In the past century, the western United States has faced rising temperatures and a decrease in precipitation. Some researchers are seeking out the ancient history of these patterns in a substance that may be unexpected to some: plant wax.
The study, coauthored by Daniel Ibarra, an assistant professor of Earth, environmental and planetary sciences and environment and society, investigated ancient plant waxes preserved in sediments from two Utah lakes: Bear Lake and Great Salt Lake.
Jessica Tierney, an author on the paper and a professor of geosciences at the University of Arizona, explained that the chemistry of this plant wax allows researchers to approximate the precipitation and evaporation rates in the western United States over the last 240,000 years — covering the most recent two glacial cycles.
“We have found evidence of large changes in rainfall patterns over the Ice Age cycles that tell us that the American West is very sensitive to global climate change,” Tierney said.
According to Tierney, while paleoclimatologists — individuals who study past climates — “don’t have a time machine,” they do have “natural archives of climate change that preserve the past changes for us.”
Tree rings are one example of this natural archive, Tierney explained. The width of the rings can provide information about the temperature and rainfall patterns of the time when the tree was growing.
Rachel So, the paper’s first author and a graduate researcher in Earth sciences at the University of Southern California, explained that the “hydrogen isotopic signature” of plant wax can help inform ancient climate patterns.
“You only have two elements in a leaf wax: carbon and hydrogen,” So said. The hydrogen that plants use to make its wax comes from “the water that it absorbed from its roots, which is usually rain,” she added.
Hydrogen isotopes are hydrogen atoms with different weights. Hydrogen atoms have just one proton in the atom’s nucleus, but some hydrogen atoms have one or more neutrons in addition to the proton.
According to So, the different proportions of hydrogen isotopes are affected by “a lot of mechanical and general chemical processes that happen in the world.”
For example, “lighter isotopes, like hydrogen that just has one proton, evaporates a lot easier than heavier ones, like the isotopes that have one proton and one neutron,” So explained. “These kinds of processes affect the isotopic signature of rainfall,” which is “preserved” in the plant wax.
Ibarra said that a key reason for analyzing these isotopes is to compare the climate patterns revealed by the plant wax to the patterns predicted by current climate models.
“The climate models actually don’t do a great job reconstructing the isotope data that we obtain from the leaf waxes,” Ibarra said.
According to Tierney, the last interglacial time is a helpful time period to study as “an analog to what we’re experiencing now with global warming,” as it was “the last time that the planet was maybe even close to as warm as it is right now.”
Glacial cycles from hundreds of thousands of years ago are still relevant because of what they reveal about water resources. Based on the rate of human civilization and industrialization, So estimates that water use has accelerated beyond the available resources.
“A lot of our water resources actually came from the last Ice Age,” So explained. “Once we basically use up all the water that was stored from the Ice Age till now, we can’t expect the world to naturally replenish these water resources because we’re no longer in an Ice Age,” she added.
