AR4): ~4mm SLE since 1990. 2012 (AR5): ~8mm SLE since 1990. 2007 (AR4): ~3mm SLE since 1990. 2012 (AR5): ~6mm SLE since 1990. A worrying observation: rates of sea level rise from ice sheets have increased very rapidly!
Speed of Sea Level Rise In general, sea level rise is a very slow process – Takes an extremely long time to melt Greenland/Antarctica In the long term, ice sheets will be a big problem, but this will take centuries to happen We’re closely monitoring for any surprises due to any “rapid dynamical changes” that could happen with the ice sheets (future topic). The CMIP5 models are doing an ok job reproducing observed sea level rise from Greenland and Antarctica (good thing!). What places are most vulnerable to sea level rise?
Population Distribution 11 of the 15 largest cities in the world are along coasts or estuaries “It was estimated that in 2003, approximately 153 million people (53 percent of the nation’s population) lived in the 673 U.S. coastal counties, an increase of 33 million people since 1980.” The population living within 1 m of sea level is unknown
Low Lying Island Nations Tuvalu: highest point is 4.5 m above sea level Maldives: Max elev = 2.5 m Average elev: 1.5 m Maldives satellite photo Some Caribbean nations are also quite vulnerable Bahamas: 80% within 1.5 m of sea level These nations could disappear!
The Netherlands are largely below sea level already! They have the “Delta Works,” a very complex system of barriers to protect them from the sea. This is one of the Seven Wonders of the modern world. Maeslant Barrier closing
Costs of Sea Level Rise Main problems will likely be from large storm surges on top of the sea level rise Example: Sandy estimated to cost $19 billion. Costs: – Wetland loss – Salinization of aquifers/crops – Constructing barriers – Relocation
Review of Sea Level Rise Hard to measure sea level: – Non-coastal measurements have only been in effect for a few decades – Have to factor out sloshing around of water due to waves, tides, ocean currents, etc – Some continents are rising/sinking!