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Chapter 10 Air Masses and Fronts

   

 

   

 

  1. Past and Present Climate

   Climate determines how we dress, what we do for recreation, and how we make a living. Climate is important for a variety of applications, weather forecasting being only one. The climate of a region is an important feature in city planning.  You would not want to locate a waste processing plant upwind of a city center.  Housing designs would benefit if architects can take advantage of solar illumination to maximize energy gains during the winter and minimize them in summer.  The climate of a region also determines agriculture of a region. The spread of certain diseases is influenced by climate conditions.

   Describing today's climates is rather straightforward because of the large number of observations we have available for analysis. Determining past climates is more of a challenge.  But there is one thing for sure, there is abundant evidence that climate of a region is not constant; it changes in space and in time. Sometimes the change is gradual, others sudden. The challenge is to discover these past changes.

   Determining past climates is like solving a mystery.  We must search for evidence that identifies the meteorological character of a given time period.  The data must offer a consistent story while obeying physical laws.  Fossils, tree rings, ocean sediments, and air bubbles trapped in glaciers all provide clues to the mystery.  This combined evidence indicates that today's climate is fundamentally different than that of a million years ago. But, knowing that climate has varied does not explain the cause of these differences.  Explaining the reasons why the climate of a region has changed is like discovering the motive behind a crime. This chapter explores some of the methods that are used to make these discoveries.  So in the words of Sherlock Holmes

   Come, Watson, come!  The game is afoot.1

 

Definition of Climate

   

Climate is the collective state of the atmosphere for a given place, over a specified interval of time.

   Climate is defined as the collective state of the atmosphere for a given place over a specified interval of time.  There are three parts to this definition. The first deals with the state of the atmosphere.  The collective state is classified based on some set of statistics.  The most common statistic is the mean, or average. Climate descriptions are made from observations of the atmosphere and are described in terms of averages (or norms) and extremes of a variety of weather parameters, including temperature, precipitation, pressure and winds.

   The second part of the climate definition deals with a location. It could be a climate the size of a cave, the Great Lakes region, or the world. In weather and climate studies we are most interested in micro-scale, regional, and global climates. The climate of a given place should be defined in terms of your purpose. For example, in Chapter 11 we used climate data to study thunderstorms. We compared thunderstorm frequency over North America with the frequency of occurrence of large hail. This comparison helped us to understand the difference between ordinary and severe thunderstorms in that location.

   Time is the final aspect of the definition of climate. A time span is crucial to the description of a climate. Weather and climate both vary with time. Weather changes from day to day.  Climate changes over much longer periods of time. Variations in climate are related to shifts in the energy budget and resulting changes in atmospheric circulation patterns.

   Many factors determine the climate of a region. The five basic climate controls on the global and region scale are latitude, elevation, topography, proximity to large bodies of water, and prevailing atmospheric circulation.  Latitude determines solar energy input. Elevation influences air temperature and whether precipitation falls as snow or rain.  Mountain barriers up wind can affect precipitation of a region as well as temperature.  Topography also affects the distribution of cloud patterns and thus solar energy reaching the surface.  The thermal stability of water moderates the temperature of regions downwind of the region. Atmospheric circulation is somewhat less regular than the previous controls.  Large-scale circulation patterns exert a systematic impact on the climate of a region, such as the position of the ITCZ or the descending branch of the Hadley cell. These controls produce variations in temperature and precipitation that bring about our changing patterns of climate and weather.

Observations

      A fundamental challenge of modern science is to predict climate. Recent concerns about global warming and the effect of greenhouse gases added to the atmosphere by humans have heightened the need to understand the natural processes that cause climate variations. To gain this knowledge climatologists have turned to the past. Let's raise some questions about current and past climates that this chapter should investigate (Figure 14.1). Fossils indicate that the global climate during the dinosaur era, some 65 million years ago, was warmer than it is today. Is there additional evidence that supports the idea that the dinosaurs lived in a climate warmer than today's? Fossil evidence also indicates that the dinosaurs quickly died off on a global scale, suggesting an environmental catastrophe of a global nature. Is there other evidence to support this environmental cataclysm that led to the demise of the thunder lizards? Pre-historic paintings of hippopotami have been found in caves located in what are now the deserts of North Africa. Today, the climates of the North African deserts could not sustain such animals. Can we classify climates by the animals that live in the region?  What is a simple way to classify climate regimes?

Temporal Scale of Climate

   The most information about the variations of a parameter over a given time period is provided by a plot of the frequency of occurrence, or the number of observations in a given interval. Figure 14.2 is a plot of the frequency of occurrence, or histogram, of the annual average temperature of Madison WI, in intervals of 5F.  The average, standard deviation, and the maximum and minimum values are also given. This type of plot is very useful, but it is difficult to plot over a large region, which makes the mean of a variable very useful. Plotting the mean over time allows us to quickly identify fluctuations in time.

   Studying climate requires caution in comparing observations from different regions of the world.  Observations must be over the same time periods. Recent climate norms are determined by averaging weather elements over a 30-year time period. The average temperature in January is the average temperature of 30 consecutive years of the temperature during January at a location.  The most recent averaging period is the period 1961 through 1990.  The previous period was 1931-1960. An exceptionally warm winter may be termed abnormal because it falls outside the typically observed temperatures during this period. 

   Climatology is concerned with averages and variations about the average value. The annual total precipitation of Portland, Oregon (USA) and Montreal, Canada is similar; Portland has 39.8 inches and Montreal has 40.8 inches. The average precipitation for a month is nearly the same for the two cities.  Portland's average monthly precipitation is 3.32 inches, while that of Montreal is 3.4 inches. Figure 14.3 demonstrates that the distribution of this precipitation throughout the year is very different for these two cities.  Portland has distinct rainy (winter) and dry (summer) seasons while precipitation is evenly distributed throughout the year over Montreal. Variation about the mean is often expressed in terms of the standard deviation.  Small standard deviations indicate little variations while large values suggest broad variations. The following statement provides more information about the annual precipitation of these two cities. The monthly mean precipitation for Portland is 3.3 with a standard deviation of 2.2; Montreal has a monthly mean precipitation of 3.4 inches with a standard deviation 0.3. 

   Extreme values (e.g., record maximum and minimum temperatures) are observations that occur only rarely.  They are of particular interest in engineering.  For example, in designing a building it is important to know the highest winds a building will have to withstand.  When burying water pipes the maximum penetration of frost must be known to avoid any possibility of bursting pipes. Record temperatures for a particular day that are reported in weather reports are additional examples of extreme values of temperature.

Spatial Scale

   As with the different scales of weather, a discussion of climate must also specify the size of the area under discussion. The different climate scales, global, regional, and microscale, are indicated in Figure 14.4

   Atmospheric circulation patterns are of critical importance in determining the climate of a location.  On a global scale, atmospheric motions transport heat from the topics towards the poles.  Evaporation over the oceans supplies the water molecules that support precipitation over land.  These circulation patterns are in large part driven by energy differences between regions of the globe.  As discussed in Chapter 7, dry climates are associated with the descending branch of the Hadley Cell while moist climates coincide with the ascending branch. On a regional scale, precipitation on the lee side of a mountain is typically less than on the windward side.  On a still smaller scale, the amount of snow downwind of a snow fence is on average larger than the amount upwind (Figure 14.4).

