I. Climate Change – Greenhouse Gases

A. Background• Greenhouse Effect

• Gases absorb heat (not light)• Natural Greenhouse Effect

• Mean planetary temperature = 15 oC vs. -6 oC• Enhanced Greenhouse Effect

• Due to GHGs emitted from human activity

I. Climate Change – Greenhouse Gases

B. Greenhouse Gases• Most important GHG is water vapor

• Accounts for ~50% of natural GHE

Greenhouse GasesGas Sources Residence

Time (y)Radiative Forcing

Relative

Influence

Carbon Carbon DioxideDioxide

FF CombustionFF Combustion

DeforestationDeforestation

Biomass BurningBiomass Burning

50-20050-200 11 63%63%

MethaneMethane Rice PaddiesRice Paddies

Cattle/TermitesCattle/Termites

LandfillsLandfills

FF ProductionFF Production

1010 2121 18%18%

Nitrous Nitrous OxideOxide

FertilizersFertilizers

DeforestationDeforestation

Biomass BurningBiomass Burning

150-170150-170 206206 6%6%

HCsHCs(incl. CFCs)(incl. CFCs)

Aerosol SpraysAerosol Sprays

FoamsFoams

RefrigerantsRefrigerants

15-65015-650 10,700 – 10,700 – 15,80015,800+

13%13%

After Petit et al. 1999

I. Climate Change – Greenhouse Gases

B. Greenhouse Gases• Long residence times indicate atmospheric

concentrations will remain high even if emissions stop

• Other factors besides GHGs may influence global climate

II. Climate Change – Other Factors

A. Cloud Cover• Reflects incoming radiation• Difficult to estimate in climate models• Effects vary in relation to altitude, thickness,

composition

B. Atmospheric Dust & Aerosols• Important factor in cool period from 1930s to 1960s

• Overwhelmed effects of rising CO2 during this period

• Dust from volcanic eruptions, drought areas• Cool period following eruption of Mt. Pinatubo in 1991

• Anthropogenic aerosols from sulfate, nitrate, black carbon, etc. (mostly from combustion)

• Also may influence cloud lifetime, precipitation

II. Climate Change – Other Factors

C. Sunspots• May affect amount of incoming radiation• Solar output varies – sunspots, solar flares

• Sunspots are magnetic storms that appear as dark patches on sun’s surface

• Number and size are maximal every 11 years• Solar output ca. 0.1% higher than normal during

maxima

http://calspace.ucsd.edu/virtualmuseum/climatechange2/06_3.shtml

II. Climate Change – Other Factors

C. Sunspots• May affect amount of incoming radiation• Solar output varies – sunspots, solar flares• Researchers have correlated minima with

“Little Ice Age” in Europe during 17th and early 18th centuries when sun was 0.25% dimmer than normal

• 20th century dominated by sunspot maxima• Some predictions that 21st century will see minima

• Estimated that sunspot variability may have contributed to half of 0.55 oC warming since 1860 and one third of warming since 1970 (Lean et al.)

http://calspace.ucsd.edu/virtualmuseum/climatechange2/06_3.shtml

II. Climate Change – Other Factors

D. Volcanism• Affects water vapor, particles, sulfides, nitrates• Generally leads to planetary cooling

• Theory about extinctions at P/T & K/T boundaries• Mt. St. Helens and Mt. Pinatubo eruptions caused

planetary cooling, and those were small eruptions

E. Photosynthesis & Transpiration• Affect CO2, water vapor

• As [CO2] rises, some plants• Photosynthesize more rapidly• Grow faster• Incorporate more CO2 into biomass

• Keep their stomata open less

• Transpiration releases water vapor into atmosphere• Less transpiration when [CO2] is higher (stomata)

II. Climate Change – Other Factors

D. Volcanism• Affects water vapor, particles, sulfides, nitrates• Generally leads to planetary cooling

• Theory about extinctions at P/T & K/T boundaries• Mt. St. Helens and Mt. Pinatubo eruptions caused

planetary cooling, and those were small eruptions

E. Photosynthesis & Transpiration• Affect CO2, water vapor

• As [CO2] rises, some plants• Photosynthesize more rapidly• Grow faster• Incorporate more CO2 into biomass

• Keep their stomata open less

• Transpiration releases water vapor into atmosphere• Less transpiration when [CO2] is higher (stomata)

II. Climate Change – Other Factors

F. Soil Characteristics• Affect heat capacity and retention• More hydrated soil leads to

• Darker color (less reflective)• More heat capacity (high heat capacity of water)

G. Albedo (Reflectivity)• Earth’s surface varies considerably (mean = 0.30-0.36)

• Ice/Snow highly reflective (0.9)• Clouds vary in reflectivity• Land generally less reflective

• Changes in land use affect albedo• Desertification increases albedo• Forest – 0.12• Grassland – 0.19• Desert – 0.30

II. Climate Change – Other Factors

H. Astronomical Factors• Earth’s axis precesses on a ~23,000 year

cycle• Axial tilt (obliquity) varies on a 41,000 year

cycle• Orbital eccentricity has a 100,000 year cycle

• Correlated with glacial periods over past 1,000,000 years

II. Climate Change – Other Factors

• Many factors affect global climate in multiple ways

• Ex – Clouds absorb re-radiated long-wavelength radiation but also may reflect incoming short-wavelength radiation

• Net effect is cooling

• Ex – Particles in the atmosphere reduce the re-radiation of long-wavelength radiation but also reflect incoming short-wavelength radiation

• Net effect is probably warming at low levels but cooling at high levels (e.g. following a large volcanic eruption)

• Uncertainty about impact of many factors