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Chapter+12+Climate+Basics1.pdf

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Department
Geography
Course
Geography 2240A/B
Professor
Philip Egberts
Semester
Winter

Description
Chapter 12 Climate - The Basics 1.0 Introduction Now that we know everything about weather, let’s move on to climate. Everyone's talking about climate change, but most who do so don't understand the basic controlling concepts very well. In this chapter, we’ll fix that! If you need to review the difference between climate and weather, go back to the Unit 3 introduction, please. Today, Earth's global climate is highly favorable to life, but it was not always so. Certainly, we know that some 21,000 years ago the global climate changed such that enormous sheets of ice covered all Canada and Northern Europe, consuming so much of Earth's water that sea levels were about 110 meters lower than today. Further back, about 100 million years ago, climate had changed such that there was no ice anywhere - even at the poles - and sea levels were obviously very high. The first part of this chapter will concentrate upon the major factors that cause Earth's climate to change, the ways we track those changes, and we'll conclude with a graphical presentation of what large-scale changes we know about. The primary aim of this whole section is to educate us sufficiently that we can better come to terms with the climate changes looming today and in the near future. Any attempt to understand climate changes must begin with a full appreciation of geologic time. Human life spans are usually measured in decades, and we feel fortunate to trace family history as far back as great-grandparents. Almost all of Earth's long history lies immensely far beyond this human perspective. By necessity, this section on climate focuses mainly on the last few hundred million years. Of course, the further back in geologic time we search, the fewer the facts available. One reason for that limitation is climate itself: the relentless action of weathering (by water and air) has eroded most of the deposits that would have helped us understand climate. Earth formed roughly 4.54 billion years ago. Figure 1 shows, on a linear scale, all of geologic time, and then the two common ways of plotting small fractions of that total: expanded liner scales and logarithmic scales. The log scale method compresses the longer periods of time and expands the shorter ones; we will make some use of both methods during this chapter. Figure 2 shows the climate changes on Earth over the last 300 million years by means of a sequence of expanded linear scales. Clearly, as we work from longer to shorter time intervals, we are looking at increased resolution of data. 2.0 Climate System Figure 3 illustrates the components of our climate system. The central part shows how interactive the components are, and we should expect that a significant variation in one of the components will result in a change to other components - thus a climate change. Throughout Earth's history, the three major causes of climate change have been: changes in plate tectonics, changes in Earth's orbit, and changes in Sun's energy (the left side of Figure 3). Climatologists use the word "forcing" when they refer to factors that cause climate changes, and they use the word "response" to refer to variation in climate produced by the forcing event. When we consider specific events in Earth's history we will consider what forcing could have produced that response, but for now, let's just review - very briefly - the properties that relate to each forcing factor. 2.1 Tectonic Processes We all know by now that tectonic activity is caused by Earth's internal heat engine, and that the geography of the planet is constantly being changed by those processes. Because these changes take place so very slowly, it's sometimes hard to see how global climate might be changed by them. On the other hand, as we'll see later, circulation of ocean currents depends greatly upon the location and arrangements of continents. 2.2 Earth Orbital Changes We'll touch on these from time to time, so it’s probably best to quickly review them here. In the early 20 Century, Milutin Milankovitch recognized and explained three forms of variation in position of Earth’s axis and its orbit around Sun. He noted that these variations can cause the amount of solar energy reaching Earth to vary by as much as 10%, thus obviously must have some effect upon climate. First, according to Milankovitch’s calculations, Earth’s orbit around Sun varies in a systematic manner over periods lasting 100,000 years. At times, Earth’s orbit is highly eccentric, at other times it is almost circular (Fig. 4). Second, Milankovitch’s calculations showed that the tilt of Earth’s axis changes systematically from 21.5 to 24.5 in a pattern that repeats every 41,000 years. As far as the northern hemisphere is concerned, a lower tilt means less radiation in the summer and more in winter (i.e. contrast between summer and winter is reduced). Third, the wobble of Earth’s axis due to the varying gravitational pull of Sun and Moon on Earth’s equatorial bulge causes changes in the direction of Earth’s tilt. This process called precession has a cycle of 26,000 years. When all of the Milankovitch factors are taken into account and a plot of solar radiation calculated from them, the resulting graph for the past million years matches the graph obtained by oxygen isotope calculations almost exactly – but the degree of variation is much smaller from the Milankovitch factor calculations than from actual measurements (Fig. 5)! [For the significance of oxygen isotope measurements, see later in this chapter.] 