ATM S 111, Global Warming:
Understanding the Forecast
DARGAN M. W. FRIERSON
DEPARTMENT OF ATMOSPHERIC SCIENCES
DAY 3: 10/08/2015
News
 Extra credit due today at 4 PM
 Submit to me via e-mail
 Article:
http://mashable.com/2015/10/05/south-carolina-floods-global-warming/
 HW due tomorrow at 11:59 PM
 Don’t forget! No make-ups
 HW 2 posted today (due next Friday)
 California enacts strict climate change target
 50% renewable energy by 2030
 Double energy efficiency of buildings by 2030
 Oil use measure failed (50% reduction in use by 2030)
New! For UW science majors:
Are you interested in better understanding Earth’s climate system and the
latest research in climate science?
The Climate Minor aims to give undergraduates a strong
interdisciplinary foundation in climate science with
opportunities to explore policy, energy, and human
dimensions of climate change and will help prepare
students for graduate study in climate related fields.
Climate Minor Components:
•
2 core classes covering foundational
climate science and quantitative methods
(6-10 credits)
•
12 elective credits covering Climate
Chemistry and Biology, the Physical
Climate, and Past Climate
•
One optional policy elective
•
Integrative Capstone Experience: learn
about ongoing research in climate through
a seminar and discussion section for
undergraduates.
It is helpful to take MATH 124 and PHYS 121,
but it is possible to complete the Climate
Minor without!
Questions?
Want more information?
http://www.uwpcc.washington.edu/
ClimateMinor
…or contact Miriam Bertram at the UW
Program on Climate Change
(uwpcc@uw.edu), or an advisor in
Oceanography, Earth and Space Sciences,
or Atmospheric Science.
Next topic: Climate Feedbacks
 Things that change when the climate gets warmer or
colder

We’ll discuss the following:
Water vapor feedback
 Ice-albedo feedback
 Cloud feedbacks

 Feedbacks are of critical importance in determining
temperature response to climate forcings

Positive feedbacks are things that amplify warming
Climate Sensitivity
 Global warming theory:
= common symbol indicating the change in a quantity
= change in temperature (in degrees C)
= radiative forcing (in W/m2)
= climate sensitivity
Climate Sensitivity
 Lots of positive feedbacks means a very sensitive
climate (large

)
Large change in temperature for even a small forcing
 Lots of negative feedbacks means small
 What are the main climate feedbacks?
 And are they positive or negative?
Water Vapor Feedback
 Water vapor feedback
 Remember water vapor is the number one greenhouse gas
 This feedback is very confidently expected to be positive
 Why? Because warmer air can hold more
moisture
Water Vapor Content
 Winters are much drier than summers
 Simply because cold temperatures means small water vapor
content
January surface water vapor content
July surface water vapor content
Water Vapor Feedback
 Basic idea:
 A warmer climate means a higher water vapor climate

20% more humid climate with 3o C temperature increase
 As with all feedbacks, water vapor doesn’t care what
the forcing is that caused the warming

Any kind of warming will result in an increase in water vapor
content
Water Vapor Feedback
 A warmer climate means a higher water vapor
climate
 Scientific uncertainty about this?

Some reasonable skeptics argue that the feedback might be
relatively weak


Arguments focus on how upper atmospheric water vapor might
change
Observations show evidence for a strong positive feedback

Water vapor increases/decreases right along with global
temperatures
Ice-Albedo Feedback
 Warming  ice melting  dark open ocean visible 
more warming
 Similar feedback is present for snow (revealing darker
land surfaces below)
Very important for
local Arctic temperatures
Not nearly as strong as
water vapor feedback
in global importance
Cloud Feedbacks
 Clouds: suspended liquid water droplets or ice
crystals in air


Don’t confuse clouds (liquid or solid) with water vapor (a gas)
Essentially, if you can see it, it’s a liquid/solid
Clouds happen when humid air cools
(often due to rising motion)
Convective clouds growing over Tiger Mountain (Prof. Dale Durran)
UFO clouds!
(actually lenticular clouds,
formed from lee waves
downwind of mountains)
Cloud Feedbacks
 Cloud feedbacks are much more uncertain than
water vapor or ice feedbacks

Partially because clouds have both an albedo effect and a
greenhouse effect
Albedo effect: clouds reflect a lot of
shortwave radiation
Greenhouse effect is strong even for
thin clouds
Low level clouds off Guadalupe, Mexico
These cause cooling (not much
greenhouse effect)
Key question: will these expand or contract
In area with warming?
Cloud Feedbacks
 Cloud feedbacks lead to the largest uncertainty in
global warming forecasts


More low clouds could lead to less warming than predicted
However, roughly equally likely, less low clouds could lead to
significantly more warming…
 Uncertainty: a reason not to act or to act quickly?
Feedbacks and Climate Sensitivity
 Climate models say that expected warming is
approximately double that expected with no
feedbacks


Warming response to doubling CO2 (we’ll likely get to this
around 2050) with no feedbacks is around 1.5o C
Models predict 3o C average response to warming with all
feedbacks acting
 There’s some uncertainty in the feedbacks though
 And it’s hard to rule out high sensitivity climates
Uncertainty in Feedbacks
 Since positive feedbacks combine, high sensitivity
climates are hard to rule out (work of Prof. Roe, ESS)
Can’t completely
rule out though
Likelihood
Most likely (3o C)
From 6,000 doubled CO2
simulations, randomly
changing climate model
parameters
Very high temperature
changes (e.g., 8o C) are
unlikely, but hard to rule
out (on the other hand, small
temperature changes like
1o C are essentially impossible)
Summary
 Radiative forcing: key method to size up
shortwave and longwave climate forcings
 Longwave forcings: greenhouse gases
 Shortwave forcings:




Solar variations
Land use changes
Soot on snow
Aerosols: key uncertainty
Summary
 Feedbacks:
 Water vapor feedback is positive
 Ice-albedo feedback is positive
 Cloud feedback is another key uncertainty
 High sensitivity climates are hard to rule out