Lecture Notes: Energy and Climate Change

Introduction

• Global Environmental Problem: The most serious environmental problem facing humanity today is global climate change.
• Causes: Increasing levels of carbon dioxide and other "greenhouse gases" due to human activities.
• Impact: Continuous increases in average global air temperatures and changing weather patterns, resulting in drought for some areas and flooding for others.
• Focus: Investigate the mechanism of the greenhouse effect, atmospheric gases causing it, and the role of suspended atmospheric particles.
• Role of Fossil Fuels: Cumulative increase in atmospheric CO₂ levels due to energy production.
• Solutions: Sustainable solutions such as burying gas emissions, using biofuels, hydrogen, wind, photovoltaics, solar power towers, or nuclear energy.
• Role of Chemistry: Chemistry and chemists play an important role in all these topics.

The Greenhouse Effect

Introduction

• Prediction: The greenhouse effect will significantly affect climates around the world.
• Terms: Greenhouse warming and global warming mean average global air temperatures are expected to increase by several degrees.
• Observation: Global warming has been underway since 1860, responsible for about two-thirds of a Celsius degree increase.
• Importance: Rapid global warming is considered our most crucial worldwide environmental problem.

Mechanism of the Greenhouse Effect

5.1 The Earth's Energy Source

• Sunlight: The Earth's surface and atmosphere are kept warm by energy from the Sun.
• Blackbody Radiation: The Sun behaves like a blackbody, emitting light efficiently.
• Peak Wavelength: For the Sun's surface temperature (5800 K), the peak wavelength is about 0.50 μm (visible light).
• Earth's Emission: The Earth emits infrared light, peaking around 13 μm.
• Energy Balance: The Earth must balance the energy it absorbs and emits to maintain a steady temperature.

5.2 Historical Temperature Trends

• Trends: Average surface temperature trends over the past 2000 years.
• Industrial Revolution: Consistent downward trend in temperature until the Industrial Revolution.
• 20th Century: Significant warming trend, producing a "hockey stick" shape.
• Fluctuations: Air temperature did not increase continuously; significant warming in 1910-1940, cooling in 1940-1970, and warming since 1970.

5.3 Earth's Energy Emissions and the Greenhouse Effect

• Earth's Emission: Emits infrared light, peaking around 13 μm.
• Greenhouse Effect: IR emitted from the Earth's surface is absorbed by atmospheric gases and re-emitted, warming the surface.
• Natural Greenhouse Effect: Responsible for the average temperature at the Earth's surface being about +15°C rather than -18°C.
• Enhanced Greenhouse Effect: Increasing concentrations of greenhouse gases lead to additional warming.

5.4 A Very Simple Model of the Greenhouse Effect

• Model: Earth with no greenhouse gases has a calculated surface temperature of -18°C.
• Real Earth: Acts as if about 60% of the energy it emits as IR is transmitted into space.
• Calculation: Results in a surface temperature of 290 K (17°C), an increase of 35 degrees by the natural greenhouse effect.

BOX 5-1 A Simple Model of the Greenhouse Effect

• Model: Earth with a single, uniform layer of air that absorbs all IR and converts it to heat.
• Calculation: Predicts Earth's surface temperature to be 303 K (30°C).
• Actual Temperature: 15°C, more consistent with a model where about one-third of the IR emitted from the surface passes through the atmosphere unabsorbed.

5.5 Earth's Energy Balance

• Energy Inputs and Outputs: Summarized in Figure 5-4.
• Total Energy: 342 W/m² present in sunlight outside the Earth's atmosphere.
• Absorbed Energy: 235 W/m² must be re-emitted into space to maintain a steady temperature.
• Greenhouse Gases: Increase the amount of IR released, achieving the balance at 390 W/m² emitted from the surface.

Molecular Vibrations: Energy Absorption by Greenhouse Gases

5.6 Types of Molecular Vibrations

• Bond-Stretching Vibration: Oscillatory motion of two bonded atoms relative to each other.
• Bending Vibration: Oscillation in the distance between two atoms bonded to a common atom.
• IR Absorption: Occurs when the frequency of light matches the frequency of an internal motion within the molecule.

5.7 Carbon Dioxide: Absorption of Infrared Light

• Absorption Spectrum: Shown in Figure 5-6.
• Peak Absorption: At 15.0 μm (OCO angle-bending vibration) and 4.26 μm (antisymmetric OCO stretch vibration).
• Effect: CO₂ collectively absorbs about half of the outgoing thermal IR in the 14-16 μm region.

5.8 Carbon Dioxide: Past Concentration and Emission Trends

• Preindustrial Levels: About 280 ppm.
• Current Levels: About 390 ppm (2010).
• Increase: Due to combustion of fossil fuels and deforestation.
• Seasonal Fluctuations: Due to the growth and decay cycle of vegetation.