2.3 Sun Energy Changes he strength of Sun has slowly increased throughout the history of the Solar System. In addition, short term variations, called sunspots, definitely result in variations in solar radiation arriving on Earth. 2.4 Climate Change Responses A useful way of thinking about how the climate system responds to forcing factors is in terms of the time it takes the climate system to react fully to some factor (response time). Figure 6 is a simply illustration of response time of a container of water to the heat provided by a Bunsen burner. After lighting the burner, the water will gradually warm toward an equilibrium point. As the water warms, the response time to application of further heat increases - which is simply illustrating the fact that the closer a system is to it's equilibrium position, the smaller the 'driving force' to push it along, and the longer it takes to change. Figure 7 relates times of forcing and response; obviously, the top figure is the case where change in forcing is so slow that climate change keeps up (that would be comparable to increasing the heat of the Bunsen burner so slowly that there was no 'lag time' in temperature change in the water). Figure 7B illustrates a forcing change of such short duration that there's essentially no response from the system. Only in C and D are forcing times and response times approximately equal (This would be roughly the case of a Bunsen burner being turned on, left on for awhile, then turned off, and left off for awhile. These 'forcing' actions cause the water to heat up, cool off, heat up, etc. In fact, the intervals may never be long enough for the water to reach either the high or low equilibrium values. If the intervals of heat-on and heat-off are fairly short, the response is really quite muted). In the real world, climate forcing rarely acts in the on-or-off mode; instead, the oscillations are smoother, such as those in Figure 8. Here are two important points to remember about forcing and response:  The rate of response of the climate system is fastest when the climate system is farthest from the equilibrium it seeks;  The system has many components with different response times; each responds to the same forcing at its own tempo. 2.5 Climate System Feedbacks Some interactions in the climate system already in operation initiate responses that may amplify whatever forcing is going one (i.e. a positive feedback), or may suppress the primary forcing (negative feedback). Here's an example: Let's say there is a decrease in the heat energy sent to Earth by Sun, and it is sufficient that ice and snow spread across regions at high latitudes that had not been covered by them before. Because snow and ice reflect far more sunlight than bare ground, any increase in the area they cover should decrease the amount of heat absorbed by Earth and further cool the climate. That's a perfect example of a positive feedback. A negative feedback might be the muting effect (reflection of energy) on warming of the atmospheric greenhouse that increasing CO -conte2t might be producing. 2.6 Heating Earth Earth's climate system is primarily driven by heat energy arriving from Sun. We already know that energy travels through space to us in the form of electromagnetic radiation, and that really only a small fraction of the total spectrum of energy is visible light. Figure 9 is a reasonable illustration of Earth's energy budget as it exists today. Many important characteristics of Earth's climate vary with latitude, such as the amount of incoming sunlight (Figure 10a) and the amounts of reflected and absorbed radiation (Figure 10b). This is probably the most appropriate point to say something about albedo; albedo is the percentage of incoming radiation that is reflected rather than absorbed by a surface. Snow and ice surfaces at high latitudes have albedos ranging from 60-90%. As you might guess, the albedo of any surface also varies with the angle at which incoming solar radiation arrives. These factors combine to make Earth's surface more reflective near poles than in the tropics. The type of vegetation covering a land region can affect its average albedo. The two major types of vegetation in the Arctic, spruce forest and tundra, interact in different ways with freshly fallen snow and produce surface with very different albedos. Snow that falls on tundra covers what little scrub vegetation exists and creates a high albedo surface that reflects most incoming solar radiation. Snow that falls on spruce forests is blown from the trees, allowing the dark-green surface of the trees to absorb most incoming radiation.
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