{"id":46,"date":"2021-03-10T18:14:21","date_gmt":"2021-03-10T23:14:21","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/physgeoglabmanual1\/chapter\/lab6\/"},"modified":"2021-04-05T22:30:02","modified_gmt":"2021-04-06T02:30:02","slug":"lab6","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/physgeoglabmanual1\/chapter\/lab6\/","title":{"raw":"LABORATORY 6: CLIMATE CHANGE \u2013 PART 1","rendered":"LABORATORY 6: CLIMATE CHANGE \u2013 PART 1"},"content":{"raw":"

LABORATORY 6<\/strong><\/span>: CLIMATE CHANGE - PART 1<\/strong><\/h2>\r\n

LEARNING GOALS<\/span><\/h1>\r\nIn this laboratory, we will examine the nature of past and future predicted climate change on the Earth.\r\n\r\nUpon completion of this laboratory you will be able to:\r\n
    \r\n \t
  1. Understand some of the mechanisms responsible for climate change.<\/li>\r\n \t
  2. Interpret data used in the investigation of the Earth's past, present, and future climate.<\/li>\r\n \t
  3. Examine and interpret the output from climate simulation models.<\/li>\r\n<\/ol>\r\n

    CLIMATE CHANGE<\/strong><\/h1>\r\nFor the climatologist, climate consists of many different variables that vary both spatially and temporally. Of these variables, the fluctuations of temperature and precipitation receive the most attention.\r\n\r\nClimatologists have concluded that fluctuations in climate, at any time scale, are produced by a complex array of external (i.e. from extraterrestrial systems) and internal (i.e. from ocean-atmosphere feedbacks) factors. The best-known long-term fluctuations are those that resulted in rhythmic glacial-interglacial periods during the last two million years. Common short-term fluctuations include the role of sunspots, volcanic eruptions, and \u00a0El Ni\u00f1o<\/strong><\/a> - <\/strong>La Ni\u00f1a<\/strong><\/a>.\r\n\r\nFigure 6.1<\/strong><\/span> summarizes the basic components of the Earth's climatic system. An example of external climate forcing is the change in the Sun's luminosity or the Earth-Sun distance, either of which would vary the amount of solar radiation received at the top of the Earth's atmosphere. Examples of internal climate forcing are changes in the concentrations of atmospheric gases, mountain building, volcanic activity, and changes in surface or atmospheric albedo.\r\n

    VARIATIONS IN THE EARTH'S ORBIT<\/strong><\/h1>\r\nThe [pb_glossary id=\"658\"]Milankovitch theory[\/pb_glossary]<\/strong> suggests that normal cyclical variations in three of the Earth's orbital characteristics is probably responsible for some climatic change. The basic idea behind this theory assumes that over time these three cyclic events vary the amount of solar radiation that is received on the Earth's surface.\r\n\r\n \r\n\r\n[caption id=\"attachment_342\" align=\"alignnone\" width=\"1024\"]\"\"

    Figure 6.1.<\/strong> <\/span>Major components of the Earth's climatic system. Image Copyright: Michael Pidwirny.<\/em>[\/caption]\r\n\r\nThe first cyclical variation is the Earth's orbital [pb_glossary id=\"659\"]eccentricity[\/pb_glossary]<\/strong> - the orbit changes from being elliptical to being nearly circular and then back to elliptical over a period of ~100,000 years (Figure 6.2<\/strong><\/span>).\u00a0The greater the eccentricity of the orbit, the greater the seasonal variation in solar energy received by the Earth. Currently, the Earth's orbit is one of relatively low eccentricity, which makes the difference in the Earth-Sun distance between [pb_glossary id=\"66\"]perihelion[\/pb_glossary]<\/strong> and [pb_glossary id=\"67\"]aphelion[\/pb_glossary]<\/strong> responsible for only ~6% variation in the amount of insolation at the top of the atmosphere. When the difference in the Earth-Sun is at its maximum (~9%), the difference in solar energy received is ~20%. (At this point you should recall the relevant material that you dealt with in Lab 1.)\r\n\r\n \r\n\r\n[caption id=\"attachment_343\" align=\"alignnone\" width=\"1024\"]\"\"

