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What’s in a Number: Making Sense of Results from the Lab and Field

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Video – Sea Change: How High Will Waters Rise?
Part 3: Interpreting the Results

The Pliomax team has explored and dated fossil Pliocene beaches from several regions, including Australia, South Africa and the U.S. East Coast. They continue searching for new sites. In Australia, the team discovered fossil shore deposits as high as 90 feet above present sea level. This observation doesn’t necessarily mean that sea level was 90 feet higher during the Pliocene because the land has also moved since the Pliocene. The Pliomax team, lead by geophysicist Jerry Mitrovica, adjusts field measurements for the confounding factors that obscure the true change in sea level.

In the three million years since the mid-Pliocene, Earth has undergone many changes. Dozens of times the planet has descended into ice ages, with miles-thick ice sheets growing over large swaths of North America, Europe and Asia. In between these long, frigid spells, these ice sheets retreated back to Greenland. Over the many millennia since the mid-Pliocene, many other geological forces have also altered the face of the planet. The great plates on which continents rest have jiggled about slightly, pulling apart in some places and pushing together in others. Squeezed and stretched and levered by the enormous force of these tectonic plates, the continents have risen up in some places and dropped down in others. Deeper below Earth’s surface, poorly understood motions of our planet’s deep interior have also pushed the continents up or pulled them down in places. Scientists have shown that continents can slide over huge hard obstacles, forcing elevated regions to migrate across the land like ripples that moves across a throw rug when dragged over an uneven floor.

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The present elevation of ancient beaches reflects in part the height of past sea level. Bit it also reflects other changes that have occurred since such beaches were formed, such as vertical movements of land. Graphic by Simon Wermuller von Elgg

No spot on Earth has been unaffected by some combination of these titanic geologic changes. Mitrovica and colleagues try to calculate the impact of these influences and produce unique adjustment factors for each site the team visits. The team applies Mitrovica’s corrections to the elevation measurements taken in the field to come up with an estimate of the true change in global sea level (which depends on the amount of water added to or removed from the ocean by changes in the size of ice sheets) since the Pliocene. For instance, if Mitrovica determines that colliding tectonic plates have raised a fossil beach, the team subtracts the amount of uplift from the present elevation of the old beach above sea level.

  • Visit Attractive Ice Sheets and learn about variations in the gravitational attraction of ice sheets.
  • Visit Deformable Planet to learn about vertical movements of land caused by advance and retreat of polar ice sheets.
  • Visit Axis Upheaval to learn about shifts in Earth’s rotational axis caused by changes in the size of polar ice sheets.
  • Visit Shifting Ground to learn about vertical shifts in land caused by movement of Earth's tectonic plates.