By Gary Griggs, Distinguished Professor of Earth and Planetary Sciences, UCSC
Sea level isn’t what it used to be, and it isn’t going to be the same in 20 or 30 years as it is today. The level of the ocean never stays the same for long, but we have developed our coasts and shorelines around the world as if it did.
There are now about 300 million people around the planet living within 3 feet of sea level, which doesn’t bode well for their future or the future of their communities and cities. Miami, New York, Guangzhou, Kolkata, and Shanghai alone have nearly $13 trillion in assets that will be at risk by about 2070 if current sea level rise trends continue.
There are natural cycles and forces that have been affecting the level of the oceans for billions of years, and we understand these pretty well. One major player is global temperature, driven primarily by the amount of solar energy we receive. This in turn affects global climate, atmospheric and ocean temperatures, and the distribution of water on the planet: how much resides as liquid in the oceans and how much is frozen in ice sheets and glaciers.
Ocean temperature itself also affects sea level because water expands as it gets warmer. Your home water heater was built with this in mind, so there is some extra space for the water as it heats up.
The distribution of continents and ocean basins and the topography of the seafloor have also played a role over the millions of years of geologic time. During major periods of seafloor volcanic activity, ocean ridges expand and increase in volume. This raises sea level just like another person getting into the hot tub.
Through a combination of these long-term natural processes that take place over millions of years, and some other random events, our planet has gone through dramatic climate changes. Fifty-six million years ago there was a huge and nearly instantaneous release of carbon from the oceans. This gave the planet a fever that lasted for over 150,000 years until all of that excess carbon was reabsorbed. Estimates of the amount of carbon released at that time are believed to be roughly equivalent to our burning all of the Earth’s coal, oil and natural gas deposits.
What released all of that carbon to the atmosphere is not completely clear, but the leading hypothesis is that most of it came from large deposits of methane (CH4) hydrate on the seafloor or within permafrost, which would have produced rapid warming. Methane is a greenhouse gas, which oxidizes to carbon dioxide over time, but is about 20 times more potent in trapping heat than carbon dioxide. Methane is a bad actor in all global warming scenarios.
The period about 56 million years ago has been called Hothouse Earth (and is also known as the Paleocene-Eocene Thermal Maximum, or PETM for short). Evidence for these exceptionally warm conditions are the preserved fossils of palm trees and crocodiles in the Arctic. The impact of increased atmospheric and ocean temperatures and melting nearly all of the world’s ice produced a rise in sea level several hundred feet higher than today.
The ocean-drilling program, which recovers long cores of sediment and rock from the deep-sea floor, has been key to discovering the evidence for Hothouse Earth. Paleoceanography is the science of studying ancient sediments for clues of past ocean conditions. It has provided the people of the planet with a glimpse of what Earth’s future may be like as our increasing world population and the unchecked use of fossil fuels continues to add more carbon dioxide and methane to the atmosphere.
Right now, modern civilization is running a massive worldwide experiment. By adding increasing amounts of greenhouse gases to the atmosphere we are slowly heating the Earth and in doing so, are melting more ice, heating up the oceans, and increasing the rate of sea-level rise.
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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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