    Figure 6.2.<\/strong><\/span> The Earth\u2019s orbit around the Sun changes from being nearly circular to highly elliptical. The period of this cycle is about 100,000 years. Image Copyright: Michael Pidwirny.<\/em>[\/caption]\r\n\r\nThe second cyclical variation results from the fact that, as the Earth rotates on its axis, it wobbles like a spinning top changing the orbital timing of the equinoxes and solstices (Figure 6.3A<\/strong><\/span>). This is known as the [pb_glossary id=\"661\"]precession of the equinox[\/pb_glossary]<\/strong> and has a cycle of ~23,000 years. Presently, the Earth is closer to the Sun in January and farther away in July (Figure 6.3B<\/strong><\/span>). Because of precession, the reverse will be true in 11,500 years and the Earth will then be closer to the Sun in July (Figure 6.3B<\/strong><\/span>). This means, of course, that if everything else remains constant, 11,500 years from now seasonal variations in the Northern Hemisphere should be greater than at present (colder winters and warmer summers) (Figure 6.3C<\/strong><\/span>).\r\n\r\n \r\n\r\n[caption id=\"attachment_344\" align=\"alignnone\" width=\"1024\"]\"\"

    Figure 6.3.<\/strong><\/span> Precession of the Equinoxes: diagram B describes the present situation; diagram C illustrates what to expect 11,500 years into the future. \u00a0Image Copyright: Michael Pidwirny.<\/em>[\/caption]\r\n\r\nThe third cyclical variation is related to the changes in the tilt ([pb_glossary id=\"662\"]obliquity[\/pb_glossary]<\/strong>) of the Earth's axis of rotation over a 41,000-year cycle. The tilt varies from about 22.0\u00b0 to 24.5\u00b0 (Figure 6.4<\/strong><\/span>). At the present time, the tilt of the Earth's axis is approximately 23.5\u00b0. When the tilt is small there is less climatic variation between the summer and winter seasons in middle and high latitudes. Winters tend to be milder and summers cooler. Warmer winters allow for more snow to fall in the high latitude regions (because warmer air allows for greater amounts of water vapour to be moved by the atmosphere). Cooler summers cause snow and ice to accumulate on the Earth's surface because less of this frozen water is melted. Therefore the net effect of a smaller tilt would be more extensive formation of glaciers in the polar latitudes. Periods of a larger tilt result in greater seasonal climatic variation in the middle and high latitudes. At these times, winters tend to be colder and summers warmer. Colder winters produce less snow because of lower atmospheric temperatures. As a result, less snow and ice accumulates on the ground surface. Moreover, the warmer summers produced by the larger tilt provide additional energy to melt and evaporate the snow that fell and accumulated during the winter months. Consequently, glaciers in the polar regions should be generally receding, with other contributing factors constant, during this part of the obliquity cycle.\r\n\r\n \r\n\r\n[caption id=\"attachment_346\" align=\"alignnone\" width=\"500\"]\"\"

    Figure 6.4.<\/strong> <\/span>The tilt of the Earth\u2019s axis can vary from a minimum of 22.0\u00b0 to a maximum of 24.5\u00b0. Image Copyright: Michael Pidwirny.<\/em>[\/caption]\r\n

    VARIATIONS IN SOLAR OUTPUT<\/strong><\/h1>\r\nUntil recently, many scientists thought that the Sun's output of radiation only varied by a fraction of a percent over many years. However, recent measurements made by satellites equipped with [pb_glossary id=\"663\"]radiometers[\/pb_glossary]<\/strong> show that the Sun's energy output may be more variable than was once thought. Measurements made during the early 1980s showed a decrease of 0.1% in the total amount of solar energy reaching the Earth over an 18 month time period. If this trend were to extend over several decades, it could influence global climate. Numerical climatic models predict that a change in solar output of only 1% per century would alter the Earth's average temperature by between 0.5 to 1.0\u00b0C.\r\n\r\nScientists have long tried to link [pb_glossary id=\"664\"]sunspots[\/pb_glossary] <\/strong>to climatic change. Sunspots are large magnetic storms that appear as dark, cooler areas on the Sun's surface. The number and size of sunspots show several cyclical patterns, reaching a maximum every 11, 80, and 200 years. The decrease in solar energy observed in the early 1980s corresponds to a period of maximum sunspot activity based on the 11-year cycle. In addition, measurements made with a solar telescope from 1976 to 1980 showed that during this period, as the number and size of sunspots increased, the Sun's surface cooled by about 6\u00b0C. Apparently, the sunspots prevented some of the Sun's energy from leaving its surface. However, these findings tend to contradict observations made on longer time scales. The middle of a cool period known as the [pb_glossary id=\"665\"]Little Ice Age[\/pb_glossary]<\/strong> (approximately from 1650 to 1750) correlated to a time of very few sunspots (the [pb_glossary id=\"666\"]Maunder Minimum[\/pb_glossary]<\/strong>), something that scientists believe repeats every 80 to 90 years (Figure 6.5<\/strong><\/span>).\r\n\r\n \r\n\r\n[caption id=\"attachment_349\" align=\"alignnone\" width=\"1000\"]\"\"

