Coral Chronicles Climate Change: Australian Reef Reveals Wetter World

Climate scientists typically use computer models to predict the effects of climate change, but University of Michigan researchers are looking to a more natural source of data: coral.

By studying coral samples from the Great Barrier Reef, the team discovered that wet-season rainfall in northeastern Queensland increased by 10% between 1750 and today, while extreme rain events more than doubled. This finding, published in Communications Earth & Environment, provides real-world evidence of climate change's impact on precipitation patterns.

"Climate scientists often say 'it's getting worse faster than expected,'" said lead investigator Julia Cole, chair of U-M's Department of Earth and Environmental Sciences. "The coral record shows this happening now."

The study, led by U-M researcher Kelsey Dyez, analyzed core samples from a coral colony near a river mouth in northern Queensland. During rainy seasons, the river carries nutrients, organic material, and sediments to the ocean, leaving traces on the coral as it grows. These faint bands, similar to tree rings, record seasonal climate variations.

"The key question is: will we see more or less rain with a warming planet?" said Dyez. "This project is crucial because it provides historical context for rainfall patterns before instrumental records existed."

To quantify rainfall changes, the researchers compared the coral data to instrumental rainfall records starting from the 1950s. This allowed them to decipher the relationship between coral characteristics and past rainfall amounts.

The coral core, collected by the Australian Institute of Marine Science, originated from a remote, protected area, minimizing the influence of human activity on the data.

"This region experiences dramatic swings between devastating floods and dry periods," said Cole. "Since northeastern Australia is agricultural, understanding rainfall changes in a warmer world is vital. People might not feel a few degrees of warming, but they suffer during droughts and floods."

The team used four techniques to reconstruct rainfall patterns. First, they analyzed the coral's luminescence under ultraviolet light. Brighter bands indicate heavier rainfall seasons due to higher organic material deposits.

Second, they measured barium content within the bands. Barium replaces calcium in the coral skeleton during high river discharge events, signifying heavy rainfall.

Third, the researchers examined the ratio of stable carbon isotopes (carbon-12 and carbon-13). A higher ratio of carbon-12 suggests more freshwater input from increased rainfall.

Finally, they analyzed the ratio of stable oxygen isotopes (oxygen-16 and oxygen-18). A dominance of oxygen-16 indicates additional precipitation and freshwater flow.

While the study focused on northeastern Australia, the researchers examined rainfall patterns across the continent. They found the increased rainfall wasn't uniform.

"Western Australia doesn't show a strong correlation, likely due to the distance," explained Dyez. "But for eastern Australia, especially Queensland where extreme rainfall events are already happening, the correlation is significant."

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