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Weather, Climate & Society
ATMO 325
Climate Projections
What is Climate Change?
• Climate change - A significant shift in the mean
state and event frequency of the atmosphere.
• Climate change is a normal component of the
Earth’s natural variability.
• Climate change occurs on all time and space
scales.
• A plethora of evidence exists that indicates the
climate of the Earth has changed.
(Next Lecture)
What is Climate Change?
• If we can’t predict the weather more than 7-10
days in advance, why should I believe that we
can provide useful 100 year climate outlooks?
Weather forecasts attempt to predict specific
weather at specific times for point locations.
A climate “forecast” attempts to replicate
changes in the statistics of weather.
- Average temperature, rainfall, etc.
- Distribution of weather events
Causes of Climate Change
• Atmospheric Composition - Anything that
changes the radiative properties of the
atmosphere (carbon dioxide, clouds, aerosols).
• Astronomical - Anything that alters the amount
or distribution of solar energy intercepted by
the Earth (solar variations, orbital variations).
• Earth’s Surface - Anything that alters the flow
of energy at the Earth's surface or changes its
distribution (snow cover, continental drift).
Global Climate Model: Take 2
Let’s model the Earth system as a planetary surface
with an absorbing atmosphere above the surface.
S0/4
(S0/4) A
(1-) FOUTSFC
A = albedo - % solar
reflected to space
FOUTATM
(1-) emitted to space
 = emissivity - %
absorbed by air
F
IN
SFC
Atmosphere TATM
FOUTSFC
Surface TSFC
FOUTATM
Global Climate Model: Take 2
A planetary surface with an IR absorbing atmosphere
above the surface.
S0/4
(S0/4)A
(1-) FOUTSFC
(1-) emitted to space
 absorbed by air
 T4ATM
Atmosphere TATM
FOUTATM=  T4ATM
(1-A) S0/4
FOUTSFC=T4SFC
Surface TSFC
1-Layer Model Summary
1. An OLR absorbing atmosphere slows the net
energy flow out from surface (relative to no atm).
2. An increase in atmosphere’s absorptivity causes
surface T to increase.
3. Radiation reaching space from the Earth is a
combination of emission from a warm surface and
colder atmosphere.
It must be equal to (1-A)S0/4 at equilibrium.
Courtesy J. Thornton UW
Global Climate Model: Take 2
Simplifying Assumptions
1. The atmosphere absorbs only Outgoing Long
wave Radiation (no absorption of solar radiation)
2. The atmosphere absorbs the same fraction
of OLR at each wavelength.
3. The atmosphere has a uniform temperature.
4. Fin = Fout for each component and whole system.
F  T
4
Courtesy J. Thornton UW
Global Climate Model: Take 2
TSFC
 1 AS0 


4

1

2




1
4
By now, this is tattooed on your brain
Courtesy J. Thornton UW
Global Climate Model: Take 2
TSFC
 239.4 


4

1

2



1
4
If  ~ 0.75, then TSFC ~ 288 K
Courtesy J. Thornton UW
How can you warm the TSFC?
TSFC
 1 AS0 


4

1

2



1
4
1. Increase the solar output S0
2. Decrease the reflectivity A
3. Increase the absorptivity 
To increase TSFC by 1˚C…
TSFC
TSFC
TSFC
S0
A