Global

   Global climate is the largest spatial scale.  We are concerned with the global scale when we refer to the climate of the globe, its hemispheres, and differences between land and oceans. Energy input from the sun is largely responsible for our global climate (Box 14.1).  The solar gain is defined by the orbit of Earth around the sun and determines things like the length of seasons.  The distribution of land and ocean is another import influence on the climatic characteristics of the Earth.  Contrasting the climate of the Northern Hemisphere, which is approximately 39% land, with the Southern Hemisphere, which only has 19% land, demonstrates this.  The yearly average temperature of the Northern Hemisphere is approximately 15.2C, while that of the Southern Hemisphere is 13.3C. The presence of the water reduces the annual average temperature.  The land reduces the winter average temperature while increasing the average temperature during summer.  As a result, the annual amplitude of the seasonal temperature is nearly twice as great for the Northern Hemisphere. The Northern Hemisphere has a large variation in the monthly mean temperature.

   The land absorbs and loses heat faster than the water.  Over land, the heat is distributed over a thin layer, while conduction, convection and currents mix the energy over a fairly thick layer of water. Soil, and the air near it, therefore follow radiation gains more closely than water.  For this reason, continental climates have a wider temperature variation.  We observed this in Chapter 3 by comparing the seasonal cycles of temperatures for different regions of the globe. 

   Table 14.1 The average temperatures of the Northern Hemisphere and Southern Hemisphere for winter, summer and the year.  The Annual Range is give as well as the differences between the Hemispheres.  Differences between the Hemispheres are caused by the differences in the distribution of land and water.


  Winter Summer Year Annual Range
NH 8.1C (46.6F) 22.4C (72.3F) 15.2C (59.4F) 14.3C (25.7F)
SH 9.7C (49.5F) 17.0C (62.6F) 13.3C (55.9F) 7.3C (13.1F)
Difference -1.6C (-2.9F) 5.4C (9.7F) 1.9C (3.5F) 7.0C (12.6F)

Regional climates

   The major factors that determine global climate also influence climate on a regional scale. Regional climates are influenced by water bodies and mountain ranges. Lakes exert a moderating influence on local climate, in a manner similar to how oceans affect larger climate.  The Great Lakes are a good example for demonstrating the impact of lakes on climate.  We saw in Chapter 7 how the Great Lakes effect snow fall.  The Great Lakes also influence the temperature of the region.  Figure 14.5 shows the average land temperature versus the average surface lake temperature in the Southern Lake Michigan region. The temperature of the water is lower than the land from mid-March to August. Largest differences occur from mid-May to early June.  The water temperature is greater than that of the land from late August to mid March, with the largest differences in late November and early-December in late autumn and winter.  Exchanges of heat and moisture above the lakes is the key to weather modification by the Great Lakes.  The influence of large water bodies on the weather of surrounding regions is most marked when the temperature differences are greatest. .

   Large mountains influence regional climates.  They provide barriers for the air. Large mountain ranges that are oriented east-west can block cold air outbreaks from reaching regions that are more southern. You can observe this by comparing the annual mean temperature of a city south of large mountain barrier with a city at a similar latitude but with no mountain barrier.  Lahore, India, (31.5N) located south of the Himalayan Mountain Range has an average temperature of 12.8C, while the temperature of Austin, TX USA (30.25N) has an average temperature of only 10.4C.

   Vegetation also affects regional climate (Figure 14.6), an observation made obvious when comparing the wind speed within a forest with the wind speed at the same height over an open field. Friction reduces the wind speed in the forest, so open areas have greater winds.  The relative humidity is usually greater in a forest than in the surrounding open country. Forests depress the summer temperatures by 1 to 2 C (2-4F) below the annual mean in their vicinity. This temperature difference is driven by heat budget differences; less solar energy reaches the forest floor than the open field.

Microclimate

   Variations in climate can be observed over a short distance. Small-scale climates are referred to as microclimates.  Examples of microscale climates include the climate of a cornfield, a house, a patio, or a sand dune.  Microclimates can be very different across a particular region. Topography, presence or absence of water, exposure to the sun, and soil conditions are important factors that determine microclimates.

   The presence or absence of snow can be a good indication of differences in microclimates.  Variations in temperature due to differences in exposure to the sun affect accumulation and melting.  South facing slopes generally retain smaller snow amounts than north facing slopes (Chapter 4).  Snowdrifts are generated by rapid changes in wind speed due to the interaction of winds with obstacles.

Determining Past Climates

      

Paleoclimatology is the study of climate of the past and the causes for observed variations.

      A fuller understanding of past climates enables scientists to better predict future climate, including the impact of humans. Uncovering the global and regional climates of the past is like a solving a mystery. We look for evidence and compile this evidence into a consistent story. Paleoclimatology is the study of climate and climate change throughout geologic time. This section discusses some of the methods paleoclimatologists use to collect evidence. 

Bubbles in ice

   

Glacier a mass of perennial ice that originates on land through the accumulation of snow.

   Bubbles trapped in ice provide windows to the past for atmospheric chemists.  Air bubbles get trapped in glaciers and ice sheets (Box 14.2) as snow gets compressed.  These trapped bubbles provide a record of the concentration of trace gases such as carbon dioxide (CO2) and methane (CH4) over the past 200,000 years.  CO2 and CH4 are trace gases and thus only occupy a small fraction of the molecules in the atmosphere.

   Methane concentrations during the last ice age were approximately 350 ppbv (parts per billion by volume). Figure 14.7 shows the concentration of atmospheric CO2 and CH4 obtained for a 2,083 meter long ice core cut from Vostk, Antarctica.  Also shown on this figure are estimates of temperature changes during this period.  The warmer temperatures are clearly related to higher concentrations of CO2 and CH4. Approximately 150,000 years ago the concentration of CO2 was less than 200 ppmv (parts per million by volume) and CH4 amounts were approximately 300 ppbv. Both these gases are greenhouse gases. Increased concentration of a greenhouse gas can lead to a warming of the atmosphere. The amount of methane has approximately doubled from about 10,000 years ago.  This was a warm period in the history of our planet and is associated with increased concentrations of the greenhouse gases CO2 and CH4.

Dust in ice

   Ice sheets also provide valuable information on the frequency of volcanic eruptions.  Strong eruptions can inject dust into the atmosphere where it is transported across the globe and then settles onto glaciers.  Snowfall then covers the dust providing a long-term record of an eruption.

   Dry conditions can lead to soil erosion and the transport of the soil by the winds in the form of dust storms.  Dust storms (Figure 14.8) from the Sahara can transport dust as far as Greenland. So, dust deposits on ice may result from a change in precipitation, or a change in wind direction that is favorable for dust transport and deposition.  Either way they indicate something happened!  Dust on ice sheets is, like many observations discussed in this section, a piece of the climate puzzle, not the complete answer.  Sediments on the ocean floor provide another clue to past climates.

Sediments

   Materials have been deposited in layers (Figure 14.9) on the ocean floor for very long periods of time.  The deeper the layer, the older the material.  These deposits can include soil from wind erosion, soil from floods, ash from volcanic eruptions, and shells of animals.  In ocean sediments, the shells of animals are primarily calcium carbonate (CaCO3), a compound that makes up limestone.  The calcium carbonate is very useful for tracking past climates by the relative amounts of different oxygen isotopes.

