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The Heartbeat of Climate. Earth’s climate engine is the Sun Small variations in solar radiation can have large climate effects Feedbacks can amplify or reduce the insolation changes. The Heartbeat of Climate. Daily Cycle. The Heartbeat of Climate. Daily Cycle Annual cycle.
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The Heartbeat of Climate • Earth’s climate engine is the Sun • Small variations in solar radiation can have large climate effects • Feedbacks can amplify or reduce the insolation changes
The Heartbeat of Climate • Daily Cycle
The Heartbeat of Climate • Daily Cycle • Annual cycle
The Heartbeat of Climate • Daily Cycle • Annual cycle • Milankovitch cycles
Milankovitch Variations • Precession - Earth’s Wobble • Obliquity - Earth’s Tilt • Eccentricity - Earth’s Elliptical Orbit
Northern Summer Northern Winter Precession of the Equinoxes Present
Precession of the Equinoxes 11,000 years from now Northern Winter Northern Summer
Precession of the Equinoxes • Earth’s wobble on its axis and the rotation of Earth’s orbit around the sun together change the season of closest approach to the sun. • These two effects have periodicities of 23,000 and 19,000 years.
Axial Tilt • Tilt of Earth’s axis varies between 22.1º and 24.5º. • Tilt changes with a periodicity of 41,000 years. • Tilt affects the poles, primarily. Low tilt makes the poles milder.
The Orbital Parameters • Axial Tilt (41,000 year period) • Precession (19,000 and 23,000 year periods) • Eccentricity (100,000 and 400,000 year periods)
First Attempt • Joseph Adhémar in 1842 made the first formal attempt at a theory of how Earth’s orbit affected climate. • Adhémar focused on precession of the equinoxes, and he argued that changes in total insolation caused glacial-interglacial oscillations. • Precession does not affect total insolation, so Adhémar’s ideas were rejected.
James Croll • In 1864, James Croll invoked combined effects of eccentricity and precession to explain glacial-interglacial oscillations. • Croll was the first to propose that the seasonal distribution of insolation could produce changes in global climate. http://www.ngdc.noaa.gov/paleo/slides/images/base/iceage07.jpg
More Croll • Croll predicted a 22,000 year cycle of ice ages during glacial epochs. • Based on the assumed importance of eccentricity, Croll argued that last ice age ended 80,000 years ago. • Croll’s emphasis on seasonality caused him to conclude that ice ages should be out of phase between hemispheres.
Northern Summer Northern Winter Precession of the Equinoxes Present
Precession of the Equinoxes 11,000 years from now Northern Winter Northern Summer
Not Quite, Croll • Croll’s carefully crafted hypothesis was rejected. • Last glacial maximum was only 18,000 years ago, not 80,000 years ago. • Glacial periods occur synchronously (at the same time) in the northern and southern hemispheres.
Milutin Milankovitch • Through the early part of the 20th century, Serbian mathematician Milutin Milankovitch worked on a complete orbital theory, including all three orbital parameters. • Milankovitch focused on summer insolation. http://www.ngdc.noaa.gov/paleo/ctl/images/iceage11.jpg
Milankovitch Theory • Milankovitch emphasised summer because he thought glacial melting was the key to persistence of ice. • Milankovitch argued that major periodicities were 19,000, 23,000, and 41,000 years. • Eccentricity was not believed to be important. http://www.geo.arizona.edu/palynology/geos462/mmilanko.gif
Testing Milankovitch • Milankovitch produced detailed latitude-specific predictions of insolation as a function of time. • To test Milankovitch theory, scientists needed to develop better tools: • Tools for determining the age of deposits; • Tools for determining climate.
http://www.ngdc.noaa.gov/paleo/slides/images/base/iceage13.gifhttp://www.ngdc.noaa.gov/paleo/slides/images/base/iceage13.gif
Determining Absolute Age • Radiometric dating • Magneto-stratigraphy
After 1 half-life, concentration of parent is 1/2 of original. After 2 half-lives, concentration of parent is 1/4 of original. After 4 half-lives, concentration of parent is 1/16 of original. Radioactive Decay
Cosmic radiation of Nitrogen 14 creates Carbon 14 via neutron capture Living things take up carbon, including carbon 14 Upon death, uptake of Carbon 14 ceases, but decay continues. Carbon 14 releases electron and reverts to Nitrogen 14 e Proton Neutron
Problems • Carbon 14 allows us to look back only about 50,000 years, because after that, 14C has decayed such that it cannot be accurately measured. • Intermediate half-life radionuclides and magnetostratigraphy save the day.
Uranium Decay Series • Much was learned about uranium in the early 20th century. • Uranium 234 with a half life of 245,000 years proved to useful for dating glacial-interglacial cycles. • In particular corals take up Uranium 234 from sea water. http://www.doh.wa.gov/ehp/rp/air/2%20%20Rad8.gif
Magnetostratigraphy • Use reversals of the Earth’s magnetic field to constrain the age of rocks. • Method extends deep into Phanerozoic history. http://www.geo.arizona.edu/palynology/geos462/pmabutlr.gif
Measuring Paleoclimate • Glacial deposits • Loess • Till • Sea level indicators • Raised Terraces • Oxygen isotopes • Temperature indicators • Plankton Species composition • Oxygen isotopes
Baoji Loess Outcrop • Loess is windblown dust deposited in front of glaciers. • Loess deposits like the one at right are cyclical. • Deposits of glacial “till” are also cyclical. http://www.ngdc.noaa.gov/paleo/slides/images/base/iceage17.jpg
Raised Reefs • Around the globe, reefs are exposed above sea level. • Some raised reefs are the product of uplift, but others have been exposed by falling sea level. • Dating raised reefs reveals periods of globally high sea level associated with interglacial periods. http://www.nd.edu/~acasad/images/wedding/honeymoon/857-999-0019.jpg
Raised Reefs Imbrie and Imbrie, 1979
The Terrestrial Record • The terrestrial record of glacial deposits and sea-level indicators was incomplete. Glaciers are good at rearranging Earth’s surface. • Nonetheless, dated features pointed strongly to a 100,000 year periodicity to glacial-interglacial oscillations. • Milankovitch thought the 100,000 cycle of eccentricity was weak. • Milankovitch theory was not widely accepted.
Turning to the Deep Sea • The deep sea sediment record is more complete than the terrestrial record. • High resolution bio- and magnetostratigraphy could push record of change back through several glacial cycles. http://www.ngdc.noaa.gov/paleo/slides/images/base/iceage14.jpg
What Forams Reveal • Foraminifera (forams) fossils are useful metres of climate. • Species assemblages respond to shifts in water temperature • Shells contain record of ocean chemistry. http://www.ngdc.noaa.gov/paleo/slides/images/base/iceage16.jpg
The 100,000 Year Cycle • The species composition and isotopic signature of deep-sea forams both showed 100,000 year periodicity. • Are the shorter-period oscillations of axial tilt and precession present? Imbrie and Imbrie, 1979
Enter Fourier Analysis • Extracting higher frequency periods is challenging due to uncertainty in absolute ages. • Hays et al in 1976 demonstrated that Fourier Analysis could be used to extract higher frequency signals. • Fourier analysis is based on Fourier’s theorem that any periodic function can be represented by a set of sine and cosine waves of various amplitude and frequency.