4S0 4 1 A 4 2   
 S0 A  
 TSFC 

 
 5472 2.8 5 
1. Increase S0 by ~20 W/m2
2. Decrease A by ~1%
3. Increase absorptivity  by ~0.02
Can GHG increases explain warming?
IPCC Fig.
IPCC WG1 Fig. 6.10
Other changes are consistent with warming
IPCC SYR 1-1
• CO2 has increased from 290 ppm in 1900
to 380 ppm today
Evidence of Warming
0.6oC warming
past century
Ahrens, Fig 13.5
Temperatures have increased 0.6 K the past 100 years
Simple approach to GHG warming
If a 33% increase in CO2 could alone
produced a 0.6 K warming, would a
projected doubling of CO2 the next
100 years produce a 1.8 K warming?
Can it possibly be that simple?
Not Really
Climate Feedbacks
Feedbacks more than double the
response of the temperature to
increasing concentrations of
greenhouse gases
Closer look at climate system reveals
couplings between physical processes
Ice/Albedo Feedback
Surface
Temperature
Snow and Ice
Cover
( +)
Planetary
Albedo
Temperature/Water Vapor Feedback
Surface
Temperature
Atmospheric
H2O vapor
( +)
Greenhouse
Effect
Temperature/Low Clouds Feedback
Surface
Temperature
Atmospheric
H2O vapor
( -)
Planetary
Albedo
Low Clouds
Temperature/Infrared Flux Feedback
Surface
Temperature
(—)
Outgoing
IR Flux
Multiple feedbacks complicate the
response of the climate system
Positive and Negative Feedbacks
• Atmosphere has a numerous checks and balances that
counteract climate changes.
• All feedback mechanisms…
Operate simultaneously.
Work in both directions.
• The dominant effect is difficult to predict.
• Cause and effect is very difficult to prove at the
“beyond a shadow of a doubt” level.
Synthesis Fig i-1
Observed distribution of temperature
changes show warming over land
IPCC SYR 1-2
Many sources are responsible for the
increase in potent GHG’s
IPCC SYR 2-1
Industrialized sectors are the biggest
per capita contributors to GHG’s
IPCC SYR 2-2
But underdeveloped sectors are among the
biggest overall contributors per GDP
IPCC SYR Fig 2-2
Attribution of RF changes (1750-2005)
WG1 SPM.2
WG1 SPM.4
Integrated world, more ecol. friendly, 2050 pop max
Integrated world, non-fossil fuel emphasis, 2050 pop max
Divided world, more ecol. friendly, slower pop rise
Integrated world, balanced fuels, 2050 pop max
Divided world, rapid unchecked pop growth
Integrated world, fossil fuel emphasis, 2050 pop max
GHG
Scenarios
IPCC SYR 3-1
Divided world, rapid unchecked pop growth
Integrated world, balanced fuels, 2050 pop max
Integrated world, more ecol. friendly, 2050 pop max
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
WG1 SPM.5
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
WG1 SPM.5
Integrated world, more ecol. friendly, 2050 pop max
Integrated world, balanced fuels, 2050 pop max
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Divided world, rapid unchecked pop growth
WG1 SPM.6
B1
A1B
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
A2
WG1 SPM.6
Precipitation
Changes (%)
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WG1 SPM.7
Projections and model consistency of relative
changes in runoff by the end of the 21st century
IPCC SYR Fig. 3.5
“Some systems, sectors and regions
are likely to be especially affected
by climate change.”
• “particular ecosystems:
- terrestrial: tundra, boreal forest and mountain
regions because of sensitivity to warming;
Mediterranean-type ecosystems because of
reduction in rainfall; and tropical rainforests
where precipitation declines
- coastal: mangroves and salt marshes, due to
multiple stresses
- marine: coral reefs due to multiple stresses;
the sea-ice biome because of sensitivity to
warming”
“Some systems, sectors and regions
are likely to be especially affected
by climate change.”
• “water resources in some dry regions at midlatitudes and in the dry tropics, due to changes
in rainfall and evapotranspiration, and in areas
dependent on snow and ice melt”
• “agriculture in low latitudes, due to reduced
water availability”
• “low-lying coastal systems, due to threat of sea
level rise and increased risk from extreme
weather events”
• “human health in populations with low adaptive
capacity.”
“Uptake of anthropogenic carbon
since 1750 has led to the ocean
becoming more acidic…”
“Increasing atmospheric CO2 concentrations lead
to further acidification.”
“Projections based on SRES scenarios give a
reduction in average global surface ocean pH of
between 0.14 and 0.35 units over the 21st
century.”
“Progressive acidification of oceans is expected to
have negative impacts on marine shell-forming
organisms (e.g. corals) and their dependent
species.”
IPCC SYR Fig. 3.6
Extreme Weather Events
“Altered frequencies and intensities of extreme
weather, together with sea level rise, are
expected to have mostly adverse effects on
natural and human systems.”
IPCC Synthesis Report
Increases in the frequency and/or the intensity of
extreme weather events is a likely consequence of
global warming.
GFDL Super Typhoon
Hurricane Intensities
Increase by 10%
Reason: Warmer Sea
Surface Temps
www.gfdl.noaa.gov
99%
90%
90%
66%
66%
66%
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
“Anthropogenic warming could lead to some
impacts that are abrupt or irreversible,
depending upon the rate and magnitude of
the climate change.
“Partial loss of ice sheets.”
“Rapid sea level rise on century time scales cannot
be excluded.”
5 in 10 chance
“There is medium confidence that approximately
20 to 30% of species assessed so far are likely
to be at increased risk of extinction if
increases in global average warming exceed 1.5
to 2.5｡C (relative to 1980-1999). As global
average temperature increase exceeds about
3.5｡C, model projections suggest significant
extinctions (40 to 70% of species assessed)
around the globe.”
“Anthropogenic warming could lead to some
impacts that are abrupt or irreversible,
depending upon the rate and magnitude of
the climate change.
90% chance
“It is very likely that the meridional overturning
circulation (MOC) of the Atlantic Ocean will
slow down during the 21st century.
“Impacts of large-scale and persistent changes in
the MOC are likely to include changes in marine
ecosystem productivity, fisheries, ocean CO2
uptake, oceanic oxygen concentrations and
terrestrial vegetation. Changes in terrestrial
and ocean CO2 uptake may feed back on the
climate system.”
Key Points: CO2 Warming
• CO2 levels are rising and will likely double by 2070.
• The greenhouse relationship between higher CO2
levels and warmer temperatures is indisputable.
• Even with perfect knowledge of future CO2 levels,
there is significant uncertainty about how much
warming would occur and how fast it would occur.
• Model results suggest ~2oC global warming, with
strongest warming in polar regions, and an overall
increase in global precipitation.
• Shifts in precipitation are much more uncertain,
as are the consequences on water resources.
IPCC Reports 2007
Intergovernmental Panel on
Climate Change
A consensus document of
of a wide sampling of the
scientific community
Available for download at
http://www.ipcc.ch/
“Most of the observed increase in
globally averaged temperatures
since the mid-20th century is very
likely due to the observed
increase in anthropogenic
greenhouse gas concentrations” -2007