   Most oxygen atoms have an atomic weight of 16.  This atomic oxygen is denoted as 16O. An oxygen atom can also have 2 additional neutrons, resulting in an atomic weight of 18 (18O). 16O is much more common than 18O. Water molecules (H2O) can incorporate both of these isotopes.  So, these two isotopes of oxygen are found in ocean water and in the shells and bones of plankton. The ratio of 18O to 16O (18O/16O) provides the clue to past climates.

   Foraminifera are microorganisms that live in the oceans and have hard shells of calcium-containing compounds, including calcium carbonate. The relative amount of the two isotopes 18O to 16O in the shells of these marine protozoans is related to the amount of continental ice.  The proportion of 18O to 16O is partly controlled by the volume of water in continental ice sheets. Since 16O is lighter than 18O, it can evaporate from the water more quickly.  The lighter water molecule tends to accumulate as snow and ice that form the glaciers.  As the ice accumulates, more of the 16O is bound in the ice sheets. So, during glacial times there is a higher concentration of 18O in the water. As foraminifera construct their shells they incorporate larger amounts of 18O than 16O because of its relative abundance.  So, as continental glaciers grow, there is less 16O in the oceans leaving higher ratios of 18O/16O in the oceans and thus in the shells. As the foraminifers die, their shells settle on the ocean floor and provide a record of the isotope ratio.  When we pull sediment cores from the ocean floor, we can obtain a record of the past 2 to 3 million years!  These cores have indicated a variation in the growth and shrinkage of ice sheets on repetitive time cycles of 100,000, 41,000 and 20,000 years. Figure 14.10 shows the departures from the average 18O/16O ratio over the past 300,000 years. Note that warm periods occur approximately every 100,000 years. 

Fossil records

   Fossils provide useful records into the past. They provide a means to track life through the ages because they are an integral part of the rocks in which they are found.  The age of the rocks can be dated. Fossils reveal ancient animal and plant life that can be used to infer climate characteristics of the past.  For example, tropical plants often have pointed tips so that the moisture can drip off the leaf.  Plant fossils that have pointed leaves indicate a moist tropical climate.  Large numbers of a given fossil also indicate favorable climate conditions for these organisms.

Water erosion

   Moving water, whether liquid or solid as in glaciers, leaves evidence of its movement. When a cold global climate warms, glaciers recede, leaving behind geological evidence of their former presence.  As they advance during cold climates, glaciers will leave scratches in hard rocks and smooth softer rocks.  As a glacier advances it pushes rocks of all sizes much like a bulldozer.  When it recedes, the rocks get left behind marking the glacier boundaries.  Called moraines, these rock deposits are recognized by the wide assortment of rock sizesfrom clay to boulders. Moraines are even found in eastern South America and in Africa south of the equator, indicating that these regions were once cold.

Tree Rings

   

Dendrochronology is the study of tree rings and is derived from the Greek words for tree and knowing the time.

   Tree rings can be used to glean information about climates of several thousand years ago. As climate classification schemes are often coupled to vegetation, it seems natural to use tree rings to peer into climates of the past. As a tree grows, the width of its trunk increases.  This growth appears as concentric rings (Figure 14.11).  The width of each ring indicates how fast the tree grew during a particular time period.  This growth is a function of available water, temperature, and solar radiation.  So, thick rings are indicative of favorable growing conditions while thin rings suggest poor growing seasons. The tree rings of different species are helpful in unraveling what caused the growth spurt or surpressed it. Figure 14.11 shows a reconstructed precipitation for Iowa using dendrochronology. This analysis indicates the dry 1930s, which were associated with the Dust Bowl. Very dry times occurred in 1820 and 1700. Some tree species are more susceptible to temperature variations while others are more sensitive to variations in water availability.  Old forests from around the world provide information on past climates. The science of studying tree rings is knows as dendrochronology.

 

Past Global Climates

   We divide Earth's history into four Eons.  Each eon, which can last billions of years, is divided into eras and periods.  An epoch of time is a subdivision of a period (Table 14.2). As we consider the climate of geological time we must discuss some geology. Continental drift, more correctly referred to as plate tectonics, is the movement of the continents. The theory of plate tectonics was proposed by Alfred Wegener who, when trying to interpret past world climate patterns, realized that the continents needed to be in different positions than they are today.  The movement of continents can be inferred by looking at a globe.  Notice how Africa and South America are like pieces of a jigsaw puzzle.  Puzzles pieces must fit together and have a matching pattern (Figure 14.12).  The same requirement must apply to fitting the continents together. Where the continents fit together, they also match in rock formation, glacier flow patterns (see Figure 14.12), fossil record, and continuity in mountain ranges.

   Approximately 300 million years ago the continents of today were merged together in one supercontinent referred to as Pangae (or Pangaea). Approximately 160 to 230 million years ago Pangae began to drift apart, eventually forming Laurasia and Gondwandaland. Laurasia consisted of what are today Asia, Europe and North America.  Gondwandaland was comprised of South America, Africa, India, Australia and Antarctica. As the continents continued to drift apart, some landmasses also collided forming today's mountain ranges, including the Himalayas and Rocky Mountain ranges. 

   If a continent drifted poleward, its climate would cool as its solar energy gains decreases. Animals and plants would have to adapt to the changing climate conditions, or move equatorward, or become extinct.  This next section gives a very brief outline of Earth's past climate (Table 14.2).  The discussion is based on evidence collected by paleoclimatogists, scientists who study ancient climates.

Precambrian Eon

   The Precambrian Eon begins with the beginning of Earth to the appearance of shelled organisms, or about 545 million years ago.  Evidence exists that suggests that ice sheets existed during the late Precambrian period.  There were probably two ice ages in the Precambrian, one about 2 billion years ago and the second about 1 billion years ago. These cold periods could be a consequence of low solar radiation, an increase in the albedo of earth due to the landmasses, or low concentrations of greenhouse gases. There is too little evidence available to determine the actual cause of this cooler climate.

Phanerozoic Eon

   The Phanerozoic Eon marks an evolution of life towards modern forms. The Phanerozoic Eon, which means visible life, is divided into the Paleozoic, Mesozoic, and Cenozoic eras, which correspond to ancient, middle and recent life, respectively.  We will discuss the climate of these eras, and their periods, separately.

        • Paleozoic Era

   The Paleozoic era was from 545 to 250 million years before present (BP). Abundant fossils appear at the beginning of the Paleozoic era.  Many of the living creatures represented by these fossils became extinct by the end of the era.  In fact, fossil evidence indicates that approximately 90% of all marine species living during this era became extinct. The trilobite (figure 14.13) is a good example of the one of the life forms that existed during the Paleozoic.

   Trilobites were crab like animals that appeared near the beginning of the Paleozoic era and became extinct at the end of the era.  The hard exteriors of these animals were more easy to fossilize than the soft-bodied organisms that existed before them. This accounts for their abrupt appearance in the fossil record. But what caused them to flourish?  Paleoclimatologists turn to plate tectonics to answer this question. During this time a new supercontinent (Pangae) was being formed. Coastal upwelling (Chapter 8) of deep ocean waters bring up the required nutrients for the formation of shells.  As the continents drifted, they formed new continental shelves that provide opportunities for upwelling and marine life to flourish.  What caused the wide spread mass extinctions that occurred at the end of this period? This extinction event is known as the "Permian crisis". Let's look at climate events leading up to the extinction.

