Effects of Airborne Particles on Climate: an Expert Elicitation

1. Effects of Airborne Particles on Climate: an Expert Elicitation M. Granger Morgan, Peter J. Adams, and David W. Keith 7 March 2006

2. Overview • Background • Radiative forcing • Aerosol (airborne particles) climate effects • Previous assessments (IPCC TAR) • Aerosols and climate uncertainty • Expert Elicitation • Design • Results • Lessons Learned 2

3. Overview • Background • Radiative forcing • Aerosol (airborne particles) climate effects • Previous assessments (IPCC TAR) • Aerosols and climate uncertainty • Expert Elicitation • Design • Results • Lessons Learned 3

4. Earth’s Energy Balance Heat (Longwave, infrared radiation) Sunlight (Shortwave, visible radiation) 235 Watts per square meter (W/m2) 235 Watts per square meter (W/m2) Perturbations to energy balance are known as “radiative forcings” 4

5. Radiative Forcings • Shortwave (incoming) or longwave (outgoing) • Both positive (warming) and negative (cooling) • Computed at various altitudes • Top-of-atmosphere (TOA): most useful metric for global average temperature • Surface: useful metric for evaporation / changes to hydrological cycle 5

6. 6 Source: IPCC Third Assessment Report

7. Overview • Background • Radiative forcing • Aerosol (airborne particles) climate effects • Previous assessments (IPCC TAR) • Aerosols and climate uncertainty • Expert Elicitation • Design • Results • Lessons Learned 7

8. Aerosols Scattering Sunlight Dust and smoke over Australia (Terra) 8

9. Aerosols Absorbing Sunlight Kuwaiti oil fires 9 photo courtesy of Jay Apt (via Steve Schwartz)

10. Aerosols and Clouds AVHRR satellite “false color” image Power plant Lead smelter Port Oil refineries Red: darker clouds (large droplets) Green: brighter clouds (small droplets) Blue: clear sky 10 Rosenfeld, Science (2000)

11. Aerosols and Clouds Aerosol Particles Cloud Droplets Clean Air Brighter, more persistent clouds Polluted Air 11

12. How direct is direct? • Direct effect: scattering/absorbing sunlight • Semi-direct effect: • aerosol absorption heats atmospheric layer • warmer air → lower relative humidity → less/no cloud • Indirect effect: modifying cloud properties • “brightness (first) effect” • “lifetime (second) effect” 12

13. Overview • Background • Radiative forcing • Aerosol (airborne particles) climate effects • Previous assessments (IPCC TAR) • Aerosols and climate uncertainty • Expert Elicitation • Design • Results • Lessons Learned 13

14. Indirect effect(s): • TAR figure shows “brightness” effect only • “lifetime” effect potentially comparable • discussion buried in text • Semi-direct effect(s): • not shown on TAR figure • postulated in 2000 • discussed in text but no global estimate given • Direct effect(s): • best understood • divided by aerosol type 14 Source: IPCC Third Assessment Report

15. Overview • Background • Radiative forcing • Aerosol (airborne particles) climate effects • Previous assessments (IPCC TAR) • Aerosols and climate uncertainty • Expert Elicitation • Design • Results • Lessons Learned 15

16. Climate Change Uncertainty • “Climate sensitivity” is a key parameter • l is “climate sensitivity” • 0.3 to 1 °C per W/m2 • 1.5 - 4.5 °C for doubling of CO2 • In climate models, representation of cloud feedback is largest source of uncertainty • In retrospective studies, knowledge of aerosol forcing is lacking global average temperature change global average radiative forcing 16

17. Aerosols and Climate Uncertainty Aerosol + GHG forcing GHG forcing High sensitivity ?? Low sensitivity 20th century T increase 17

18. Aerosols and Climate Uncertainty • Uncertainty in aerosol forcing makes testing climate models against 20th century temperature record almost meaningless • Nevertheless all climate models do this test and claim good agreement as “validation” of their model • Aerosol forcing is a “tunable” parameter • High sensitivity models ↔ Strong aerosol cooling • Low sensitivity models ↔ Weak aerosol cooling 18

19. hourly daily monthly annual decadal century Challenges • Need to characterize particle • mass/number concentration • size distribution: ~10 nm to 10 mm • chemical composition: >hundreds compounds • mixing state • interactions with clouds • Highly variable in space and time: intra-hemispheric mixing Mean aerosolresidence Mean CO2 residence NH/SH mixing 19

20. Overview • Background • Radiative forcing • Aerosol (airborne particles) climate effects • Previous assessments (IPCC TAR) • Aerosols and climate uncertainty • Expert Elicitation • Design • Results • Lessons Learned 20

21. Expert Elicitation • Granger Morgan “unofficially” invited by IPCC to survey expert opinion • Not intended to replace peer-reviewed scientific studies in literature • Usefulness • reveal agreement/disagreement between experts • little systematic work on uncertainty in aerosol forcing 21

22. Elicitation Methodology • Administered by mail • 52 experts invited from broad base of expertise types • Aerosols, clouds, and climate • Modeling, experimental • Global to micro scale • 29 agreed • 2 said they lacked expertise • 3 did not complete • 24 useable responses • Participants acknowledged but responses are anonymous 22

23. Elicitation Methodology • Six parts • Direct: scattering/absorption of sunlight • Semi-direct: change in clouds as absorbing aerosols heat atmosphere • Cloud brightness (first indirect): smaller droplets → brighter clouds • Cloud lifetime (second indirect): smaller droplets → less precipitation • Total: net effect of above at top-of-atmosphere • Surface: net effect of above at surface 23

24. Elicitation Methodology For each part/effect: • list top factors contributing to uncertainties • estimate radiative forcing probability distributions • upper/lower bounds • “counterfactual” question • 5/95% confidence intervals • 25/75% confidence intervals • best estimate • probability uncertainty will (in 20 years) • increase • shrink by 0-50% • shrink by 50-80% • shrink more than 80% 24

25. Overview • Background • Radiative forcing • Aerosol (airborne particles) climate effects • Previous assessments (IPCC TAR) • Aerosols and climate uncertainty • Expert Elicitation • Design • Results • Lessons Learned 25

26. Best understood • Responses broadly consistent with IPCC TAR

27. One respondent: “semi-direct effect is positive by definition” • Absorbing aerosols above marine stratocumulus increase reflectivity via dynamical effects – “still semi-direct”? • Forcing or feedback?

28. Most experts mostly in 0 to -2 W m-2 range of IPCC TAR • A minority suggest possible effects of -3 to -4 W m-2

29. Omitted from IPCC TAR • Many reflect “conventional wisdom” of 0 to -2 W m-2 • Significant minority give wider uncertainties • Believers in positive – an enlightened minority?

30. “Forward” modeling: estimate forcing based on aerosol physics • “Reverse” modeling: estimate aerosol forcing as that needed to match historical temperature trends

32. Conclusions • IPCC TAR assessment ok for what was reported • Significant uncertainties (cloud lifetime and semi-direct) unreported • Field is not “mature”: new physical mechanisms being uncovered/studied, significant chances of uncertainty increasing • Terminology is ambiguous (as well as confusing) • Lines between “forcings” and “feedbacks” blurred • Aerosols are part of the (irreducible?) climate uncertainty 32