DARGAN M. W. FRIERSON DEPARTMENT OF ATMOSPHERIC SCIENCES DAY 3: 10/08/2015 ATM S 111, Global Warming: Understanding the Forecast
Jan 21, 2016
DARGAN M. W. FRIERSONDEPARTMENT OF ATMOSPHERIC SCIENCES
DAY 3 : 10 /08 /2015
ATM S 111, Global Warming: Understanding the Forecast
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 ([email protected]), 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
positiveWhy? 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 aswater 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
Convective clouds growing over Tiger Mountain (Prof. Dale Durran)
Clouds happen when humid air cools(often due to rising motion)
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, MexicoThese cause cooling (not much greenhouse effect)Key question: will these expand or contractIn 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)
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)
Lik
elih
ood
Most likely (3o C)
Can’t completely rule out though
Summary
Radiative forcing: key method to size up shortwave and longwave climate forcings
Longwave forcings: greenhouse gasesShortwave 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