Coral study: Previous warming period inspired sea level rise in fits and starts

According to evidence revealed by a coral reef in the Gulf of Mexico, the planet's oceans didn't rise steadily during previous periods of global warming, but in fits and starts.

At the end of the last ice age, between 20,000 and 10,000 years ago, Earth's glaciers finally began to retreat and melt. As temperatures warmed and glaciers shrunk, meltwater filtered into the oceans and sea levels rose.

New analysis of fossil reefs a few dozen miles off the coast of Corpus Christi suggests corals experienced several accelerated bursts of sea level rise before being inundated completely.

Currently, climate change models treat sea level rise linearly. The latest findings -- detailed in the journal Nature Communications -- suggests a higher degree of variability.

"Our results offer evidence that sea level may not rise in an orderly, linear fashion," Jeff Nittrouer, a coastal geologist at Rive University, said in a news release.

"Sea level rose quite fast, paused, and then shot up again in another burst and so on," added Rice marine geologist AndrÃ© Droxler. "This has profound implications for the future study of sea-level rise.

Coastal reefs live in symbiosis with algae. Coral relies on the microorganisms to turn sunlight into food. Reefs can adapt using a type of growth pattern called backstepping when sea levels rise gradually, but reefs become drowned when levels rise dramatically.

Fossil reefs now 195 feet beneath the surface of the Gulf of Mexico once grew among shallow waters. Their growth patterns can still be analyzed, revealing their attempts to adapt to rising seas some 19,000 years ago.

"The coral reefs' evolution and demise have been preserved," said Rice graduate student Pankaj Khanna. "Their history is written in their morphology -- the shapes and forms in which they grew."

Using 3D imaging technology, researchers surveyed the structures of the coral reefs. They found a series of terraces, evidence that reefs grew vertically during pauses between the bursts of sea level rise.

Researchers found terraces at reefs many miles apart were similarly aligned, and they expect more geologic evidence around the globe will reveal similar signs of stop-and-go sea level rise. As more evidence becomes available, researchers will need to modify their sea level rise prediction models.

"Based on what we've found, it is possible that sea-level rise over decadal time scales will be a key storyline in future climate predictions," Khanna said.

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