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Understanding uncertainties and feedbacks

CLIM 101: Weather, Climate and Global Society. Understanding uncertainties and feedbacks. Jagadish Shukla. Lecture 15: 22 Oct, 2009. Reading for Week 8 Lecture 15 Understanding uncertainties and feedbacks. GW Chapter 3, 5. CLIM 101: Weather, Climate and Global Society.

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Understanding uncertainties and feedbacks

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  1. CLIM 101: Weather, Climate and Global Society Understanding uncertainties and feedbacks Jagadish Shukla Lecture 15: 22 Oct, 2009

  2. Reading for Week 8Lecture 15Understanding uncertainties and feedbacks • GW Chapter 3, 5

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

  4. 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

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

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

  8. 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

  9. 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)

  10. The IPCC AR4

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

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

  13. 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)

  14. Clouds: Still the Largest Source of Uncertainty

  15. Center of Ocean-Land-Atmosphere studies Climate Model Fidelity and Projections of Climate Change J. Shukla, T. DelSole, M. Fennessy, J. Kinter and D. Paolino Geophys. Research Letters, 33, doi10.1029/2005GL025579, 2006

  16. Increase in Surface Temperature Observations Predictions with Anthropogenic/Natural forcings Predictions with Natrual forcings 1.0º C IPCC 2007

  17. Projected Future Warming Figure 9.13, IPCC TAR

  18. What is in store for the future and what has already been committed 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. CO2 Eq 3.4oC = 6.1oF 850 2.8oC = 5.0oF 600 1.8oC = 3.2oF 0.6oC = 1.0oF 400

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

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

  21. 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

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

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

  24. 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

  25. A System Without Negative Feedbacks Example “Runaway Greenhouse Effect”, T  H2O  T Catastrophic Warming! Temperature Time

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

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

  28. Feedbacks in the Biosphere • The plankton multiplier in the ocean (positive) • (Colder  Stronger Ocean Biological Pump  Remove ATM CO2) • 2. Carbon dioxide fertilization, plant growth (negative) • 3. Effect of higher temperatures on respiration (positive) • 4. Reduction of forest growth because of climate change (positive) • 5. Increased greenhouse gases due to increase of fires (positive) • 6. Release of methane from wetland and permafrost (positive)

  29. Feedbacks in the Climate System • Water vapor feedback • Cloud-radiation feedback • Ice-albedo feedback • Climate-Carbon Cycle feedback

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

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

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

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

  34. Understanding and Attributing Climate Change Center of Ocean-Land-Atmosphere studies • 1. Equilibrium Climate Sensitivity (ECS) and Transient Climate Response (TCR) • Definitions • Model ECS and TCR—the role of feedbacks • 2. Detection and Attribution • Detection and Attribution of What? • Modeling with and without anthropogenic forcing • 3. Understanding?

  35. Equilibrium Climate Sensitivity (ECS) and Transient Climate Response (TCR) Center of Ocean-Land-Atmosphere studies • Definition: The ECS is the full equilibrium surface temperature response to a doubling of CO2 • Definition: The TCR is the surface temperature response at CO2 doubling for a 1%/yr increase of CO2 (i.e. at year 70) • a. ECS and TCR are basically model concepts • b. TCR < ECS • c. ECS is a measure of the feedbacks in the system: • Recall:

  36. Center of Ocean-Land-Atmosphere studies Climate Model Fidelity and Projections of Climate Change J. Shukla, T. DelSole, M. Fennessy, J. Kinter and D. Paolino Geophys. Research Letters, 33, doi10.1029/2005GL025579, 2006

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