Honors 1360 Planet EarthLast time:Measuring Earth’s Hydrosphere

Obs: Gravity changes (allow us to see “hidden” groundwater; can also separate steric from mass sea level change)

The Cryosphere:Obs: Surface landforms including moraines, cirques, U-shaped valleys, ancient lakeshores, shallow sediments …Hyp: All tell us about past glacial cycles

Obs: 40,000 to 100,000 year natural cycles similar to periodicity of Earth-sun orbital changes…Hyp: Milankovitch Cycles: Summer insolation at high northern latitudes controls glacial/interglacial cycles!

Today:• The Cryosphere• Cryosphere Now

15 October 2008

Read for Thur: 351-363

Glacial-Interglacial Cycles Through Pliocene-Pleistocene

These are Natural cycles of climatic variation!

40,000–100,000 year cycles → Changes in Earth Insolation?

Milankovitch cycles: Variations in

(1) Eccentricity (“ellipticity”) of Earth orbit around the sun (T = 100,000 years)

(2) Obliquity (tilt of Earth’s spin axis relative to plane of orbit) (T = 41,000 years)

(3) Precession (changes in orientation of the spin axis) (T = 26,000 years)

The important factor: How much insolation does high latitude (particularly northern!) receive in summer?

Planetary orbits are ellipses with Sun at one focus:

Current Earth distance to Sun changes by ~3.4%Recall S = S0(R/R0)2

1.0342 = 1.069 (i.e. ~7% total change over a year!)

Min-Max distance varies between 0% to 12%

N

SJan 3Jul 4

Tilt of the Earth gives us seasons:

Precession determines what season in the year we are closest(Now, northern hemisphere has less insolation in summer,

more in winter, and a shorter winter than southern hemisphere)

• Glaciers form on continents• At latitudes 40-80º, very sensitive to seasonality • Right now, a very large fraction of continental area is in that range of northern hemisphere!

--Major deglaciations occur when insolation is increasing, but clearly more going on here!

Prediction: If high-latitude insolation controls glaciation, expect a one-to-one correlation between the two…

Ice Cores, Palaeotemperature Proxies, and “Natural”Climate Variations:

• Drill into an undisturbed part of a glacier & retrieve core

• Chronology from patterns of accumulation, ablation, melt

• Measure water chemistry, gas chemistry in trapped bubbles with mass-spectrometer (e.g., 18O); dust & particulates

• Use statistics to estimate pre-instrumental climate & temperature

How can you use statistics to infer temperature from proxy?

Year1900 1950 2000

18O

Temperature

18O

Example:

Take slope and intercept of aline that best fits T versus 18Oand use that relationship tocalculate earlier T from 18O!

Hansen et al. PNAS 2006concludes “Global mean temperature in 2005 was the warmest in 120,000years”…

Mann et al.,Eos 2003concludes four different palaeo-temperature proxies are in general agreement (but with large uncertainties)

• Ice volume and orbital forcing “wiggles” don’t match one-for-one, BUT• Orbits are the metronome for glaciation as evidenced by similar spectral power!• Temperature and CO2 DO match closely!

Hence, orbits are the metronome but CO2 calls the tune!

Ice core data suggest CO2 increase lags behind temperature

What does this imply about the process taking place?

Feedbacks:

Ice increases albedo → cooling

Observe CO2 in step with glaciations: Hypothesize * Improved uptake of CO2 by ocean organisms?

Observe increased sulfuric acid in glacial times (in S) * Marine algae produce more sulfide when water is salty?

Satellite Measurements ofOcean Chlorophyll

EarthClimateSystem

Solar Energy

Ice Albedo (+)

Bio-CO2 (+)

Bio-SO2 (+)

Because of feedbacks, small changes in input (solar) lead tolarge changes in the system state:

In chaos/system theory, like Lorenz’ butterfly effect…

Earth climate history for the past 2 Myrs suggests two stable points (glacial, interglacial) with unstable transitions