DARGAN M. W. FRIERSON DEPARTMENT 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  Article: 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? ClimateMinor …or contact Miriam Bertram at the UW Program on Climate Change or an advisor in Oceanography, Earth and Space Sciences, or Atmospheric

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/m 2 ) = 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 3 o 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 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, 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 CO 2 (we’ll likely get to this around 2050) with no feedbacks is around 1.5 o C  Models predict 3 o 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., 8 o C) are unlikely, but hard to rule out (on the other hand, small temperature changes like 1 o C are essentially impossible) Likelihood Most likely (3 o C) Can’t completely rule out though

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