“what a climate model is, and what uncertainty means”

Noah S. Diffenbaugh

Department of Environmental Earth System Science and Woods Institute for the Environment

Stanford University

“what a climate model is”

General Circulation Model (“GCM”)

Australia Government BOMLaws of Physics!!!

Two basic parts:

-“Dynamics”- Atmospheric circulation- Ocean circulation

-“Physics”- Radiation- Clouds- Precipitation- Land surface fluxes- and more…..

Global Radiation Balance

From Marshall and Plumb, Elsevier, 2008

Te = [S0(1-ap) / 4σ] 1/4

Te = emission temp

S0 = solar “constant”

ap = planetary albedo

σ = Stefan-Boltzman constant

Water Vapor in Air

Santer et al., 2007

Observed and Modeled Water Vapor Content

• important for greenhouse effect• important for atmospheric circulation• important for extreme events

Atmosphere and Ocean Circulation

“and what uncertainty means”

Observed “Global Warming”

Keeling Curve

Global Warming Art

IPCC, 2007

• Observed warming does not occur without human forcings

“The Future”

From the perspective of climate model experiments

IPCC SRES Scenarios

Raupach et al., 2007

IPCC, 2007

• spread within each scenario

• spread between scenarios

Climate Feedbacks

Santer et al., 2007

http://internationalspacestationpictures.com/pictures-of-earth-from-space.php

NASA

Hawkins and Sutton, 2009

“Internal Variability”

“Model”

“Scenario”

Diffenbaugh and Scherer, 2011

Diffenbaugh and Scherer, 2011

Date of permanent exceedence of late-20th century maximum

Diffenbaugh and Ashfaq, 2010

U.S.A. Summer Soil-Moisture/Temperature/Precipitation

Coupling2030-2039 minus 1980-1999

CBS News

Joplin Tornado – May 22, 2011

Brooks et al. 2003

Severe Thunderstorm Environments

• Convective Available Potential Energy (CAPE)

Brooks et al., 2003

Pole-equator temperature gradient

Thermal Wind Equation

S6

• Vertical Wind Shear

Trapp et al., 2007

Trapp et al., PNAS, 2007

From the perspective of the climate system

Giorgi, Diffenbaugh, et al., 2008

ClimateChangeUncertainty

How do we make decisions when:

1. we don’t know the level of climate forcing in the future?

2. we don’t understand every process that could be important for climate change impacts?

3. The phenomena that matter most for climate change impacts may exceed the limits of predictability?