When climate scientists look into the future to determine what the effects of climate change might be, they use computer models to simulate potential outcomes, such as how precipitation will change in a warming world.

But scientists at the University of Michigan are looking for something more concrete: corals.

Researchers examining coral samples from the Great Barrier Reef found that between 1750 and today, wet season precipitation in this part of the world increased by nearly 10% and the rate of extreme rain more than doubled as the global climate warmed. The results are published at: Contact Earth and Environment.

“Climate scientists often say: ‘I knew it would be bad, but I didn’t think it would happen this fast.’ But that’s actually what we see in these coral records,” said principal investigator Julia Cole, chair of UM’s Department of Earth and Environmental Sciences.

“Future studies tend to use climate models and those models can produce different results. Some might say it’s raining more, some might say it’s raining less. We’re showing that at least in north-east Queensland there’s definitely more rainfall, it’s definitely more variable, and that’s already happening.”

The study, led by UM researcher Kelsey Dyez, analyzed core samples from a coral colony in an estuary in northern Queensland, Australia. Precipitation entering the river during the summer rainy season collects nutrients, organic matter and sediments, which are then carried to the mouth of the river and discharged into the ocean, washing away the coral colony.

When corals swim in this fresh water, they pick up geochemical signals from the river and record them in their carbonate skeletons. Coral core samples show faint bands of lighter and darker material. These bands represent each wet and dry season the coral experiences. The tapes also contain information about the climate of each season, just as tree rings record climate patterns over the years they grow.

“We want to know if we will get more precipitation when we warm the Earth. Less rainfall? Maybe different parts of the world will react differently?” said Diez. “This project is particularly important because we can put this warming and changes into context. We can record precipitation for a period of time before we have instrumental records for that part of the world.”

To determine exactly how much rain falls in each rainy season and how many extreme rain events occur in each season, the researchers compared instrumental rainfall records starting in the 1950s with corresponding years in the corals. This gave the researchers a calibration period they could use to determine the relationship between the properties of the corals and the amount of precipitation that fell each rainy season while the corals were alive (back to 1750).

The coral core was collected by the Australian Institute of Marine Science from a remote area in north-east Queensland. The land surrounding the river’s watershed is also within a conservation area; This means that nutrients and sediments carried into the river by rain are unlikely to be produced by human activities.

“In recent years this region has experienced a major transition between floods that damaged communities and drier periods,” Cole said. said. “Because north-east Australia is an agricultural region, rainfall changes in a warmer world have real impacts. People may not feel a few degrees Celsius of warming, but they really suffer from drought or flood.”

To reconstruct precipitation, researchers used four different indicators. The researchers first looked at the brightness of the bands in the coral. When they shine a black light on coral, organic compounds within the coral cause it to fluoresce. The brighter the band fluoresced, the more organic compounds moved along the river and accumulated on the coral; this reflected the heavy rainfall season.

The researchers also measured how much of the element barium was present in each band. The coral skeleton is made of calcium, but barium can replace the calcium once it settles into the skeleton. The more barium in the tape, the more river flow flows over the corals.

The researchers then looked at stable carbon isotopes (carbon-12 and carbon-13) in the coral. The higher the ratio of these two isotopes, in favor of carbon-12, the more water must have flowed down the river due to more rainfall.

Finally, the researchers examined the stable isotopes of oxygen (oxygen-16 and oxygen-18). The fact that the ratio of these two isotopes is in favor of oxygen-16 is a sign that additional rainfall and fresh water are coming from the river.

Since the coral records were located off the coast of northeastern Australia, the researchers wanted to understand whether similar rainfalls were experienced across Australia. Looking at instrumental rainfall records across Australia, researchers found that the increase in rainfall was not uniform across Australia.

“This actually has little to do with Western Australia. This is too far. However, there is a significant correlation for most of eastern Australia. And that’s where a lot of people live,” Dayez said. “It’s particularly strong in Queensland, which is getting a lot of heavy rainfall right now.”