5.9 Carbon Dioxide: Atmospheric Lifetime

• Lifetime: Complex, not decomposed chemically or photochemically.
• Temporary Sinks: Dissolution in surface seawater or absorption by growing plants.
• Permanent Sink: Dissolution in deep ocean waters, taking hundreds of years.

5.10 Carbon Dioxide: Inputs and Outputs

• Anthropogenic Emissions: 7.7 Gt of carbon annually from fossil-fuel combustion and cement production.
• Sinks: Oceans and biomass absorb about half of the emissions.
• Accumulation: About 4.1 Gt annually in the atmosphere.

5.11 Water Vapor: Its Infrared Absorption and Role in Feedback

• Absorption: Through H-O-H bending vibration, peaking at about 6.3 μm.
• Feedback: Increased water vapor due to global warming causes additional warming.
• Positive Feedback: The operation of a phenomenon produces a result that itself further amplifies the result.

Other Greenhouse Gases

5.13 Methane: Absorption and Sinks

• Importance: Next most important greenhouse gas after CO₂ and water.
• Absorption: H-C-H bond-angle-bending vibrations absorb at 7.7 μm.
• Lifetime: About a decade.
• Dominant Sink: Reaction with hydroxyl (OH) radicals.

5.14 Methane: Emission Sources

• Anthropogenic Sources: Energy production, agriculture, waste disposal.
• Natural Sources: Plant decay, wetlands, rice paddies.
• Other Sources: Landfills, biomass burning, ruminant animals, fossil fuel extraction.

5.15 Methane: Concentration Trends and Possible Future Increases

• Historical Levels: About 0.75 ppm preindustrial, doubled to about 1.8 ppm currently.
• Future Increases: Possible increase due to temperature rises from the enhanced greenhouse effect.

5.16 Nitrous Oxide

• Importance: Significant greenhouse trace gas.
• Structure: Linear NNO.
• Absorption: Bending vibration at 8.6 μm, bond-stretching vibration at 7.8 μm.
• Lifetime: About 120 years.
• Anthropogenic Sources: Fertilizers, manure decomposition, fossil-fuel combustion.

5.17 CFCs and Their Replacements

• Importance: Potential to induce global warming due to persistence and strong IR absorption.
• Absorption: C-F bond stretch at 9 μm.
• Lifetime: Long residence times.
• Replacements: HCFCs and HFCs with shorter lifetimes and less efficient IR absorption.

5.18 Sulfur Hexafluoride

• Importance: Good absorber of thermal IR, long-lived in the atmosphere.
• Usage: Insulating gas in electric utilities and semiconductor industry.
• Concentration: 7.0 ppt (2010), increasing linearly.

5.19 Tropospheric Ozone

• Importance: Natural greenhouse gas with a short residence time.
• Formation: Pollution from power plants, motor vehicles, forest fires, grass fires.
• Absorption: Symmetric-stretching vibration in the atmospheric window region.

The Climate-Modifying Effects of Aerosols

5.20 The Interaction of Light with Particles

• Reflection: Atmospheric particles can reflect incoming sunlight back into space.
• Absorption: Some particles can absorb certain wavelengths of light.
• Albedo: Fraction of sunlight reflected by a surface.
• Sulfate Aerosols: Reflect sunlight back into space, increasing Earth's albedo and cooling the surface.

5.21 Aerosols and Global Warming

• Cooling Effect: Concentrated in the Northern Hemisphere due to industrial activity.
• Lifetime: Short, removed efficiently by rain.
• Future Trends: Unclear, depends on sulfur dioxide emissions and controls.
• Dimethyl Sulfide (DMS): Produces sulfate aerosols, potentially tempering global warming.

Global Warming to Date

5.22 Allocation of Warming to Natural and Anthropogenic Factors

• Greenhouse Gases: CO₂ > CH₄ > O₃ > CFCs > N₂O.
• Aerosols: Currently cancel about 40% of the net warming from greenhouse gases.
• Uncertainty: Aerosol effect has the largest uncertainty.

5.23 Global Warming: Geography

• Temperature Increases: Not evenly spread around the globe.
• Arctic Region: Most significant warming, sea ice disappearing.
• Sulfate Aerosols: Keep eastern U.S. and Canada cooler.

5.24 Global Circulation Models

• Predictions: Reproduce the climate of the last century and a half quite well.
• Clouds: Net feedback from increased cloudiness is uncertain.
• Low-Lying Clouds: Net cooling effect.
• High-Lying Clouds: Net warming effect.

5.25 Signs of Climate Change

• Precipitation: Increased in most areas, decreased in others.
• Extreme Weather: Becoming more common.
• Winters: Becoming shorter.
• Ice Cover: Shrinking fast.
• Coral Reefs: Warming water is killing much of the coral.
• Mosquito-Borne Diseases: Reaching higher altitudes.
• Rising Seas: Threatening to engulf Pacific islands.

