GPHY 314 Chapter 7: Why is the Climate Changing?

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29 Dec 2020
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CHAPTER 7: Why is the Climate Changing?
Continental Drift
Earth’s continents moving, can substantially alter arrangement of continents across surface
o Can lead to large changes in climate through several mechanisms
Location of continents determines whether ice sheets form
o Most important requirement for growth of an ice sheet is summer temperatures cool
enough that snow falling during winter doesn’t melt during following summer
Most favorable for land at high latitudes with least sunlight
o Ice sheets reflect sunlight so formation increases albedo, increasing reflection of solar
radiation back to space and cooling planet
o Loss of ice sheet will warm climate through same mechanism
Location of continents determines ocean circulation
o Oceans carry huge amounts of heat from topics to polar regions, so changing circulation
can alter relative temperatures of tropics and polar regions
30 million years ago Antarctic Peninsula separated from southern tip of South
America, opening Drake Passage
Isolated Antarctica and allowed winds and water to flow unhindered,
reducing transport of warm winter and air from tropics, cooling Antarctic
and helping build Antarctic ice sheet
Continental ice drift can also indirectly affect climate by regulating atmospheric carbon dioxide
o Movement can change pattern of rainfall and expose new rock to atmosphere, changing
locations and rate of weathering, altering the amount of carbon dioxide in atmosphere
40 million years ago Indian subcontinent collided with Asia, forming Himalayas
and adjacent Tibetan Plateau
Led to changing wind patterns, bringing heavy rainfall onto vast expanse
of newly exposed rock -> One of reasons planet has been cooling
The Sun
Since late 1970s, clear eleven-year cycle during which solar constant varies by about 0.1%
o Has little influence on climate because thermal inertia of water keeps temperature from
varying quickly, so it doesn’t respond to rapid changes in heating
In order for sun to be responsible for recent warming, there would need to be sustained, long-term
increase in solar constant over past few decades
o But measurements show no evidence of this
Another reason to discount sun as explanation is that increase in solar output would warm entire
atmosphere, but this isn’t happening
^Conclude rapid warming of past few decades not caused by brightening of sun
Sun’s influence on climate prior to middle of 20th C more difficult to determine because no
satellite measurements of solar constant
o Instead, output for this period inferred indirectly from other measurements i.e. number of
sunspots, chemical proxies (carbon-14 content of plant material)
o Most recent analyses suggest sun has brightened over past few hundred years, potentially
explaining some of gradual warming of 18th, 19th, 20th C
The Earth’s Orbit
If earth moved closer to sun, solar constant would increase even if brightness of sun didn’t
change -> Relevant since earth’s orbit not a perfect circle
o Ellipse whose eccentricity (ratio of length to width) varies with time
o Over course of 100,000 years, orbit cycles between orbit that is slightly more elliptical
and more circular
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