DARGAN M. W. FRIERSONDEPARTMENT OF ATMOSPHERIC SCIENCES

DAY 4 : 04 /08 /2010

ATM S 111, Global Warming: Understanding the Forecast

Assignments

Should have read “Who’s Responsible” p.32-42

For next class, read “Extreme Heat” p. 45-57HW #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 warmingToday:

Finish up forcings and feedbacks Classifying emissions of greenhouse gases

Climate Forcings vs Climate Feedbacks

Climate forcings: Things that change global temperatures directly

Climate feedbacks: Things that respond to temperature changes (but

themselves affect temperature too!)

Radiative Forcing

Radiative forcing is calculated as the change in shortwave in or longwave out due to the particular climate forcing Positive radiative forcing = increased shortwave in or

decreased longwave out Positive radiative forcing warming

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 spaceRadiative 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)

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

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

Aerosol “direct effect”

Aerosol direct effect: haze (here from east coast)

reflects sunlight back to space.

Aerosol indirect effect is influence on cloud

formation

Summary of Shortwave Climate Forcings

Radiative forcings for 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

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 hereBest 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

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

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

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 increaseAs 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

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

Ice/Snow Feedbacks

Can you think of a feedback involving ice/snow?

Arctic sea ice floating on the ocean

Ice-Albedo Feedback

Warming ice melts dark open ocean visible more warming

Similar feedback is present for snow (revealing darker land surfaces below)

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

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?

Lapse Rate Feedback

Main negative feedback: “lapse rate feedback”

Height (km

)

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 asexpected (resolved by Professor Qiang Fu and Celeste Johanson at UW): If it had been true, this would have meant we’d expect more warming!

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 simulations randomly changing modelparameters

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

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