    Figure 6.5.<\/strong><\/span> Variations in monthly average sunspot numbers for about 400 years. Data from 1749 is continuous. Data before 1749 is sporadic and individual observations are marked with an X. \u00a0Image Source:     Wikimedia Commons<\/a>, Robert A. Rohde. This image is licensed under the Creative Commons<\/a>Attribution-Share Alike 3.0 Unported<\/a> license.<\/em>[\/caption]\r\n\r\nDuring periods of maximum sunspot activity, the Sun's magnetic field is strong, and vice versa<\/em>. The magnetic field of the Sun also reverses every 22 years, during a sunspot minimum. Some scientists believe that the periodic droughts on the Great Plains of the United States are in some way correlated with this 22-year cycle.\r\n

    GREENHOUSE ENHANCEMENT<\/strong><\/h1>\r\nOver the past three centuries, the concentration of carbon dioxide has been increasing in the Earth's atmosphere because of human influences. Human activities like the burning of fossil fuels, conversion of natural prairie to farmland, and deforestation have caused a net release of carbon dioxide into the atmosphere. From the early 1700s, carbon dioxide has increased from 280 parts per million to 380 parts per million as measured in 2005. Most scientists believe that higher concentrations of carbon dioxide in the atmosphere will enhance the greenhouse effect making the planet significantly warmer. Most scientists believe that the average temperature of the Earth has increased by about 0.6\u00b0Celsius over the last century because of the enhancement of the greenhouse effect. Further, most computer climate models suggest that the average temperature of the planet will warm up by another 1.5 to 4.5\u00b0Celsius if carbon dioxide reaches the predicted level of 600 parts per million by the year 2050 to 2100.\r\n

    RECONSTRUCTING PAST CLIMATES<\/strong><\/h1>\r\nA wide range of evidence exists to allow climatologists to reconstruct the Earth's past climate. This evidence can be grouped into two general categories: direct measurements and indirect measurements. Direct measurements include the various instrumental measurements done at meteorological stations. These instrumental meteorological records have been very important in detecting recent global warming due to greenhouse effect enhancement. Indirect measurements of climate can be made from a number of natural processes that in some way record temporal fluctuations in some climatic variable. It is very important to realize that indirect measurements of this sort are just a surrogate or proxy of the climate variable of interest.\r\n

    METEOROLOGICAL INSTRUMENT RECORDS<\/strong><\/h1>\r\nCommon climatic elements measured by instruments include temperature, precipitation, wind speed, wind direction, and atmospheric pressure. However, many of these records are temporally quite short as many of the instruments used were only created and put into operation during the last few centuries or decades. Another problem with instrumental records is that large areas of the Earth are not monitored. Most of the instrumental records are for locations in populated areas of Europe and North America. Very few records exist for locations in Less Developed Countries (LDCs), in areas with low human populations, and the Earth's oceans. Over the last 50 years many meteorological stations have been added inland areas previously not covered. Another important advancement in developing a global record of climate has been the recent use of remote satellites to gather climate data.\r\n

    BIOLOGY PROXY DATA \u2013 TREE RINGS<\/strong><\/h1>\r\nIt is well known that the age of a temperate forest tree can be calculated by counting the number of annual growth rings from center to bark. The last complete ring before the bark usually represents the tree growth that occurred the previous year. On closer observation of a series of such rings it becomes obvious that the width of these yearly rings varies. This variation has been correlated to changes in some climatic variable that affects the tree's growth. Andrew E. Douglass, working in Arizona, was one of the first scientists to examine the causes of variations in tree ring widths. Douglass suggested that the variance in ring widths was caused by sunspot activity that in turn had an influence on climate at the Earth's surface. Today, the science of dendroclimatology<\/em> is a well-established method of deriving [pb_glossary id=\"668\"]proxy climate data[\/pb_glossary]<\/strong>.\r\n\r\n(a) Underlying principles and concepts of dendroclimatology<\/strong>\r\n\r\nTree rings have unique properties that make them an important source of past climate information:\r\n