Sea-level changes through geological history
Sea-level has been close to its present level for the past 6000 years, before which it was lower and fluctuating, last achieving its present position about 120,000 years ago. About 15,000-16,000 years ago, sea-level was 130-140 m below its present position. For the past 500,000 years it has been lower than today about 90% of the time.
These major changes coincide with the latest Ice Age, the later half of the last 1.65 million years of geological time, represents the last 10,000 years when most of the icesheets have melted. Sea-level falls coincide with periods of glaciation whereas the rises occur during interglacials — the warmer times between ice advances, like the present day.
The onset of the Ice Age began about 40 million years when surface waters in the southern oceans suddenly cooled and the deep ocean basins quickly filled with water ~10°C cooler than before that sank because of its increased density. By about 15 million years ago, the Antarctic Icecap had formed, accelerating production of cold waters.
About 6'5 million years ago, sea-level fell by as much as 50 m, probably associated with expansion of the icecap in Antarctica. This might have caused the Mediterranean Sea to dry up over ~1,000 years, producing vast salt deposits, preserved in the sediments of the sea floor.
About 5 million years ago there followed a brief warming trend and sea-level rose again leaving shallow marine sediments inland of modern coastlines around much of the world. Fossil floras and faunas show that climates were generally warmer than today — Iceland had a temperate climate; southern England was subtropical.
Let us take a case of geological period when the climate was very close to the current climate.
Carboniferous period:
Life was at its full bloom at the age of Carboniferous period (360 MM years -300 MM years).Insects, plants, dinos and many species evolved at this time. It is associated with all the green forest correlatable to present coal deposits. Climate of this period matched today's climate.
Similarities with our Present World
Average global temperatures in the Early Carboniferous Period were hot- approximately 20° C (68° F). However, cooling during the Middle Carboniferous reduced average global temperatures to about 12° C (54° F). This is comparable to the average global temperature on Earth today!
Similarly, atmospheric concentrations of carbon dioxide (CO2) in the Early Carboniferous Period were approximately 1500 ppm (parts per million), but by the Middle Carboniferous had declined to about 350 ppm — comparable to average CO2 concentrations today!
Earth’s atmosphere today contains about 380 ppm CO2 (0.038%). Compared to former geologic times, our present atmosphere, like the Late Carboniferous atmosphere, is CO2- impoverished! In the last 600 million years of Earth’s history only the Carboniferous Period and our present age, the Quaternary Period, have witnessed CO2 levels less than 400 ppm.
Global Temperature and Atmospheric CO2 over Geologic Time
There has historically been much more CO2 in our atmosphere than exists today. For example, during the Jurassic Period (200 mya), average CO2 concentrations were about 1800 ppm or about 4.7 times higher than today. The highest concentrations of CO2 during all of the Paleozoic Era occurred during the Cambrian Period, nearly 7000 ppm — about 18 times higher than today.
The Carboniferous Period and the Ordovician Period were the only geological periods during the Paleozoic Era when global temperatures were as low as they are today. To the consternation of global warming proponents, the Late Ordovician Period was also an Ice Age while at the same time CO2 concentrations then were nearly 12 times higher than today– 4400 ppm. According to greenhouse theory, Earth should have been exceedingly hot. Instead, global temperatures were no warmer than today. Clearly, other factors besides atmospheric carbon influence earth temperatures and global warming.
I will be presenting what is written on the rocks and what is the opinion.
Our Future Written in Stone
Today the Earth warms up and cools down in cycles. Geologic history reveals similar cycles were operative during the Carboniferous Period. Warming episodes caused by the periodic favorable coincidence of solar maximums and the cyclic variations of Earth’s orbit around the sun are responsible for our warm but temporary interglacial vacation from the Pleistocene Ice Age, a cold period in Earth’s recent past which began about 2 million years ago and ended (at least temporarily) about 10,000 years ago. And just as our current world has warmed, and our atmosphere has increased in moisture and CO2 since the glaciers began retreating 18,000 years ago, so the Carboniferous Ice Age witnessed brief periods of warming and CO2-enrichment.
Following the Carboniferous Period, earth witnessed predominantly desert-like conditions, accompanied by one or more major periods of species extinctions. CO2 levels began to rise during this time because there was less erosion of the land and therefore reduced opportunity for chemical reaction of CO2 with freshly exposed minerals. Also, there was significantly less plant life growing in the proper swamplands to sequester CO2 through photosynthesis and rapid burial.
It wasn’t until Pangea began breaking up in the that climates became moist once again. Carbon dioxide existed then at average concentrations of about 1200 ppm, but have since declined. Today, at 380 ppm our atmosphere is CO2-impoverished, although environmentalists, certain political groups, and the news media would have us believe otherwise.
What will our climate be like in the future? That is the question scientists are asking and seeking answers to right now. The causes of “global warming” and climate change are today being popularly described in terms of human activities. However, climate change is something that happens constantly on its own. If humans are in fact altering Earth’s climate with our cars, electrical powerplants, and factories these changes must be larger than the natural climate variability in order to be measurable. So far the signal of a discernible human contribution to global climate change has not emerged from this natural variability or background noise.
Understanding Earth’s geologic and climate past is important for understanding why our present Earth is the way it is, and what Earth may look like in the future. The geologic information locked up in the rocks and coal seams of the Carboniferous Period are like a history book waiting to be opened. What we know so far, is merely an introduction. It falls on the next generation of geologists, climatologists, biologists, and curious others to continue the exploration and discovery of Earth’s dynamic history– a fascinating and surprising tale, written in stone.
