90

60

30

040 80 1200 160

Mea

n an

nual

tem

pera

ture

(°F)

Mean annual precipitation (inches)

To change phase, H2O molecules need energy to break bonds and escape from the liquid

Evaporation

Hotter means more water vapor in atmosphere

evaporation rates dependent not only on temperature but also wind speed

Condensation

Most condensation falls as rain

Clouds need cloud condensation nuclei (CCNs)

Nucleation

AutomobilesIndustryBiomass burning

cloud

Precursor gases:

Rainout

Evaporation

Growth

Dry deposition

(sulfur dioxide,nitrogen oxides)

Cloud uptake

Aerosol CyclesAerosol Cycles

[adapted from Jakob, 1999]

Aerosols and Climate: Direct Effect

Direct Effect:

Scattering and

absorption by

particles

photo: SeaWifs website

[Durkee et al., 2000]

Aerosol indirect effect

Aerosol Indirect Effect

Anthropogenic aerosol example using ship emissions:

(Johnson et al., 1996)

Cloud Formation in a Clean and Polluted Cloud Formation in a Clean and Polluted AtmosphereAtmosphere

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).

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

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

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

Surface temperature changesSurface temperature changes from 1750 to 1990from 1750 to 1990Aerosols only (°C)Aerosols only (°C)

volatile orga

nic ca

rbon

molecules

CCN formation

Cloud Formation

Cloud Albedo

Precipitation/Water Stress

Surface Temperature

Hydrological Cycle

• 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