   During the Cambrian period there was evidence of a wide spread ice age. The evidence for this ice age lies in the moraines of Greenland, Scotland, China, Scandinavia, South America, southern Africa and Australia.  There was a sudden blooming of life after this ice age ended. The first evidence of fish coincides with the Ordovician period. Evidence of plants and animals emerging onto land came in the Middle to Late Ordovician, about 400 million years BP.  This suggests that the ozone layer was thick enough for life to move out of the oceans.  This would lead to profound changes in atmospheric chemistry. As plants flourish the concentration of oxygen increases due to photosynthesis.  Plants would also curtail land erosion, reduce the surface albedo, and thus change the energy budget of the planet.

   Most evidence indicates that the Devonian period was warm and dry. The appearance of seeds and leaves occurred during the Devonian Period, indicating the presence of plants on land. Animals soon followed the appearance of the plants.  Evidence for animal life is found in burrows preserved in sedimentary rocks that are 450 million years old. The first amphibians and trees appeared in the Late Devonian Period which heralded the coming of the next geological period-the Carboniferous Period.

   Coal beds formed during the Carboniferous Period provide the evidence for paleoclimatogists.  The hot and humid climate supported abundant swamps needed to form coal. The hot and humid climate is consistent with evidence that the landmasses were located in the tropics. Amphibians flourished during the Carboniferous and Permian Periods. Reptiles appeared during the middle of the Carboniferous era.  The ice sheets began to recede in the Permian period and deserts began to expand as climates once again shifted.  The dry times of the Permian Period extended into the Triassic Period, the beginning of the Mesozoic Era.

   The rock record indicates that at the end of the Paleozoic there was a major global climate change.  Areas with lush vegetation became desserts. The world was cold during the late Paleozoic era of 300 million years ago. Increased volcanic activity, associated with the moving continents, could be a key in this climate change. During the late Paleozoic, glaciation occurred in southern Africa, South America and Australia at a time when land masses merged in a single large continent in high southern latitudes. As the continents merged, species once separated now became competitors for survival.

        • Mesozoic Era

   The Mesozoic Era, between the period 250 to 65 million years BP had a warm climate.  There is no evidence that suggests the presence of ice sheets during the entire Mesozoic Era. Even Greenland and Antarctica were subtropical type climates. Average air temperatures were probably 15F (8C) warmer than today. Deep-water sediments indicate that water temperatures were about 15 to 20 C warmer than today. This is consistent with evidence that concentrations of CO2 where higher in the Mesozoic Era than today. The increased amounts of CO2 resulted from out-gassing by volcanoes. The active volcanoes were associated with the fragmentation of Pangea.

   The Mesozoic Era was the age of dinosaurs, which first appeared in the Triassic period.  Tropical and subtropical climates supported dinosaurs. Much of the Mesozoic Era had a single landmass, Pangaea, that extended from high southern to high northern latitudes.  One large landmass probably facilitated strong heat transport by the oceans, which led to an even distribution of global temperatures. Towards the end of the Mesozoic Era mountain building began as Pangaea broke up and landmasses drifted northward. The weather became cooler and drier and the continents continued to drift northward.

   Fossil evidence suggests that the middle Cretaceous was also warmer than the present. During this time period the climate was warm and moist.  Plants flourished and would eventually supply the large amounts of carbon needed to form oil and coal deposits. 

   The Cretaceous period ended quickly about 65 million years ago.  At the end of this period there was another climate change and another mass extinction. The dinosaurs were only one of the species that became extinct.  Approximately 75% of the total number of living species became extinct. The cause of this mass extinction is not positively known. A well-defined clay deposit marks the transition from the Cretaceous era to the Tertiary period.  This boundary is known as the K-T boundary.  This K-T boundary contains large amounts of iridium, a valuable clue in determining how this rapid climate change occurred.

   The iridium content of Earth's crust is small compared to the iridium in asteroids.  The iridium rich K-T layer suggests the impact of a large asteroid. In 1990, evidence of a collision with an asteroid 65 million years ago was found near the Yucatan Peninsula (Figure 14.14). Named for a local village, the Chixulub crater is a 112-mile wide impact crater visible in gravity and magnetic field data. The crater size is consistent with a 6 to 12 miles wide asteroid! Such an impact would result in devastating environmental consequences.  Fires would immediately ignite and spread outward from the impact point.  The impact would inject large amounts of soil throughout the atmosphere. Fine aerosols would remain in the stratosphere and block out sunlight. The larger particles would be thrown into the stratosphere and eventually return to Earth due to gravity.  As the particles returned to Earth, they would heat up and eventually become molten.  Their impact would start additional fires far from the impact site. The impact would cause a heating of the atmosphere generating large amounts of nitrous oxides.  As these nitrogen oxides reached the stratosphere, they would destroy the ozone layer.  Nitrogen oxides that did not make it to the stratosphere would become acid rain, further stressing plant life. Huge waves would be generated by the impact. There is evidence that around this time a tsunami 1000m in height hit the Caribbean and moved hundreds of miles up the Mississippi valley.

   A large meteor impact would generate a great deal of heat, dust and debris and a layer of fine dust particles that would shroud the planet for several weeks or months.  This dust would scatter more sunlight out to space and prevent it from reaching the surface.  The reduced sunlight would cause a climatic cooling and inhibit photosynthesis causing plants to die. Plants are an important part of the food chain without which many species would perish. Fine dust and soot in the 1 cm thick world-wide K-T boundary layer indicates that the resulting darkness would cut-off photosynthesis and reduce visibility to the point where even scavengers would have a hard time surviving. The combination of the effects of fires, acid rain, ozone loss, and lack of sunlight would be stressful for many species and may explain the rapid extinction of so many species at once.

Cenozoic Era

   The early Cenozoic Era was about 3C warmer than today; however, the Cenozoic Era is characterized by a climate that was cooling. This era has two periods: the Tertiary and the Quaternary. The Tertiary, which began 65 million years ago and ended with the Ice Age 1.6 millions years BP. A prolonged cooling between approximately 58 and 35 million years ago led to the formation of glaciers in Antarctica.  The Quaternary period includes the present time back to the Ice Age.

   Much of the United States was located at subtropical latitude at the beginning of the Cenozoic Era. The continent slowly drifted northward causing a cooling.  Based on pollen records, the Utah region was largely forested 20 million years ago.  These forests were replaced by grasslands and are now deserts.

   About 10 million years ago glaciers formed in the mountains of the Northern Hemisphere.  Approximately 2 to 4 million years ago the cooling resulted in ice over lowlands as well as the oceans of the North Pole.  There were two centers of ice sheets that grew and receded during this period.  One center was in the Canadian Rockies; the second center was located at the Laurentian Shield of eastern Canada.  There were several major advances of the continental ice sheets during the Cenozoic Period.  The geological evidence left during glacial advances and retreats (Figure 14.15) indicates this.

