Understanding uncertainties and feedbacks

Jagadish ShuklaJagadish Shukla

CLIM 101: Weather, Climate and Global Society

Lecture 15: 22 Oct, 2009

Reading for Week 8Reading for Week 8Lecture 15Lecture 15

Understanding uncertainties and feedbacks

• GW Chapter 3, 5

CLIM 101: Weather, Climate and Global Society

Uncertainty Uncertainty and Feedbackand 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 = 1.01Slope = 1.82

Slope = 1.02Slope = 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 ForcingClimate models without volcanic Forcing

Domingues et al. 2008

ThSL: Thermosteric sea level change(density changes induced by temperature change)

OHC - ocean heat content

Climate models withClimate models with volcanic Forcingvolcanic Forcing(0-700 m)

Domingues et al. 2008

ThSL: Thermosteric sea level change(density changes induced by temperature change)

Global mean sea level (deviation from the 1980-1999 mean)

Uncertainty in estimated long-term rate of sea-level change

Based on tide gaugesBased 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. PaolinoGeophys. Research Letters, 33, doi10.1029/2005GL025579, 2006

Climate Model Fidelity and Projections of Climate ChangeClimate Model Fidelity and Projections of Climate Change

IPCC 2007

1.0º C1.0º C

Increase in Surface TemperatureIncrease in Surface Temperature

ObservationsPredictions with Anthropogenic/Natural forcingsPredictions with Natrual forcings

Projected Future Warming 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 2100.

1.8oC = 3.2oF

2.8oC = 5.0oF

3.4oC = 6.1oF

CO2 Eq

850

600

4000.6oC = 1.0oF

CLIM 101: Weather, Climate and Global Society

Uncertainty Uncertainty

CLIM 101: Weather, Climate and Global Society

FeedbackFeedback

Positive vs. Negative Positive vs. Negative FeedbackFeedback

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

Tem

per

atu

re

If no feedbacks present

With positive feedbacks

Effect of Positive Effect of Positive Feedback (1)Feedback (1)

Effect of Positive Feedback Effect of Positive Feedback (2)(2)

Time

Tem

per

atu

re

If no feedbacks present

With positive feedbacks

The Need for Negative The Need for Negative FeedbacksFeedbacks

• Positive feedbacks are destabilizing - they tend to drive the system away from equilibrium

• Negative feedbacks are required to restore equilibrium

A System Without Negative A System Without Negative FeedbacksFeedbacks

Time

Tem

per

atu

re

Catastrophic Warming!

Example “Runaway Greenhouse Effect”, T H2O T

The Way Physical Systems Usually BehaveThe Way Physical Systems Usually Behave

Time

Tem

per

atu

re Warming Accelerating

Warming Decelerating

Feedbacks - SummaryFeedbacks - 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 Feedbacks in the BiosphereBiosphere

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 Feedbacks in the Climate SystemSystem

1.Water vapor feedback

2.Cloud-radiation feedback

3.Ice-albedo feedback

4.Climate-Carbon Cycle feedback

Ice-Albedo Feedback Ice-Albedo Feedback (1)(1)

Cooling

Albedo Increases

Absorption of sunlight

decreases

Ice Increases

Ice-Albedo Feedback Ice-Albedo Feedback (2)(2)

Warming

Albedo Decreases

Absorption of sunlight

increases

Ice Decreases

Water Vapor Feedback Water Vapor Feedback (1)(1)

Warming

Evaporation from the Oceans Increases

Atmospheric Water Vapor Increases

Stronger Greenhouse Effect

Water Vapor Feedback Water Vapor Feedback (2)(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 ChangeUnderstanding and Attributing Climate Change

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:

Equilibrium Climate Sensitivity (ECS) and Equilibrium Climate Sensitivity (ECS) and Transient Climate Response (TCR)Transient Climate Response (TCR)

Center of Ocean-Land-Atmosphere studies

Center of Ocean-Land-Atmosphere studies

J. Shukla, T. DelSole, M. Fennessy, J. Kinter and D. PaolinoGeophys. Research Letters, 33, doi10.1029/2005GL025579, 2006

Climate Model Fidelity and Projections of Climate ChangeClimate Model Fidelity and Projections of Climate Change

THANK YOU!

ANY QUESTIONS?

Center of Ocean-Land-Atmosphere studies