ATM S 111, Global Warming: Understanding the Forecast

1. ATM S 111, Global Warming: Understanding the Forecast Dargan M. W. Frierson Department of Atmospheric Sciences Day 4: 04/08/2010

2. Assignments • Should have read “Who’s Responsible” p.32-42 • For next class, read “Extreme Heat” p. 45-57 • HW #1 due Friday night 11:59 PM!!! • Enrollment key = “albedo” • John Seinfeld lecture tonight, 7:30 PM, Kane 110 • Atmospheric Aerosols: Unsung warriors in the battle against global warming • Today: • Finish up forcings and feedbacks • Classifying emissions of greenhouse gases

3. Climate Forcingsvs Climate Feedbacks • Climate forcings: • Things that change global temperatures directly • Climate feedbacks: • Things that respond to temperature changes (but themselves affect temperature too!)

4. Radiative Forcing • Radiative forcing is calculated as the changeinshortwavein or longwaveout due to the particular climate forcing • Positive radiative forcing = increased shortwave in or decreased longwave out • Positive radiative forcing  warming

5. Radiative Forcing of Greenhouse Gases • Several greenhouse gases have increased since preindustrial times • They increase the greenhouse effect and prevent longwave radiation from escaping to space • Radiative forcings of these gases: Carbon dioxide: 1.66 W/m2 Methane: 0.48 W/m2 Nitrous oxide:0.16 W/m2 CFCs: 0.32 W/m2 Total greenhouse gas radiative forcing: about 3 W/m2 (includes a few other gases too)

6. Shortwave Forcings • Changes in strength of the Sun, or things that reflect away solar radiation • Some changes have led to warming, some to cooling • Solar changes are relatively small • More soot (especially on top of snow) has led to warming • Cutting down forests to make farmland or pastures had led to cooling • Aerosols (small particles, especially from air pollution) have led to a large cooling

7. Air Pollution Aerosols • Air pollution particles block out sunlight too • Sulfates from dirty coal burning are particularly important (sulfate aerosols) • This is the same stuff that causes acid rain • These are a big effect • One of the main uncertainties in our understanding of climate

8. Aerosol “direct effect” Aerosol direct effect: haze (here from east coast) reflects sunlight back to space. Aerosol indirect effect is influence on cloud formation

9. Summary of Shortwave Climate Forcings • Radiative forcingsfor shortwave agents in current climate vs preindustrial • Remember CO2 radiative forcing is currently: 1.66 W/m2 • Solar radiation changes: 0.12 W/m2 • Land cover changes: -0.20 W/m2 • Soot on snow: 0.10 W/m2 • Aerosol direct effect: -0.50 W/m2 • Aerosol indirect effect (clouds): -0.70 W/m2 • All of the above shortwave forcings have significant scientific uncertainty associated with them though! • We just don’t know these values very accurately

10. Total Radiative Forcing • CO2: 1.66 W/m2 • Total GHG: about 3 W/m2 • Shortwave forcings: about -1.3 W/m2 • With significant scientific uncertainty here • Best guess of total forcing: 1.6 W/m2 • The Earth has been warming over the last 150 years • Not that hard to say that it’s due to greenhouse gases • Greenhouse gases have dominated the radiative forcing • We’ll discuss other methods of “attribution” later in the class • The patterns of warming also match that of GHG warming and not other causes

11. Local Aspects of Many Climate Forcings • CO2 is still the main problem • And it is global (essentially the same concentration everywhere) • Hence “global warming” is an appropriate name • Many of the other climate forcings are much more localized though • Soot on snow, land use, aerosols all tend to be localized • Hence “climate change” is a better term when including these

12. Next: 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 a forcing

13. 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

14. 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?

15. 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

16. 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

17. 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

18. Water Vapor Feedback • A warmer climate means a higher water vapor climate • No legitimate arguments out there that water vapor could act as a negative feedback • Some reasonable skeptics argue that the feedback might be only weakly positive • 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

19. Ice/Snow Feedbacks Can you think of a feedback involving ice/snow? Arctic sea ice floating on the ocean

20. Ice-Albedo Feedback Warming  ice melts  dark open ocean visible  more warming Similar feedback is present for snow (revealing darker land surfaces below)

21. Cloud Feedbacks • Cloud feedbacks are much more uncertain • Partially because clouds have both an albedo effect and a greenhouse effect High (thin) Clouds Warm Low (thick) Clouds Cool

22. 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?

23. Lapse Rate Feedback Main negative feedback: “lapse rate feedback” Upper atmosphere warms faster than lower atmosphere in climate models This is where longwave radiation to space comes from Funny skeptic argument: upper atmosphere wasn’t warming as fast as expected (resolved by Professor Qiang Fu and Celeste Johanson at UW): If it had been true, this would have meant we’d expect more warming! Height (km)

24. 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

25. Uncertainty in Feedbacks Since positive feedbacks combine, high sensitivity climates are hard to rule out (work of Prof. Roe, ESS) From 6,000 simulations randomly changing 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) Likelihood

26. Summary • Radiative forcing: key method to size up shortwave and longwave climate forcings • Longwaveforcings: greenhouse gases • Shortwave forcings: • Solar variations • Land use changes • Soot on snow • Aerosols: key uncertainty

27. 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