GEOL 1301 Chapter Outline Chapter 19: Climate

1. THE CLIMATE SYSTEM - involves interactions between the earth's atmosphere, solid earth, biosphere, hydrosphere, and cryosphere (the frozen part of the hydrosphere).
   
   
2. WORLD CLIMATES - result from the combined effects of spatial and seasonal variations of temperature, precipitation, and to a lesser extent the other elements of climate. However, variations in the elevation and nature of the earth's surface also have significant influence.
   
   
3. CLIMATE CLASSIFICATION: The best-known scheme for classifying climates was developed by Wladimir Köppen, who used plant assemblages as reflections of climate to define the major categories. The five major divisions in the Köppen system are:
   1. Humid tropical (A): winterless, all monthly mean temperatures above 18°C (64°F).
   2. Dry (B) climates: precipitation rate less than potential evaporation rate.
   3. Humid middle-latitude climates with mild winter (C): coldest month averages below 18°C (64°F) but above -3°C (27°F).
   4. Humid middle-latitude climates with severe winter (D): coldest month averages below -3°C (27°F), and warmest month averages above 10°C (50°F).
   5. Polar climate (E): summerless, with warmest month averaging less than 10°C (50°F).
   
   
4. HUMID TROPICAL (A) CLIMATES
   1. The Wet Tropics (Af and Am) make up about 10% of the earth's land area, mostly confined to 5° to 10° north or south of the equator. Diminishing rainfall marks the boundary more frequently than decreasing temperature.
      1. The tropical rain forest (Af) is warm and experiences year-round precipitation of at least 6 cm (2.4 in) per month. Commonly, the temperature averages 25°C (77°F) year-round, and annual precipitation is greater than 200 cm (79 in).
      2. Monsoon (Am) climates have a short dry season.
   2. Tropical Wet and Dry (AW) climate characterizes a tropical grassland or savanna, which is distinguished from other A-type climates by having a pronounced dry season alternating with a rainy season.
   
   
5. DRY (B) CLIMATES - cover about 30% of the earth's land surface (more than any other climatic group). They are in the only one of the five major Köppen groups (A-E) that is defined on the basis of moisture rather than temperature. The boundary between dry and humid conditions is calculated (see Table 18.1, p. 495) from the precipitation and the potential evaporation (based on temperature). If R is defined as the annual precipitation in cm, and T is the average annual temperature in °C, then PE, the potential evaporation, is calculated as; 2×T, if 70% of the rain falls during the cooler 6 months of the year; 2×T + 28, if 70% of the rain falls during the warmer 6 months of the year; or as 2×T + 14, if neither half of the year has 70% or more of the total annual rain.
   1. Low-Latitude Deserts (BWh) and Steppes (BSh) - are centered in belts that follow the Tropics of Cancer (23.5° North) and Capricorn (23.5° South), thus lie under the subtropical high-pressure atmospheric belts.
   2. Middle-Latitude Deserts (BWk) and Steppes (BSk) - tend to be rainshadow deserts, situated leeward or downwind from a moisture-trapping mountain range, or isolated in the deep interior of a large landmass, far from any moisture source.
   
   
6. HUMID MIDDLE-LATITUDE CLIMATES WITH MILD WINTERS (C CLIMATES)
   1. Humid Subtropics (Cfa): The southeastern U.S. is a major area of this type of climate, characterized by a hot, humid summer and a definite cold (but mild) winter.
   2. Marine West Coast (Cfb): This climate type is observed on the west coast of North America from about Seattle to coastal Alaska, and much of northwestern Europe. Onshore winds dominate the weather, so that winters are mild and summers are comparatively cool.
   3. Dry-Summer Subtropics (Csa, Csb): This is sometimes known as a Mediterranean climate, since that area exemplifies it. In the U.S. it can be observed in California and Oregon: Los Angeles is more Csa, whereas San Francisco looks more like Csb.
   
   
7. HUMID MIDDLE-LATITUDE CLIMATES WITH SEVERE WINTERS (D CLIMATES) - These are known mainly in the interiors of the northern-hemisphere continents, since in the southern hemisphere this latitude belt is dominated by oceans.
   1. Humid Continental (Dfa, Dwa, Dfb, Dwb): Areas of these climate types are cut off from west-moving maritime air masses, so that there is less moderating effect on annual temperature ranges. Consequently, winters tend to be severe but summers are hot. Summers tend to be wetter than winters, because of easier access of mT (maritime tropical) air and the moisture it brings. Annual rainfall totals decrease toward the continental interior (that is, with distance from the ocean).
   2. Subarctic (Dfc): Also known as the taiga climate after the northern forest type, the subarctic climate is characterized by extreme cold, but with a brief summer interval. Thus these regions exhibit the largest annual temperature ranges that can be observed on the earth.
   
