DARGAN M. W. FRIERSONDEPARTMENT OF ATMOSPHERIC SCIENCES

DAY 3 : 04 /06 /2010

ATM S 111, Global Warming: Understanding

the Forecast

Reading Assignments

From last time: Make sure you’ve read Rough Guide p. 20-31 “The

Greenhouse Effect” The Big Picture The Outlook What Can We Do About It

Also class notes from this section should be reviewed carefully

Next reading assignment: Rough Guide p.32-42 “Who’s Responsible” for

ThursdayAlso remember HW #1 is due Friday night

Outline of This Lecture

Review of: How the greenhouse effect works

What are the main greenhouse gases? And which are changed by human activity?

Forcings and feedbacks Forcings due to different gases Positive feedbacks which lead to more warming Negative feedbacks which lead to less warming

Atmosphere with No Greenhouse Effect

If there was no atmosphere, longwave radiation from the surface would escape directly to space & Earth’s temperature would be 0o F (-18o C)

Missing piece: the greenhouse effect All longwave radiation doesn’t escape directly to

space

-18o C (0o F)

The Greenhouse Effect

Greenhouse gases block longwave radiation from escaping directly to space The extra downward longwave radiation from above

warms the surface

Extra downward radiation due to greenhouse gases

15o C (59o F)

The Greenhouse Effect

Greenhouse effect is intuitive if you pay attention to the weather! Cloudy nights cool less quickly

In the desert, temperatures plunge at night! No clouds & little water vapor in the desert: little

greenhouse effect

Gases in the Atmosphere

The main constituents of our atmosphere are not greenhouse gases Nitrogen (N2, 78%), oxygen (O2, 21%), argon (Ar,

0.9%) don’t contribute to the greenhouse effect“Trace gases” are important in the

atmosphere! Ozone makes up less than 1 out of every million

molecules, but absorbs nearly all the harmful UV radiation from reaching Earth

Greenhouse gases are a small fraction of the atmosphere

That small amounts can make a big difference is somewhat intuitive too – just a little bit of dust can block out the Sun…

The Main Greenhouse Gases

The main greenhouse gases are: Water vapor (H2O) Carbon dioxide (CO2) Methane (CH4) Nitrous oxide (N2O) Ozone (O3) Chlorofluorocarbons (the ozone depleting chemicals

which have been banned) and HFCs (their replacements)

We’ll discuss these individually…

Water Vapor

Gas form of water AKA humidity Not the same as clouds – clouds are tiny droplets of

water suspended in airThe number one greenhouse gas!

Powerful because there’s a lot of itNot controlled by humans!

It’s a feedback not a forcing (next topic) Observed to be increasing with global warming

Carbon Dioxide

CO2: The primary contributor to the anthropogenic greenhouse effect (63% so far)

Increases primarily due to fossil fuels (80%) and deforestation (20%) Preindustrial value: 280 ppm Current value: 386 ppm

Very long-lived 100 year lifetime in the

atmosphere Some emissions stay in the

atmosphere for thousands of years

Methane

CH4

Natural gas like in stoves/heating systems

Much more potent on a per molecule basis than CO2

Only 1.7 ppm though – much smaller concentration than CO2

Natural sources from marshes (swamp gas) and other wetlands

Increases anthropogenically due to farm animals (cow burps), landfills, natural gas leakage, rice farming

Methane

The lifetime of CH4 is significantly shorter than carbon dioxide Breaks down in the atmosphere in chemical reactions Lifetime of methane is only 8 years

Methane concentrations have been leveling off in recent years, possibly due to drought in wetlandsat high latitudes

Global Warming Potential

CO2 lifetime > 100 yearsMethane lifetime = 8 years

But methane is a much stronger greenhouse gasHow to put these on similar terms? Global

warming potential (or GWP) Global warming potential is how much greenhouse

effect emissions of a given gas causes over a fixed amount of time (usually 100 years) Measured relative to CO2 (so CO2 = 1)

Methane’s global warming potential is 25 Much more potent than CO2 (25 times more powerful)

even though it doesn’t stay as long

Nitrous Oxide

N2O Laughing gas

Also more potent on a per molecule basis than CO2

Global warming potential: 310Comes from agriculture, chemical industry,

deforestationSmall concentrations of

only 0.3 ppm

Ozone (O3)

Don’t confuse ozone with global warming! The ozone depletion problem is essentially solved

And the ozone layer will be fully recovered in 50 years or so

Ozone (O3)

Don’t confuse ozone with O-Zone Ozone: O3, gas in the atmosphere O-Zone: Moldovan pop band

Ozone

Ozone (O3) occurs in two places in the atmosphere In the ozone layer very high up

This is “good ozone” which protects us from ultraviolet radiation & skin cancer

Near the Earth’s surface “Bad ozone”: caused by air pollution

Bad ozone is a greenhouse gas, and is more potent on a per molecule basis than CO2

But it’s very very short-lived Fun fact: Global warming potential for ozone is not usually

calculated – rather it’s wrapped into the GWPs of the other gases that lead to its chemical creation

CFCs

CFCs or chlorofluorocarbons are the ozone depleting chemicals Have been almost entirely phased out

CFCs are strong greenhouse gases Their reduction likely saved significant global

warming in addition to the ozone layer!Some replacements for CFCs (called HFCs)

are strong greenhouse gases thoughGlobal warmingpotentials of up to 11,000!

The Natural Greenhouse Effect

Contributions to the natural greenhouse effect: H2O (water vapor): 60% CO2 (carbon dioxide): 26% All others: 14%

These numbers are computed with a very accurate radiation model First running with all substances, then removing each

individual gas

The Unnatural Greenhouse Effect

Increasing levels of CO2 and other greenhouse gases leads to a stronger greenhouse effect With more greenhouse gases, it becomes harder for

outgoing radiation to escape to spaceIt’s like this same picture from before, but more.

More radiation is trapped before it gets out to space.

Longwave radiation is emitted from a higher (and colder) level on average.

The Unnatural Greenhouse Effect

Contributors to the “anthropogenic” greenhouse effect Numbers for the whole world up to this point:

Carbon dioxide: 63% Methane: 18% CFCs, HFCs: 12% Nitrous oxide: 6%

The Anthropogenic Greenhouse Effect

Contributors to the “anthropogenic” greenhouse effect based on current US emissions Numbers for the US

based on current (2008)emissions

CO2 is the big problem in the US currently.

Note how much lower the HFCs are than on the previous slide. This is b/c we basically don’t emit CFCs any more.

From US EPA 2010 report (draft)

Summary

Greenhouse gases: Number one is water vapor Number two is CO2

Global warming potential: total warming caused over a fixed time period

Methane, nitrous oxide, & HFCs are also current concerns with global warming