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Uncertainty and Feedback

CLIM 101: Weather, Climate and Global Society. Uncertainty and Feedback. CLIM 101: Weather, Climate and Global Society. Uncertainty. Sources of Uncertainty: Observations. Instrument error Sparse, infrequent measurements - inadequate sampling or sampling bias

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Uncertainty and Feedback

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  1. CLIM 101: Weather, Climate and Global Society Uncertainty and Feedback

  2. CLIM 101: Weather, Climate and Global Society Uncertainty

  3. Sources of Uncertainty: Observations • Instrument error • Sparse, infrequent measurements - inadequate sampling or sampling bias • Observing system change over time • Mixing direct measurements and proxy measurements

  4. Cooling  Increased post-WWII pollution in NH • ------- Little change ---- Variability due to solar changes, volcanism • Warming  Increasing GHG

  5. Slope = 1.02 Slope = 1.67 Slope = 1.01 Slope = 1.82 Synthetic time series example: Need large samples to avoid “end effects” in estimating linear trends

  6. URBAN HEAT ISLAND EFFECT full US Historical Climatology Network (USHCN) data USHCN data without the 16% of the stations with populations of over 30,000 within 6 km in the year 2000 • UHI and changes in land use can be important for DTR at the regional scale • The global land warming trend is unlikely to be influenced significantly by increasing urbanization. USHCN data for the 16% of the stations with populations over 30,000 Full USHCN set minus the set without the urban stations

  7. observations in each 1° grid box at 250 m depth

  8. Global mean sea level (deviation from the 1980-1999 mean) Uncertainty in estimated long-term rate of sea-level change Based on tide gauges Based on satellite altimetry Range of model projections (SRES A1B scenario)

  9. Global Sea Level  base period  Reconstructed fields since 1870 Coastal tide gauges Satellite altimetry

  10. Global annual ocean heat content w.r.t. 1961-1990 mean for the 0 to 700 m layer Update of Levitus et al. (2005) … shading represents 90% confidence interval Update of Ishii et al. (2006) … error bar represents 90% confidence interval Update of Willis et al. (2004; 0 to 750 m) … error bar represents 90% confidence interval

  11. Sources of Uncertainty: Models • Input data (forcing) uncertainty • Differing assumptions with respect to relevant processes • Differing estimates of model parameters • Intrinsic unpredictability • Unpredictability of external phenomena (e.g. volcanoes)

  12. Climate models without volcanic Forcing OHC - ocean heat content ThSL: Thermosteric sea level change (density changes induced by temperature change) Domingues et al. 2008

  13. (0-700 m) Climate models withvolcanic Forcing ThSL: Thermosteric sea level change (density changes induced by temperature change) Domingues et al. 2008

  14. The IPCC AR4

  15. IPCC SRES Emission Scenarios(IPCC Special Report on Emission Scenarios) Pg (Petagram) =1015 g = Gt (Gigaton)

  16. What is in store for the future and what has already been committed CO2 Eq Global warming will increase if GHGs concentration increase. Even if GHGs were kept constant at current levels, there is a “commitment” of 0.6°C of additional warming by 2100. 3.4oC = 6.1oF 850 2.8oC = 5.0oF 600 1.8oC = 3.2oF 0.6oC = 1.0oF 400

  17. Projected Future Warming Figure 9.13, IPCC TAR

  18. Clouds: Still the Largest Source of Uncertainty

  19. CLIM 101: Weather, Climate and Global Society Feedback

  20. Ice-Albedo Feedback (1) Cooling Ice Increases Albedo Increases Absorption of sunlight decreases

  21. Ice-Albedo Feedback (2) Warming Ice Decreases Albedo Decreases Absorption of sunlight increases

  22. Positive vs. Negative Feedback • Something triggers a small system change • The system responds to the change • Feedback • Positive Feedback: The response accelerates the original change • Negative Feedback: The response damps the original change

  23. Water Vapor Feedback (1) Warming Evaporation from the Oceans Increases Atmospheric Water Vapor Increases Stronger Greenhouse Effect

  24. Water Vapor Feedback (2) Cooling Evaporation from the Oceans Decreases Atmospheric Water Vapor Decreases Weaker Greenhouse Effect Water Vapor Feedback is Positive

  25. Effect of Positive Feedback (1) With positive feedbacks Temperature If no feedbacks present Time

  26. Effect of Positive Feedback (2) If no feedbacks present Temperature With positive feedbacks Time

  27. The Need for Negative Feedbacks • Positive feedbacks are destabilizing - they tend to drive the system away from equilibrium • Negative feedbacks are required to restore equilibrium

  28. A System Without Negative Feedbacks Catastrophic Warming! Temperature Time

  29. The Way Physical Systems Usually Behave Temperature Warming Accelerating Warming Decelerating Time

  30. Response to Oscillatory Energy Source Normal Behavior

  31. Response to Oscillatory Energy Source Response with weakened negative feedbacks – increased amplitude

  32. Feedbacks - Summary • Positive feedbacks tend to increase the amplitude of the system response • Negative feedbacks tend to reduce the amplitude of the system response

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