Lecture 26

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Chapter 23

One of the key themes of this course is that the Earth continually changes. Much change is due to plate tectonics; the proximity of the Earth to the Sun, and to gravity. Gravity, and the Sun's energy, combine to drive erosion. Plate tectonics builds mountains, moves continents. Geologic and biological phenomenta interact and cause further change.
Gradual change occurs over millions to billions of years. Catastrophic change is nearly instantaneous in the context of geological time (i.e. seconds to thousands of years).
Change can be unidirectional - i.e. doesn't repeat, or cyclical - repeats over and over.
Most of the carbon in the Earth's atmosphere was supplied by volcanoes. Carbon can be removed from the atmosphere in several ways - e.g. by dissolving in seawater or being absorbed by photosynthetic organisms. Approx. 63 billion tons of carbon is removed from the atmosphere each year by life forms. Carbon (in CO2) can also be removed from the atmosphere by chemical weathering at the Earth's surface. Some of this carbon is returned to the atmosphere (e.g. via respiration, or by the decay of dead organisms). Other carbon is stored over the long-term (e.g. in fossil fuels or methane hydrates). CO2 and methane concentration in the atmosphere plays a key role in determining Earth's climate as they are greenhouse gases.
Global Climate Change:

The stratigraphic record (e.g. coal deposits indicate warm climate, abundance of vegetation; glacial till indicates considerably lower temperatures)
Paleontological evidence (e.g. the assemblages of species present in the fossil record can be a diagnostic indicator of climate).
Oxygen-isotope ratios: The ratio of two isotopes of oxygen (O18/O16) in glacial ice indicates the atmospheric temperature in which the snow formed. If the ratio is high, the temperature was higher; if lower, the temperature was lower. Ice coring to almost 3 km in the Arctic and the Antarctic (spanning 400,000 years) has revealed many cycles of air temperature.
Bubbles in ice: trap air at the time the ice formed. This air can be analyzed for CO2 content. The CO2 record has been extended back 240,000 years.
Growth rings: Each ring represents 1 year of growth; the thickness of the ring indicates the speed of growth in one year.
Human history: Clues to Earth's past climate can be found in our recorded history.

The Earth was considerably warmer 100 million years ago than it is today. Data indicates it started to cool down about 80 million years ago. Why this happened isn't entirely clear, but could have involved a number of factors including the positions of continents (as discussed in the previous lecture), by volcanic activity (e.g. long-term global increase in volcanic activity can contribute to long-term global warming by increasing the concentration of greenhouse gases in the atmosphere), the uplift of land surfaces (uplifted land subjected to more chemical weathering, which absorbs CO2), formation of coal, oil or organic shale (removes CO2 from the atmosphere), life (an abundance of lifeforms that absorb CO2 can reduce the global atmosphere temperature).

Fluctuations in solar radiation and cosmic rays: The energy output by the Sun isn't exactly constant and depends on the sunspot cycle. Every 9 to 11.5 yrs a large number of sunspots (likely magnetic storms) appear on the Sun, and this affects the radiation output by the Sun. The energy flux to the Earth can affect the climate.
Earth's orbit and tilt - as discussed in the previous lecture
Volcanic emissions - discussed CO2 in the previous lecture. Also, volcanic aerosols in the atmosphere can change the albedo of the Earth - or the pctg. of solar energy that is reflected back into outer space.
Ocean currents and greenhouse gases - already discussed in the previous lecture.

Could be due to sudden surge in volcanic activity, or by the impact of a large asteroid. Much evidence indicates massive eruptions in Siberia could have caused the greatest mass extinction event at the end of the Permian period. Other evidence points to an asteroid impact on the Yucatan peninsula as the cause of the Cretaceous-Tertiary mass extinction.

The Earth's human population now doubles every 44 years. Currently it is over 6 billion. With the population increasing, and the standard of living increasing, we use a staggering amount of resources and have had a significant effect on the Earth system.
One effect, of several, is pollution. This includes smog, water contamination, acid runoff, acid rain, radioactive materials, ozone depletion. The burning of fossil fuels, incl. coal, hydrocarbons, adds greenhouse gases, carbon dioxide (CO2) and methane (CH4), to the atmosphere. In 1800 the mean concentration of CO2 in the atmosphere was 295 ppm (parts per million). By 2000 this concentration had reached 370 ppm. The concentration of methane has also increased. The global mean temperature has risen approx. 0.9 degrees between 1880 and 2000. Ocean temperatures have also risen.
Global warming might cause:

a shift in climate belts (temperate climates would move to higher latitudes; desert regions would expand)
stronger storms (due to increased ocean temperature, e.g. more intense hurricanes)
a rise in sea-level (due to melting of polar ice sheets and the thermal expansion of ocean water).

What the Earth does (e.g. plate tectonics) is clearly beyond our control. What we do in managing our resources, using our resources, is not beyond our control. The evidence clearly shows that human activity (e.g. burning fossil fuels) has had a measurable effect on the Earth's climate, and this problem will not go away given current trends. This is a critical issue we have to tackle, preferably sooner than later.