The Earth was hotter than it is today.
By
Dr. Nitish
Priyadarshi
Geologist.
Email:
nitish.priyadarshi@gmail.com
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 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.
Climate change is changing the world. Either it is in the
form of temperature rise or in the form of severe floods. Many times question
arises in my mind whether this climate change is the outcome of present human
activities on the earth or it has happened in early geological ages too. Answer
is “yes” climate change has occurred several times from the beginning of the
earth formation. Evidences are preserved in from of rocks, sediments, and
fossils. Studying
the records of past climate change will fill you like reading thriller novel in
which every chapter is full of suspense and thrill. Every chapter of this novel
denotes different geological periods with different stories of climate change.
Change is the only constant in the history of the earth.
Since its creation around four billion years ago, our home planet has
constantly been subjected to changes brought about by the interplay of internal
of forces and external influences. The enormous lithospheric plates are
continually shifting, reshaping the continents. Volcanic eruptions and earthquakes
are the visible results of this process. Seen from the perspective of the
history of the earth, our planet, as we know it today, is merely a snapshot in
time. Everything is in a state of flux. Everything- including the climate- is
locked in a continuous process of change, giving rise to favourable and
unfavourable conditions to which all life on earth-human, plant and animal-
must constantly adopt.
My article is about the climate conditions of the Mesozoic.
The term “Mesozoic” was introduced by John Phillips in 1840
for the rock formations containing remains of “middle forms” of life. The
Mesozoic Era that began at about 230 million years ago and closed at about 65
million years ago represents less than half the duration of the Palaeozoic Era.
The Mesozoic Era has been further subdivided into Triassic, Jurassic and
Cretaceous periods.
The lower boundary of the Mesozoic is set by the Permian–Triassic
extinction event, during which approximately 90% to 96% of marine species and
70% of terrestrial vertebrates became extinct. It is also known as the
"Great Dying" because it is considered the largest mass extinction in
the Earth's history. The upper boundary of the Mesozoic is set at the Cretaceous–Paleogene
extinction event (or K–Pg extinction event), which may have been caused by an
asteroid impactor that created Chicxulub Crater on the Yucatán Peninsula.
Towards the Late Cretaceous, large volcanic eruptions are also believed to have
contributed to the Cretaceous–Paleogene extinction event. Approximately 50% of
all genera became extinct, including all of the non-avian dinosaurs.
Pangaea was characterized by extreme climate variations
because it was such a huge continent. The coastal areas near Equator had a
monsoon climate with summer rain, while its interior areas were dry and
desert-like. Pangaea was a many times larger continent than Eurasia, most
likely with an even more pronounced continental climate. One cannot doubt that
winters in central Pangaea must have been very harsh experiences, and summers must
have been insufferable glowing hot. There were no actual ice ages during
Mesozoic. Pangaea was the largest continent ever, and it was surrounded by an
equally huge ocean that was the Panthalassic Sea, which covered the rest of
Earth's surface. It is believed that the shores of Pangaea must have been
exposed to very strong monsoon winds.
Three of the five largest mass extinctions in Earth history
are associated with the Mesozoic: a mass extinction occurred at the boundary
between the Mesozoic and the preceding Paleozoic; another occurred within the
Mesozoic at the end of the Triassic Period; and a third occurred at the
boundary between the Mesozoic and subsequent Cenozoic, resulting in the demise of
the dinosaurs.
Perhaps the largest number of palaeogeographic
reconstructions have been made for the Mesozoic Era. In many parts of the
world, the Era began with a new phase of sedimentation . Pangaea, the
supercontinent of the Palaeozoic Era was gradually torn apart during the
Mesozoic Era. Fragmentation of the Pangaea began with the opening of
proto-Atlantic and proto-Indian oceans. On the basis of the palaeo-magnetic
evidence, it has been suggested that that this break- up began with the
separation of North America and Gondwana Land in Late Triassic Epoch. The Mesozoic was a time of significant tectonic,
climate, and evolutionary activity.
