Tipping Points in Earth's Climate System

1. Gaje Buchanan Atmospheric Sciences “Tipping Elements in the Earth's Climate System” (2008) and “Early Warning of Climate Tipping Points” (2011) Primarily by Timothy M. Lenton Tipping Points in Earth's Climate System

2. What Is a Tipping Point, and Why Is It Important? • A tipping point is defined as a critical threshold at which a tiny perturbation can qualitatively alter the state of development of a system • In other words, a small change can have a large, long-term set of consequences: “little things can make a big difference” • Knowing a system's tipping point and when it may occur can help us avoid reaching the tipping point entirely, or help us better prepare for the effects of reaching a tipping point so that we can mediate the negative effects

3. Determining a “Policy-relevant” Tipping Element • A tipping element is any system which has a component that could potentially be a tipping point • Note: A tipping point need not be a climate variable • A “policy-relevant” tipping element restricts possible tipping elements down to those that are under influence of human activities that can be affected by political policy. • Time period of 100-1,000 years, in which consequences of decisions made within this century are observed within this millennium.

4. Method of Determining Policy-Relevant Tipping Elements • “Tipping Points in the Earth System” workshop • Consisted of 36 leading experts, and 52 members of the international scientific community • Their goal: compile a list of possible future tipping points

5. Policy-relevant Tipping Elements

6. Policy-relevant Tipping Elements

7. The Policy-relevant Tipping Elements • Arctic Sea-Ice • As sea-ice melts, it exposes darker ocean, which has a lower albedo • Positive ice-albedo feedback has dominated thinning and shrinkage, leading some to believe a tipping point has already been reached • Many models indicate threshold for Summer Arctic sea-ice loss • Conclusion: Summer ice-loss threshold may be close, if not already passed

8. The Policy-relevant Tipping Elements • Greenland Ice Sheet • Deglaciation lowers ice altitude, increasing surface temperature and causing a positive feedback loop • IPCC: local warming threshold of ~1.9-4.6ºC • Poor modeling of GIS leads to great uncertainty in time frame • Conclusion: threshold is accessible this century, with lower limit of collapse being 300 years

9. More Policy-relevant Tipping Elements • West Antarctic Ice Sheet • Collapse possible if disintegration of ice shelves and acceleration of ice streams allows warmer ocean water to intrude beneath, or just from pure surface heating • ~5ºC local warming to exceed melting point in Summer on major ice shelves • WAIS requires ~8ºC to reach Summer melting point • Conclusion: qualitative change possible this millennium, worst case scenario collapse in 300 years

10. More Policy-relevant Tipping Elements • Atlantic Thermohaline Circulation • Can collapse from sufficient intrusion of freshwater into North Atlantic • Most models show collapse of convection with sufficient freshwater forcing • Irreversible in some models • Freshwater input due to Anthropogenic forcing is poorly understood, causing uncertainty • Conclusion: many models simulate clear passing of tipping point this century with a qualitative change this millennium

11. Yes, More Policy-relevant Tipping Elements • El Niño Southern Oscillation • Increased ocean heat uptake due to global warming could cause permanent deepening of the thermocline in the eastern equatorial pacific • This could cause more frequent El Niños with greater amplitude and duration • No currently identified tipping point value • Conclusion: required warming is achievable this century, and qualitative change is possible this millennium, but threshold is uncertain

12. Still Not Done With Policy-relevant Tipping Elements • Indian Summer Monsoon • Increases in aerosol forcing and/or land-use change cause increases in albedo, which weakens the monsoon • Model indicates collapse at planetary albedo of ~.5 but increases in CO2 stabilizes the monsoon • Lenton and Co. believe brown haze and land-use change are poorly modeled • Conclusion: Lenton believes that ISM meets the criteria for a tipping element

13. Really? More Policy-relevant Tipping Elements? • Sahara/Sahel and West African Monsoon • Affected by sea surface temperatures, especially anti-symmetric patterns between the hemispheres • Greenhouse gas forcing is expected to increase interhemispheric temperature gradient, causing increased Sahel rainfall • Conclusion: Meets the criteria for a tipping element, and is a rare example of a beneficial potential tipping element

14. The Last Policy-relevant Tipping Element • Amazon Rainforest • A lot of Amazon precipitation is recycled, and deforestation simulations generate a 20-30 percent reduction in precipitation, lengthening of dry season, and increase in Summer temperatures • ~3-4ºC global warming may cause Amazon dieback because of more persistent El Niño, which causes Amazon basin drying • Conclusion: ultimate fate may be decided by interplay between land-use change, response of regional precipitation and ENSO cycles, though it does meet the tipping element criteria

15. Fooled You! This Is Actually The Last Policy-relevant Tipping Element • Boreal Forest • Complex system between tree physiology and wildfires • Dieback could be caused by many things, and the mechanisms that drive forest interaction are poorly understood • Conclusion: dieback could occur at ~3ºC of global warming, but there is much uncertainty

16. Well That Was Boring, Why Should I Care? • Knowing these tipping elements and their possible tipping points may help us mediate risks or avoid tipping points entirely

17. How Tipping Elements Relate to Each Other

18. Methods of Early Warning • Bifurcation • As a point approaches this, perturbations to the system causes larger deviations and longer recovery times • Once this is reached, there is no recovery • This can be measured

19. Bifurcation Diagram

20. Methods of Early Warning • Noise-induced transitions • A natural variability caused non-linear change, i.e. a tipping point is passed due to natural causes, and not from anthropogenic forcing • Very unpredictable and shows almost no warning signs • However, if we can analyze the slowest decay rate in the system we may be able to use that as an indicator of stability

21. Methods of Early Warning • Palaeorecord tests • Using a system's past behavior as it approached an abrupt transition to help identify its behavior as it approaches a possible tipping point

22. Example of a Palaeorecord Test

23. Limitations of Early Warning • False alarms and missed alarms • Poorly interpreted data and uncertainties in general understanding

24. Improving Early Warning Systems • Bifurcation is the best prospect for early warning, but mostly for “fast” systems with little internal memory • For example, the monsoon, because anthropogenic forcing is slow relative to its timescale • However, it demands higher resolution data • Improvement of real-time monitoring

25. Conclusion • In short, early warning of tipping points is feasible, but we need more research in nearly every facet • It's crucial that we discover these tipping points so that we can prevent further, possibly irreparable, damage to our planet

26. Questions • Due to apathy, I will not be taking questions • I'm joking, please ask me questions!