About Us
Contact Us

                                                                                                                                                                Printer friendly format

Click on the picture to enlarge

Shift in terrestrial biomes, glaciers and sea levels in eastern

North America 50-60 million years ago was the most recent

globally-warm period when the Arctic was ice-free and subtropical forests extended northward to Greenland. 18,000 years ago marks the height of the Pleistocene glaciation before glaciers began retreating. In the last 18,000 years, tundra environments shifted nearly 2,500 miles northward, and the familar white pine and northern hardwood forests of the Gulf of Maine watershed shifted north at least 500 miles to their present position. Paleoclimatic reconstruction is an inexact science and the biome boundaries are approximate.Data primarily from Dott and Prothero (1994), "Evolution of the Earth", McGraw Hill, Inc.

 Graphic by Ethan Nedeau

Environmental changes from a geologic perspective


By Ethan Nedeau

When I see lobster traps on people’s lawns, my geologic self thinks it silly—it will be millennia before lobsters roam this landscape again. I hope the people aren’t melting butter in anticipation. But I admire their foresight and blaring reminder that this land is not timeless and that someday, geologic tides will wipe our slate clean. Or maybe, for some people, lobster traps are just decorations.

The predictability of the Earth’s spin and the moon’s rotation allows us to know almost precisely where the ocean’s water line will be. Tide charts provide the exact time of high and low tides along the Gulf of Maine’s coastline. Geologic tides—the rise and fall of ocean levels over thousands or millions of years in response to global climate change—are less predictable and harder to comprehend than the familiar lunar tides. The vast time scale that geologic tides operate on challenges our imagination. But geologists think of time in terms of millions or billions of years. Earth is between four and five billion years old, and eons (one million years) are the preferred unit of measure for major geologic events.

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
the Eocene epoch’s warming went extinct in the ensuing cold period.

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