natural and human influences on climate change natural climate influence only human climate...

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