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GEOL 1302, Week 7 Notes

by: Theresa Nguyen

GEOL 1302, Week 7 Notes GEOL 1302

Marketplace > University of Houston > GEOL 1302 > GEOL 1302 Week 7 Notes
Theresa Nguyen
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To determine a cause for the present-day warming, we examined all of the natural process that are capable of changing our climate. Among these were tectonic motions solar variations, and orbital va...
Intro To Global Climate Change
yunsoo choi
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This 5 page Class Notes was uploaded by Theresa Nguyen on Sunday October 16, 2016. The Class Notes belongs to GEOL 1302 at University of Houston taught by yunsoo choi in Fall 2016. Since its upload, it has received 6 views.

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Date Created: 10/16/16
Chapter 7: Why is the climate changing? Part I Why is the climate changing?  The Earth’s climate is changing. - The evidence is so overwhelming, no one disputes this.  Instead, much of the most heated argument is over the cause of the warming:  Is it caused by human activity, or is it natural? - Today, we address this question.  Attributing the cause of a trend is more difficult than identifying the trend. - Our strategy is to examine the mechanisms that have changed climate in the past and determine if they could be the cause of the recent warming.  You will see that a careful review of all of the possible causes of the recent warming yields the conclusion that the most likely explanation is the increase in greenhouse gases in our atmosphere. 1. Movement of the continents  Earth’s continents are moving. - Not fast - move at about the same rate that your fingernails grow – but over tens of millions of years, this movement, also referred to as tectonic motion, substantially alters the arrangement of the continents across the Earth’s surface.  Such changes can directly lead to large changes in the climate.  The location of continents helps determines whether ice sheets form. - Ice sheets form because of cold summer temperatures. - If snow that falls during the winter does not melt during the following summer.  Land at high latitudes is the most favorable location for cold summers (and winter snow).  Ice sheets matter to the climate because ice reflects sunlight, so the formation of an ice sheet increases planetary albedo, cooling the planet.  Through the same chain of logic, the loss of an ice sheet will warm the climate.  For example, if Antarctica moved toward the equator over the next 100 million years, loss of the Antarctic ice sheet would be likely, thus leading to significant warming.  In addition, the location of the continents determines the ocean circulation.  The oceans carry huge amounts of heat from the tropics to the high latitudes, so changing ocean circulation can change the temperatures of the tropics and polar regions. Drake’s Passage  Before separation of continents from Antarctica, flow diverted towards equator, bring warm water to high latitudes.  Drake’s passage opened 25-20 Mya.  Intense glaciation and global cooling 13 Mya.  Ocean-GCMs suggest opening of Drake’s passage did not have large impact on climate. India-Asia collision and Tibet  Movement of the continents can change the pattern of rainfall and expose new rock to the atmosphere.  Which changes the locations and rate of chemical weathering – and therefore the amount of carbon dioxide in the atmosphere.  For example, 40 million years ago the Indian subcontinent collided with the Asian continent, forming the Himalayas and the adjacent Tibetan Plateau.  Changing wind patterns brought heavy rainfall onto the vast expanse of newly exposed rock in these features, and the resultant chemical weathering drew down atmospheric carbon dioxide over a period of tens of millions of years. Movement of the continents  Thus, movement of the continents can indeed change the climate – but could this be responsible for the rapid warming of the past few decades? - The answer is no.  It takes millions of years for continental movement to cause significant climate change.  Continental movements cannot significantly modify the climate over decades or centuries. 2. The Sun  If the Sun brightens, the climate will warm. - We might therefore wonder if the recent warming of the climate can be explained by an increase in the output of the Sun.  This is a reasonable question because it is well known that the Sun’s output varies on many time scales. - Over the Sun’s 5-billion-year life is has slowly gotten 30%.  Since the late 1970s, instruments on satellites have been measuring the solar constant, over this period, the most significant observed variation is an 11year cycle, by which total solar energy output varies approximately 0.1 percent.  