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