Causes of Global Climate Change: Global Cooling

Volcanic Eruptions

• Tambora, Indonesia - 1815 "year without a summer"
• Mount Toba, present day Sumatra - 73,000 years ago accelerated glaciation

Milankovitch cycles

Processional of equinoxes - every 23,000 years

• A measure of the slow clockwise motion of the equinoxes along the ecliptic due to the motion of the earth's axis of rotation around the pole of the ecliptic; the angular movement of the spin axis of an object around an axis fixed in space.
  
  So, as the Earth spins, it wobbles a bit. N Pole currently points towards Polaris (the North Star) but shifts to point towards Vega over 23,000 years.
  
  
  image source: NPS Milankovitch Cycles

Obliquity - every 41,000 years

• The degree to which the axis tilts varies from 21.5͍° to 24.5° over 41,000 years. Currently our tilt is at 23.5°. This tilt affects insolation - the amount of sunlight an area receives. More tilt: poles receive more sunlight. Less tilt: poles receive less sunlight
  
  
  
  image source: NPS Milankovitch Cycles

Eccentricity - 100,000 to 400,000 year periods

• The amount that the earth's revolution deviates from a circular path. The more circular path, all points on it are perihelia and the more stable the climate. More elliptical path, more pronounced perihelion and aphelion and the less stable the climate will be.
  
  
  image source: NPS Milankovitch Cycles

Changes in deep ocean circulation patterns

Deep-ocean currents form as a response to density differences in the water. Factors creating a dense mass of water include:

• Cold sea water temperature
• Increase in salinity
• click here for a shockwave animation of brine exclusion in sea ice!
• click here for a flash video of the thermohaline current!

The thermohaline current brings cold, nutrient rich water from the poles to the equator. This helps to moderate our planet's temperature. Influx of fresh water from melting ice caps dilutes salinity, therefore decreasing density, and current shuts off.

"DRAMATIC CHANGES IN THE NORTH ATLANTIC—Subpolar seas bordering the North Atlantic have become noticeably less salty since the mid-1960s, especially in the last decade. This is the largest and most dramatic oceanic change ever measured in the era of modern instruments. This has resulted in a freshening of the deep ocean in the North Atlantic, which in the past disrupted the Ocean Conveyor and caused abrupt climate changes. (B. Dickson, et. al., in Nature, April 2002)  WHOI "


A Long Record Of Abrupt Climate Changes

Image and text from: WHOI Ocean and Climate Change Institute

The Younger Dryas - about 12,700 years ago, average temperatures in the North Atlantic region abruptly plummeted nearly 5°C and remained that way for 1,300 years before rapidly warming again.

The 8,200-Year Event — A similar abrupt cooling occurred 8,200 years ago. It was not so severe and lasted only about a century. But if a similar cooling event occurred today, it would be catastrophic.

The Medieval Period — An abrupt warming took place about 1,000 years ago. It was not nearly so dramatic as past events, but it nevertheless allowed the Norse to establish settlements in Greenland.

The Little Ice Age — The Norse abandoned their Greenland settlements when the climate turned abruptly colder 700 years ago. Between 1300 and 1850, severe winters had profound agricultural, economic, and political impacts in Europe. (R.B. Alley, from The Two-Mile Time Machine, 2000)

What Happens if the Conveyor Shuts Down?

Click here for the video "If Too Much Fresh Water Enters the North Atlantic" from WHOI
Text from: Abrupt Climate Change: Should We Be Worried? By Robert B. Gagosian

Scenario 1: Conveyor slows down within next two decades.
Such a scenario could quickly and markedly cool the North Atlantic region, causing disruptions in global economic activity. These disruptions may be exacerbated because the climate changes occur in a direction opposite to what is commonly expected, and they occur at a pace that makes adaptation difficult.


Scenario 2: Conveyor slows down a century from now.
In such a scenario, cooling of the North Atlantic region may partially or totally offset the major effects of global warming in this region. Thus, the climate of the North Atlantic region may rapidly return to one that more resembles today’s—even as other parts of the world, particularly less-developed regions, experience the unmitigated brunt of global warming. If the Conveyor subsequently turns on again, the “deferred” warming may be delivered in a decade

Effects of Global Cooling

• Temperatures
  • Europe and North America ~20°C colder than present
  • Tropical regions ~ 2°C colder than present
• Development of ice caps
  • Lowers sea levels
  • Causes land bridges to form
  • Changes to flora and fauna
    
    
    Comparison of the ice sheets during the Last Glacial Maximum vs. our climate today.
    
    
    Image source: Global land environments since the last interglacial

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