Page 55 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 1 essentials of Geography 19
▼Figure 1.11 Global impacts of Mount Pinatubo’s eruption. The 1991 Mount Pinatubo eruption affected the earth–atmosphere system on a global scale. As you read Geosystems, you will find references
to this eruption in many chapters. A summary of the impacts is in Chapter 13. [Inset photo by Dave Harlow, USGS.]
A thin aerosol cloud affects 42% of globe (20° S to 30° N)
Effects on the Earth–atmosphere system:
Aerosol cloud causes colourful twilight and dawn skies worldwide.
Winds spread the ash cloud westward
15° N 120° E
Diffuse sunlight increases, causing a slight enhancement of photosynthesis and plant growth
in the atmosphere), Chapter 6 (satellite images of the spread of debris by atmospheric winds), Chapter 11 (temporary effect on global atmospheric temperatures), Chapter 13 (volcanic process), and Chapter 19 (effects on net photosynthesis). Instead of simply describing the eruption, we see the linkages and global impacts of such a volcanic explosion.
Earth’s Dimensions
We all have heard that some people in the past believed Earth was flat. Yet Earth’s sphericity, or roundness, is not as modern an idea as many think. For instance, more than two millennia ago, the Greek mathematician and philoso- pher Pythagoras (ca. 580–500 b.c.) determined through ob- servation that Earth is spherical. We do not know what observations led Pythagoras to this conclusion. Can you guess at what he saw to deduce Earth’s roundness?
Evidence of Sphericity Pythagoras might have no- ticed ships sailing beyond the horizon and apparently sinking below the water’s surface, only to arrive back at port with dry decks. Perhaps he noticed Earth’s curved shadow cast on the lunar surface during an eclipse of the Moon. He might have deduced that the Sun and Moon are not really the flat disks they appear to be in the sky, but are spherical, and that Earth must be a sphere as well.
Earth’s sphericity was generally accepted by the educated populace as early as the first century a.d. Christopher Columbus, for example, knew he was sail- ing around a sphere in 1492; this is one reason why he thought he had arrived in the East Indies.
Earth as a Geoid Until 1687, the spherical-perfection model was a basic assumption of geodesy, the science that determines Earth’s shape and size by surveys and mathematical calculations. But in that year, Sir Isaac Newton postulated that Earth, along with the other planets, could not be perfectly spherical. Newton reasoned that the more rapid rotational speed at the equator—the part of the planet farthest from the central axis and therefore the fastest moving—produces an equatorial bulge as centrifugal force pulls Earth’s surface outward. He was convinced that Earth is slightly misshapen into an oblate spheroid, or, more correctly, an oblate ellipsoid (oblate means “flattened”), with the oblateness occurring at the poles.
Earth’s equatorial bulge and its polar oblateness are today universally accepted and confirmed with tremen- dous precision by satellite observations. The unique, ir- regular shape of Earth’s surface, coinciding with mean sea level and perpendicular to the direction of gravity, is described as a geoid. Imagine Earth’s geoid as a sea-level surface that extends uniformly worldwide, beneath the
Atmospheric reflectivity (albedo) increases 1.5%
Reduced sunlight at Earth’s surface decreases average temperatures in the Northern Hemisphere by 0.5 C°
On June 15, 1991, 13–18 million tonnes of ash and sulfuric acid mist is blasted into the atmosphere.