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?