Climate Tipping Points

1. Climate Tipping Points

2. 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

3. “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

4. 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)

6. 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

7. http://ocean.mit.edu/~cwunsch/papersonline/thermohaline.pdf

8. 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

9. 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

10. 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

11. Importance of MOC • Heat transport • “Global Conveyer Belt” • 50% of poleward energy transport

12. 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

13. Last Ice Age Interglacials (rapid changes) Present day Observations http://oceanworld.tamu.edu/resources/ocng_textbook/chapter13/chapter13_01.htm

14. 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

15. Heat content, 0-700m, deviation from 1961-1990 average http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter5.pdf

16. Linear trends (1955-2003) in ocean heat content, 0 – 700 m layer

18. Salinity trends, 1955-1998. Red = more salt, blue = less

19. 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?

20. 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

21. 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

23. moulin

25. 1987 – 2007 temperature trend (wikipedia)

26. Greenland Tipping Point Warming melts ice sheet Altitude of ice lowers More ice melts

27. 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

28. 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

29. 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)

31. 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

33. 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

34. Antarctica Geography

35. 5.4 million sq miles – 1.5 times the size of the United States

36. July January April December September

37. West Antarctic Ice Sheet (WAIS) • Marine based • Bedrock is below sea level • Weight depresses bedrock by 0.5 – 1 km

39. 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/269436 From the journal Nature (2009)

40. 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

41. 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

42. https://www.cresis.ku.edu/research/data/sea_level_rise/jpeg/southeastern_us/southeastern_us_6m.jpghttps://www.cresis.ku.edu/research/data/sea_level_rise/jpeg/southeastern_us/southeastern_us_6m.jpg

45. West Antarctic Ice Sheet in decline East Antarctic Ice Sheet Stable

46. Snowball Earth – Tipping point

47. 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

48. 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?