LECTURE 15: EARTH: SURFACE AND ATMOSPHERE
- What is continental drift? What causes it?
- What happens when plates slide against each other, run\
into each other, or spread apart?
- What is the composition of the Earth's atmosphere?
Why doesn't it contain hydrogen or helium?
Why does it contain oxygen?
- What factors control the Earth's surface temperature?
- What is the greenhouse effect?
- How is human activity changing the Earth's climate?
- Earth's surface 70% ocean, 30% land.
- Continents "fit together" like jigsaw pieces.
- Especially true if use continental shelf edges instead of shorelines.
- Suggests: used to be joined together, have drifted apart.
- Radical idea, confirmed by studies in 1950s and 1960s showing
spreading at mid-ocean ridges.
- Can now measure motions of continents with radio astronomy,
- Earth's surface consists of 16 plates, floating on oozing, "plastic"
layer of mantle.
- Motions driven by convection ("boiling") in hot mantle.
- Where plates spread apart: hot magma wells up, fills gap.
Ocean floor being "repaved" by flow from mid-ocean ridges.
- Where plates slide against each other: stick-slip earthquakes,
sudden jumps up to several meters. Example: San Andreas fault.
- Where plates collide head on:
- Subduction: one plate slides below other.
- Crust buckling: surface buckles, creating mountain ranges.
- "Super-continents" form, break apart, reform, timescale of hundreds
of millions of years.
- Total mass of atmosphere ~ 10-6 Mearth.
- Composition: 78% nitrogen (N2), 21% oxygen (O2),
1% water vapor (H2O).
- Traces of other gases, mostly carbon dioxide (CO2).
- Pressure at sea level: 105 newton/m2 =
- Pressure caused by weight of atmosphere above.
- Drops by factor of two with each 5 km of altitude.
- Complex layered structure with different heating and cooling mechanisms.
Why no hydrogen and helium?
- At given temperature, lighter molecules/atoms move faster.
- N2 at 300K, typical speed 0.3 km/sec.
- Hydrogen and helium faster, ~ 1 km/sec.
- Escape speed for Earth gravity: 11 km/sec.
- Fastest moving hydrogen and helium atoms escape.
- Over time, others speed up, all escape.
- Primordial atmosphere: N2, CO2, H2O
released by volcanic activity.
- H2O condensed into oceans.
- CO2 dissolved in water, stored in rocks.
- What's missing?
- Oxygen produced by photosynthesizing organisms.
- Began to appear 2 billion years ago, reached present fraction
400 million years ago.
- Free oxygen in planetary atmosphere is a potential sign of life.
- Can look for this signature in planets around other stars, if
we can measure their spectra.
Surface of Earth well insulated from hot material below.
Temperature of Earth's surface determined by heating from Sun.
- Suppose Earth were a perfect blackbody.
- Heated by absorbing all sunlight that falls on it.
- Cooled by emitting blackbody radiation given its temperature.
- Calculate equilibrium temperature at which heating and cooling balance.
- Result: 280 K, 45 degrees F.
- Pretty good explanation! BUT
- Clouds, oceans, land reflect 40% of Sun's light, not available for heating.
- Redo the calculation: -20 F. Wrong!
THE "GREENHOUSE" EFFECT
Why is surface warmer than above argument implies?
- Solar energy absorbed by land and ocean.
- Re-radiated as 300 K blackbody radiation, peaks at 10,000 nm.
- Atmosphere is partly opaque at these wavelengths, absorbs
radiated energy from surface.
- Absorption from water, carbon dioxide, methane.
- New prediction: 65 F. Good!
- This phenomenon is called the "greenhouse effect": atmospheric
absorption of infrared radiation from Earth's surface, raising
- Not how a real greenhouse works.
We rely on the greenhouse effect to keep the Earth's surface at
a livable temperature.
You can have too much of a good thing.
- Primary greenhouse gas is carbon dioxide, CO2.
- Plants absorb CO2, produce oxygen.
- Human activity (deforestation, burning of
oil and coal) has raised atmospheric CO2 by 30%.
- Going up rapidly now, few percent per decade.
- Average temperatures are rising, as predicted based on CO2
- Measurable directly and through effects on glaciers, ice shelves,
- Temperature now higher than at any time since last ice age.
What will happen next?
Many significant uncertainties in climate modeling,
though these are getting smaller as models improve.
Bottom line: We are engaged in a risky global chemistry experiment.
- Two limiting cases:
- Self-regulation, e.g. from more clouds and plant growth.
Earth stabilizes at slightly higher temperature.
- Runaway greenhouse: melting ice caps, more water vapor absorbing
more infrared radiation, more CO2 from plant death.
- As warming accelerates, changes in weather patterns
(temperatures, rainfall, droughts) could occur fairly quickly.
- Significant sea level changes could occur on timescales of decades.
- Re-absorbing CO2 takes decades, so by the time bad
effects appear, it may be too late to reverse them.
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Updated: 2005 May 8[dhw]