Wrap-up of Lecture 2. Aerosol Forcing and a Critique of the Global Warming Consensus

Transcription

1 Wrap-up of Lecture 2 Massive research efforts over the past two decades have allowed aerosol scientists to put boundaries on the "direct" and "cloud-albedo" forcings. [IPCC 2007, Fig SPM.2] When climate models include negative forcing by aerosols, they get a better fit to the 20th century temperature record (-0.1 to -0.9) W/m (-0.3 to -1.8) W/m 2 GHGs + aerosols GHGs Aerosol Forcing and a Critique of the Global Warming Consensus by Tad Anderson tadand@uw.edu Outline Current GW consensus: "detection and attribution" The problem of aerosol forcing Fitting the temperature record > exaggerated confidence Implications for the Global Warming Theory Goal To illustrate the vigorous criticism within science that is, ultimately, our best reason for confidence

2 Note: Scientific Consensus on GW = IPCC IPCC: Intergovernmental Panel on Climate Change Chartered in 1988 under UNEP and WMO (United Nations Environment Program, World Meteorological Organization) Purpose - assess knowledge of human-induced climate change, impacts, options - issue regular reports to support policymaking Working Groups WG I. Scientific Assessment WG II. Impacts WG III. Mitigation (or, prevention) Key references for this lecture Anderson et al., 2003, "Climate forcing by aerosols: A hazy picture," Science, 300, Kiehl, 2007, "Climate model response and sensitivity," Geophys. Res. Lett., 34, doi: /2007GL Schwartz et al., 2007, "Quantifying climate change: Too rosy a picture?", Nature Reports: Climate Change, 1, History First comprehensive report in 1990 Subsequent major reports: 1995, 2001, 2007

3 Current global-warming consensus Current consensus detection and Warming of the Earth s surface attribution 1. Is predicted by climate models forced with GHG s 2. Has in fact been detected 3. Match between prediction and observation is sufficiently good that attribution has been claimed Conceptual framework for this consensus: T s = F (Eq. 6.1 of IPCC, 2001) F: change in planetary energy balance (W/m 2 ) T s : resulting change in surface temperature (K) : climate sensitivity 1. Warming is predicted by climate models 1a. Unequivocal, positive forcing from greenhouse gases (GHGs) [IPCC 2007, WG-I, Fig. SPM.1] 1b. Substantial enough to cause significant warming (according to climate models) Jim Hansen (1941- ) 10,000 5,000 0 Years before 2005 [adapted from Hansen et al., 1988, J. Geophys. Res., 93, ]

4 2a. Warming has been detected 2b: Warming appears to be beyond natural variability IPCC 2007, WG-I, Fig. SPM.3 IPCC 2001, WG-I, SPM, Fig 1b

5 3a. Attribution to human causes: IPCC a. Attribution to human causes: IPCC 2001 GHGs + aerosols GHGs Natural Climate Influence Human Climate Influence All Climate Influences IPCC 1996, WG-I, Technical Summary, Fig. 15 IPCC 2001, WG-I, SPM, Fig. 4

6 3b. Attribution to human causes: IPCC 2007 Note: pattern matching does lend additional confidence in the attribution claim Note: Warming so far is modest; importance of GW theory comes from predicted warming if we carry on with BAU emissions predictions IPCC 2007 WG I Fig. SPM.4 proxy instrumental

7 Problem with the attribution argument Industrial-era Forcings: IPCC 2007, Fig. SPM.2 Attribution: When we put known forcings into our climate models, we are able to reproduce the magnitude and pattern of industrial-era temperature changes. Total Forcing is uncertain by a factor-of-four! Problem: The forcings (especially aerosol forcings) are not known! Industrial-era total: 1.7 ( ) W/m 2

8 B&H: aerosol dominates uncertainty Aerosol forcing dominates the uncertainty in total forcing. Aerosol vs GHG forcing probability distribution of climate forcings Aerosol Non-Aerosol reference: Boucher and Haywood, 2001, Clim. Dyn. 18, 297.

9 Aeosol forcing is visible from space Aerosol forcing is visible from space Direct, clear-sky forcing over the ocean Aeosol forcing is visible from space Aerosol forcing is visible from space "Ship Tracks" example of indirect aerosol forcing Mechanism: > particles from ship > more and smaller cloud droplets > more droplet surface area > clouds above ship are "brighter" - i.e. reflect more sunlight Jan 21, 2008, off coast of British Columbia Photo credit: NASA Earth Sciences & Image Analysis Laboratory, Johnson Space Center NASA Earth Observatory

