Linda: Understanding sea level rise in the past & future

We’ve all heard that sea level is on the rise! According to NOAA, sea level around the world is rising at about an eighth of an inch per year1. Doesn’t sound like much, does it? But a higher sea level means that floods will reach farther inland, people who live near the coasts may lose their homes, and entire low-lying islands may be submerged.

A question often asked in relation to the various effects of climate change is, Has this happened before? And the answer is, Yes! Throughout Earth’s history, sea level has risen and fallen in events called “transgressions” and “regressions.”(Miller et al., 2011). But how do scientists know where sea level was before humans existed?

One way to tell where sea level used to be is by finding the traces of ancient coral reefs. Reefs form just off coastlines in shallow waters. When sea levels rise, corals follow, growing on top of older corals to keep pace with the sunny surface. Coral growth in the Caribbean and the South Pacific has revealed that after the melting of the last ice age, which was over 10,000 years ago, sea level rose at a rate of 40mm per year! (Miller et al., 2011) That’s 10 times faster than the current rate, which is just over 3mm per year.

Another way to track sea level change is with isotopes. An isotope is a form of an atom (such as hydrogen, oxygen, or carbon) that has a different weight due to a varying number of neutrons, one of the types of particle that make up atoms. Some oxygen atoms have a mass of 16 atomic units, while others have a mass of 18. (Yes, believe it or not, not all oxygen atoms weigh the same!) So, oxygen-16 atoms are lighter than oxygen-18 atoms. When ice sheets grow, water with the more oxygen-16 is incorporated into the ice, and more oxygen-18 remains in the ocean.

Marine organisms also use oxygen (in carbonate, CO3) to help build their shells. When the organism dies, its shell sinks, is buried, and can later be discovered as a fossil. This fossilized shell still retains the same oxygen isotopes it had when it was formed. So, scientists can reconstruct times that have more or less ice. Scientists have analyzed carbonate-containing shells all over the world to determine when Earth was cold and icy, and when it was warm and ice-free. By looking at oxygen isotopes, scientists have found that the growth of the Laurentide Ice Sheet in the most recent Ice Age caused sea level to drop by 120m — that’s almost 400 feet (Miller et al., 2011). 

Earth’s current sea level rise is nothing new - but that doesn’t mean it won’t have consequences. Almost 40% of the population in the United States lives along coastlines.

So, Earth’s current sea level rise is nothing new - but that doesn’t mean it won’t have consequences. Almost 40% of the population in the United States lives along coastlines, within reach of floods and storms. Sea level rise is not a threat just to people, but to vital infrastructure — major roadways, tunnels, and bridges for transportation, oil wells and power plants, even crucial groundwater reserves. Sea level rise is a serious issue on a global scale that needs to be addressed before its impacts become irreversible.

To learn more:

NOAA. Last revised December 2016. Is sea level rising? National Ocean Service. Retrieved from <>.

Miller, K., Mountain, G., Wright, J., and Browning, J. 2011. A 180-million-year record of sea level and ice volume variations from continental margin and deep-sea isotopic records. Oceanography 24(2):40-53.


Linda Berryman is graduating this spring with an Environmental Science & Management degree, and a minor in Geology, from UC Davis. This post was written as part of Geology 116: The Oceans class at UC Davis.


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