   During this global Ice Age the global temperature decreased by about 5C.  Tropical regions cooled by about 1 to 2C (1.5 to 3.6F) while the interiors of continents in the midlatitude regions were about 10C cooler. Weather conditions where drier and there was less precipitation. Because the landmasses were closer the equator, the temperature gradient was strongest close to the equator. The large pole to equator temperature gradient probably resulted in stronger winds.

      • The Quaternary Period

   The Quaternary period marks the last 1.8 million years.  This is the time period in which Homo sapiens developed. The advance and retreat of the glaciers during this period is inferred from evidence of the glacial deposits.  This advance and retreat is also supported by 18O/16O isotope ratios. The advance and retreat of the glaciers occurred in both hemispheres, telling us that this was a global phenomenon and not a regional climate change. The Quaternary Period has two epochs, the Pleistocene and the Holocene Epochs. The Holocene Epoch starts with the recent glacial retreat of about 10,000 years ago and is considered an interglacial period.

   As we discuss the climate record of the past 1.8 million years, we no longer need to consider continental drift or mountain building as they are essentially the same as today. The cooling trend that began about 53 million years ago peaked in the Ice Age, or the Pleistocene Ice Age.  The Ice Age however, was not a time of complete ice coverage.  During this cold period glaciers both expanded (a glacial climate) and receded (an interglacial climate).  Interglacial periods typically lasted about 10,000 years. During these times Greenland and Antarctica remained ice covered.

   The last great ice age occurred during the Pleistocene. There were two main pulses of glacial episodes, one about 115,000 years BP and the second around 75,000 years BP. The first glacial period added ice to the polar caps.  The second glacial period increased the ice coverage of North America and Eurasia. The extent of the ice sheet is shown in Figure 14.16. There is evidence that the Laurentide ice sheet in North America extended into what is now southern Illinois.  The ice sheets were about 4000 meters thick. During this extensive ice age, water was locked-up in the large ice sheets and as a result sea level was about 120 m lower than today. Large regions of continental shelf were then dry land. The lower sea levels allowed migrations of land animals between Asia and North America using the Bering land bridge. The glacier reached its maximum coverage about 18,000 years ago when approximately 30% of Earth was covered by glaciers. Compare that to today's coverage of approximately 2.25%.

   Figure 14.17 shows the change in the average temperature over the past 18,000 years. The maximum coverage of the glaciers 18,000 years ago is consistent with a cold period during this time. A relative cold period also existed between about 11,000 and 10,000 years ago and is known as the Younger Dryas (named for the reappearance of a polar wildflower (Dryas octopetala) in Europe).  The Younger Dryas was marked by the advance of some ice sheets in Scotland and Scandinavia, and cooling of regions in the North Atlantic.  The cause for this cooling during the Younger Dryas is unknown. It has been suggested that the melting of the North American Laurentide ice sheet changed circulation patterns of the deep ocean.  Northward transport of heat by warm ocean currents could cause the regional cooling associated with the Younger Dryas.

   About 11,000 years ago the glaciers rapidly melted. Glaciers made one last advance about 10,000 years ago and then their latest retreat. Rapid melting of glaciers caused sea levels to rise at a rate of 1 centimeter per year (Figure 14.18). 

   During the early Holocene, from 10,000-5,000 years ago, the Northern Hemisphere was warmer than today. The midlatitude temperatures were about 2.5C higher and the sea level was several meters higher. Northern Africa was very wet between about 9,000 and 6,000 years ago.  Cave drawings of hippopotami, and various hoofed animals in the Sahara and Saudi Arabia support this.  Sediments in the Arabian Sea indicate that precipitation over India was greater than present.

   The warm climatic period in the Halocene between 900 and 1200 AD is referred to as the Medieval Climatic Optimum.  The medieval warm period resulted in thriving vineyards in the British Isles. The northern polar ice caps retreated towards the North Pole enough for the Norse to sail across the Atlantic and settle in North America, Greenland, and Iceland.  By 930 AD the Vikings established a settlement in Iceland, just south of the Arctic Circle.  Around 980 AD, two Norse settlements were made in Greenland. These settlers lived by raising cattle, sheep, and goats in addition to hunting harp seals in spring and caribou in autumn. After this climatic optimum, the climate became cooler and drier in the subtropics.

   After 1200 AD, the climate cooled again, which resulted in famines and plagues. Winters were colder by the end of the Roman Empire.  Evidence for a colder climate is given by the height of snow line in the mountains (Figure 14.19). The snow line is the lower limit of the snowcap on high terrain at any given time. A low snow line indicates a cooler climate than a high snow line. The sea ice advanced, isolating the colonies of Greenland settled previously. It appears that the settlers suffered during the Little Ice Age, which prevented agriculture as the coastal regions of Greenland froze. Wetter summers also made it difficult to dry hay for livestock and the colder, longer winters made it difficult for young livestock to survive.  The migration of harp seal and caribou also changed with the new weather conditions. By about 1500 the Norse settlements were empty. Climate change played a role in the demise of these Norse settlements but there might have been other factors.  The Inuit were able to survive in this harsh climate, perhaps because their hunting skills were more appropriate for the changing hostile conditions.  This cooling heralded a new climate region known as the Little Ice Age.

   

Little Ice Age is a period of expansion of mountain glaciers in the Alps, Alaska, Iceland, and Norway during the 16th and 17th centuries.

   The period between about 1400 and 1850 is called the Little Ice Age.  The coldest period, with the greatest advance of mountain glaciers, occurred around 1750.  In geological terms, the Little Ice Age started and ended very quickly. Around 1570 Europe was 1 to 2C cooler than today. The Thames River in London froze over 11 times in the seventeenth century, and twice as many times between 1564-1565 and 1813-1814. The Thames River has not frozen over in the last 100 years!

   The Little Ice Age can be documented by dating moraines and ice core analysis, and dendroclimatology.  In addition, historical documents reveal the extent of the Little Ice Age. Records indicate that the 1590s and 1690 were about the coldest time recorded in Europe. Such information tells us that although the Little Ice Age was global, the dates of glacial advance varied with location.  The European Alps saw an advance a century before the glaciers in Norway.   The year 1816 was an extreme year in the Little Ice Age for North America and northwestern Europe.  In New England the corn crop was all but destroyed as crops froze.  The hay crop was so small that few animals could be kept over winter. Part of this cooling was likely due to the eruption of Mt. Tambora (See chapter 4)

      From 1850 to about 1940 temperature rose worldwide and the glaciers retreated again.  The warm summers of the 1930s contributed to the Dust bowl era of the Great Plains.  Since the 1970s there has been a steady warming of the global mean temperature. The five warmest years of the 20th century all occurred in the 1990s. For climatologists, the main question associated with this warming is the cause, not whether a warming is occurring.  Discussion of this question centers on the role of human activities in this warming versus natural climate variations.  We present these issues in the next chapter. Next we consider the climate of today

Climate Classification: Precipitation and temperature

   There are many different regional climates across the world.  To make sense of this variability we devise classification schemes in which important characteristics of a phenomenon are grouped into classes that have things in common. Classification is a process common to all sciences.

   The goal of classification is to identify and group together things that have similar characteristics.  Climate classifications describe the world's climates.  The problem with classifying climates is that there are few sharp dividing lines between different climates.  There are gradual transitions from one climate to another.

