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

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

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

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

observations in each 1° grid box at 250 m depth

full US Historical Climatology Network (USHCN) data USHCN data for the 16% of the stations with populations over 30,000 USHCN data without the 16% of the stations with populations of over 30,000 within 6 km in the year 2000 Full USHCN set minus the set without the urban stations 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. URBAN HEAT ISLAND EFFECT

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

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

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)

The IPCC AR4

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

Climate models with volcanic Forcing (0-700 m) Domingues et al ThSL: Thermosteric sea level change (density changes induced by temperature change)

Global mean sea level (deviation from the 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)

Clouds: Still the Largest Source of Uncertainty

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

IPCC º C Increase in Surface Temperature Observations Predictions with Anthropogenic/Natural forcings Predictions with Natrual forcings

Projected Future Warming Figure 9.13, IPCC TAR

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 o C = 3.2 o F 2.8 o C = 5.0 o F 3.4 o C = 6.1 o F CO 2 Eq o C = 1.0 o F

CLIM 101: Weather, Climate and Global SocietyUncertainty

Feedback

Positive vs. Negative Feedback 1.Something triggers a small system change 2.The system responds to the change 3.Feedback Positive Feedback: The response accelerates the original change Negative Feedback: The response damps the original change

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

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

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

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

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

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

Feedbacks in the Biosphere 1.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)

Feedbacks in the Climate System 1.Water vapor feedback 2.Cloud-radiation feedback 3.Ice-albedo feedback 4.Climate-Carbon Cycle feedback

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

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

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

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

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? Understanding and Attributing Climate Change Center of Ocean-Land- Atmosphere studies

Definition: The ECS is the full equilibrium surface temperature response to a doubling of CO 2 Definition: The TCR is the surface temperature response at CO 2 doubling for a 1%/yr increase of CO 2 (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: Equilibrium Climate Sensitivity (ECS) and Transient Climate Response (TCR) Center of Ocean-Land- Atmosphere studies

J. Shukla, T. DelSole, M. Fennessy, J. Kinter and D. Paolino Geophys. Research Letters, 33, doi /2005GL025579, 2006 Climate Model Fidelity and Projections of Climate Change

THANK YOU! ANY QUESTIONS? Center of Ocean-Land- Atmosphere studies