“These floods made headlines because they were unprecedented. You have to think about how intense extreme rainfall puts a unique strain on infrastructure,” adds Bhattacharya. “It’s worth thinking about how we plan infrastructure for more intense and extreme rainfall in places that don’t get a lot of rainfall today, like the desert southwest.”
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 the story, said Jessica Tierney, a University of Arizona researcher who was part of the study. “The only real evidence we have for how our planet behaves in a warmer climate comes from the geological past,” she adds. “That tells us exactly what happened.”
Leaf waxes also predate climatic records of Antarctic ice cores, which only date back about a million years and require a climate capable of supporting ice. One study used leaf waxes glimpse the climate of a warmer Spain 15 to 17 million years ago. Another watched the South West Africa humidity history for 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 co-author, came up with the idea of studying the Pliocene on a bus at a conference for young researchers.
Their analysis began with marine sediment collected decades ago by the research vessel Joides Resolution, which travels the oceans drilling cores up to 10 km below the surface. The samples used for the study were taken off the coast of California: one off the Baja Peninsula at over 2,600 meters depth and one in the East Cortes Basin at 1,700 meters depth. In the Pliocene, leaf waxes would have been carried west by 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 high-temperature pressurized solvent that extracted the waxes. Next, they measured the isotopic composition of hydrogen and carbon using an isotope ratio gas chromatograph mass spectrometer, which separated the waxes by their molecular weight.
“The hydrogen used to make the wax comes from the rainwater that the plant uses to grow. You can think of isotopes as a fingerprint,” says Tierney. “These isotopes actually plot what kind of precipitation you have, which is pretty cool. They can also plot the amount of winter precipitation versus summer precipitation. So that’s pretty powerful.
For the second part of the study, the climate modeler Ran Feng, a professor in the Department of Geosciences at the University of Connecticut, ran simulations to determine how sea temperatures influenced the stronger Middle Pliocene monsoons. Feng found that when sea temperatures – in an area stretching from Alaska off Baja California – were warmer compared to the generally warmer tropical waters off Central America, they created conditions conducive to stronger monsoons in the southwest. The warmer local air acts as a heat pump, drawing in the relatively cooler tropical air and warming it, attracting humidity. “So it creates this loop,” she says. “That’s why it’s able to conduct moisture in the southwestern regions of North America.”
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