   
8. POLAR (E) CLIMATES - are cold year-round, although the sun will be in the sky for much of the time in the summer months, it never gets far above the horizon and contributes little heating. Although precipitation tends to be low, the evaporation rate is also low, so that water is commonly present (if only as ice for much of the year).
   1. Tundra (ET): Plant life is abundant, except for trees: the permafrost that is common in such areas inhibits deep root development. Organic matter decays slowly, thus can build up thickly in marshes or bogs.
   2. Ice cap (EF): Approximately 9% of the earth's surface is ice covered. This corresponds to virtually all of Greenland and Antarctica, plus small patches at high elevations elsewhere on the earth. All monthly mean temperatures are lower than 0°C (32°F).
   
   
9. HIGHLAND (H) CLIMATES - tend to be cooler and moister than nearby areas at that lie at lower elevations. Orographic lifting also tends to cause precipitation on the windward side of a mountain range, so that the leeward side of the range tends to be dryer, and the lowlands downwind may qualify as a rainshadow desert. Steep slopes and variable facing directions of slopes produce strong "micro-" or local climate variations, controlled by the amount of sunlight or the amount of rainfall striking the slope.
   
   
10. HUMAN IMPACT ON GLOBAL CLIMATE: Natural mechanisms that may cause climate change include such things as: cyclical variations in the earth's orbit (discussed in Chapter 5); changes in ocean circulation due to plate tectonics (discussed in Chapter 7); and volcanic emissions (discussed in Chapter 8). Most attention is currently focused on the connection between fossil-fuel burning and strengthening of the greenhouse effect. But there is evidence that humans have already been influencing global climate, for millennia prior to the start of the industrial revolution, by their agricultural practices.
    1. Carbon Dioxide, Trace Gases, and Global Warming
       1. Carbon dioxide and water vapor, both naturally present in the air, transmit visible light but trap infrared light from the earth's surface, warming the troposphere (greenhouse effect).
       2. Without some greenhouse effect, earth would be uninhabitably cold. However, human activities since the industrial revolution and perhaps millenia before, have had the effect of building up the abundance of carbon dioxide.
          1. Burning of fossil fuels (coal, petroleum, natural gas) produces carbon dioxide that is released to the atmosphere.
          2. Deforestation by burning (and use of wood as fuel) contributes additional carbon dioxide, and simultaneously reduces the ability of the environment to photosynthesize the carbon dioxide back to oxygen.
       3. Several different sets of data, including ice-core measurements and continuous 40-year analyses of air composition, show that an increase of carbon dioxide concentration is in progress. Temperature records have been debated more, but also tend to show that climate is warming and has been for some reason, if not human influence, over the last century or so.
       4. Other "trace" gases contribute to the greenhouse effect:
          1. Methane occurs naturally in swamps, wetlands, rice paddies, and in digestive systems of many animals. It also is produced as natural gas from wells. It is so strong as a greenhouse gas that it is better to burn it, as the resulting carbon dioxide produces less greenhouse effect.
          2. Nitrous oxide forms from breakdown of some fertilizer components and as a byproduct of running internal combustion engines.
          3. Chlorofluorocarbons (CFCs) include Freon and related compounds, which have now been taken out of production, but will not disappear immediately because they are long-lasting under some circumstances, and recycling is allowed.
    2. Some Possible Consequences of Greenhouse Warming - Modelling of future climate requires state-of-the-art computing power, proper assumptions, and accurate, detailed input data. Models that have been developed to date are unable to show local details, and not all of them agree on key points. At any rate, most of them predict that the tropics will warm slightly, and the warming effect will increase toward the poles. Rainfall patterns will be shifted, and water availability will change due to this and the evaporation-rate changes.
       1. Water resources and agriculture: alteration of water distribution patterns could render some current agricultural areas unproductive, while making other barren areas productive. Crops that can grow in a given region probably will also change.
       2. Sea-level rise: Melting of glaciers and icecaps adss water to the ocean. This, combined with thermal expansion of the ocean water, will cause increased flooding and possibly force abandonment of some coastal areas.
       3. New weather patterns: Predicted changes in weather include:
          1. Increased frequency and intensity of hurricanes
          2. Shifting in the paths of cyclonic storms, altering precipitation patterns and distribution of severe weather.
          3. Changes in intensity of heat waves and droughts.

Last revised x/x/x 2003 by M.A.Jordan