Climate has changed from “Hot to Cold” and Cold to Hot”- a brief history.
DR. NITISH PRIYADARSHI.
In recent years, the scenario of future global environment is haunting the man as the present environmental changes (e.g. global warming) pose considerable danger to his own existence and environment. He is presently struggling to understand as to what will be the nature and extent of these changes in the next hundred years. In order to understand the processes of changes and the effects they are likely to have on the future environment of the biosphere, we should develop a historical perspective- a perspective based on global environmental changes preserved in the rocks of the planet earth.
The history of earth’s climate is characterized by change. Times of glaciation on the earth have been followed by warm intervals and the duration in years of both cold and warm intervals has varied by several orders of magnitude.
The solid earth accumulated about 4700 m.y. ago from a cloud of cosmic particles and gaseous materials and as they collected gravitationally a hot planetary nucleus formed.
The first atmosphere of the earth, then, contained hydrogen, helium, neon, argon and various other lighter and inert gases, none of which is abundant in the present atmosphere. Most of these on liberation to the air now either escape earth’s gravitational pull because of their low densities or are bound up in minerals by chemically reacting with them.
Climates before 3800 million years ago:
Geologic evidence for paleoclimates of this interval is scant; fossil remains and unaltered sedimentary rocks are not known from the record. According to different research reports climate before 3800 m.y. ago were probably warm, perhaps warmer than present. Greenhouse effects contributed to this. Cooling effects of unknown magnitude were also operative. A steady cooling may have taken place over the interval in response to progressively decreasing carbon dioxide concentrations.
Climates between 3800 and 2400 million years ago:
From different research for this interval, a scenario can be constructed in which warm and wet climates characterize the early part, and in which a gradual cooling takes place as a consequence of changes in atmosphere and hydrospheric composition, to culminate in the glacial episode of the middle Precambrian. Evidence for earth’s earliest glaciation is best documented for an interval in the middle Precambrian. Still earlier episodes of glacial activity may have occurred, but glaciation may have been of such extremely local extent or short duration that glacial strata have not been detected.
Climates between 2300 and 950 million years ago:
The general distribution of the sedimentary rocks provides several clues to climates which followed the earliest glaciations. For the 2300 to 950 m.y. interval, abundant evidence exists that many carbonates are of warm water origin.
Oxygen and hydrogen isotopic compositions of middle Precambrian Cherts suggest high ground-surface temperatures, as in early Precambrian.
Climate between 950 to 615 million years ago:
A great deal of information exists that much op the earth was glaciated during the late Precambrian, particularly in the time span from 950 to 615 m.y. ago. The 950 m.y. figure represents the approximate age of the oldest among a series of tillites which signal the end of the long period of carbonate sedimentation which began about 2000 m.y. ago.
Climate in Paleozoic:
The Paleozoic encompasses about 350 m.y. and geographically restricted glaciations occur in each of the six periods of the era.
The broad trend of Paleozoic climate was from relative warmth which followed the late Precambrian glaciations, to a long and widespread glacial interval near the end of the era.
The sudden and repeated extinction events of trilobites provides clues about the climate changes in the Cambrian.
All of the continents were close to the equator and the trilobites were adapted to warm waters presumably. It has been suggested that the extinction of the trilobites was associated with a cooling of the ocean waters.
This hypothesis is supported by the fact that it was only the deeper dwelling trilobites that survived the extinctions. This is probably because they were already adapted to cold conditions since they lived in deep (cold) waters. Nonetheless, the case of temperature change as the cause remains unproven.
What is particularly problematic about any cooling idea to explain the extinction is evidence that suggest that atmospheric CO2 was much higher in the early Paleozoic Era. This evidence is in the form of various mineral types whose presence is a sensitive indicator of atmospheric CO2 levels.
During the Ordovician Life expanded in diversity tremendously. There were extensive reef complexes in the tropics. The early Ordovician was thought to be quite warm, at least in the tropics and cooled considerably at the end of the period.
