Transcript Lecture 16 - University of California, Irvine

Lecture 16
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Observations of climate change
Feedback mechanisms
Air pollution
The stratospheric ozone hole
Changing land surfaces
Greenhouse gases and global warming
– Global warming and H2O
– Climate modeling
• Climate change assessments
Global average surface temperature
+ive/-ive feedback mechanisms
• Feedbacks occur when one change leads
to some other change that can act to
reinforce or inhibit the original change
• Reinforcing, +ive feedback, e.g. water
vapor feedback, ice/albedo feedback
• Inhibiting, –ive feedback. Example CO2
and photosynthesis – end result is
equilibrium
Ice albedo temperature feedback
Air pollution (aerosols and gases that are
harmful to life)
• Natural sources e.g., from volcanoes, soils
• Anthropogenic sources
– Carbon monoxide
– Lead
– Sulfur dioxide, sulfur trioxide (+H2O)
– Sulfuric acid
– Nitric oxide, nitrogen dioxide
– hydrocarbons, (volatile organic compounds)
– ozone
The stratospheric ozone hole
• Ozone produced photochemically in the
stratosphere
• Ozone plays a fundamental role in the
radiation budget and dynamics of life
• Minimum amount of ozone is observed in
spring in SH (October). Why?
• Discussed ozone depletion as a result of
release of CFCs.
• The winter atmosphere above Antarctica is
cold, -90C.
• Strong temperature gradient from pole to
mid-latitudes. From thermal wind
arguments, this results in strong westerly
winds encircling the South Pole, so called
polar vortex
• Results in isolation of SP air, keeping it
very cold and chemically isolated
• Polar stratospheric clouds can form
• Add sunlight and the result is rapid
destruction of ozone
Total column ozone over Halley Bay station
Notice
change
after 1975
Oct monthly mean ozone concentration
’87, ’89, ’90, ‘91
Why not a NH ozone hole?
• The NH polar vortex is continually being
bombarded with Rossby waves that have
propagated from the troposphere in the
NH. This disturbs the vortex and does not
allow the same chemical isolation (nor as
strong a vortex) as occurs in the SH.
Stratospheric polar vortex
WMO Scientific assessments of ozone
depletion: 2002, 1998, ’94, ‘91, ‘89, ‘85
Life cycle of CFCs: Cl and Br compounds in
the atmosphere cause the ozone depletion
over Antarctica
Changing land surfaces
• Desertification: spreading of a desert
region because of a combination of
climate change and human impacts on the
land. Examples: overgrazing,
deforestation without reforestation,
diversion of water away from a formerly
fertile region, farming on land with
unsuitable terrain or soil
• The Sahel or sub-Sahara (14-18N)
• Partly due to natural variability – depends
on the northernmost location of the ITCZ
for rain. Shifts by 100 km from year to year
• Enhanced by biogeophysical +ive feedback
mechanism
The incredible shrinking Aral Sea
1973
1987
2000
Urban heat island effect
Notice the warmth of Pittsburgh compared to
the surrounding rural area
Greenhouse gases and global warming
• H2O, CO2, CH4, CFCs are all gh gases
• Humans have changed the amount of gh
gases in the atmosphere over the past
century by 25%
• Based on the radiation concepts of the
greenhouse effect, temperature should
have increased a lot too. Hard to quantify
• Complex system
Global warming and atmospheric water
• Human activities add little H2O to the
atmosphere directly
• Saturation water pressure increases very
rapidly with temperature
• Since H2O is a strong gh gas this will lead
to still warmer temperatures, still more
water vapor etc
• On the other hand cloudiness may also
increase, which would be a cooling effect
Global warming and atmospheric H2O,
continued
• More aerosols can mean more CCN, more
droplets can form in clouds and the clouds
may reflect more
• Indirect aerosol effect on climate
• Contrails are airplane-induced clouds
Stratus clouds off California, ship tracks can
be seen in the clouds since their reflectance
is enhanced by increased aerosols
Climate modeling
• GCMs, global climate models consist of an
atmospheric model, coupled to an ocean
model, coupled to a sea ice m., land
surface model
• Give statistical estimates of future
conditions
• Sensitivity studies to understand
processes
Schematic of atmospheric processes
included in a GCM