   A challenge in designing a climate classification scheme is that climates fluctuates and transition zones often exist between two very different climate regions, making sharp boundaries difficult to establish. One of the simplest climate classification schemes is based on solar illumination. This approach does have sharp boundaries.

   Temperature and precipitation are two important climate variables.  These two parameters typically define the type of vegetation that can grow in the region.  It is therefore useful to classify climate according to these variables. The most widely used climate classification schemes are those of K�ppen.  Vladimir K�ppen developed his classification system from 1918 to 1936. He used vegetation and temperature as a natural indication of the climate of a region. There have been improvements made to the original K�ppen scheme, particularly by Trewartha and Horn.  The current classification scheme has six main groups, each designated with a letter: Tropical Moist (A), Dry (B), Moist with Mild Winters (C), Moist with Severe Winters (D), Polar (E), and Highland (H). Figure 14.20 organizes these climate groups by their temperature and precipitation characteristics.  Climates A, C, D, E, and H are based on temperature while B climates focus on precipitation differences. 

   

Climographs are plots of climatic data. Climographs usually consist of two climatic elements plotted through an annual cycle.

   Division of the six main categories provides additional classifications. The next sections provide more detail on the climate in each major group.  Figure 14.21 shows the global distribution of this classification scheme. We will use climographs to depict the characteristic monthly mean temperature and precipitation of each of these climates.

Tropical Humid Climates (A)

   The mean monthly temperature of Tropical Humid climates is high, no lower than 65F (18.3C) with small annual variations, typically less than 18F (10C).  Killing frosts are absent in A type climate regions. The diurnal variation in temperature in A climate regions is often larger than the annual variation.

        • Tropical Wet, dry and monsoon (Af, Aw and Am)

   While Tropical Humid climates have abundant rainfall (typically more than 100 cm or 39 in per year) they can have different precipitation patterns. A type climate zones are therefore subdivided into three subtypes: tropical wet climates (Af), tropical wet-and-dry climates (Aw), and tropical monsoon (Am).  Examples of these climates are Iquitos, Peru (Af), Asunicion Paraguay (Aw), and Manaus, Brazil (Am). Figure 14.22 shows the annual temperature and precipitation patterns of these cities.

   The tropical wet climates, Af, have temperature that is distributed fairly uniformly throughout the year.  Total precipitation over a year averages between 6.9 -10 inches (17.5 - 25.0 cm). The precipitation amount for each month is at least 2.4 inches (6 cm). Af also has a diurnal precipitation pattern, with most thunderstorms occurring in the afternoon, triggered by solar heating of the surface. Vegetation in Af climates is very lush, such as the tropical rainforests of Brazil and the Congo.

   Tropical wet-and-dry climates, Aw, have a dry season. Summers are wet and winters are dry in Aw climates.  This seasonal rainfall is linked to the season migration of the InterTropical Convergence Zone.  There is a cool season in Aw climates, which occurs during winter. Analysis of Figure 14.19 shows that Aw often border Af.  The vegetation of Aw climates is typically savanna or tropical grasslands with scattered deciduous trees, as in the grasslands of Africa.

   Tropical monsoon climates, Am, are monsoon climates that have a short dry season. Monthly average temperatures of Am climates are uniform throughout the year. These climates tend to occur in regions that have seasonal onshore winds to supply an ample supply of moist air.  Orographic lifting also helps to enhance the precipitation of Am regimes. 

Dry Climates (B)

   Dry climate zones (B) are located in regions where evaporation exceeds precipitation. Rainfall is highly variable in these B climate zones.  Most of the land regions of the world are designated as B climate zones!  The descending branch of the Hadley cell or a rain shadow caused by mountain barriers causes lack of precipitation in many of the B climate zones. 

        • Semiarid and Desert (BS, BW)

   There are two subtypes of the B climate: steppe or semiarid (BS) and arid or desert (BW). Inspection of the climate zone map indicates that the BS are situated between humid climates and desert climates and are thus transition zones. 

   Dry climates span from the tropics to the poles.  The mean annual temperature is a function of latitude. We must distinguish dry climates according to temperature. So, BSh and BWh are warm dry climates, typical of tropical regions. Figure 14.23 plots the temperature and precipitation of Dakar, Senegal and Cairo Egypt, examples of BSh and BWh climates, respectively. The precipitation peak over Dakar results from the ITCZ.

   Hollywood's portrayal of deserts is usually one of a hot sweltering day, with an intense sun and large sand dunes. The cities of Dakar and Cairo tend to conform to this description.  But not all dry climates are hot tropical deserts. BSk and BWk climates are cold dry climates of the higher latitude regions (Figure 14.24).  BSk and BWk climates typically have more precipitation, and less evaporation, then their counterparts, BSh and BWhBSk and BWk are therefore typically more humid than the hot tropical desert climates, but both BSk and BWk have small enough amounts of precipitation to be classified as dry climates.  BWk climates are often located in the rain-shadows of large mountain ranges or the interior of continents. BWk climates have warm to hot summers and cold winters.  BSk climates are mid-latitude steppe.  BSk climates have similar annual temperatures to the BWk; the difference between the two climates is in total annual precipitation.  BSk typically have more precipitation than BWk climates.

Moist Subtropical Mid-latitude Climates (C)

   Moist subtropical and mid-latitude climates are characterized by humid and mild winters.  At least eight months of the year have temperatures above 50F (10C), with the coolest month below 65F (18.3C) and above 27F (-3C).  Geographically, the subtropics lie between the tropics and the middle latitudes; however, subtropical climates also often lie in the middle latitude regions.  This is where the largest annual temperature ranges are observed as tropical and polar air masses govern the weather at different times of the year.  In the tropics, seasons are distinguished by wet and dry cycles; in the middle latitudes seasons are distinguished by annual variations in temperature.  In the tropical regions plants go dormant with a lack of precipitation.  In subtropical climates, plant go dormant due to low temperatures.

   There are three major sub-groups, the marine west coast (Cfb and Cfc), humid subtropical (Cfa and Cwa), and the Mediterranean (Csa and Csb).

Marine West Coast (Cfb, Cfc)

   Summers and winters of marine West Coast climates are typically mild with no dry season.  The Cfb regime has a warm summer while the Cfc has a cool summer (Figure 14.25). Cfb and Cfc climates are usually near the coast. The characteristic temperature and precipitation is determined by the advection of air over ocean currents. This moderates the annual range in temperature. When cool water is up wind, the summer high temperatures are primarily moderated, while warmer ocean currents lead to milder winter temperatures. The coldest month of the year has an average temperature above freezing, making snowfall rare. The name of this climate type, marine west coast, suggests that these climates lie along the west coasts of continents. However, these climates are also found along southeastern Australia and southeastern Africa.

Humid sub-tropical (Cfa, Cwa)

   Humid sub-tropical climates have hot summers (Figure 14.26).  These climate regimes occur in the mid-latitudes regions.  Daytime high temperatures typical of this regime are in the 80 to 90F range.  The humid conditions, dewpoints in the 70s, keep the low temperatures in the evening from getting very cold. Winter temperatures are mild. While mean temperatures may be above freezing in winter, it is not uncommon for the temperatures to drop below 32F (0C).  Precipitation in humid sub-tropical climates is plentiful, 30 to 100 in (75 to 250 cm) per year.  Summer precipitation is usually associated with convection and mid-latitude cyclones bring the winter precipitation.  Cfa climates are wet all year round while Cwa regions have a brief dry season in the winter. Summer precipitation in both climates is primarily convective. 