Despite the tremendous expansion of life during the Ordovician Period there was a devastating mass extinction of organisms at the end of the Ordovician. This extinction was one of the greatest mass extinction ever recorded in Earth History.
The more likely cause is that the Earth cooled, particularly the oceans where most of the organisms lived during the Ordovician (Remember there were no land plants and no evidence of land organisms yet). All the extinctions occurred in the oceans.
Early Silurian warming was quickly followed by gentle cooling until the middle Devonian, and warm and dry conditions characterized the interval between the late Silurian and Early late Devonian.
During the Silurian Period the first land plants appeared. Marine organisms once again expanded in diversity following the extinction of so many families in the late Ordovician.
By the Devonian fish were a common part of the marine biological communities. Particularly important were the jawed fish. These were predators and they must have had quite an impact on the marine communities during the Devonian.
The first fossil evidence of insects and terrestrial trees comes from Devonian age rocks.
The Devonian is thought to have been quite warm. Evidence of this comes from the extensive amount of tropical-like reefs. The climate is also thought to have been quite dry. Evidence of this comes from extensive evaporite (salt deposits) that have been found dispersed much more broadly than any time in the earlier Paleozoic.
The early Carboniferous continued warm although humidity increased, and marked cooling in the late Carboniferous led to glaciation.
The Permian is one of the most interesting climate intervals because of the variety of climatically significant rocks which it contains. Asia appears to have been subjected to relatively wet climates through most of the Permian, as were large regions of Gondwanaland following the glaciations. Coals are known from short distances above glacial deposits in most Gondwana continents, suggesting that expansion of the seas, owing to post-glacial transgressions, led to high humidity in relatively high latitudes.
As the Permian opened, the Earth was still in the grip of an ice age, so the polar regions were covered with deep layers of ice. Glaciers continued to cover much of Gondwanaland, as they had during the late Carboniferous . At the same time the tropics were covered in swampy forests.
Towards the middle of the period the climate became warmer and milder, the glaciers receded, and the continental interiors became drier. Much of the interior of Pangea was probably arid, with great seasonal fluctuations (wet and dry seasons), because of the lack of the moderating effect of nearby bodies of water. This drying tendency continued through to the late Permian, along with alternating warming and cooling period.
Examination of the history of climate for single continents and super continents leads to the conclusion that during the Paleozoic, Europe and North America were subjected to only mild changes in climate while Gondwanaland went through several episodes of glaciation and variable states of humidity.
Climate in Mesozoic:
The Mesozoic Era (200 m.y.) presents excellent evidence for warm and dry climates. The initial Triassic climates were closely similar to those of the latest Permian, i.e. cool and humid, and were followed by a warm, drying-out period which may have lasted until the late Jurassic. The middle Triassic apparently was a time of great latitudinal expansion of evaporite deposition and of reef building. For this reason, mid- Triassic climate are considered to have been relatively warm and, possibly, the most arid in earth history.
During late Triassic global climate was warm. There was no ice at either North or South Poles. Warm Temperate conditions extended towards the poles.
Rapid global warming at the very end of the Permian may have created a super- “Hot House” world that caused the great Permo-Triassic extinction. 99% of all life on earth perished during the Permo-Triassic extinction.
Jurassic Period climate:
There are no proven glacial deposits of Jurassic age. During early and middle Jurassic climate the Pangean Mega-monsoon was in full swing. The interior of Pangea was very arid and hot. Deserts covered what is now the Amazon and Congo rain forests. China, surrounded by moisture bearing winds was lush and verdant. During the late Jurassic the global climate began to change due to breakup of Pangea. The interior of Pangea became less dry, and seasonal snow and ice frosted the polar regions.
Evidence from oxygen isotopes in late Jurassic belemnites indicates maximum temperatures of surface sea water of about 14° C at 75° S latitude. If correct, this would be at least 7° C warmer than present day temperatures, and a warm earth accordingly is implied.