Percentage change in monthly values of energy in (Ein). - Seasonal changes in the Earth–Sun distance have been removed.  Because of the enormous thermal inertia of the oceans, the climate does not respond much to these 11-year variations.  In order for the Sun to be responsible for the recent warming, there would need to be a long-term increase in the solar constant over the past few decades. - The measurements show no evidence of this.  In addition, an increase in solar output would warm the entire atmosphere. - This is not happening. - Rather, measurements from weather balloons and satellites show that the stratosphere has cooled over the past few decades.  Thus, we can conclude with high confidence that the rapid warming of the past few decades is not caused by a brightening of the Sun.  The Sun’s influence on climate before the 1970s, is more difficult to determine. - Solar output for this period must be inferred indirectly from other measurements.  The most recent analyses of these records suggest that the Sun has brightened over the past few hundred years, and this can potentially explain at least some of the gradual warming of the 18th, 19th, and early 20th centuries.  This has led to a positive radiative forcing with a magnitude of approximately +0.12 W/m^2, which is minor compared to radiative forcing from greenhouse gases. 3. The Earth’s orbit  The solar constant is determined not just by the energy emitted by the Sun, but also by the Earth–Sun distance.  If the Earth moved closer to the Sun, then the solar constant would increase even if the brightness of the Sun did not change.  This is relevant because the Earth’s orbit is not a perfect circle: It is an ellipse whose eccentricity varies with time.  Over the course of 100,000 years or so, as the orbit becomes more eccentric, the average Earth–Sun distance increases and the average amount of solar energy falling on the Earth decreases.  For the Earth’s orbit, the change in eccentricity causes the annual average solar constant to vary by approximately 0.5 W/m2. - This change in the solar constant will lead to changes in the Earth’s climate.  Other aspects of the Earth’s orbit can also vary, such as the timing of the perihelion.  Today, the Earth is closest to the Sun during January, when it is wintertime in the northern hemisphere.  In 11,500 years, the Earth will be closest during July, and in 23,000 years it will again be January.  Another important variation is the tilt of the Earth (also known as the obliquity). - Today, the Earth’s spin axis is tilted 23.5° from vertical.  Over the next 41,000 years, the Earth’s tilt will complete a cycle through a range of tilt angles from 22.3° to 24.5°.  Changing the date of closest approach to the Sun or the tilt of the Earth does not change the Earth–Sun distance, so it does not change the solar constant.  Rather, these changes change how sunlight is distributed over the planet, in both latitude and season.  For example, increasing the tilt of the planet increases the amount of sunlight hitting the polar regions and decreases the amount hitting the tropics, which alters the climate.  Orbital variations regulate how sunlight is distributed over the planet, so they play a role in regulating these temperatures.  These orbital variations and the climate effects that follow are often referred to as Milankovitch cycles, after Serbian mathematician Milutin Milankovitch, who was the first one to recognize that the ice-ages cycles occurred at exactly the same frequency as variations in the Earth’s orbit.  While these orbital variations are critical in ice ages, are they responsible for the warming of the past few decades? - They are almost certainly not.  These orbital variations are so slow that it takes at least thousands of years to make any significant change in the amount of or distribution of incoming sunlight. - The warming of them past century has been much too fast to be caused by these slow orbital variations. - The warming must be due to other causes. 4. Internal Variability  Changes in the output of the Sun or in the Earth’s orbit are examples of forced variability.  Changes driven by the internal physics of the system rather than external changes in the planet’s energy in or energy out, are often referred to as internal variability.  The best-known example of internal variability in our climate is the El Niño/ Southern Oscillation (ENSO). - El Niño events, which make up the warm phase of ENSO, occur every few years and last a year or so.  During El Niño events, the Earth warms several tenths of a degree Celsius. - El Niño’s opposite is La Niña.  During La Niña events the Earth cools several tenths of a degree. - These events cause a temporary change year, but no long-term changes in the climate.


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