Mediterranean (Csa, Csb)

   Mediterranean climates are characterized by little precipitation in the summer (Figure 14.27).  This particular climate classification has a peak in precipitation during winter.  The lack of precipitation in summer is associated with the presence of a high-pressure system that moves into the region and stays.  Summer temperatures range from hot to mild and winter temperatures are mild. When located along a coast, winter temperatures are very mild. Winter temperatures can drop below freezing if far from the modifying influences of a large body of water. 

Severe Mid-latitude Climates (D)

   The severe mid-latitude climates (D) are located in the eastern regions of continents. So, the temperature range of the D climate regimes is generally greater than C climate types, which tend to be located on the west side of continents. The average temperature of the coldest month of a D type climate regime must be less than 27F (-3C). These climate types typically have snow on the ground for extended periods of time. There a two basic D climate types, humid continental and subarctic. These subgroups are further divided into groups based on precipitation and summer temperature.  The second letter f indicates that the climate has no dry season, while a second letter of w indicates a dry season in winter. 

   For D climates, a third letter of a, b or c indications a hot summer, a warm summer, or a cool summer, respectively. A hot summer climate has a warmest month of about 72F (22C) with at least four months about 50F (10C).  Warm summer are defined to have at least four months with average temperatures above 72F, but the warmest month has a temperature less than 72F. Cool summer climates have only one to three months with a mean temperature greater than 50F (10C). Finally, a D type climate with d as a third letter indicates an extremely severe winter with a cool summer.

Humid Continental (Dfa, Dfb, Dwa, Dwb )

   Humid continental climates have a large range in temperature; each has severe winters and cool-to-warm summers.  The climates in the subgroup denoted by an f (e.g. Dfa, Dfb) do not have a clear dry season. Examples of the Dfa and Dfb climates are Fargo, North Dakota, USA (Dfb) and Vladivostok, Russia (Dwb) (Figure 14.28).  Both cities have a large annual temperature range.  Vladivostok has a strong summer time maximum in precipitation, while Fargo's monthly averaged precipitation is more evenly distributed throughout the year. 

Subarctic (Dfc, Dfd, Dwc, Dwd)

   Subarctic climates have a very large range in annual temperature.  Winters are very long and cold.  Summers are brief and cool. Fairbanks, Alaska, USA (Dfc) and Verkhoyansk, Siberia (Dfd) are examples of subarctic climate regimes (Figure 14.29).  Both have very cold winters, monthly averaged temperatures below freezing.  Monthly averaged temperatures for both cities are below freezing for 5 months. In this climate regime, monthly mean temperatures that are below freezing can occur for up to seven months! Precipitation is greater in summer than winter for both cities. The poleward displacement of the mid-latitude cyclones leads to this maximum precipitation in summer.

Polar Climates (E)

   Polar climates (E) occur poleward of the Arctic and Antarctic circles. Polar climates are extremely cold and have little precipitation (Figure 14.30).  The mean temperatures of polar climates are less 10C (50F) for all months. This cut-off temperature is the minimum temperature for tree growth. Precipitation, mostly frozen, is less than 25 cm (10 in) of melted water. They have a marked seasonal temperature cycle that corresponds to the solar input.

   A distinction is made between two polar climate types: tundra (ET) and ice caps (EF).  This distinction is made based on the warmest month being warmer (ET) or colder (EF) than 0C (32F).  Greenland and the Antarctica Plateau are examples of EF climates. EF climate zones have essentially no vegetation while tundra occupies ET climate zones.  The vegetation of a tundra is primarily mosses, lichens, flowering plants, and some woody shrubs and small trees.  ET regions have a layer below the surface that is perennially frozen, a condition referred to as permafrost.  During the summer, enough energy is received so that the top layer of soil thaws.  This causes the tundra to become wet and swampy.  About a meter below the surface the ground is still frozen.  This frozen layer may extend to hundreds of meters.

   Precipitation in Polar climates is very low, sometimes less than the tropical deserts.  However, these regions are not considered deserts because precipitation exceeds evaporation.

Highland Climates (H)

   The elevation above sea level is an important climate variable. Highland climates (H) characterize the type of climates associated with high mountainous terrain. These climate zones are complex and driven by changes in latitude, altitude, and exposure. A wide variety of climates are exhibited in H climate zones.  As noted in Chapter 3, temperature usually decreases with height in the troposphere. The temperature of a mountain location will strongly depend on the slope angle and aspect, which influences the amount of solar energy received. A common feature of a highland climate is the large diurnal temperature variation.  Rapid daytime heating and nighttime cooling results because of the thin dry air.

   Mountains have a large variation in precipitation. The amount depends on the orientation of the highland, atmospheric moisture and prevailing wind directions.  The leeward side is a rain shadow while the windward side can have heavy rainfall.

 

Summary

   Climate varies from location to location, and with time. Climatology and paleoclimatology attempt to organize the complex and varied climates of the world through classification systems. It is quite amazing that, through scientific evidence, we have a general idea of Earth's climate since the beginning of Earth's history.

   Evidence indicates that during most of the Earth's past 500 million years the Earth was a more genial climate than today. This mild climate was interrupted by occasional ice ages.  Climatic zones where present, but were less distinct than they are today.  Several theories have been put forth to explain the change between glacial and interglacial periods. Continental drift is an important part of understanding very long-term climate changes and was discussed in this chapter. Additional theories, variations in the Earth's orbit, changes in the output of the sun, and changes in atmospheric composition, are discussed in the next chapter.

   Earth's history is divided in eons. The first geological eon is the Precambian.  The small amount of geological evidence indicates that glaciers existed during this time. The Paleozoic period saw the beginning of life, glaciers in the Southern Hemisphere and expanding deserts.  Hot and humid periods during the era provided the plant life that eventually created today's extensive coal and oil fields. The Mesozoic era was the age of the dinosaur and there is no evidence of ice sheets during this warm period in geological history. The current geologic era is the Cenozoic era, which began about 65 million years ago.  For the past 20,000 years, records of climate are rather complete. Over this period of time the Earth's climate changed from a period of extreme glaciation to a warm interglacial period.  Evidence points to a cold period between AD1400 and AD1650, and is known as the Little Ice Age.  This period also includes the Medieval Climatic Optimum (900-1200AD), when the climate was warm. Over the past 20 years, global temperatures have steadily increased.  Some of this increase may be a result of human activity, as discussed in the next chapter.

   Geological time is classified by eras based on recognizable events in Earth's history. Today, the K�ppen based climate classification is the most widely used scheme for mapping climates of the world. The K�ppen scheme is just one method of classifying climate. Different schemes could be developed based on the purpose of the user. For example, the K�ppen scheme would not be very useful for wind energy applications. A scheme could therefore be devised that is based on wind speed and direction. For hydrological applications we might want a scheme based on the type of precipitation, not just monthly mean precipitation.  One could also develop a classification scheme based on the energy budget of a region.