Climate in Cretaceous:
The Cretaceous must be recognized as time of great warmth over the globe. This conclusion derives from oxygen isotopes, paleobiogeography and rock distributions, all of which indicate that temperatures were higher than now over the full range of latitude from equator to poles. No ice existed at the poles. Dinosaurs migrated between the Warm Temperate and Cool Temperate zones as the season changed.
How did the earth was such warmer at Mesozoic? Was there an increase in the radiation received from the Sun? Or, can the warm globe be explained by some strictly terrestrial cause?
Climate in Tertiary (65 m.y.):
The period of time which elapsed between the end of the Cretaceous and the present time.
Paleocene, Eocene and Oligocene apparently experienced cool changes which were both more frequent and more intense than those of the Mesozoic. Over this interval between about 65 and 22.5 m.y. ago, long episodes of relatively slight warming were punctuated be severe and abrupt drops in temperature leading to successively cooler regimes.
According to other opinion, the climate during the Paleocene was much warmer than today. Palm trees grew in Greenland. Global climate during the late Eocene was warmer than today. Ice had just begun to form at the South Pole. India was covered by tropical rain forest.
During the Oligocene, ice covered the South Pole but not the North Pole. Warm Temperate forests covered Northern Eurasia and North America.
The climate during the Miocene was similar to today’s climate but warmer. The gradual reduction in average temperature was continued throughout this time. We can assume that relatively warm climates were succeeded by cool climates which continued into the early Pliocene. As a result of this cooling, ice volume in Antarctica would have been about 50% greater than at present.
During the Pliocene times the continuing drop in average temperature caused the extinction of many groups of mammals and migration of other forms to warmer regions.
Pleistocene climate was characterized by repeated glacial cycles. It is estimated that, at maximum glacial extent, 30% of the Earth’s surface was covered by ice. Deserts on the other hand were drier and more extensive. Rainfall was lower because of the decrease in oceanic and other evaporation.
To observe a Holocene environment, simply look around you. The Holocene is the name given to the last 10,000 years of the Earth’s history- the time since the end of the major glacial epoch, or “ice age”. Since then, there have been small-scale climate shifts- notably the “Little Ice Age” between about 1200 and 1700 A.D.- but in general, the Holocene has been a relatively warm period in between ice ages.
Humanity has greatly influenced the Holocene environment. The vast majority of scientists agree that human activity is responsible for “Global Warming”, an observed increase in mean global temperatures that is still is going on. Habitat destruction, pollution and other factors are causing an ongoing mass extinction of plant and animal species. According to some projections, 20% of all plant and animal species on Earth will be extinct within the next 25 years.
The inhabitants of Mumbai or Riyadh might dispute on the fact that the earth is currently in the grip of a glacial episode. True, the present is an interval of relative warmth, an interglacial period, but for the past several million years the planet has been colder, on average, than it has over much of its history.
The examples of changes of global environment and the associated mass extinctions in the geological past clearly indicate that ecosystem is quite sensitive to environment changes and also has a capacity to regrow. Environmental factors, whether natural or man made, become ecologically disruptive when they cross threshold limits. Ecological viability, on the other hand, allows evolution to resume when extreme destructive natural factors relent during times of normalcy.
What ever may be the truth but it is true that the climate of the earth is changing from the time of its birth from hot to cold and cold to hot. Earlier too the earth has passed through global warming due to natural causes but this time we the humans are culprits for the changes. When man-made factors are added to the natural ones, the ecosystem may be damaged beyond repair.
· L.A. Frakes, 1979. Climates throughout geologic time. Elsevier publication, New York.
· J.D. Macdougall, 1996. A short history of planet earth. John Wiley and Sons, New york.
· D.G.A. Whitten and J.R.V. Brooks, 1983. The Penguin dictionary of geology. Penguin Books Ltd. England.
· The proceedings of the 94th Indian Science congress, Part II ,2007.
Dr. Nitish Priyadarshi
Geologist and Environmentalist