Plants make waxes on their leaves composed of carbon and the hydrogen drawn from rainwater. When the plant dies, those waxes turn into dust that floats on the wind, then drifts down to form layers preserved in marine and terrestrial sediments. Trapped within those sediments is a timeline tracing pictures from prehistoric times: which vegetation flourished, or the intensity of the rainfall. “Plants lock in the environments that existed when they were growing,” says Tripti Bhattacharya, a professor in Syracuse University’s Department of Earth and Environmental Sciences who led a recent study examining the Southwestern monsoon season in the Pliocene, 3 million years ago. The story of what happened then is the story of what will happen to a warming world. Summer rainfall in the Pliocene created wetter conditions and a landscape of lakes—not the deserts of today. And now, she notes, atmospheric carbon dioxide levels are once again what they were in the Pliocene: “It’s a snapshot of the world we’re likely to go toward if we don’t curb emissions.” The leaf waxes reveal that the Pliocene had intense monsoons during the summer, shifting the narrative of previous research that attributed wetter conditions only to winter rainfall. “Our paper presents the first direct evidence that monsoon changes caused wet conditions in the middle Pliocene,” Bhattacharya says. The Southwest currently has a monsoon season that typically runs from June to September and stretches from Western Mexico into Arizona, Nevada, New Mexico, and some parts of California, Colorado, and Utah. These rains can be severe: Earlier this summer, a so-called 1,000-year-storm flooded the driest place in North America, Death Valley, stranding hundreds of visitors and causing extensive damage. In Las Vegas, two people drowned and a casino flooded during what was called the worst monsoon in a decade. In Arizona, the monsoon flashed through the state during July and August. Monsoons today will likely follow the Pliocene pattern and intensify, but also expand their range in Southern California. (Bhattacharya says additional data from other sedimentary deposits would help map the expansion in more detail.) Their extremes, she adds, will likely match what was normal during the Pliocene. That’s bad news for a region not built to withstand the storms. The advantage of using proxies like leaf wax to understand climate is that while predictive computer models are helpful, they can disagree. There’s no denying history, says Jessica Tierney, a University of Arizona researcher who was a part of the study. “The only real evidence we have of how our planet behaves in a warmer climate is from the geological past,” she adds. “It tells us exactly what happened.” Leaf waxes also predate climate records from Antarctic ice cores, which go back only about a million years and require a climate that can support ice. One study used leaf waxes to glimpse the climate of a warmer Spain some 15 to 17 million years ago. Another looked at the moisture history of Southwest Africa for the past 3.5 million years. Bhattacharya began using them while working as a postdoctoral fellow in Tierney’s lab. Five years ago, she and Ran Feng, a coauthor, came up with the idea of studying the Pliocene while riding a bus during a conference for young researchers. Their analysis started with marine sediments collected decades ago by the research vessel Joides Resolution, which roams the oceans drilling cores from as deep as 6 miles below the surface. The samples used for the study were taken off the coast of California: one off the Baja peninsula from a depth of more than 2,600 meters, and one from the East Cortes Basin at a depth of 1,700 meters. During the Pliocene, leaf waxes would have been transported west on the wind to become part of this marine sediment. The team got a cube of each core, freeze-dried them, and ran them through “a glorified espresso machine,” says Bhattacharya, using a solvent under pressure at high temperatures that extracted the waxes. Then they measured the hydrogen and carbon isotope composition using a gas chromatograph-isotope ratio mass spectrometer, which separated the waxes by their molecular mass. “The hydrogen that’s used to make the wax is coming from rainwater that the plant uses to grow. You can think of isotopes as like a fingerprint,” Tierney says. “These isotopes actually trace the kind of rainfall you have, which is pretty cool. They can also trace the amount of winter rainfall versus summer rainfall. So, it’s pretty powerful.” For the second part of the study, climate modeler Ran Feng, a professor at the University of Connecticut’s Department of Geosciences, ran simulations to determine how sea temperatures influenced the stronger monsoons of the mid-Pliocene. Feng found that when marine temperatures—in an area that extends from Alaska to off the coast of Baja, California—were higher relative to the usually warmer tropical waters off Central America, they created conditions for stronger monsoons in the Southwest. Warmer local air acts like a heat pump, drawing the relatively cooler tropical air and warming it, pulling in moisture. “So it creates this loop,” she says. “That’s why this is able to drive moisture into the Southwest North America regions.” Monsoons will help with drought as the Southwest dries. But they will be stronger, dropping inches of rain in a short time and causing more frequent flooding. “The monsoon accounts here in Arizona for about 60 percent of our rainfall for the year,” Tierney says. “It’s an important source of water in the desert. It does, in certain hydrological systems, recharge the groundwater. But the flip side of that is that these monsoon storms can be so intense and so quick that a lot of the water can end up running off into the watersheds and off the landscape. So, it’s not always the case that it recharges groundwater.” Those storms also threaten the built environment, and as the climate has changed, design standards for infrastructure like roads, bridges, dams, and stormwater systems have not kept pace. The National Oceanic and Atmospheric Association’s Atlas 14 reports for the US Southwest rely only on historical rainfall amounts, not a changing future, for its projections. The agency’s Southwest study was released in 2004 and last revised in 2011. There is another troubling connection between more intense monsoons and disasters: wildfire. Stronger rainfall, Bhattacharya says, increases the growth of fuel loads by encouraging plant growth. Subsequent droughts set the stage for bigger fires. “We think a stronger monsoon season creates unanticipated hazards from fire and flooding,” she adds, noting that more research will bring the picture into focus. “We’re planning to go further and study this in the Pliocene to see how fire and flooding respond to a warmer climate.”
