|Les changements climatiques au cours des âges
géologique du Nouveau-Brunswick ne recoupe quun cinquième du temps géologique.
Cependant, cette histoire est suffisante pour relever dimportants changements
climatiques. Afin de mettre en perspective les changements climatiques créés par
lhomme, il faut se rappeler que le climat planétaire change continuellement depuis
des millions dannées.
Lhistoire compte déjà des périodes deffet de serre et des périodes
glaciales. Un puissant effet de serre dans latmosphère du lointain passé est la
cause probable de lévolution de la vie. Par ailleurs, la composition de
latmosphère na pas toujours été identique à celle daujourdhui.
En se penchant sur lhistoire de la planète, les scientifiques réussissent à
mieux comprendre les changements climatiques actuels. En guise dexemple, les
températures dans les provinces maritimes ont subi une chute de 5 °C il y a plus de 10
000 ans, au cours dune période connue sous le nom de « Younger Dryas ».
Climate Change Across the Eons
Randall F. Miller, Ph.D.
New Brunswick Museum
New Brunswick's geologic history records a little more than a fifth of geologic time.
Even so, the story of our province shows we live on an ever-changing planet. For
geologists, an understanding of earth history has always meant considering a principle
developed in the early 19th century: "The Present is the Key to the Past". As we
look ahead and try to understand the potential for environmental change in the next
century, it is equally important to realize that the past is the key to the future.
"Land plants did not evolve until 430 million years ago."
Climate change is nothing new. The recent meeting
on climate change that resulted in the Kyoto Protocol (Kyoto Protocol to the United
Nations Framework Convention on Climatic Change) is not examining some new phenomenon
never before experienced on earth; the Earth always changes. Plate tectonics shift
continents, uplift mountains and create new oceans. The surface we live on is dynamic.
Climate has swung from greenhouse to icehouse conditions. The evolution of life is linked
to the composition of the atmosphere and to the temperature and climate on the planet's
Today the atmosphere is a blend of 78% nitrogen, 21% oxygen, and 1% carbon dioxide,
methane, water, ammonia and other trace gases. It has not always been this way. The
earth's first atmosphere was likely lost to space. The present atmosphere may have formed
from gases contained in the colliding planetesimals or from volcanic gases. The primitive
atmosphere was rich in the "greenhouse" gases; carbon dioxide and nitrogen with
trace amounts of methane, ammonia, sulphur dioxide and hydrochloric acid. They trap
infrared radiation from the sun and warm the surface of the planet. Four billion years ago
the sun's power was only 75% of what it is today. A super "Greenhouse Effect" in
the early atmosphere was likely responsible for keeping the early earth warm enough for
life to evolve.
Rocks 3.5 billion years old contain the first fossils of simple (prokaryotic) cells
that lack a nucleus. These cells were the only life forms until about two billion years
ago, when (eukaryotic) cells with a nucleus appeared. Oxygen only began to accumulate over
the last two billion years. The present level may have been reached as early as 1.5
billion years ago, the result of the photosynthetic activity. Increased oxygen supported
the development of "oxygen-loving" organisms and created the ozone layer which
filters out ultraviolet radiation harmful to life. Without this protection, life would not
have evolved on land. As oxygen in the atmosphere increased, so did the development of
more complex life forms.
Land plants did not evolve until 430 million years
ago. For plants the move from water to land was difficult, involving a number of
adaptations. Plants developed a waxy outer coating to prevent water loss, pores to allow
gas exchange, a vascular system to circulate water and food throughout the plant, and new
reproductive structures that could work on land. The most crucial steps in plant evolution
happened in the Devonian Period. Plants evolved specialized parts - stems, leaves and
roots - and some grew to a size that rivaled modern trees. Since the late 1800's New
Brunswick has been an important place for the study of Devonian plants. The fossils found
in New Brunswick represent a diverse flora that began to turn the landscape green 380
million years ago. Living among the plants, the first land animals began to evolve. Some
of the oldest land animals in North America come from New Brunswick.
