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To put geologic time in perspective, geologists have compared the Earth’s age to a calendar year—if Earth formed on New Year’s morning, the first recognizable fossil animals do not appear until mid-November, dinosaurs go extinct on Christmas Day and all of human recorded history occurs in the final seconds of New Year’s Eve. The Atlantic Ocean is less than 250 million years old—about the second week of December in the geologic year. At that time, ancestors of modern insects, fish, amphibians and reptiles were already living on land and in the water. Dragonflies were launching aerial attacks on hapless insects over 200 million years before the Atlantic Ocean or Rocky Mountains were formed. Earthquakes
and volcanoes remind us that the ground beneath us moves; nearly 800,000
earthquakes are recorded around the world each year, and though catastrophic
earthquakes and volcanoes are considered rare in our lifetimes, they occur
every second of geology’s metaphoric calendar year. On a geologic
time scale, old-growth forests are as ephemeral as crabgrass that sprouts
the day after you weed the garden, and our lakes are like rainwater puddles
on pavement that dry soon after the storm passes. Only 20,000 years ago, northeastern North America looked much like northern Greenland looks today, glaciers up to a mile thick covered the landscape. When glaciers were at their peak, sea levels were over 120 meters lower than they are now because an enormous portion of the Earth’s water was locked up in glacial ice. The famed fishing grounds that helped shape the economic and cultural history of the region—especially Georges Bank, Grand Bank and Browns Bank—were tundra environments where wooly mammoths and mastodons roamed. Pollen grains, freshwater peat and mastodon teeth have been found far out on the continental shelf in water up to 120 meters deep, as far as 130 kilometers from the modern coastline. New England’s forests, lakes and rivers are less than 15,000 years old. As glaciers retreated, seawater flooded much of Maine as far inland as Bingham and Millinocket. This was caused by two processes—relative sea level rise (transgression) resulting from melting of glacial ice, and compression of the edge of the continent caused by the immense weight of the glaciers. It took thousands of years for the land to rebound after glaciers retreated, and in the meantime, seawater flooded much further inland than it would have from transgression alone. Wave-pounded shorelines were located in central interior Maine for thousands of years; most of Maine’s human population now lives in what was the bottom of the sea only 12,000 years ago. Ever since the land rebounded and the shoreline retreated toward its present position, the terrestrial landscape changed from a subtidal marine environment, to tundra, to boreal forest, to the mixed forest we see today. Though geologic tides are natural phenomena, there is currently much debate on how human-accelerated global warming might hasten the incoming geologic tide. Scientists have estimated that the sea level rose an average of one meter (three feet) per century from 17,000 to 6,000 years ago, and since then has risen at a slower rate of 12 to15 centimeters (five to six inches) per century. Sea levels rose 10 to 20 cm (four to eight inches) in the 20th century, and global climate change models predict an additional rise of 9 to 88 cm (four to 35 inches) by 2100. We may be accelerating the sea level rise at a pace similar to what it was over 6,000 years ago. The rise is due mainly to melting of high-elevation glaciers and polar ice caps, as well as thermal expansion of ocean water due to global warming. In an extreme scenario, if all polar ice caps melted, including Antarctic ice sheets that contain 90 percent of the world’s ice, sea levels would rise by as much as 65 meters (200 feet) and flood 15 percent of the Earth's land area. This
has happened before. Sea levels have risen and fallen thousands of times
since oceans first formed. The most recent globally warm period occurred
in the early Eocene epoch, about 55 million years ago. The globe was completely
ice free at that time. Dinosaurs, crocodiles and turtles inhabited areas
that are now deeply covered in glacial ice, and subtropical forests extended
as far north as Hudson Bay. Greenland was about 15°C warmer than it
is today. Vast amounts of modern coastal lands were underwater during
this geologic high tide. Within 15 to 25 million years, the warm climate
deteriorated, major glaciations in the Polar regions locked up Earth’s
water and there was a sharp drop in sea levels of over 150 meters. Much
of the warm-water marine life that evolved during Considering
environmental changes from a geologic perspective can be comforting in
some ways, because it reminds us that change is a natural phenomenon.
Global climate is changing. In fact, it has never been static. Ice ages
will come and go, oceans will rise and fall and ecosystems will be created
and destroyed. We talk metaphorically about the futility of building sand
castles on a beach, yet our cities are in the geologic intertidal zone
of the world’s Humans are certainly contributing to the rapid climate
changes of the last 100 years, but there is uncertainty about the role
of natural phenomena such as solar activity. Humans may not necessarily
be throwing the global ecosystem out of equilibrium, but may be accelerating
a natural warming cycle that began 20,000 years ago. There were some cooling
periods during that time, such as the Little Ice Age between 1550 to 1850,
but the overall warming trend over the last two millennia is undeniable. In some ways, we have hit the fast-forward button, causing things to happen in decades that might ordinarily happen in centuries. Slow changes often go unnoticed, but fast changes may raise alarm. Modern science has developed instruments that make precise measurements of everything from atmospheric chemistry, to sea-surface temperatures, to algal production in remote waters. We use sophisticated computer models to make predictions and our scientific understanding to surmise consequences. For the first time ever, a species is causing global change that has the intelligence and sophistication to measure that change, and it is beginning to take steps to reverse those changes. © 2005 The Gulf of Maine Times |