Geoengineering Earth's Climate to Combat Global Warming

Introduction

• Geoengineering: Massive-scale schemes to alter the climate of the entire planet.
• Methods: Reflecting sunlight back into space or removing carbon dioxide from ambient air.

5.26 SRM: Using Metal Reflectors in Space

• Proposal: Tens of trillions of small silicon disks in orbit around the Earth.
• Cost: In the trillions of dollars, decades to implement.

5.27 SRM by Increasing Sulfate Aerosols in the Stratosphere

• Proposal: Artificially increase the concentration of sulfate aerosol particles in the lower stratosphere.
• Effectiveness: High, scores high on affordability and speed of implementation.
• Safety: Low, likely to affect rainfall and monsoon patterns.

5.28 Precipitation and Stratospheric Ozone in a Geoengineered World

• Precipitation: Reduction in regional rainfall.
• Stratospheric Ozone: Some decrease due to ozone-destroying chemical reactions.

5.29 Geoengineering by Ground-Level Systems

• Cloud Whitening: Increasing reflectivity of marine clouds.
• Roof and Road Whitening: Increasing albedo of the built environment.
• Desert Reflectors: Covering desert areas with reflective material.
• Feasibility: Uncertain, could affect weather patterns.

5.30 SRM Schemes Summary

• Sulfate Aerosols: High effectiveness, affordability, and speed of implementation, but low safety.
• Ground-Based Reflectors: Low effectiveness, affordability, and safety.

Atmospheric Residence Time Analysis

• Concentration Change: Depends on the rate of input and the efficiency of sinks.
• First-Order Relationship: Rate of elimination is directly proportional to the concentration.
• Steady-State Concentration: Achieved when the rate of elimination equals the rate of intake.
• Half-Life: Time required for half of the substance to decay.
• Residence Time: Average amount of time one of its molecules exists in air before it is removed.

Review Questions

1. Sketch a plot showing the main trends in global air temperature over the last century and a half.
2. What is the wavelength range, in μm, for infrared light? In what portion of this range does the Earth receive IR from the Sun? What are the wavelength limits for the thermal IR range?
3. Explain in terms of the mechanism involved what is meant by the greenhouse effect. Explain what is meant by the enhancement of the greenhouse effect.
4. Explain what is meant by the terms symmetric and antisymmetric bond-stretching vibrations, and by angle-bending vibrations.
5. Explain the relationship between the frequency of vibrations in a molecule and the frequencies of light it will absorb.
6. Why don't N₂ and O₂ absorb thermal IR? Why don't we consider CO and NO to be trace gases which could contribute to enhancing the greenhouse effect?
7. What are the two main anthropogenic sources of carbon dioxide in the atmosphere? What is its main sink? What is fixed carbon?
8. Is water vapor a greenhouse gas? If so, why is it not usually present on lists of such substances?
9. Explain what is meant by positive and negative feedback. Give an example of each as it affects global warming.
10. What is meant by the term atmospheric window as applied to the emission of IR from the Earth's surface? What is the range of wavelengths of this window?
11. What reaction is the dominant tropospheric sink for methane?
12. What are four important trace gases that contribute to the greenhouse effect?
13. What are the six most important sources of methane?
14. What are the three most important sinks for methane in the atmosphere? Which one of them is dominant? What is meant by the term clathrate compound?
15. Is the enhancement of the greenhouse effect by release of methane from clathrates due to increased temperature an example of feedback? If so, is it positive or negative feedback? Would an increase in the rate and amount of photosynthesis with increasing temperatures and CO₂ levels be a case of positive or negative feedback?
16. Explain in chemical terms what is meant by nitrification and denitrification. What are the conditions under which nitrous oxide production is enhanced as a by-product of these two processes?
17. What are the main sources and sinks for N₂O in the atmosphere?
18. Are the proposed CFC replacements themselves greenhouse gases? Why is their emission considered to be less of a problem in enhancing the greenhouse effect than was that of the CFCs themselves?
19. By which two mechanisms does light interact with atmospheric particles?
20. Explain how sulfate aerosols in the troposphere affect the air temperature at the Earth's surface, by both the direct and indirect mechanisms.
21. List four important signs, other than increases in average air temperature, that global warming is occurring.
22. Define the terms geoengineering and solar radiation management.
23. Describe how the use of sunlight reflectors in space could reduce global warming.
24. Describe how the scheme of increasing stratospheric sulfate concentration could reduce global warming. Why is particle size an important factor? What schemes have been suggested by which the sulfate could be delivered to the stratosphere?
25. What would the likely effects of solar radiation management be on (a) rainfall levels and (b) stratospheric ozone recovery?
26. How is the residence time of a substance related to its rate R of input/output and to its total concentration C?