The
climate of the Mesozoic was varied, alternating between warming and cooling
periods. Overall, however, the Earth was hotter than it is today. We can also
say that “Earth’s climate during the Mesozoic Era was generally warm, and there
was less difference in temperature between equatorial and polar latitudes than
there is today”. The Mesozoic is characterized by a warm and
humid Earth. Oxygen levels were very similar to today’s levels, thanks to
the large coniferous forests that proliferated in the early Triassic period but
were replaced by Angiospermae forests in the early Cretaceous.
We
pass from a dry and arid climate at the beginning of the Triassic as we leave
an extinction that left the continents dry, to still warm climates but with
greater humidity at the beginning of the Jurassic and remain so until the end
of the Cretaceous. It is thought that in the Cretaceous, the North Pole was
able to hold ice in winter.
Dinosaurs first appeared in the Mid-Triassic, and became the
dominant terrestrial vertebrates in the Late Triassic or Early Jurassic,
occupying this position for about 150 or 135 million years until their demise
at the end of the Cretaceous. Birds first appeared in the Jurassic (however,
true toothless birds appeared first in the Cretaceous), having evolved from a
branch of Theropod dinosaurs.
Pangaea began breaking up at the end of the Triassic. Between
North America and Africa, volcanoes were spewing out large amounts of alkaline
lava, which can be found on both continents. It is assumed that this volcano
disaster initiated another round of extinction of up to 80% of Earth's species,
and thus paved the way for the dinosaurs that came to dominate the Earth in the
next hundred million years. Some believe that the root cause of species
extinction was that the intense volcanic activity volcanoes emitted so much CO2
that the Earth's temperature rose dramatically so that the heat was the real
killer that wiped out so many species.
Jurassic began 213 million years ago and lasted until 144
million years before present, a total of 69 million years. The gradual break-up
of Pangaea that began in the late Triassic, continued in Jurassic. The climate
of the new and smaller continents was milder and more rainy. Hot and humid
tropical breezes blew through dense forests of ferns, cycads, ginkgo trees and
various conifers.
Since large parts of the Earth's surface were covered by
water and green plants, planet's albedo decreased, and more of the Sun's energy
was absorbed as heat. The increased atmospheric moisture content also helped to
increase the temperature, because water vapor is a greenhouse gas.
There is strong evidence that the sea surface level in the
World increased during the Jurassic, which meant that many low-lying areas were
transformed into shallow coastal sea, and in the same time the climate became
more oceanic, that is wetter, warmer and with much less seasonal variation.
The Cretaceous period began 145 million years ago and lasted
until the dinosaur extinction 65 million years ago. During this period
prevailed almost everywhere on Earth a warm and humid climate. The thick layer
of chalk, which was created by the microscopic calcareous algae that have been
found in abundance throughout the Earth's oceans, gave its name to the period.
Up until then, chalk deposits had been restricted to shallow coastal waters.
By the middle of the Cretaceous period about 100 million years ago, the mean- temperature on the planet's surface was between 6 and 12 degrees higher than it is today. The annual mean temperature in the Arctic was about 10 degrees, which is about 20 degrees warmer than today. In the tropical regions, the surface water temperature was approx. 5 to 10 degrees higher than the present.
By the middle of the Cretaceous period about 100 million years ago, the mean- temperature on the planet's surface was between 6 and 12 degrees higher than it is today. The annual mean temperature in the Arctic was about 10 degrees, which is about 20 degrees warmer than today. In the tropical regions, the surface water temperature was approx. 5 to 10 degrees higher than the present.
The environment was unusually warm and polar ice caps did
not yet exist. This played a large part in evolution and is a key factor behind
the flourishing of the dinosaurs. During the Triassic period the climate was
generally dry, which changed near the Jurassic period as oceans began to rise
due to mounting layers of magma covering the seafloor. As a result, flooding
overtook many parts of the exposed land. This allowed the climate to change
with increased humidity and it continued that way even into the Cretaceous
period. However, the climate began to cool during the Cretaceous although
temperatures may have risen again near the end of the Mesozoic.
Understanding how the Earth responded to past
extreme warming and CO2 input can help us prepare for how the planet will
respond to current, human-caused climate change.
References:
K. Ravindra, 1988. Fundamentals of historical geology and
stratigraphy of India. Wiley eastern limited, New Delhi.
