Global Climate Change The sky is falling! The sky is falling!
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Global Climate Change • Radiative Equilibrium, Solar and Earth Radiation • Atmospheric Greenhouse Effect • Greenhouse Gases • Global Climate Change ➡ Predictions: Temperature, Sea Level ➡ Feedback Effects ➡ Alternative Causes ➡ Historical Climate Trends
• Remediation: Greenhouse Gas Emission Reduction 2
Radiative Equilibrium
Absorption
Emission
Radiative Equilibrium: amount of EMR absorbed equals amount of EMR emitted
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Sun and Earth are both near-blackbodies Sunlight I Terrestrial radiation
λ
Sun, 6000K: emits mostly visible light Earth, 255K: emits mostly infrared radiation
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Atmospheric Absorption • Earth’s atmosphere is a selective absorber (not a BB) ➡ Transmits visible light (such as sunlight) ➡ Absorbs infrared radiation (such as terrestrial radiation)
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The Atmospheric Greenhouse Effect
S
E
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The Atmospheric Greenhouse Effect
S
E
A
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Greenhouse Gases • These are gases that are transparent to visible light, partially opaque to infrared radiation • Earth’s major greenhouse gases: Water vapor
Methane
Nitrous oxide
Carbon dioxide
CFCs
Ozone
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Global Warming Potential (GWP) —related to amount of predicted warming (“radiative forcing”) from a unit increase in gas concentration GWPs relative to carbon dioxide: Carbon dioxide
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Methane Nitrous oxide
21 310
Chlorofluorocarbons 4000–20000 9
Water Vapor (H2O) • Sources ➡ Evaporation of oceans, land surface water ➡ Anthropogenic emissions (insignificant)
• Sinks ➡ Condensation, precipitation Largely out of the control of humans; at higher air temperatures, water vapor content is often higher than at lower temperatures 10
Carbon Dioxide (CO2) • Sources ➡ Aerobic respiration ➡ Outgassing by land and oceans ➡ Fossil fuel combustion
• Sinks ➡ Green plant photosynthesis ➡ Dissolution in oceans
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Carbon Cycle Atmosphere
Plants
Animals
Fossil Fuel
Land
Oceans Deep Ocean Sediments
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Anthropogenic Carbon Dioxide Inventory Cement Production
2%
Deforestation 25% Fossil Fuel Combustion 73%
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Methane (CH4) • Sources ➡ Cellulose digestion/breakdown ➡ Organic decay
• Sinks ➡ Chemical reactions ➡ Soil uptake Recent rapid increase due to food production and gas field leaks
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Nitrous Oxide (N2O) • Sources ➡ Soil bacteria ➡ Oceanic outgassing ➡ Combustion, atmospheric chemical reactions
• Sinks ➡ Chemical reactions in stratosphere Very slow increase over time (until recently); linked to agricultural production
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Chlorofluorocarbons (CFCs) • Sources ➡ Industrial/consumer leakage, direct emission (all anthropogenic)
• Sinks ➡ Chemical reactions in stratosphere Concentration trend dependent on production of CFCs and regulation in developed and less-developed countries
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Global Change • Climate change due to anthropogenic greenhouse gas emissions • Assume: carbon dioxide mixing ratio doubles in next 50–100 years
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Sea level rises from: Polar ice cap melt Thermal expansion of sea water Vegetation loss from drought, weather changes Uncertainties: Effect of non-CO2 greenhouse gases with higher GWPs Emission forecasts Feedback effects Limited numerical forecast models
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Feedback Effects Enhance
Cause
Effect Suppress
• Positive feedback: the effects enhance or reinforce the cause, resulting in increased effect (the “vicious circle”) • Negative feedback: the effects suppress the cause, decreasing the effect (“selfcorrecting” or “thermostatic”)
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Albedo • Reflectivity of a surface ➡ Higher albedo = less absorption of EMR ➡ Less EMR absorption = lower temperatures
Overall albedo of Earth-atmosphere system: 30% 20
Feedbacks • Sea level and ice cover changes ➡ Melting ice delays onset of atmospheric warming ➡ Rising sea level and receding ice cover decreases albedo — enhancing warming (positive feedback)
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• Water vapor and cloud cover changes ➡ Warmer air can contain more water vapor—positive feedback ➡ Cloudiness may increase when water vapor content increases
• Precipitation patterns altered — vegetation coverage changes ➡ Affects amount of carbon dioxide sink capacity ➡ Affects albedo 22
Other Possible Causes of Climate Change • Sunspot cycle (approx. 22 years long) ➡ Longer cycle = lower solar intensity ➡ Intensity variation less than 0.5%
• Natural variations in solar activity (flares, prominences, general increase/decrease in intensity) ➡ Small and unpredictable ➡ General intensity change usually over geologic time
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Earth’s Climatological Record
Natural carbon dioxide concentration and temperature appear correlated…do humans really affect global temperature? 24
Year to Year Variation
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What We “Know” • Humans are increasing the atmospheric carbon dioxide content • About 0.5–1.0°C increase in global temperature during the last 100 years • The major greenhouse gas concentrations will continue to rise • Increasing greenhouse gas concentrations tend to warm the planet
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What is “Likely” • Scientific analysis shows a high likelihood of a connection between anthropogenic increases in greenhouse gas concentrations and the observed warming trend
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What is Not Certain • How much warming will occur in the future and how quickly • Feedback of warming on other climatic features (e.g., precipitation) and the Earth System • Need more information about natural variations, solar variation, impacts of clouds and land use, relative contribution of human activities vs. natural influences, future greenhouse gas emissions 31
Reduction of GH Gas Emissions and Accumulation • Why bother? • Purpose: risk reduction ➡ Decrease fossil fuel usage ➡ Reduce forest clearing ➡ Increase plant biomass (?)
• Carbon dioxide controlled by these measures, but what about methane and nitrous oxide? ➡ Difficult—agricultural link 32
Non-Fossil Energy Sources • • • • • • • •
Biomass Nuclear Fission Geothermal Heat Solar Heat Collection Wind Power Hydroelectric Power Photovoltaic Cells Hydrogen Fuel Cells 33
Non-Fossil Energy Sources • • • •
Biomass Nuclear Fission Geothermal Heat Solar Heat Collection
Heat
Steam
Electricity
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Non-Fossil Energy Sources
• Wind Power • Hydroelectric Power
Kinetic energy
Electricity
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Non-Fossil Energy Sources [Energy]
Electricity
• Photovoltaic Cells • Hydrogen Fuel Cells 36
Regulated Carbon Markets • Set emissions quota, divide into chunks (“emission credits”), allocate to polluters • Unused credits can be sold in open market ➡ Depending on pollutant, ability to sell or buy may be regulated
• Quota reduced year-to-year • Subject to political abuse 37
Engineering Climate Change (or Unchange) • Problem: Carbon dioxide emission rate exceeds carbon dioxide sink rate Natural
Anthropogenic
Sources
Natural
Sinks
• Solutions: ➡ Decrease source rate ➡ Add anthropogenic sinks ➡ Alter the physical environment 38
• Carbon Sequestering ➡ Grow biomass, then bury it deep in the ground ➡ Pump carbon dioxide to bottom of ocean, letting pressure liquify the CO2 ➡ In both techniques, the point is to keep the carbon dioxide in the ground/ocean, and out of the atmosphere
CO2
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• Altering the physical environment: Increase planetary albedo ➡ Install mirrors in the atmosphere or on Earth’s surface ➡ Distribute light-colored dusts in the air
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