Fire / Feu

Les changements climatiques au cours des âges

L’histoire géologique du Nouveau-Brunswick ne recoupe qu’un cinquième du temps géologique. Cependant, cette histoire est suffisante pour relever d’importants changements climatiques. Afin de mettre en perspective les changements climatiques créés par l’homme, il faut se rappeler que le climat planétaire change continuellement depuis des millions d’années.

L’histoire compte déjà des périodes d’effet de serre et des périodes glaciales. Un puissant effet de serre dans l’atmosphère du lointain passé est la cause probable de l’évolution de la vie. Par ailleurs, la composition de l’atmosphère n’a pas toujours été identique à celle d’aujourd’hui.

En se penchant sur l’histoire de la planète, les scientifiques réussissent à mieux comprendre les changements climatiques actuels. En guise d’exemple, les températures dans les provinces maritimes ont subi une chute de 5 °C il y a plus de 10 000 ans, au cours d’une période connue sous le nom de « Younger Dryas ».

A Long View:
Climate Change Across the Eons

Randall F. Miller, Ph.D.
New Brunswick Museum
September 1998

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.

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"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 thin skin.

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 episodes."

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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.