Southwest regions of North America have experienced a variety of extreme weather conditions recently, including sweltering heatwaves, burning wildfires, and monsoon rainstorms that result in flash floods and mudslides. A group of scientists from Syracuse University, the University of Connecticut, the University of Arizona, George Mason University, and Harvard University are searching for environmental hints from millions of years ago to predict what the southwestern climate may resemble in the future as a result of global warming. Scientists believe that warmer temperatures may result in heavier and more widespread summer rainfall throughout the southwest United States by examining ancient climate data.
Due to increased aridity from higher temperatures, subtropical regions like southern North America are getting drier as a result of global warming. However, throughout the summer, excessive precipitation is occasionally brought on by rising temperatures. An intensifying monsoon is what’s causing this. Just this past summer, Death Valley and other regions renowned for their lack of rainfall experienced catastrophic flooding as a result of the monsoon’s influence in southern California.
The Thonis Family Professor of Earth and Environmental Sciences at Syracuse University and the study’s principal investigator, Tripti Bhattacharya, looked at a previous period in Earth’s history that featured a powerful North American summer monsoon. Despite having carbon dioxide levels similar to today, the North American southwest was unexpectedly rich in lakes and plants and animals that require a moister environment 3 million years ago.In a recent publication, the team makes the case that wetter circumstances in the past may have been caused by a greater monsoon during the middle Pliocene, with consequences for the present and the future.
Finding Answers in Ancient Leaf Waxes
Bhattacharya and Ph.D. candidate in Earth and environmental sciences Claire Rubbelke examined Pliocene-era leaf waxes preserved in ocean sediment cores from Baja California and southern California to learn how the monsoon altered in the middle Pliocene. These waxes’ hydrogen isotopic composition reflects historical variations in the monsoon. Since rain is the source of the hydrogen required to make leaf wax, determining the hydrogen content can provide information about past precipitation totals. By passing solvents through sediments at high temperatures and pressures, researchers may extract the waxes from the leaves. They then separate the waxes by their molecular masses using a device known as a gas chromatograph-isotope ratio mass spectrometer to perform the isotopic measurements.
In contrast to earlier research that claimed that Pliocene hydroclimate changes were only the result of winter, not summer, rainfall, Bhattacharya notes that “with information encoded in leaf waxes, we found that the Pliocene featured a stronger summer monsoon in western Mexico, stretching all the way to where southern California is now.” The middle Pliocene experienced rainy circumstances as a result of monsoon shifts for the first time, according to our work.
Driven by Temperature Changes
Ran Feng, a professor in the Department of Geosciences at the University of Connecticut and the study’s second author, is an expert in climate modeling. He ran simulations to see how sea surface temperatures would have affected the strength of the North American Monsoon in the mid-Pliocene. Her team discovered that the Pacific’s water temperatures were set up in such a way as to move more moisture from the tropics to the subtropics. Particularly, the gradient of temperature between the subtropics and the tropics, which is what drives the strengthening of the North American monsoon, was lessened.
One of the key determinants of monsoon intensity is temperature. Rainfall favorability is decreased as a result of descending motion over several monsoon zones in southern North America due to warmer weather in the eastern equatorial Pacific. However, when the California margin is warmer than the eastern equatorial Pacific, as it can be now during marine heat wave occurrences, more tropical moisture enters the subtropics, bringing more precipitation from the North American monsoon.
We may learn how our world behaves in warm climates by examining the environment of the mid-Pliocene, claims Feng. The process we uncovered here is already in action during current maritime heat wave occurrences, and we predict that it will become more common in the future with a warmer climate and maybe more frequent marine heat wave events.
Their findings provide more evidence that greater temperatures on the California edge contribute to improving the atmosphere’s suitability for monsoon rainfall—basically, by supplying more energy to feed monsoon storms.
According to Bhattacharya, floods during the summer months in southwestern North America will probably get worse in the future. We think our work is a good example of how previous climate dangers may be used to forecast future ones.
According to Jessica Tierney, a professor of geosciences at the University of Arizona and a co-author of the paper, these projected periods of “Pliocene-like” rainfall in southwestern North America will have an impact on ecosystems, infrastructure for people, and water supplies.
More rain will fall in the southwest United States as a result of a stronger monsoons, which is beneficial for the area’s ongoing drought, according to Tierney. Unfortunately, a lot of the rain that falls during monsoons storms does so rapidly, rushes off the land, and can result in catastrophic floods that put populations in danger.
Their research provides evidence that a warmer climate with conditions similar to the middle Pliocene brings with it the potential for an enlarged and more intense monsoons, even though precise forecasts regarding future North American monsoons remain questionable. The intense summer monsoons conditions that currently plague the Southwest of North America may soon become more common due to present trends in global warming and human-caused climate change.
Claire Rubbelke, a doctoral student in Earth and Environmental Sciences at Syracuse University; Scott Knapp, a doctoral student in atmospheric, oceanic, and earth sciences at George Mason University; and Minmin Fu, a doctoral student at Harvard University, are additional writers on the paper.
Three National Science Foundation grants—OCE-1903148, OCE-2103015, and EAR-2018078—were used to finance Bhattacharya’s research.
