DARGAN M. W. FRIERSON DEPARTMENT 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 “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 “Who’s Responsible” for Thursday Also 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 0 o F (-18 o C) Missing piece: the greenhouse effect  All longwave radiation doesn’t escape directly to space -18 o C (0 o 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 15 o C (59 o 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 (N 2, 78%), oxygen (O 2, 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 (H 2 O)  Carbon dioxide (CO 2 )  Methane (CH 4 )  Nitrous oxide (N 2 O)  Ozone (O 3 )  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 air The number one greenhouse gas!  Powerful because there’s a lot of it Not controlled by humans!  It’s a feedback not a forcing (next topic)  Observed to be increasing with global warming

Carbon Dioxide CO 2: 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 CH 4  Natural gas like in stoves/heating systems Much more potent on a per molecule basis than CO 2  Only 1.7 ppm though – much smaller concentration than CO 2 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 CH 4 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 wetlands at high latitudes

Global Warming Potential CO 2 lifetime > 100 years Methane lifetime = 8 years  But methane is a much stronger greenhouse gas How 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 CO 2 (so CO 2 = 1)  Methane’s global warming potential is 25  Much more potent than CO 2 (25 times more powerful) even though it doesn’t stay as long

Nitrous Oxide N 2 O  Laughing gas Also more potent on a per molecule basis than CO 2  Global warming potential: 310 Comes from agriculture, chemical industry, deforestation Small concentrations of only 0.3 ppm

Ozone (O 3 ) 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 (O 3 ) Don’t confuse ozone with O-Zone  Ozone: O 3, gas in the atmosphere  O-Zone: Moldovan pop band

Ozone Ozone (O 3 ) 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 CO 2  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 though Global warming potentials of up to 11,000!

The Natural Greenhouse Effect Contributions to the natural greenhouse effect:  H 2 O (water vapor): 60%  CO 2 (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 CO 2 and other greenhouse gases leads to a stronger greenhouse effect  With more greenhouse gases, it becomes harder for outgoing radiation to escape to space It’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 CO 2 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 CO 2  Global warming potential: total warming caused over a fixed time period  Methane, nitrous oxide, & HFCs are also current concerns with global warming