Abrupt Climate Change

1. Abrupt Climate Change Kalee Anderson Atmospheric Sciences major

2. Articles The role of the thermohaline circulation in abrupt climate change Peter U. Clark, Nicklas G Pilas, Thomas F. Stocker & Andrew J Weaver Abrupt Climate Change R.B. Alley, J Marotzke, W.D Nordhaus, J.T. Overpeck, D.M Peteet, R.A. Pielke Jr., R.T. Pierrehumbert, P.B Rhines, T.F Stocker, L.D Talley, J.M Wallace

3. Introduction • What do we mean by abrupt climate change? • A persistent transition of climate (over subcontinental scale) that occurs on the timescale of decades • “Technically, an abrupt climate change occurs when the climate system is forced to cross some threshold, triggering a transition to a new state at a rate determined by the climate system itself and faster than the cause.” • Why is understanding abrupt climate change so important?

4. Introduction • Current models don’t simulate past changes with great accuracy • Public debate (regarding climate change) has focused more on greenhouse gas emissions rather than the impact of large changes

5. Abrupt Climate Change • Describes the scientific foundation for a research agenda focused on abrupt climate change and identify areas in which the possibility of climate change has a bearing on the current policy debate about human induced climate change

6. What has climate done? • Long term stabilizing feedbacks have maintained Earth-surface conditions for about 4 billion years, however data indicate that over times of 1 yr to 1 million years, the dominant feedbacks have amplified climate perturbations. • Examples?

7. What has climate done? • Even a slow forcing can trigger an abrupt change • Where do human concerns come in?

8. What has climate change done? • 20th century warming • -4 degrees C or more in places on the Atlantic side of the Arctic (1920’s) • Dust Bowl • Abrupt drought regimes infrequent in US, but more common elsewhere (the Sahel)

9. Abrupt Pacific shift possibly related to ENSO • On the Atlantic side, abrupt shifts in the THC • Records show ice-age events were larger and even more widespread than those of the Holocene or previous interglacials

10. Why climate changed abruptly • Triggers, Amplifiers, and sources of persistence • Like a canoe???

11. Why climate changed so abruptly? • Triggers • Drying of the Sahara during the latter part of the Holocene • Ice-age Dansgaard-Oeschger (DO) oscillations linked to time/orbital forcing • Triggers can be fast, slow, or in between

12. Why climate changed so abruptly? • Amplifiers • Abundant • Can produce large changes with minimal global forcing • Examples: Drying/cooling

13. Why climate changed so abruptly? • Sources of Persistence • For DO oscillations, the thermohaline circulation of the oceans is implicated in the persistence

14. Impacts on Ecological/Economic Systems • FCCC has focused attention on anthropogenic forcing instead of abrupt climate change • Most systems have ability to adapt to a changing enviroment • Slow vs. Abrupt changes?

15. Outlook • Research coupling economic/climate models have progressed over the past decade, but there is virtually no linked research on abrupt climate change • Not only magnitude, but also the rates of human forcing of the climate system are crucial issues for abrupt climate change

16. Results

18. Results

21. The role of THC in abrupt climate change • Data and models both suggest that abrupt climate change during the last glaciation originated through changes in the Atlantic THC in response to small changes in the hydrological cycle.

22. How does the ocean affect climate?

23. Thermohaline Circulation (THC) • What is it? • Differences in the water’s density and salinity driving deep-ocean currents thousands of meters below the surface (NOAA)

25. THC in the Atlantic • How it occurs in the Atlantic today? • Responsible for much of the total oceanic poleward heat transport in the Atlantic • May have many speeds.Why?

26. Most, but not all coupled GCM projections of the 21st century climate show a reduction in the strength of the Atlantic overturning circulation with increasing concentration of greenhouse gases

27. Past changes in THC • Characterized by transitions between 3 possible modes • Modern Mode, Glacial Mode, & Heinrich Mode

28. Modern mode • Characterized by formation of deep water in the Nordic Seas and it’s subsequent flow over the Greenland-Scotland ridge

29. Glacial mode • NADW probably formed through open-ocean convection in the subpolar North Atlantic, sinking to depths of less than 2500 m.

30. Heinrich mode • Antarctic-derived waters filed the North Atlantic basin to depths as shallow as 1,000 m

31. Atmospheric Radiocarbon • The magnitude of related changes in the rate of the overturning of circulation and deep ocean ventilation remains controversial • Atmospheric radiocarbon offers great promise for identifying past changes in the THC

32. Beryllium (Be) • Cosmogenic radionuclide • Polar ice core records of Be can be used to estimate past changes in cosmic radiation because Be is rapidly removed from the atmosphere to the ice surface by precipitation

33. Beryllium (Be) • The Be record from the GISP2 ice core suggests that changes in cosmogenic production rates are an unlikely cause of 1st order changes in atmospheric radiocarbon between 15 & 22 kyr. • Small age offsets indicate that the GISP2 timescale is too old in this interval by about 60 years (see later figure)

34. North Atlantic Deep Water (NADW) • Any increase in formation of deep or intermediate waters in the Southern Ocean and North Pacific are now too old and therefore will have a lesser impact on the atmospheric radiocarbon budget than changes in NADW

35. Mechanisms of past abrupt climate change • Reconstructions of changes in the freshwater flux from ice sheets around the North Atlantic show good agreement with past changes in the THC and North Atlantic climate (becomes important in understanding abrupt climate change!)

36. Changes in Atlantic THC • Millennial scale changes that involved transitions from modern to glacial modes of NADW are manifested as dramatic fluxes of North Atlantic climate referred to as Dansgaard-Oeschger (DO) events • Each has a characteristic pattern of abrupt warming, followed by gradual cooling

37. The D-O signal • DO events may be amplified expressions of an ongoing persistent and stable climate cycle • Paleoclimate records identify additional atmospheric responses to North Atlantic climate (which would further amplify and transmit the DO signal)

38. The D-O signal cont… • Includes changes in the strength of trade winds and associated oceanic upwelling, the position of the ITCZ with effects on water vapor transport for the Atlantic to the Pacific basin, the strength of the Asian monsoon, SST’s of the Pacific warm pool, and ventilation of the North Pacific

39. Modeling abrupt change • Faces challenges • Disparity in timescales between transient states and mode changes • A compromise in model complexity due to long integrations and sufficient model resolution to capture abruptness • Uncertainties in initial conditions…calibration problems in proxy data

40. Results • Some modeling experiments find that during the next few centuries, the THC moves to an off state in response to increased gg’s • The fate of the THC largely depends on the response of air-sea heat and freshwater fluxes to the increased load of gg’s

41. Results • Thresholds and the stability properties of the THC depend fundamentally on the mean climate state.

46. Summary • Abrupt climate change will always be accompanied by uncertainty • Any future abrupt climate change may have large/unanticipated impacts. • Improved understanding may increase lead time for mitigation and adaptation

47. Summary • Continued efforts are needed to simulate past abrupt climate changes that so remarkably affected the global climate system

48. Future research? • Progress towards mechanistic understanding of abrupt climate change • More complete models and estimates of impacts could make response strategies more effective • What can we do in response to abrupt climate change?

49. Questions?