Mike Warner
Department of Atmospheric Sciences
Day 1: June 25, 2013
ATM S 111, Global Warming:
Understanding the Forecast
Last time…
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Shortwave radiation
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Radiation from the Sun
Mostly visible light
Mostly gets absorbed at the surface or reflected back to space
Longwave radiation
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Radiation emitted by the Earth
Infrared (invisible)
Greenhouse effect makes it harder for longwave radiation to
escape to space
Outline
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Today: How a tiny fraction of the atmosphere controls its
temperature
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What are the main greenhouse gases?
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And which are changed by human activity?
Connections to the ozone depletion problem
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Ozone itself and the chemicals that caused it
Atmospheric composition
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Our atmosphere is mostly
nitrogen (N2, 78%),
oxygen (O2, 21%),
and argon (Ar, 0.9%)
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But these are not greenhouse gases
Molecules with 1 atom or 2 of
the same atoms only aren’t greenhouse gases
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The primary gases in our atmosphere are thus transparent to
longwave radiation
Greenhouse Gases
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Polyatomic molecules are greenhouse gases
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Water vapor (H2O)
Carbon dioxide (CO2)
Methane (CH4)
Nitrous oxide (N2O)
Ozone (O3)
Chlorofluorocarbons (the
ozone depleting chemicals which
have been banned)
The fact that they can rotate and vibrate means
they can absorb the right frequencies of longwave
Greenhouse Gases
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All greenhouse gases are a rather small fraction of the
atmosphere!
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“Trace gases” have a remarkable effect on the
atmosphere
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Water vapor has the highest concentration: 0.4%
CO2: 0.04%
Methane: 0.0002%
E.g., ozone is less than 0.00001% of the atmosphere, but
absorbs essentially all harmful UV-B and UV-C radiation
Let’s discuss each gas separately
Water Vapor
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Gas form of water
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AKA humidity
Not the same as clouds – clouds are tiny droplets of water
suspended in air
The number one greenhouse gas!
Not controlled by humans!
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It’s a feedback not a forcing (topic of the next lecture)
Observed to be increasing with global warming
Carbon Dioxide
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CO2
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The primary contributor to the anthropogenic (humancaused) greenhouse effect
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It’s what we breathe out, what plants breathe in
Over 60% of the anthropogenic greenhouse effect so far
Increases primarily due to
fossil fuel burning (80%)
and deforestation (20%)
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Preindustrial value: 280 ppm
Current value: 399 ppm
Carbon Dioxide
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CO2 will also be the main problem in the future
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It’s extremely long-lived in the atmosphere
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Around 50% of what we emit quickly gets taken up by the
ocean or land
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We’ll discuss this more later
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Most of the rest sticks around for over 100 years
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Some of what we emit will still be in the atmosphere over
1000 years from now!
Methane
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CH4
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Much more potent on a per molecule basis than CO2
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Only 1.7 ppm though – much smaller concentration than CO2
Natural sources from marshes (swamp gas) and other
wetlands
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Natural gas like in stoves/heating systems
Video of methane release from tundra
lakes in Alaska & Siberia
Increases anthropogenically due
to farm animals (cow burps),
landfills, coal mining, gas leakage,
rice farming
Methane
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The lifetime of CH4 is significantly shorter than carbon
dioxide
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Breaks down in the atmosphere in chemical reactions
Lifetime of methane is only 8 years
Methane leveled off for a few years
(droughts in high latitude wetlands?)
Starting to rise again though?
1984
2012
Global Warming Potential
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CO2 lifetime > 100 years
Methane lifetime = 8 years
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But methane is a much stronger greenhouse gas
How to put these on similar terms? Global warming
potential (GWP)
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Global warming potential is how much greenhouse effect
emissions of a given gas causes over a fixed amount of time
(usually 100 years)
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Measured relative to CO2 (so CO2 = 1)
Methane’s global warming potential is 25
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Much more potent than CO2 even though it doesn’t stay as long
Nitrous Oxide
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N2O
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Also more potent on a per molecule basis than CO2
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Laughing gas
Global warming potential: 310
Comes from agriculture, chemical industry, deforestation
Small concentrations of
only 0.3 ppm
Ozone (O3)
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Don’t confuse ozone with global warming!
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The ozone depletion problem is essentially solved
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And the ozone layer will be fully recovered in 50 years or so
Ozone (O3)
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Don’t confuse ozone with O-Zone
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Ozone: O3, gas in the atmosphere
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O-Zone: Moldovan pop band
Ozone
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Ozone (O3) occurs in two places in the atmosphere
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In the ozone layer very high up
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Near the Earth’s surface
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This is “good ozone” which protects us from ultraviolet radiation & skin
cancer
“Bad ozone”: caused by air pollution
Bad ozone is a greenhouse gas, and is more potent on a per
molecule basis than CO2
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But it’s very very short-lived
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Global warming potential for bad ozone is wrapped into the other gases
which lead to its chemical creation
CFCs
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CFCs or chlorofluorocarbons are the ozone depleting
chemicals
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CFCs are strong greenhouse gases
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Have been almost entirely phased out
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
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Contributions to the natural greenhouse effect:
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H2O (water vapor): 60%
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CO2 (carbon dioxide): 26%
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All others: 14%
The Unnatural Greenhouse Effect
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Increasing levels of CO2 and other greenhouse gases leads
to a stronger greenhouse effect
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With more greenhouse gases, it becomes harder for outgoing
radiation to escape to space
It’s like this picture from
before, but more…
The Unnatural Greenhouse Effect
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Contributors to the “anthropogenic” greenhouse effect
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Numbers for the whole world up to this point:
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Carbon dioxide: 63%
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Methane: 18%
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CFCs, HFCs: 12%
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Nitrous oxide: 6%
The Anthropogenic Greenhouse Effect
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Contributors to the “anthropogenic” greenhouse effect
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Numbers for the USA
based on current (2008)
emissions
HFC/CFCs are much lower than on
previous slide (no more CFC emissions)
CO2 is the big problem in the US now
From US EPA 2010 report (draft)
Summary
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Greenhouse gases:
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Number one is water vapor
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Number two is CO2
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Also methane, nitrous oxide, CFCs
Global warming potential: way to compare different
greenhouse gases to CO2
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Many other GHGs are stronger, but CO2 remains the biggest
problem