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 (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

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