Natural and Human

Influences on Climate Change

Natural climate influence only

Human climate influence only

All Climate Influences

Sources of Climate Variability

• Orbital forcing• Solar output• Volcanic eruptions• Earth system feedbacks

– GHGs– Land surface, cryosphere, etc.– Oceans

• Humans

Orbital Forcing

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

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)

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

Theory vs. Observations?

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.

Solar Output

Swedish Royal AcademyNovember 2002

Sunspot Numbers (C14 and Be10 proxy

• 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)

Observations of Climate Variability

• Instrumental observations (~1700s - present):– Decadal climate

variability– Centennial climate

variability

• Proxy records:– Multi-centennial,

millennial, multi-millennial

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)

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