 

        • Terminology

   You should understand all the following terms. Use the glossary and this Chapter to improve your understanding of these terms.

   Cenozoic Era

   Climate

   Climographs

   Dendrochronology

   Dry Climates

   Highland Climates

   Isotopes

   K�ppen scheme

   Little Ice Age

   Medieval Climatic Optimum

   Mesozoic Era

   Moist Subtropical Midlatitude Climates

   Paleozoic Era

   Pangae

   Precambrian Eon

   Polar Climates

   Severe Midlatitude Climates

   Tropical Humid Climates

   Younger Dryas 

 

   

        • Review Questions
  1. How might you design a climate classification scheme that is based on the frequency of air mass types?
  2. What type of parameters should be included in a climate classification scheme that would be useful to farmers?
  3. Describe the climate where you live.
  4. What is the difference between weather and climate?
  5. What features separate a tropical climate from the climate of Polar Regions?
  6. What type of parameters should be included in a climate classification scheme that would be useful to city planners?
  7. How would you classify the climate of the dinosaur era in terms of the K�ppen classification scheme?
  8. A million years from today, human activities might be used to determine climate signal. Describe how a human activities of today, could leave behind a useful signature of today's climate?
  9. Explain why the deeper ice layer on a glacier is, the older it is.  Is this always the case for rock layers?
  10. Why is a layer of coal an indicator of a time with a warm climate, rather than a cold climate?
  11. What is the Little Ice Age?
  12. Why are lower sea levels associated with ice ages?
  13. What is continental drift and why is it important in climate change studies.
  14. Describe one theory on how the dinosaurs become extinct.
  15. What might cause a mass extinction today?
  16. Why would the average global precipitation be lower during the height of the ice age?
  17. How can human activities result in a climate change?
  18. Based on the number of craters on the moon, we expect to find 400 large craters (50 km wide) on Earth. There are only approximately 160 known impact craters, with only about 14 craters larger than 50 km, have been found on Earth so far.  Why do we find so few craters on Earth?
 

 

   

        • Web Activities

   Web activities related to subjects in the book are marked with subscript.  Activities include:

   Identifying climate regimes

   Identifying past climates

   Practice multiple choice exam

   Practice true/false exam 

 

   Box 14.1 Variations in Solar Output

      Recent measurements of the energy output of the sun indicate that the output varies slightly.  The sun's energy output is correlated to sunspot activity.  Sunspots are magnetic storms on the sun that appear as dark spots on the sun's surface (include figure of sunspot).  Since sunspots are magnetic storms, the sun's magnetic field also varies with sunspot activity.  Like magnets, sunspots have a polarity.  The polarity of the sunspots reverses every 11 years, so the sun's magnetic cycle is 22 years. 

      The number and size of the spots reaches a maximum every 11 years.  Sunspot activity (see figure) peaked in 1980 and 1991 and is expected to peak again in 2002. Recently, a minimum number of sunspots occurred in 1975, 1986 and 1997. Large number of sunspots reduces the solar output by only 0.1%.  This change in total solar energy output is not very large, particularly in terms of modifying the weather of the troposphere. There is however, anecdotal and empirical evidence that links the sunspot cycle with Earth's climate.  For example, a period of reduced solar activity was observed during 1645 to 1710.  During this time there were few, if any sunspots.  This is period is called the Maunder Minimum and it occurred at the same time as the Little Ice Age.  There are also fluctuations in weather patterns of the Northern Hemisphere that coincide to an approximate 22-year cycle. For example, there is a periodic 20-year drought in the Great Plains of the United States. So, any correlation between sunspot number and climate fluctuations is likely to include some feedback process. As of yet, scientific explanations of the 22-year correlation are have not been fully proven.

      The peak in sunspot activity affects the solar wind that bombards our upper atmosphere with high-energy particles.  At a peak of sunspot cycle the upper atmosphere can reach temperature of 1225C (2240F), whereas during a sunspot minimum the temperature is only 225C (440F).  It is not fully understood how these upper atmospheric changes can affect the troposphere.

 

    Box 14.2 Glaciers and ice bergs

      Glaciers form on land when accumulation of ice and snow in winter exceeds summertime melting. As the snow accumulates, ice crystals compact under the pressure and trapped air is expelled.  Eventually, this forms larger ice crystals and the glacial ice compacts and has a blue appearance.  This blue color arises from the fact that ice weakly absorbs red light, while scattering the blue.

      Glaciers are classified into three groups, ice sheets, mountain glaciers, and piedmont glaciers.  Ice sheets are continental sized glaciers that flow in all directions. Ice caps are small ice sheets.  Mountain glaciers are confined by topography, which determines the direction of flow.  Piedmont glaciers spread out from flat terrain.

   When the ice gets to be about 30 meters thick, its weight causes it to flow downhill. How fast it flows is a function of how steep the land is and the size of the glacier.  The speed can range from a few centimeters per day to ten meters each day. On land, the front edge of a glacier melts when it reaches a region with above freezing temperatures.  If the glacier ends in the ocean, great blocks of ice can break away and become ice bergs.  This process is called calving. 

   The accompanying figure is a satellite image of a calving process that occurred in Antarctica during the spring of 2000.  This iceberg is approximately 185 miles long and 23 miles wide—a surface area about twice that of the state of Delaware!  It is about a quarter of a mile thick, with all but 100 feet lying below the water. The iceberg, called B-15, contains approximately 3.4 trillion gallons of water! Moving towards the ocean at a rate of about a half a mile per year, this glacier calves this size iceberg every 50 to 100 years. In 1956 an iceberg that was about 208 miles long by 60 miles wide broke off!  

   A satellite view of the calving of an iceberg off the Ross iceshelf in Antarctic.  This iceberg is about twice the size of the state of Delaware.

    

 

   


Era Period Epoch MYBP

(beginning of period)

Features Climate Features
Cenozoic Quaternary Holocene 0.01 Age of Mammals  
Little Ice Age (1450-1850 AD)

Medieval Climatic Optimum (900-1200AD

Pleistocene 1.8 The Ice Age
Tertiary Pilocene 5.3  
Miocene 23.8  
Oligocene 33.7  
Eocene 54.8  
Paleocene 65 Beginning of a cooling period, leading to glaciers in Antarctica
Mesozoic Cretaceous 144 Mass extinction at the end of the Mesozoic period 

Age of Dinosaurs 

Break-up of Pangae, increasing volcanic activity.

intraglacial climates - no glaciers during the Mesozoic Era
Jurassic 206  
Triassic 250  
Paleozoic Permian 286 Final assembly of Pangae Glaciers in southern Africa, South America and Australia receded, desert regions expanded.
   Carboniferous 360 Extensive coal formation hot and humid
   Devonian 410 First amphibians warm and dry
     Silurian 440    
   Ordovician 505 Beginning formation of Pangae 

Primitive fish

 
   Cambrian 544   Wide spread ice age
Pre-cambrian         Evidence of ice sheets.

   Table 14.1 A sketch of Earth's geological periods. 


   

 

   

 

   

 

   

 

   1 The Return of Sherlock Holmes, The Adventure of the Abbey Grange, (1904) by Sir Arthur Conan Doyle,

 

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