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,

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 latitudesMore 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 gaugesblack = 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 shelfhas 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 locationWhat controls it?

Thermal expansion of the oceanThe exchange of seawater/ice with lake/soil water or land iceTidesOcean currentsAtmospheric windsTsunamisTectonic activity

Some locations are rising/fallingex: recovery from the weight of all that ice

duringthe 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 icesheet 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 everywhereSea 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 20th 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 increasingin recent decades

How do we know this?

Monitoring the ocean

Many instruments are usedto measure ocean heatcontent

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

Argo floats

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 currentlycontribute ~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 NationsTuvalu: highest pointis 4.5 m above sea level

Maldives:Max elev = 2.5 mAverage elev: 1.5 m

Maldives satellite photo

Some Caribbean nations are also quitevulnerable

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!