10 Aerosol forcing estimates: two independent methods 1. Forward calculations (study the aerosol) aerosol mass from chemical transport model mass-dependent optical and cloud-nucleating properties from measurements satellite data as constraint 2. Inverse calculations (fit the T-record) assume observed warming is the response to a substantial, positive forcing multiple runs of simple climate models with varying values for the poorly known input parameters (aerosol forcing, climate sensitivity, ocean heat diffusivity) assign probabilities based on fit to simplified versions of the temperature record [ref: Anderson et al., 2003, Science, 300, ] Aerosol forcing estimates: two independent methods 1. Forward calculations (study the aerosol) A. Boucher and Haywood, Clim. Dyn. 18, 297 (2001) B. Kiehl et al., JGR, 105, 1441 (2000) (sulfate only) C. Hansen and Sato, Proc. NAS 98, (2001) D. Hansen et al., Proc. NAS 95, (1998) E. Schwartz and Andreae, Science 296, 2139 (1996) F. Menon et al., J Atmos Sci 59, 692 (2002) (indirect only) 2. Inverse calculations (fit the T-record) G. Wigley and Raper, Science 293, 451 (2001) H. Harvey and Kaufmann, J. Clim. 15, 2837 (2002) I. Gregory et al., J. Clim. 15, 3117 (2002) J. Andronova and Schlesinger, JGR 106, (2001) K. Knutti et al., Nature 416, 719 (2002) L. Forest et al., Science 295, 113 (2002) [ref: Anderson et al., 2003, Science, 300, ]

11 Aerosol forcing estimates - results Comparison of Forward and Inverse methods Applications - references Aerosol forcing applications forward calcs inverse calcs applications Applications in climate studies: M. Crowley, Science 289, 270 (2000) (attribution) N. Mitchell et al., 2001 (IPCC Chap 12) (attribution) O. Cubasch et al., 2001 (IPCC Chap 9) (projection) P. Tett et al., 2002, J. Geophys. Res. (attribution) Q. Meehl et al., 2003, J. Climate (diagnosis) [ref: Anderson et al., 2003, Science, 300, ] M. - fits paleo temperature record with solar and volcanic forcings - shows recent warming is anomalous and is explained by the known anthropogenic forcings N. - review of detection/attribution studies for industrial era - shows that pattern of 100-year warming is very well explained by the known anthropogenic and natural forcings O. - IPCC climate change projections for various emission scenarios P. - new detection/attribution study for industrial era Q. - climate diagnostic study; exploring natural variability, climate sensitivity, etc. [ref: Anderson et al., 2003, Science, 300, ]

12 Comparison of Aerosol Forcing Methods and Applications IPCC attribution Follow-up studies since Kiehl Schwartz et al., 2007 IPCC climate projection Given uncertain in aerosol forcing, modelers are picking values that allow them to fit the temperature record. Danger: We may get the "right" answer for the wrong reasons. Figure from: Anderson et al., 2003, Science, 300, Jeff Kiehl, Climate Modeler, National Center for Atmospheric Research Steve Schwartz, Aerosol Scientist, Brookhaven National Lab.

13 Comparison of IPCC Climate Models Total forcing compensates for climate sensitivity Total forcing is controlled mostly by aerosol forcing Schwartz et al., 2007 implied uncertainty in climate model simulations How can model uncertainty be so small if uncertainty in total forcing (an input to the models) is a factor of four? Kiehl, 2007, "Climate model response and sensitivity," Geophys. Res. Lett., 34, doi: /2007GL IPCC 2007 WG I Fig. SPM.4

14 Schwartz et al., 2007 Current paradigm sidenote on attribution Hansen 1981 By the way fitting the T-record is not a new result uncertainty in climate model simulations if uncertainty in aerosol forcing is included Source: Hansen et al. 1981, Science, 213, Note: very different set of forcings! Figure 2 from Schwartz et al., 2007, Nature Reports: Climate Change, 1, (modified from IPCC 2007 WG I Fig. SPM.4; error bars added)

15 IPCC "Attribution" argument reconsidered Epilog 1: 1988 prediction based on GW Theory Simple but misleading argument The fit shows we understand the causes A more defensible argument i. The fit shows we have a plausible explanation. ii. Studies of alternative explanations (e.g. sun, natural variability) show that none of these is plausible. Therefore, rising GHGs is the only plausible explanation for the recent, observed warming. Observations: 5-year running mean Reference period: Jim Hansen (1941- ) predictions from Hansen et al. (1988) simplified and smoothed for clarity

16 Epilog 1: 1988 prediction based on GW Theory Epilog 2: Large, positive forcing in 2100 for BAU emissions is unequivocal past century this century natural Observations: 5-year running mean Reference period: Jim Hansen (1941- ) odd situation: We know forcings in the future better than we know forcings at present or in the past. A1F1 B1 predictions from Hansen et al. (1988) simplified and smoothed for clarity

17 Wrap-up Current global warming consensus (i.e. the IPCC) places heavy emphasis on the "detection and attribution" argument Confidence in this argument is exaggerated, according to myself and several other scientists Why? Climate models "fit" the temperature record by using aerosol forcing as a "tuning knob". Thus, the fit is better than the actual knowledge going into the models. Better arguments exist for confidence in the GW Theory: - it led to a successful prediction (in 1988) of recent warming - GHG's are a plausible explanation for recent warming - there are no other plausible explanations - climate forcing in 2100 (and beyond) will definitely be positive and very large if we carry on with BAU emissions The END Questions / Comments??? Tad Anderson tadand@uw.edu