Mass extinction of at least half the known living species on Earth took place six times during the past 550 million years, according to fossils and geologic evidence that exist in ancient rocks and sediments. One, 65 million years ago, was initiated by a large asteroid collision that created a huge impact crater, and which blasted out an enormous amount of ultra-microscopic particles that formed dust clouds which blocked sunlight for years all over Earth. This prevented photosynthesis and killed off many plants and many animals as well. Another mass extinction that took place 250 mya, was due to massive volcanic-ash eruptions that also spread long-lasting sunlight-blocking atmospheric dust clouds all over Earth which destroyed myriad forms of life that had been alive then.

There is evidence in the geologic rock record that indicates the remaining four mass extinctions (occurring 350, 450, 500, and 550 million years ago) resulted from rapid major world-wide climate changes having taken place that effected large world-wide temperature fluctuations back and forth from tropical to frigid --- with correlated major worldwide continental glacial meltings and advances, and with matching oceanic sea-level rises and falls of hundreds of feet.

Many minor extinctions also occurred in the past. The latest one involved profound warm and cold climate changes that took place during the Pleistocene Ice Age over the last few million years. The changes caused significant environmental disruption that affected survival and extinction of innumerable species on land and sea. Minor-extinction temperature fluctuations (which may help us understand why major-extinction climate changes occurred) happened periodically over approximately 100,000-year cycles. They were caused by three shorter planetary-motion deviations known as Milankovic cycles --- named after Milutin Milankovic, a brilliant Serbian mathematician who first calculated their periods in the early 20th century. He theorized that they were influencing changes in the amount of solar energy reaching our planet which was the major factor in causing the recent Pleistocene series of climate fluctuations.

The three Milankovic cycles are: Precession, a 26,000-year periodic wobble of Earth's axis that is due to the sun's gravity constantly (but unsuccessfully) attempting to pull the plane of Earth's equator into the ecliptic (the plane of Earth's orbit) --- Obliquity, a 42,000-year periodic change in the angle of Earth's axis-tilt from 221/2 o to 241/2o (the current tilt of the axis is 231/2o) --- and Eccentricity, a 100,000-year periodic change in the shape of Earth's slightly elliptical orbit from more elliptical to less elliptical.

Both Precession and Eccentricity factors create changes in the average distance of Earth from the sun. The greater the distance, the less solar energy reaches Earth, which causes climate cooling. Correspondingly, the smaller the distance, the more solar energy reaches Earth --- which causes climate heating. Interestingly, the 100,000-year average periodic climate-change cycle is asymmetrical --- with the warming phase lasting 20,000 years, while the cooling phase lasts 80,000 years, or four times longer. A graph of the warming-cooling cycle resembles a playground slide, with a steep ascend side and a gentle descend side. We are presently in a warming phase that began 18,000 years ago.

According to Milankovic, the natural warming phase has another 2,000 years before the next natural cooling phase commences. But sudden cooling or warming excursions (rapid short-term reversals) of a warming or cooling phase) may lengthen or shorten the 2,000 years we have left before the next natural cooling phase is expected to begin. Moreover, the anthropogenic (human-related) Global-Warming factors that exacerbate the undesirable negative effects of the natural warming trend which now exists, are increasing world temperatures from the Tropics to the Poles.

In the Tropics, human-induced Global Warming is causing more numerous and more intense hurricanes that are creating havoc in many parts of the world. In the normally frozen polar regions, glaciers and sea-ice are rapidly melting and causing major environmental changes that threaten the existence of Frigid Zone wildlife. Elsewhere, urban human populations and edifices crowded into coastal regions of Temperate and Tropical zones will be inundated as worldwide sea-levels rise approximately 400 feet after all land snows and glaciers have melted.