4 Earth’s Energy Balance Sunlight (Shortwave, visible radiation) 235 Watts per square meter (W/m 2 ) Heat (Longwave, infrared radiation) 235 Watts per square meter (W/m 2 ) Perturbations to energy balance are known as “radiative forcings”
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
14 Source: IPCC Third Assessment Report Indirect effect(s): TAR figure shows “brightness” effect only “lifetime” effect potentially comparable discussion buried in text Direct effect(s): best understood divided by aerosol type Semi-direct effect(s): not shown on TAR figure postulated in 2000 discussed in text but no global estimate given
16 Climate Change Uncertainty “Climate sensitivity” is a key parameter is “climate sensitivity” 0.3 to 1 °C per W/m °C for doubling of CO 2 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
17 Aerosols and Climate Uncertainty High sensitivity Low sensitivity GHG forcing 20 th century T increase Aerosol + GHG forcing ??
18 Aerosols and Climate Uncertainty Uncertainty in aerosol forcing makes testing climate models against 20 th 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
19 NH/SH mixing intra- hemispheric mixing Challenges Need to characterize particle mass/number concentration size distribution: ~10 nm to 10 m chemical composition: >hundreds compounds mixing state interactions with clouds Highly variable in space and time: centurydecadalannualdailymonthlyhourly Mean CO 2 residence Mean aerosol residence
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
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
23 Elicitation Methodology Six parts 1.Direct: scattering/absorption of sunlight 2.Semi-direct: change in clouds as absorbing aerosols heat atmosphere 3.Cloud brightness (first indirect): smaller droplets → brighter clouds 4.Cloud lifetime (second indirect): smaller droplets → less precipitation 5.Total: net effect of above at top-of-atmosphere 6.Surface: net effect of above at surface
24 Elicitation Methodology For each part/effect: a)list top factors contributing to uncertainties b)estimate radiative forcing probability distributions upper/lower bounds “counterfactual” question 5/95% confidence intervals 25/75% confidence intervals best estimate c)probability uncertainty will (in 20 years) increase shrink by 0-50% shrink by 50-80% shrink more than 80%
Best understood Responses broadly consistent with IPCC TAR
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?
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
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?
“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