Lecture 18

Astronomical Influences on Earth's Climate

1. Eccentricity of Earth’s orbit around the sun (varies over period of ~ 100,000 years)
2. Precession (Period of ~ 26,000 years)
3. Inclination of Earth’s axis versus orbital plane (Period of ~41,000 years)
4. Milankovitch Hypothesis: Changes in all three of these aspects are responsible for interglacial warming.

Natural disasters

1. Nuclear winter theory is analogous to some natural catastrophic events: Volcano eruptions, Asteroid impacts, Bush fires, Oil fires
   
2. Volcanoes
3. Energy Released by a single volcano can be comparable to a global nuclear war:

   Kilauea, Hawaii: 1952	43 MT
   Krakatau, Indonesia: 1883	150 MT
   Tambora, Indonesia: 1815	20,000,000 MT

Click to Enlarge. The 1815 Mount Tambora eruption. The red areas are maps of the thickness of volcanic ashfall.
Image courtesy of Indon, Some rights reserved.

Effects on Sunset

1. Within six months following an eruption, ejected gaseous sulfur dioxide is converted into sulfuric acid.  There is an Increase of stratosphere temperature by ~ 4 degrees, and a decrease of hemisphere temperature by ~ 0.2 degrees.
2. Most effects come from the scattering of photons on sub-micron sized particles in the dusty stratosphere & troposphere. Often, spectacular sunrise and sunset colors come from sulfuric particles and dust:
3. The vivid red sky in Edvard Munch’s painting “The Scream” was inspired by the twilights in Norway.

Asteroids

1. Asteroid impact is a relatively frequent event over the age of the earth, 47 are recorded on the North American continent alone.
2. Evidence for the Impact Hypothesis - Walter Alvarez finds high levels of iridium in a clay later at the Cretaceous-Tertiary (K-T) boundary.  Such levels of iridium are only found in extraterrestrial material.  In addition, many layers have been found elsewhere.  Furthermore, a large crater of the right age has been found on the Yucatan Peninsula (now named the Chicxulub Event).  The Chicxulub event corresponds to 70 Million Megaton TNT, which is roughly 7000 times the maximum arsenal of US and Soviet Union.
3. Asteroid Classifications
   1. City Buster: 15-m meteoroid => 8.5·105 tons TNT. Serious local consequences, though atmosphere provides partial shield. Hydrogen-bomb scale, but without the radioactivity.
   2. People Buster: 1-km asteroid => 250,000 MT. No atmospheric shield. Hemispheric-scale effects. At threshold for global effects. Significant fraction of all humans killed.
   3. Planet Buster: 10-km diam asteroid => 250 million MT. global effects. Ejected, vaporized rock and water fill atmosphere => global winter => major extinction of life forms, including virtually all humans.
4. Asteroid Example: Tunguska, Siberia, 1908
   At 7:17 AM on the morning of June 30, 1908, a mysterious explosion occurred in the skies over Siberia. It was caused by the impact and breakup of a large meteorite, at an altitude of roughly six kilometers in the atmosphere. Realistic pictures of the event are unavailable. However, Russian scientists collected eyewitness accounts of the event.  Seismic vibrations were recorded as much as 1000 km (600 mi) away.  At 500 km (300 mi), observers reported "deafening bangs" and a fiery cloud on the horizon.  About 170 km (110 mi) from the explosion, the object was seen in the cloudless, daytime sky as a brilliant, sunlike fireball; thunderous noises were heard.  At distances around 60 km, people were thrown to the ground or even knocked unconscious; windows were broken and crockery knocked off shelves.  Reindeer herders were asleep in their tents in several camps about 30 km (20 mi) from the site. They were blown into the air and knocked unconscious.

Nuclear Winter

1. Some scientists predict that smoke and debris rising from the massive fires of a nuclear war could block sunlight for weeks or months, cooling the earth's surface and producing climate changes that could, for example, negatively affect world agricultural and weather patterns.  The first study suggesting nuclear winter was conducted in 1982 by Paul Crutzen and John Birks.  This study was later followed by R.P. Turco, O.B. Toon, T.P. Ackerman, J.B. Pollack, and Carl Sagan in 1983 by first 1D model simulations.  This study attributed atmospheric and environmental changes to burning forests.  Carl Sagan and others conducted additional studies and found that soot from cities and dust from the explosions themselves were also climatically significant and could cool the surface of the earth. 
2. In 1983, R.P. Turco, O.B. Toon, T.P. Ackerman, J.B. Pollack, and Carl Sagan (referred to as TTAPS) published a paper entitled Global Atmospheric Consequences of Nuclear War (Click here to Read) which is the foundation that the nuclear winter theory is based on.  The theory states that nuclear explosions will set off firestorms in the cities and surrounding forest areas. The small  particles of soot are carried high into the atmosphere. The smoke will block the sun's  light for weeks or months. The land temperatures would fall below freezing.  This combination of reduced temperatures and reduced light levels would have catastrophic ecological consequences.  Average light levels would be below the minimum required for photosynthesis during the first 30-40 days after the explosions, and most fresh water would be frozen. The TTAPS study concluded that "the possibility of the extinction of Homo Sapiens cannot be excluded." This effect is similar to what may have killed the dinosaurs.
3. Scientific Studies on Nuclear Winter
4. 1. U.S. National Academy of Sciences
      
   2. The Royal Society of Canada
      
   3. U.S. Department of Defense
   4. USSR
5. Reduction of sun light follows absorption law:
   
   
6. Impact on temperatures - TTAPS study assumed that a 5000 MT first strike with 0.1-0.33 Mt of dust per 1Mt of TNT exploded. The model estimates 225 MT smoke emission and 960 MT dust emission. The optical absorption parameter indicated almost 99% absorption, which would coincide with drops in temperature by up to 40 degrees Celsius.
7. Impact on food supply - Foodstuffs are vulnerable to even a 5-10 degree average change, depending on selectivity. The “Little Ice Age” of the 16th – 17th centuries was only an average of one degree less than mean temperatures.
8. Latitude effects - 3-6 month cooling period, depending on weather and rainfall conditions. Only limited spread to Southern hemisphere except for wind related changes in Antarctic regions
9. Revised Studies - 3D model simulations (Malone et al. 1986), taking into account more complex atmospheric parameters, reduce the absorption coefficient predictions. These also take into account removal of dust by wind and rainfall. Significantly lower dust emission of < 200 MT.  In this, the average summer temperature in the Northern hemisphere would drop by only 10 degrees.