By 300 million years ago Laurentia and Gondwana
had been brought together to produce a supercontinent called Pangea. New Brunswick was
about 500 kilometers south of the equator and attached to the landmass that would become
Europe. Climatic conditions were cool and humid, and lush vegetation accumulated in swampy
areas. The Carboniferous Period, represented in central and eastern New Brunswick, is
known as the "Coal Age" when climate, large swamps and abundant plants
contributed to the creation of coal fields. Oddly, our use of coal, created in a tropical
climate hundreds of millions of years ago, is contributing to climate warming today.
The supercontinent Pangea slowly drifted north for
100 million years. New Brunswick crossed the equator into the northern hemisphere.
Following a period of relative quiet, plate tectonic activity and rifting 225 million
years ago began to break up the continents once again. Complete separation of the
continents to produce the Atlantic Ocean left New Brunswick attached to North America. The
spreading of the Atlantic Ocean continues today. During all that time New Brunswick has
been slowly moving away from Europe and Africa at a rate of about 3 centimeters per year.
"Over the last 2 million years there have been many glacial-interglacial
The Quaternary Period, our Period, is only two million years old. It encompasses the
history of human evolution and includes the present. The Quaternary is usually known as
the Ice Age, since it includes the most recent period of widespread glaciation.
Astronomical forces, called Milankovitch cycles, are believed to be the pacemaker of
glaciation. Three cycles (23,000, 41,000 and 100,000 years in length respectively) affect
the wobble of the earth's axis, the tilt of the axis and the eccentricity of the earth's
orbit, and control the amount of sunshine at high latitudes. About 18,000 years ago
glacial ice reached its maximum extent in North America, covering almost all of Canada,
including the Maritimes.
At the end of the last ice age New Brunswick was
partially flooded by the ocean as the weight of glacier ice pushed the land down. Today,
fossil clams, snails and more rarely sea urchins, crabs, brittlestars and walrus occur in
the ice age sediments along our coastline. The last glaciers melted from New Brunswick
about 10,000 years ago bringing to a close the most recent episode of glaciation. The
current warm period is an interglacial, a time between major ice buildup. Over the last 2
million years there have been many glacial-interglacial episodes. The last deglaciation in
the Maritimes includes an important record of rapidly changing climate. Detailed
information comes from microscopic fossils. Grains of tree pollen accumulate in the
millions in the bottom of lakes. By taking cores scientists show a record of vegetation
changing from tundra to forest over thousands of years. Studies of fossil fly larvae and
beetles are used to build detailed temperature records.
That the planet's climate can change rapidly, say
several degrees centigrade over a matter of decades, is not new. It happened most recently
during a time known as the Younger Dryas. About 10,800 (radiocarbon) years ago as average
temperature in the Maritimes was slowly warming, a rapid cooling plunged the region back
toward the ice age. Average annual temperature dropped as much as 5 C in as little as a
decade. Glaciers re-advanced across the landscape and fossils show that the developing
forest reverted to tundra. The cold climate lasted for hundreds of years and was
experienced all around the North Atlantic. Scientists are very interested in the Younger
Dryas event sice it is one of the best examples of rapid climate change, similar to the
magnitude of future climate change predicted for some models of greenhouse warming.
Even the thought that humans have never
experienced this kind of rapid climate change would be wrong. The late-glacial period is
the time humans first populated the Maritimes. Early peoples produced stone tools, that
have been found at two sites near Kingsclear and Quaco in New Brunswick and at Debert,
Nova Scotia where thousands of artifacts and evidence of encampments have been found.
Humans in the Maritimes lived through the Younger Dryas cooling.
At the end of the 20th century our society became aware of the potential for
environmental change from global warming, ozone depletion and other changes to our planet.
The predicted changes, many as a result of human activity, are not outside the range of
events that have already influenced our planet. What is new is that humans may be driving
climate change, and over the coming decades you and your children may experience rapid
changes in climate not experienced since the end of the last ice age.