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Lecture Earth: Atmosphere and Climate.
Earth's atmosphere. Weather and climate. The greenhouse effect. Cosmic influences on the biosphere.
The Main Point
The Earth has a relatively thin but dynamic atmosphere that warms the d i t h th t th surface via the greenhouse effect and makes life possible.
Reading:
Chapters 10.1, 10.2, 10.6
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Earth's Atmosphere
Composition of the Earth's Atmosphere
Gas Symbol Permanent Nitrogen Oxygen Argon Neon Helium Hydrogen Xenon N2 O2 Ar Ne He H2 Xe Variable Water Vapor Carbon Dioxide Methane Nitrous Oxide Ozone (Troposphere) Ozone (Stratosphere) Particulates H20 CO2 CH4 N2O O3 O3 0 to 4 .0364 .00018 .000031 .000004 .0012 .000001 364 1.8 0.31 0.04 12 0.01 78.08 20.95 0.93 0.0018 0.0005 0.00006 0.000009 18 5 0.6 0.09 Percent PPM
Origin of the Atmosphere
Three main possibilities:
(1) Primordial: Formed along with the rest of the Earth ~4.6 billion years ago. This hypothesis is hard to reconcile with our current understanding of planet formation. (2) Internal: Gases released by volcanic outgassing over th I t l G l db l i t i the history of the Earth. This is likely the main source of atmospheric gases for terrestrial planets. (3) Extraterrestrial: Volatiles (materials that evaporate at relatively low temperatures ) delivered from impacts of icy comets and hydrated asteroids. Evaporation or sublimation of volatiles is another source of atmospheric gas.
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Highly oxidizing atmosphere ("Non-Equilibrium"). Surface pressure: 1 bar (~ 1 kg/cm2). 1 bar = 10 m of water. Oceans equivalent to 300 bars!
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"Weather" vs. "Climate"
Atmospheric Gains
Atmospheric Losses
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Day-to-day, quasi-random fluctuations in atmospheric circulation and precipitation are called weather. Patterns of weather observed over decades to centuries are the climate. Weather changes quickly; climate slowly. Climate changes have occurred throughout Earth's history (how do we know this?)
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Evidence for Past Climate Change
Geologic evidence for massive glaciations: glacially-carved landforms, transported boulders, till, moraines, etc. Fossil evidence: periodic fluctuations in warm vs. cold water organisms. Variations in Oxygen isotopes: 16O evaporates easier than 18O. When ice caps form, ocean water contains more 18O. Isotopic composition of seashells formed in seawater therefore provides a record of how much ice is present on Earth. Greenland and Antarctic ice cores show variations in temperature, dust. Last ice age was ~20,000 years ago. Others at 150,000; 250,000; 350,000... Caused by changes in Earth's orbital parameters?
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Will the Climate Change in the Future?
Yes!
Naturally (b fl ll (by fluctuations in Earth's orbital parameters?) i i h' bi l ?) AND Anthropogenically (i.e. human influences)
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The Earth's Surface Temperature
A simple problem with a confusing answer!
What is the "equilibrium" temperature of Earth?
Amount of incoming energy must balance outgoing energy. Incoming: Sunlight @ 1368 W/m2 = S0 (solar constant).
Earth "intercepts" an amount = RE2 S0. Some fraction, A, is reflected into back space (A 0.3). So the fraction (1-A) is absorbed by the ground and heats it up. Therefore the heat input to Earth is (1-A) RE2 S0 W/m2. Outgoing: An object at temperature T emits T4 W/m2. Remember the Stefan-Boltzmann Law from Lecture 6. Earth's entire surface emits 4RE2 T4 W/m2. So to balance, (1-A) RE2 S0 = 4RE2 T4.
The Greenhouse Effect
Some of the infrared (heat) radiation that would normally escape to space is absorbed by certain gases in the Earth's atmosphere. This is the greenhouse effect, responsible for warming the Earth by more than 30C above its equilibrium temperature. Carbon dioxide (CO2) and water vapor (H2O) are Earth's main greenhouse gases. Without the greenhouse effect, we would not be here! With too much greenhouse effect, we would not be here!
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Solution: T = 255 K = -17C = 0F = DAMNED COLD! So WHY AREN'T EARTH'S OCEANS FROZEN?
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Greenhouse Effect
An example of human influence
Concentration of greenhouse gases in the atmosphere
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Mars T ~ 5C Earth T ~ 35C Venus T ~ 500C!!
Atmospheric carbon dioxide measured from Mauna Loa, HI
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Why is CO2 important?
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How do we know the change is due to human activity?
This figure is from the 2007 report of the Intergovernmental Panel on Climate Change (IPCC). The panel is a highly respected intergovernmental body composed of governments and hundreds of scientists. The scientific evidence for human induced climate human-induced change is very compelling. Further warming depends on our emission choices. The most optimistic and most pessimistic IPCC predictions yield carbon dioxide concentrations ranging from two to three times preindustrial levels by 2100.
Another example of human influence
The Ozone Hole 1979 1998
Low O3
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High O3 Why is Ozone important?
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The Ozone Layer
Creation of ozone blocks high energy UV radiation from getting to Earth's surface. surface Makes life possible!
Cl and other O3 destroying phases last hundreds of years in the atmosphere!
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The Ozone Layer
Destruction of ozone allows more UV to get to the surface: Health risks!
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Cosmic Influences on the Biosphere
Impacts have occurred on Earth (Lec. 11). These catastrophic events appear to have had serious consequences for life on Earth!
Severe climate changes: "impact winter". li h i i Mass extinctions. Probably influenced the origin of life itself!
~65 million years ago, a 10km class asteroid or comet crashed into the Earth, raising a huge cloud of dust that lowered global temperatures for years and probably led to the extinction of the dinosaurs!
Much more detail in course Parts 3 & 4...
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Summary
Earth has a dynamic N2 & O2-rich atmosphere. Earth's volatiles: primordial or externally-derived? Weather vs. Climate. vs Climate CO2 & H2O are critical greenhouse gases, warming Earth above the freezing point of water. Ozone protects the biosphere against Solar UV. Climate changes! (Natural and Anthropogenic).
Ice Ages, Global Warming, Impacts.
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Next Lecture...
Surface of the Moon:
General Properties of the Moon. Lunar Geology.
Reading:
Chapters 2.3, 9.3, 10.3
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