Ancient leaf wax holds clues to future climate

Researchers dilute sediment cores with a variety of solvents. The samples are forced through a column of silica gel, which traps the unwanted chemicals and leaves the alkanes they want to measure. The dark line at the bottom of the liquid in the middle three columns is where some extra chemicals are getting stuck, while other chemicals can traverse through the gel to drip into vials at the bottom. (Credit: Syracuse U.)

Researchers have used chemicals from preserved plant matter to pinpoint the processes responsible for changes in past rainfall and drought in southwestern Africa.

Their findings, published in Geophysical Research Letters, may have implications for the future.

In September 2023, extreme rains struck South Africa’s Western Cape province, flooding villages and leaving a trail of destruction. The catastrophic devastation is just one recent example in a string of extreme weather events that are growing more common around the world. Fueled by rising sea surface temperatures from global warming, torrential storms are increasing both in frequency and magnitude. Concurrently, global warming is also producing the opposite effect in other instances, such as mega-drought that recently threatened the water supply of Cape Town in southwestern Africa to the point where residents were at risk of running out of water. This one-two punch of weather extremes is devastating habitats, ecosystems, and human infrastructure.

In the study led by Claire Rubbelke, a PhD candidate in earth and environmental sciences at Syracuse University, and Tripti Bhattacharya, professor of earth and environmental sciences, researchers zeroed in on the Pliocene epoch (~3 million years ago)—a time when conditions were very similar to today’s. Despite warmer temperatures, many parts of the world, including southwestern Africa, experienced dramatic increases in rainfall over land, likely caused by warmer than normal sea surface temperatures. This mimics a modern event called a Benguela Niño, where researchers believe shifting winds cause warm waters to move southward along the coast of Africa causing enhanced rainfall over typically arid regions.

“In the present day, the intensity and location of extreme precipitation from Benguela Niño events appear to be influenced by both Atlantic and Indian Ocean sea surface temperatures,” says Rubbelke, who is a member of Bhattacharya’s Paleoclimate Dynamics Lab. “During the Pliocene, it appears that these Benguela Niño-like conditions may have been a permanent feature.”

The team’s work was inspired by collaborator and study coauthor Natalie Burls, associate professor at George Mason University. Burls, an oceanographer and climate scientist from South Africa who received a PhD at the University of Cape Town, has long been intrigued by the way geological evidence from past warm climates in Earth’s history can help researchers make sense of future rainfall and drought conditions.

“This study, which explored how past warm climates can inform us on what to expect in the future as our planet warms, brings to the fore the important role of ocean warming patterns,” says Burls. “It’s important to understand how these patterns determine the response of the hydrological cycle over southwest Africa to global warming.”

To study the impact of global warming on precipitation from millions of years in the past, the team analyzed “molecular fossils” in the form of ancient leaf waxes. “These are compounds produced by leaves to protect themselves from drying out,” says Bhattacharya. “They get shed from leaf surfaces and find their way to ocean sediments, where we can extract them and study their chemical composition.”

Plants use hydrogen from rainwater to produce the waxy outer coating on their leaves, which survives in ocean sediment for millions of years. The leaf wax functions as a time capsule preserved in ocean sediment.

After transporting the millions-year-old sediment from Africa to their lab in Syracuse, Rubbelke and Bhattacharya used heat and pressure to extract lipids (e.g. fat molecules), and then used a variety of solvents to isolate the exact class of molecules that they were looking to measure. From those molecules, they determined the number of different types of hydrogen present.

“When we measure the amount of heavy and light isotopes of hydrogen in the waxes, it reveals different physical processes like increased rainfall, or how far the water vapor travels,” says Rubbelke. “We can therefore identify changes in these processes by looking at long-term changes of hydrogen.”

By comparing their data to climate models, they verify how well those models capture past climate change, which can in turn improve the accuracy of those models to predict future rainfall. As Bhattacharya notes, this is critical because climate models often disagree on whether certain regions will get wetter or drier in response to global warming.

“We are using real world data from the ancient geologic past to improve our ability to model rainfall changes as the planet warms,” she says.

The study’s third author, Ran Feng, assistant professor of earth sciences at the University of Connecticut, helped analyze the comparison data and specifically examined the proposed mechanism that explains the Pliocene wet conditions in southwest Africa. She says many features of ongoing climate change are reincarnations of the past warm climates.

“In our case, we have shown that sea surface temperature pattern surrounding South Africa is key to explaining the past hydroclimate conditions of this region,” notes Feng. “Looking into the future, how this sea surface temperature pattern may evolve has profound implications to the environmental changes in South Africa.”

Rubbelke, whose interest in paleoclimate research started in high school while studying ice cores and oxygen isotopes, says that the work she is doing alongside Bhattacharya at Syracuse is particularly fulfilling because they are contributing valuable data to an area where there is currently a knowledge gap.

“This research is really cool because not a lot of paleoclimate records from the Southern Hemisphere exist, compared to the Northern Hemisphere at least,” says Rubbelke. “I feel like I’m really contributing to an international research effort to rectify that.”

As to whether the future will be wetter or drier in southwestern Africa, the team’s results suggests that both are possible, depending on where extreme sea surface temperatures are occurring.

While not much can be done to reverse global warming, short of cutting the use of fossil fuels completely, the researchers say this study illuminates the need for vulnerable communities to have the tools and resources to adapt to these seemingly more frequent extreme weather events.

“A key aspect of helping vulnerable communities involves improving our ability to predict hydroclimate extremes, “says Bhattacharya. “Our study directly speaks to this need, as we show that sea surface temperature patterns strongly influence climate models’ ability to predict changes in rainfall in southwestern Africa.”

Bhattacharya and Rubbelke’s research on this project had support from the National Science Foundation.

Source: Syracuse University