AEONS: CLIMATE NOTES

X. CLIMATE NOTES

Over 4.6 billion years, there have been six supercontinents – contiguous aggregations of material separate from ocean, on top of the earth’s crust. Precipitation increased with each supercontinent break-up, because there was more orographic interaction between moist air currents and land masses.

Earth's initial atmosphere was mostly carbon dioxide and methane, from prevalent vulcanism and gaseous releases during tectonic movements. Carbon dioxide and newly-eroded materials from land and rivers provided nutrients for early plant cells, which through photosynthesis generated new oxygen to the atmosphere. Atmospheric oxygen increased in stages with each supercontinent, from 1% to 21% of current times.

Today, the natural plant-animal, carbon dioxide-oxygen cycle is as important and essential to our atmosphere as it was at the dawn of planetary life. Current levels of carbon dioxide are not uniform in all parts of the globe; they vary between 300 and 440 parts-per-million. This was initially reported as comparable to levels observed in four 140,000-year cycles in the 1998 Antarctic ice cores. Runaway planetary temperatures from this source have not been detected, either in earth's prehistory, or on other planets.

The total area of land above sea-level has increased throughout prehistory, generally leading in itself to cooler temperatures, owing to energy reflection rather than absorption into the oceans. However, from various causes, global temperatures have fluctuated throughout prehistory. Most of the time, the earth was warmer at all latitudes, except during the five glacials, including the present glacial (“ice age”).

The five glacials include an extreme “snowball earth” during the Huronian glacial in pre-Cambrian times, when ice extended to the subtropics. At the end of the Archaean aeon, the earth was twice as warm as present, owing to initial volcanic formation, as well as volcanic and radiation activity. For other reasons, the Mesozoic was 10°C warmer in places than at present.

Earth’s climate is affected significantly by both internal and external forces. “On-board” factors include:

aggregation and global location of continental land-mass;

wind currents and rains based on the foregoing;

events beneath and in the earth’s crust;

volcanic dust and emissions;

variable albedo depending on surfaces and ground cover;

oceanic dynamics and currents; and

variable atmospheric composition and density.

External forces include:
earth’s axial rotational eccentricity (21,000 year);

orbital precession (100,000 year);

orbital plane drift (400,000 year);

variations within the sun’s galactic orbit;

variations in solarization as the sun moves through galactic arms of dust;

gravitational positioning of other planets; and

the sun’s own internal life processes.

This last includes a progressive increase in solar radiation by about 4% every 500 million years; and the sun goes through energy output cycles at 11-year and 160-year intervals. (Sunspot and solar minimum cycles).

Glacials have been present for roughly 25% of the last 3.1 billion years; earth’s surface has been ice-free for the other 75% of this time. Our current period, the Quaternary, (that is, the Pleistocene and Holocene epochs), began 2.6 million years ago with the start of the earth’s fifth glacial, (also named the Quaternary).

We live at the apparent end of this most recent of five major glacials in earth’s history, but it may be merely a warm chron or "interglacial", to be followed by a colder one. It seems likely that such variations are caused by combinations of the factors listed above.

Antarctic ice cores from 1998 indicate that during the last 800,000 years, there have been four glacial chrons —at very regular intervals of 140,000 years, with similar levels of atmospheric elements; and it is possible that a fifth cold cycle would begin within 20,000 years. "... temperatures from the end of the Younger Dryas Period to the beginning of the Holocene some 12,500 years ago rose about 20 degrees Fahrenheit in a 50-year period in Antarctica, much of it in several major leaps lasting less than a decade." (University of Colorado).

The current warm interglacial chron began quite clearly 12,500 years ago. It marked the the end of the "paleolithic" (old stone age)- a reference to the emergence of humans from a universal cave-dwelling culture, into the "neolithic" (new stone age); and then a "post-neolithic" (after 6000 years ago). The post-neolithic was characterized by group activities such as metal-working, pottery, animal husbandry, agriculture, and written language.

The associated melting of the northern ice-cap showed itself in the development of great rivers and lakes, such as the Great Lakes in North America; and the relatively sudden formation of the North Sea separating the British Isles from the rest of Europe, around 6000BC.

Humans have never experienced a glacial-free earth, having evolved over the last 200,000 years of a 200-million-year glacial age. It is possible, though not certain, that a glacial chron will return in about 20 millennia, but that still leaves us with learning to accommodate to a warmer earth in the interim. Development of presently frigid regions will come about, in exchange for an avoidance of more torrid areas. Across the world, countries must exercise greater care and responsibility with all aspects of earth's resources, and with the manufacture or production of artificial materials.

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