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Natural and Human
Influences on Climate Change
Natural climate influence only
Human climate influence only
All Climate Influences
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Sources of Climate Variability
• Orbital forcing• Solar output• Volcanic eruptions• Earth system feedbacks
– GHGs– Land surface, cryosphere, etc.– Oceans
• Humans
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Orbital Forcing
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Orbital forcing: eccentricity
• Currently, difference in distance between aphelion (farthest from sun) and perihelion (closest to sun) is only 3%; ~ 6% variation in W*m^-2.
• When orbit is most eccentric, ~ 25% variation between aphelion and perihelion
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Orbital forcing: obliquity
• Less tilt = solar radiation more evenly distributed between winter and summer.
• However, less tilt = increased difference in radiation between equator and poles
• Currently, middle of range (23 degrees)
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Orbital forcing: precession
• Wobble of the spin of the earth around its axis causes solstices to vary in their coincidence with apehelion and perihelion.
• Winter solstice occurred when the earth was furthest from the sun ~11,000 ybp. + greater seasonal variability
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Theory vs. Observations?
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Imbrie and Imbrie (chillin’ for 23k) vs. Berger and Loutre (cookin’ for 50k) vs.
Archer and (!) Ganopolski (CO2, 500kyr)
• The amount of solar radiation in the northern hemisphere at 65°N seems to be related to occurrence ice ages.
• Astronomical calculations show that 65°N summer insolation should increase gradually over the next 25,000 years.
• No declines in 65°N summer insolation sufficient to cause an ice age are expected in the next 50,000 - 100,000 years.
• Archer: CO2 may be sufficient to dampen orbital variation for 500 kyr.
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Solar Output
Swedish Royal AcademyNovember 2002
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Sunspot Numbers (C14 and Be10 proxy
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• Royal Observatory of Belgium record: observations from around the world since 1749; other data spotty.
• Note 11 year cycles (peak to trough = -1.3 Wm-2
• Note variability: lower numbers associated with lower solar luminosity• Maunder and Dalton minima associated with obvious northern hemisphere climatic variability• 1900-1950 increase associated with warming in early 20th century (16-36% of warming<1950,
none since 1970)
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Observations of Climate Variability
• Instrumental observations (~1700s - present):– Decadal climate
variability– Centennial climate
variability
• Proxy records:– Multi-centennial,
millennial, multi-millennial
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Instrumental Observations• Human records (both measurements and other
observations documented the LIA).
• Other observations only for the Medieval warm period.
A Scene on the Ice (Hendrick Avercamp)
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LIA spatially, temporally variable
• Any of several dates ranging over 400 years may indicate the beginning of the Little Ice Age:
• 1250 for when Atlantic pack ice began to grow
• 1300 for when warm summers stopped being dependable in Northern Europe
• 1315 for the rains and Great famine of 1315-17
• 1550 for theorized beginning of worldwide glacial expansion
• 1650 for the first climatic minimum
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