Climatic Ice Cycles
Bill O’Neill biophil@bainbridge.net <mailto:biophil@bainbridge.net>
It’s all fitting together! Don’t you love it when that happens? For the last couple of years I’ve been auditing courses at the UW (under the ACCESS program for folks over 60, www.washington.edu/students/reg/access <http://www.washington.edu/students/reg/access>), making connections with scientific disciplines other than my own. A lecture Paul Middents presented a couple of weeks ago, in his latest series of classes for BPAA, happily coincided with a course I’m taking at UW this term, Climatic Extremes: Causes and Effects.
Ice Margin, GreenlandDepartment of Geophysics, Niels Bohr Institute, University of Copenhagen www.glaciology.gfy.ku.dk/ngrip/
Paul described the work of the Serbian astronomer, Milutin Milankovitch, who had calculated the perturbations of Earth’s relationship to the Sun, which he predicted (in the 1920-30s) could account for rece
nt ice ages. Fifty years later (Milankovitch died in 1958), his predictions were verified - first through ocean sediment cores and later by drilling deep into Greenland and Antarctic remnants from the ice ages. The Milankovitch theory anticipated that oscillations in the tilt of Earth’s axis--caused by the gravitational pull of the large planet --would, every 41,000 years, minimize the amount of solar energy (insolation) warming the summers at the northern latitudes (Arctic Circle) where continental glaciers originate. During these periodic insolation minima, centuries of relatively cool summers didn’t provide sufficient heat to completely melt preceding winters’ snowfall, permitting accumulation of layer-upon-layer, year-after-year. Such tepid summers occur when the Earth’s angle of tilt (obliquity) approaches its minimum--about 22o inclination from a perpendicular to Earth’s orbital plane--in contrast to its present 23.5o inclination or to the maximum inclination of over 24o. At the times of those minima, summers are cooler and winters are warmer. If the Earth’s inclination were 0o, there would be no seasons at all. In addition to changing its tilt, Earth’s axis wobbles slowly in a circle as the Sun and Moon tug on the Earth’s equatorial bulge, shifting the season when Earth most closely approaches the Sun as it traverses its annual elliptical orbit. This precession of the seasons (equinoxes) has a complicated variation pattern which restarts approximately every 19,000 or 23,000 years. Today, the Northern Hemisphere has summer when Earth is distant from the Sun (aphelion), but 10,000 years ago perihelion occurred in summer. Furthermore,
the elliptical orbit of the Earth undergoes eccentricity changes over a period of 100,000 years. The main effect of this eccentricity cycle is to influence the impact of precession --since our orbit is now nearly round, precessional effects are insignificant.
Ice Core, GreenlandDepartment of Geophysics, Niels Bohr Institute, University of Copenhagen www.glaciology.gfy.ku.dk/ngrip/
All of these cycles seem to be manifested in cores drilled through seafloor sediments (dating back 4+ million years, MYr) and, in amazing detail, by ice cores obtained from continental ice on both Greenland and Antarctica. Up to 2.75 MYr ago oxygen isotope-ratios of fossils in sea sediments indicate gradual cooling, but show no evidence of ice-rafted debris associated with glaciation. From 2.75 MYr ago until 0.9 MYr ago, small ice sheets apparently grew, then melted at cycles of 41,000 and 23,000 years, since ocean sediment cores contain layers of glacial sand (ice-rafted debris) typical of icebergs “calved” off continental ice. Finally, it appears larger ice sheets grew and melted in cycles close to 100,000 years in length, especially over the last half MYr. Furthermore, evidence of extremely intense tropical monsoons (which flooded portions of North Africa and Arabia at intervals in the past) appears on a 23,000-year cycle.
Apparently, extremes of obliquity and precession of the summer equinox combine to initiate accumulation of ice, leading into an ice age. Evaporation of sea water favors the lightest molecules of H2O; so when that water vapor condenses, snows and becomes part of a continental ice mass, it contains less 18O than sea water. Conversely, the sea water incorporated into fossil shells retains the excess 18O and the relative disposition of that fractionated oxygen (18O/16O) when recovered from drilled cores provides historical records of ice volume (complement of sea volume and sea level) and temperature. Changes of marine oxygen isotope ratios (d18O) indicate that > 50 glacial maxima occurred over the past 3 MYr. Some threshold condition was crossed less than a million years ago that made conditions especially favorable for major glaciations.
In the last 500,000 years maximum d18O values not only increase (cooler temps.), but are spaced farther apart, in a distinct 100 KYr rhythm. It seems doubtful that eccentricity is its direct cause, since Earth’s orbit is now so nearly circular. Obliquity and precession vary latitudinal (but not global) insolation by as much as 12%, enough to make quite a difference with respect to ice accumulation. Both 41 K and 23 K year cycles persist into the major glaciation period. They show up exquisitely in the ice cores recovered from two sites, 20 miles apart in Greenland, and an Antarctic core drilled over Lake Vostok (halted 400’ above it to avoid contamination of that “pristine” lake which may serve as a model for Europa). Each core is about two miles long, and the stories of their recovery and interpretation are told in two fascinating recent books, Two-mile Time Machine and The Ice Chronicles, and a 1997 paper in Nature on the Russian work. Besides d18O records, all these ice cores provide histories of CO2, CH4 and temperature.
Fast responding parts of Earth’s climate system are forced by ice volume, which is itself slow-responding. Hence, temperature changes lag thousands of years behind changes in insolation. Ice-driven responses are manifested in the oceans, wind & soils (seen as salt & dust in cores) and vegetation (seen as pollen in cores), even at great distances from the ice sheets. Surprisingly, the Antarctic ice (which provides a vivid historical record) has done little to actually change climate in either hemisphere, since it covers Antarctica even
during interglacial intervals. Ice volume causes CO2 levels to change, and CO2 exerts positive feedback, amplifying the formation or melting of ice. Most striking is the rapidity of the termination phase in which continental ice sheets melt completely over 10,000 years, after taking 90,000 years to accumulate. It appears that since 900 KYr ago something in Earth’s climate system has become sensitive to modulation of the precessional signal resulting in the 100 KYr cycle.