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Climate Tipping Points
What is a ‘Tipping Point’?
• A moment in a non-linear system where a small perturbation leads to a rapid change to a new state
• Linked to positive feedback loops– Ice albedo feedback– Water vapor feedback– Will not necessarily lead to ‘runaway’ effects
“Known” Climate Tipping Points
• Shut down of the meridional overturning circulation (MOC)
• Melting of Greenland and Antarctic ice sheets
• Melting of the Arctic sea ice
The MOC
• Artist formally known as “Thermohaline Circulation”
• Large scale ocean circulation that transports mass– Temperature– Salinity
• Supplies heat to higher latitudes– Mild northern Europe climate
• Strength of MOC has varied over time– Responds to temperature– Responds to salinity changes (function of wind)
The MOC
• Used to be thought to be driven by density changes
• Wind drives the surface currents (e.g. Gulf Stream)
• Large cooling, evaporative forcings make surface water unstable*– Sinks in North Atlantic
*http://ocean.mit.edu/~cwunsch/papersonline/thermohaline.pdf
http://ocean.mit.edu/~cwunsch/papersonline/thermohaline.pdf
The MOC
• We now view the MOC as mechanically driven by the surface wind field– Changes driven by changes in the wind field– Cools surface– Evaporates water (becomes more saline)– Determines where deep convection occurs– Produces turbulence in the deep ocean
Importance of MOC
• Controls dynamics of ocean– Contrast between warm surface waters and
cold deep water
• Similar flux amounts to surface currents– Much slower, but much larger volume
• Fluxes influence Earth’s heat budget and climate– Decades to millennia
Importance of MOC
• CO2 storage– Oceans store 50x more carbon than
atmosphere– More CO2 dissolves in cold water than warm
water – deep water is major CO2 reservoir– MOC determines how much new CO2 can be
dissolved into the ocean• temperature of deep water• rate of deep water replenishment• Increased ventilation and/or warming of deep
layers could lead to massive CO2 release to atmosphere
Importance of MOC
• Heat transport– “Global
Conveyer Belt”– 50% of poleward
energy transport
Importance of MOC
• Climate Fluctuations– Correlation of Greenland temperature with the
strength of the MOC– When ice bergs are produced, freshwater
injection stabilizes column– Slows MOC, polar front surges southward– Can cause rapid climate changes
http://oceanworld.tamu.edu/resources/ocng_textbook/chapter13/chapter13_01.htm
Present day Last Ice Age Interglacials (rapidchanges)
Observations
What is happening to the MOC now (IPCC)?
• Oceans are warming– 0.10°C rise from surface to 700 m (1961-
2003)– Slight cooling since 2003
• Salinity changes are occurring– Freshening in the subpolar latitudes– Salinification in shallow tropical and
subtropical oceans
http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter5.pdf
Heat content, 0-700m, deviation from 1961-1990 average
Linear trends (1955-2003) in ocean heat content, 0 – 700 m layer
Salinity trends, 1955-1998. Red = more salt, blue = less
What is happening to the MOC now (IPCC)?
• Changes in strength of MOC inconclusive– Shorter term variability (North Atlantic
Oscillation)– Inadequate long-term observations
• Climate model projections– Gradual decrease in MOC as consequence of
warming and freshening of N. Atlantic– Tipping point? Will it shut down?
Greenland Ice Sheet
• Covers 81% of the island• 2480 km long, 750 km
wide• Contains 8% of Earth’s
fresh water• 2nd largest ice body in
world (Antarctica)• Mean ice altitude is 2.1
km• Ice is about 110,000 years
old
What is happening to the ice sheet now?
• Record melting in recent years– 16% increase in the
area exposed to melting from 1979-2002
• Estimated melting (2003-2008) – 239 km3 per year (satellite estimated, Science Daily 2008)
http://www.sciencedaily.com/releases/2008/09/080930081355.htm
moulin
1987 – 2007 temperature trend (wikipedia)
Greenland Tipping Point
Warming meltsice sheet
Altitude of icelowers
More ice melts
Greenland Tipping Point
• 3°C warming will lead to irreversible melting (IPCC)– Threshold will be reached in the coming
decades
• Bamber (UK) - 6°C– Used more sophisticated model– Noted that ice sheet existed 125,000 years
ago when temperatures were 5°C higher than today
Greenland Tipping Point
• Hansen (NASA)– 3°C assumes linear response of ice sheet– Ice melt has been non-linear in the past– Could result in even faster melting
• Some lost ice is replaced by enhanced precipitation over Greenland
Abrupt Climate Change in the past
• Younger Dryas– ‘Recent’ event (12,800 years before present)– Dramatic cooling that lasted about 1,200
years– Dramatic warming at end (up to 10°C in 10
years over Greenland)
Cause of Younger Dryas
• Couple of theories
• Theory 1 – Massive injection of fresh water into the Atlantic– Melting of Lake Agassiz
• Theory 2 – Cosmic Event– Large North American meteor strike
Antarctic Ice Sheet
• 90% of Earth’s total ice volume
• 70% of Earth’s total fresh water
• 2 miles thick at pole
• Sea level rise of ~60 m if it were to completely melt
• Surrounded by sea ice
Antarctica Geography
5.4 million sq miles – 1.5 times the size of the United States
January April July
September December
West Antarctic Ice Sheet (WAIS)
• Marine based– Bedrock is below
sea level
• Weight depresses bedrock by 0.5 – 1 km
WAIS Tipping Point?
• WAIS is inherently unstable– Has collapsed in the past (found from sea
floor drilling)– 40,000 year cycle, 38 times in the past 5
million years*– Atmospheric CO2 ~ 400 ppm at the time of
the last collapse
• Resulting sea level rise: ~7 meters
*http://www.digitaljournal.com/article/269436From the journal Nature (2009)
WAIS Tipping Point?
• Time scale*– Surrounding ice shelves: centuries– WAIS collapse: within 1,000 years
• Forcings– Main melt from warmer ocean– 5°C rise in ocean temperature enough to do it
• Much larger EAIS would continue to melt but would not collapse
Penn State University modeling study
Other Impacts of WAIS melt
• More complicated sea level changes– Falling sea level close to Antarctica, higher
rises farther away (gravitational attraction)
• Isostatic rise will dump even more water into the ocean many years after the melt
• Shift in Earth’s rotation axis– 500 meters if entire sheet melts– Will move water from southern hemisphere
toward northern hemisphere
http://www.sciencedaily.com/releases/2009/02/090205142132.htm
https://www.cresis.ku.edu/research/data/sea_level_rise/jpeg/southeastern_us/southeastern_us_6m.jpg
East AntarcticIce Sheet Stable
West AntarcticIce Sheet in decline
Snowball Earth – Tipping point
James HansenFeb. 15, 2009 (London Observer)
• “...coal is the single greatest threat to civilization and all life on our planet”
• “potential for explosive changes”• “irreversible effects”• “...greatest danger hanging over our children
and grandchildren is the initiation of changes that will be irreversible on any time scale that humans can imagine”
• “The trains carrying coal to power plants are death trains”
http://www.guardian.co.uk/commentisfree/2009/feb/15/james-hansen-power-plants-coal
Tipping Point = Point of No Return?
• Depends on the time scale– Arctic sea ice (5 – 10 years)– Meridional overturning circulation (few
decades)– CO2 concentrations (many centuries)
• Already past a point of no return?
– WAIS?