By Gary Griggs, Distinguished Professor of Earth and Planetary Sciences and Director of the Institute of Marine Sciences at UCSC
The level of the ocean around the Earth, and therefore the location of the shoreline, is inextricably tied to climate. As climate changes, so does sea level.
Throughout the history of the oceans, which goes back about 3.5 billion years, give or take a few million, climate has constantly changed and, in response, sea level has gone up and down. As seawater warmed, it expanded. As the Earth warmed, ice sheets and glaciers melted and retreated, adding more water to the oceans and moving the shoreline inland.
This wasn’t a big deal a hundred million years ago, or even ten thousand years ago. Animals living along coasts migrated away from the shoreline. Habitats, whether intertidal, estuary or marsh, gradually responded and re-established themselves. There were no coastal communities or big cities with condominiums, houses, hotels and other businesses. There were no sewage treatment facilities, power plants, ports or harbors, highways or bridges, refineries, or other infrastructure at the ocean’s edge. Today, however, it’s a very different story.
The last several million years of Earth history were dominated by the Ice Ages. These were multiple periods, on roughly 100,000-year cycles, when the climate got cooler and glaciers and ice sheets expanded. Thousands of feet of ice covered much of North America, from Seattle, extending down into the mid-West, and east to Cape Cod.
Where did all that water that was locked up in the ice sheets and glaciers come from? There was only one source for that much water- the oceans.
The last Ice Age or glacial epoch ended about 18,000 years ago. At that time, the Earth was about nine degrees Fahrenheit cooler than today. In response, ten million cubic miles of seawater were evaporated and transferred to the continents where it formed ice sheets and glaciers. Taking all of that water out of the ocean dropped the sea level about 350-400 feet. At Santa Cruz, the shoreline during an ice age would have been about ten miles offshore of today’s beaches. If you were in good shape, you could have hiked the 30 miles out to the Farallons off of San Francisco. That was all dry land at the time, although you might have encountered some large animals in California you don’t see today, cave bears, saber tooth tigers, dire wolves, and giant sloths, to name a few.
The climate then gradually warmed as the irregularities of the Earth’s orbit around the Sun increased our total solar energy input. Once the Earth starts to warm, there are some other well-recognized feedbacks that amplify this heating.
A warmer ocean can hold less carbon dioxide so more of this greenhouse gas leaves the ocean and enters the atmosphere. Warming also leads to the melting of more sea ice, which is happening today in the Arctic Ocean. As we replace ice, which reflects a lot of solar energy, with ocean, which absorbs heat, the oceans get warmer and give off more carbon dioxide.
In addition, permafrost, or ground that is normally frozen and which covers about 25% of the Earth’s surface in northern latitudes, begins to thaw with a warming Earth. The permafrost contains large amounts of decaying vegetation and methane, a very potent greenhouse gas, which is released to the atmosphere.
The effect of these additional greenhouse gases in the atmosphere, carbon dioxide and methane, is like putting another blanket on the bed- heat is retained and we gradually move from an Ice Age into an interglacial period. This is what took place repeatedly over the past two million years, with Earth’s climate acting like a pendulum, swinging back and forth between glacial and interglacial periods, or cold and warm intervals.
And while we often talk today about the greenhouse effect, or the impact of putting more greenhouse gases into the atmosphere as though it started with human occupancy of the planet, its been around for billions of years.
Carbon dioxide, methane, ozone, and nitrous oxide are all natural occurring greenhouse gases in the atmosphere. They have produced a far more livable planet, at least for us, than would have otherwise been possible. The average temperature on Earth over the last century was about 60 degrees Fahrenheit. Without naturally occurring greenhouse gases, the average temperature would be zero degrees, or, you got it, well below freezing.
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Editor’s Note:
This is a new column by Gary Griggs, Distinguished Professor of Earth and Planetary Sciences and Director of the Institute of Marine Sciences at UCSC written for Times Publishing Group Inc.
Gary has been studying the coast of California for 47 years and has recently published his 7th and 8th books: The California Coast from the Air- Images of a Changing Landscape (with partner Deepika Shrestha Ross), and Our Ocean Backyard-Collected Essays. The California Coastal Commission and Sunset Magazine named him as one of California’s Coastal Heroes in 2009.
“Lines in the Sand” is written by Gary to help us coastal dwellers understand what is happening and is predicted to happen, which may be well within the lifetimes of many of our readers.
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