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90
60
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040 80 1200 160
Mea
n an
nual
tem
pera
ture
(°F)
Mean annual precipitation (inches)
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To change phase, H2O molecules need energy to break bonds and escape from the liquid
Evaporation
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Hotter means more water vapor in atmosphere
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evaporation rates dependent not only on temperature but also wind speed
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Condensation
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Most condensation falls as rain
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Clouds need cloud condensation nuclei (CCNs)
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Nucleation
AutomobilesIndustryBiomass burning
cloud
Precursor gases:
Rainout
Evaporation
Growth
Dry deposition
(sulfur dioxide,nitrogen oxides)
Cloud uptake
Aerosol CyclesAerosol Cycles
[adapted from Jakob, 1999]
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Aerosols and Climate: Direct Effect
Direct Effect:
Scattering and
absorption by
particles
photo: SeaWifs website
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[Durkee et al., 2000]
Aerosol indirect effect
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Aerosol Indirect Effect
Anthropogenic aerosol example using ship emissions:
(Johnson et al., 1996)
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Cloud Formation in a Clean and Polluted Cloud Formation in a Clean and Polluted AtmosphereAtmosphere
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This cloud has only few cloud droplets, hence, reflects less sunlight (darker cloud).
This cloud has more cloud droplets, hence, reflects more sunlight (lighter cloud).
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Low Clouds
Very thick water clouds reflect large amounts of sunlight
Very near the surface, temperature of the cloud effectively the same as surface. Infrared radiation is therefore about the same - almost like the cloud wasn’t there!
NET EFFECT: Cooling
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High CloudsThin, cold ice clouds reflect less sunlight
Extremely cold, emits infrared at colder temperatures, prevents warmer surface infrared from escaping to space
NET EFFECT: Warming
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Surface temperature changes from 1750 to 1990Surface temperature changes from 1750 to 1990
Greenhouse gases only Aerosols only
Greenhouse gases and aerosols Global and annual mean changes:Greenhouse gases only: 1.7 °CAerosols only: -0.9 °CGreenhouse gases and aerosols: 0.6 °CObserved temperature increase over the last 140 years: 0.6 °C
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Surface temperature changesSurface temperature changes from 1750 to 1990from 1750 to 1990Aerosols only (°C)Aerosols only (°C)
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volatile orga
nic ca
rbon
molecules
CCN formation
Cloud Formation
Cloud Albedo
Precipitation/Water Stress
Surface Temperature
Hydrological Cycle
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• Elevated CO2 stimulates photosynthesis
• Trees grow faster in elevated CO2 and are bigger at the end of the experiment
• N concentrations are reduced• No large changes in structure• Stomatal conductance often is
lower
We know how trees respond to elevated CO2
[CO2]Photosynthesis
There is a wealth of data from many CO2 enrichment studies demonstrating physiological responses of seedlings and young trees
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