Lecture 6 background

Lecture 6 Background

Climate, Glaciation, and Stability

Ricklefs 6th ed. Chapter 3, p. 52-53

-Greenhouse effect: When the sun’s energy that reaches the earth’s surface is absorbed by vegetation, soil, and surface water, this heat is radiated toward space and absorbed by gases in the atmosphere.

-The greenhouse effect is beneficial to life by maintaining tolerable temperatures on earth. However, human activities have intensified this natural phenomenon.

-Mauna Loa study: Measurement of CO2 concentrations started being recorded in the 1958 at Mauna Loa, where there is relatively good air quality. With concentrations of 316 ppm (parts per million) in 1958, it has been found that there are strinking increases in the past decades: 352 ppm in 1990 and 384 ppm in 2007.

-Global warming models: Varying predictions of global temperature change exist. During the course of the 20th century, average surface temperatures on earth had increased 0.74 degrees Celsius. This temperature increase does not occur uniformly at all places on earth; humid tropical regions are more stable, and northern high latitudes experience the starkest increases.

-Global warming as a positive feedback mechanism: Warmer temperatures melt snow and ice, less snow and ice will result in lower albedo, and lower albedo will cause the earth to absorb more radiation from the sun, further increasing temperatures. Additionally, higher soil temperatures and thawing permafrost will cause more CO2 to be released in the atmosphere, due to increased respiration rates.

Ricklefs 6th ed. Chapter 4, p. 61-75

-Earth’s physical environment varies widely. In general, the climate is cold and dry toward the poles and hot and wet toward the equator.

-Global temperature and precipitation patterns are caused by solar radiation. Near the equator, the sunlight is more intense, as the sun strikes the earth at a higher angle, and thus a smaller area. Near the poles, the sun reaches the earth at a lower angle, and reaches a greater area.

-The earth is tilted at 23.5°, which causes seasonal variations in temperature (think northern summer-southern winter and vice versa). The solar equator, which is the latitude directly under the sun’s zenith (highest position) shifts due to the earth’s tilt, and ranges from 23.5°N to 23.5°S. This shift in the solar equator causes seasonal patterns of precipitation in the tropics.

-When evaporation and condensation tendencies in the atmosphere are balanced, equilibrium water vapor pressure has been reached. Above this equilibrium level, excess water vapor condenses and leaves the atmosphere as precipitation. Below this level, water evaporates from wet surfaces into the atmosphere. The equilibrium water vapor pressure increases with temperature, so warm air holds more water vapor than cold air.

-Hadley, Ferrel, and Polar cells are circulation patterns driven by differential heating of the earth’s atmosphere by the sun at different latitudes. Hadley cells have warm moist air rising in the equator, while Ferrel and Polar cells have air rising in the temperate region at 60°N and 60°s. Both Hadley and Ferrel cells have sinking air around 30°N and 30°S. Polar cells circulate toward the poles, with the air sinking around 90°N and 90°S.

-The rising of the warm air in the Hadley cells around the equator is also known as the intertropical convergence. As moist tropical air rises at this convergence, it cools and water vapor condenses, resulting in precipitation. At 30°N and 30°S, where the Hadley and Ferrel cells have sinking air, these regions are known as the subtropical high-pressure belts. As air sinks, its temperature and equilibrium water vapor pressure both increase. This causes the climate to be dry, and this is evident in the world’s deserts all located within the subtropical high-pressure belts.

-The intertropical convergence shifts throughout the year, following the shift of the solar equator. This causes seasonal variation in the tropics, as the convergence zone brings precipitation.

-Due to the rotation of the earth and the conservation of momentum, air currents moving north to south or vice versa in the Northern Hemisphere will veer right. In the Southern Hemisphere, the air will tend to the left. This is known as the Coriolis effect.

-Oceans currents redistribute heat, by bringing cold surface water toward the tropics and warm water towards the poles. Due to the Coriolis effect, the west coasts of continents are cold (path by which cold water goes toward the equator), and east coasts of continents tend to be warmer.

-When surface currents move apart, upwelling occurs. This is where deeper water moves upward towards the surface. Deep water tends to be nutrient-rich, and thus many upwelling regions have high biological productivity.

-Thermohaline circulation, also known as the ocean conveyor belt, moves water between major ocean basins. Based on ocean waters’ varying temperatures and densities (as a result of salinity), water sinks and rises and distributes heat from the tropics to higher latitudes. This ocean circulation has a strong influence on global climate conditions, as scientists believe that the thermohaline circulation shut down 12,700 years ago and caused drastic climatic changes.

-Due to its high specificity of heat, water gains and loses heat slowly. When seasonal temperature changes exist, temperate lakes will experience periods of vertical mixing and layered water columns. In the winter, water that is less dense floats to the surface and freezes, leaving the water below warmer and thus unfrozen. As the weather warms, the surface water also warms and sinks down, bringing oxygen down and forcing cooler water and lake-bottom sediments up. This causes nutrient cycling among the layers. In the summer, surface layers warm up much faster than deeper layers, and creates a thermocline, at which the temperature of the water changes abruptly. In the fall, the surface waters cool faster than the deeper waters, and the upper layers become denser and sink down, mixing the vertical layers. This mixing of the layers in the fall is similar to that of the spring and are known as the fall and spring overturn, respectively.

-El Niño Southern Oscillation (ENSO): This occurs on a 2-10 year cycle. During normal years, the cold Peru current flows along the South American coast and warms as it continues westward. Warm air then rises in the Western Pacific and loops back eastward to sink at the high pressure point on the South American coast. In an El Niño year, two loops form over the Pacific, with rising air at the low pressure region in the central Pacific. The rising air then travels both east and west and sinks along the Eastern Pacific and Western Pacific. Warm water collects along the North American coast. El Niño years bring cool, wet weather to the southern US and Mexico and warm, wet conditions to the northern regions. La Niña events accentuate normal conditions and bring colder waters to the eastern Pacific.

-Historical climate: Lake sediments can be used to determine past climatic patterns. Using oxygen isotope ratios, approximate temperature cycles can be determined.

Local example of land rebounding from glacial weight.
Beaver Island provides a nice local example of the effects of glaciers advancing and retreating. The island is in the northeastern part of Lake Michigan. The hills are the oldest part of the island because they stood 5-20 meters above the water level back when the melting glaciers formed Lake Algonquin. Later, as the glaciers continued to melt, the weight of ice on the land around the edges of the hills was reduced and the land rebounded to form the shoreline of the island during the Nippissing period (the name given the lake changes in each era). Geologists theorize that the inland lakes on the Beaver Island are in glacial channels formed during the Algonquin period that then became turned into independent lakes as the land rose upward in the Nippissing.. The current shoreline of the Lake Michigan era is one more step down. This too is land that rose as ice weight was removed. The island is rather like a wedding cake with three layers, except for a steep section along the western ridge where the Nippissing shelf is missing.
Peter Blair cites a lot of information about the ice albedo effect and El Niño and La Niña events in the Climate Denial Crock of the Week videos (yeah, I know I've written about them in two other articles, but just because I recommend them so much!)
[http://www.youtube.com/user/greenman3610] Click on the links to all videos there!

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