I Climate Change Greenhouse Gases A Background Greenhouse

I. Climate Change – Greenhouse Gases A. Background • Greenhouse Effect • • • Gases absorb heat (not light) Natural Greenhouse Effect • Mean planetary temperature = 15 o. C vs. -6 o. C 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 Gases Gas Carbon Dioxide Methane Nitrous Oxide HCs (incl. CFCs) Sources Residence Radiative Relative Time (y) Forcing Influence FF Combustion Deforestation Biomass Burning 50 -200 1 63% Rice Paddies Cattle/Termites Landfills FF Production 10 21 18% Fertilizers Deforestation Biomass Burning 150 -170 206 6% Aerosol Sprays Foams Refrigerants 15 -650 10, 700 – 15, 800+ 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 • • • B. Reflects incoming radiation Difficult to estimate in climate models Effects vary in relation to altitude, thickness, composition Atmospheric Dust & Aerosols • Important factor in cool period from 1930 s to 1960 s • • Overwhelmed effects of rising CO 2 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

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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 17 th and early 18 th centuries when sun was 0. 25% dimmer than normal 20 th century dominated by sunspot maxima • • • Some predictions that 21 st century will see minima Estimated that sunspot variability may have contributed to half of 0. 55 o. C warming since 1860 and one third of warming since 1970 (Lean et al. )

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II. Climate Change – Other Factors D. Volcanism • • Affects water vapor, particles, sulfides, nitrates Generally leads to planetary cooling • • E. Theory about extinctions at P/T & K/T boundaries Mt. St. Helens and Mt. Pinatubo eruptions caused planetary cooling, and those were small eruptions Photosynthesis & Transpiration • • Affect CO 2, water vapor As [CO 2] rises, some plants • • • Photosynthesize more rapidly Grow faster Incorporate more CO 2 into biomass Keep their stomata open less Transpiration releases water vapor into atmosphere • Less transpiration when [CO 2] is higher (stomata)

II. Climate Change – Other Factors D. Volcanism • • Affects water vapor, particles, sulfides, nitrates Generally leads to planetary cooling • • E. Theory about extinctions at P/T & K/T boundaries Mt. St. Helens and Mt. Pinatubo eruptions caused planetary cooling, and those were small eruptions Photosynthesis & Transpiration • • Affect CO 2, water vapor As [CO 2] rises, some plants • • • Photosynthesize more rapidly Grow faster Incorporate more CO 2 into biomass Keep their stomata open less Transpiration releases water vapor into atmosphere • Less transpiration when [CO 2] is higher (stomata)

II. Climate Change – Other Factors F. Soil Characteristics • • Affect heat capacity and retention More hydrated soil leads to • • G. Darker color (less reflective) More heat capacity (high heat capacity of water) 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 years

II. Climate Change – Other Factors • Many factors affect global climate in multiple ways Ex – Clouds absorb re-radiated longwavelength radiation but also may reflect incoming short-wavelength radiation • • • Ex – Particles in the atmosphere reduce the reradiation of long-wavelength radiation but also reflect incoming short-wavelength radiation • • Net effect is cooling 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